Romain Gojon
email: gojon@kth.se

Christophe Bogey
email: christophe.bogey@ec-lyon.fr.

Azimuthal organisation of turbulent structures in underexpanded impinging round jets

The azimuthal organisation of the turbulent structures in underexpanded round jets impinging on a flat plate have been investigated using compressible large eddy simulation. The jet shear-layer region, as well as the region of the wall jets created on the plate after the impact are considered. The jets are characterized by a Nozzle Pressure Ratio of 4.03, a fully expanded Mach number of 1.56, and a Reynolds number of 6 × 10 4 . The distance between the nozzle and the plate varies from 4.16r0 to 9.32r0. The jets generate acoustic tones due to a feedback mechanism. In this paper, the near pressure and density fields of the jets are analysed using Fast Fourier Transform on the nozzle exit plane, the plate, and an azimuthal plane. The amplitude and the phase fields on these sections at the tone frequencies are represented. Similar organisations of the turbulent structures are found in the jet shear layers and the wall jets. Thus, axisymmetric and helical arrangements of the structures in the shear layers lead to concentric and spiral distributions of the structures on the plate, respectively. In particular, for one of the jets, a spiral shape and concentric rings, associated with two tone frequencies generated simultaneously, are observed on the flat plate in the pressure and density phase fields. Moreover, the convection velocity of the turbulent structures on the plate is evaluated from the phase fields. Several diameters away from the jet axis, the velocities found for the present jets compare well with those found experimentally in the phase-averaged distributions of fluctuating pressure for impinging ideally expanded jets using fast-response Pressure-Sensitive Paint. Near the jet axis, given that the present jets are underexpanded, differences are observed, due to the presence of shock cell structures in the jets. Finally, the convection velocity of the turbulent structures on the wall are estimated from cross-correlations of radial velocity. The values obtained compare well with those determined from the phase fields.

I. Introduction

Very intense tones have been observed experimentally by Powell 1 and Wagner 2 , among others, in the acoustic field of high subsonic and supersonic jets impinging on a flat plate. Powell 1 suggested that such tones are generated by an aeroacoustic feedback mechanism between the turbulent structures propagating downstream from the nozzle to the plate and the acoustic waves propagating upstream from the plate to the nozzle.

For subsonic impinging round jets, the tone frequencies can be predicted by the model proposed by Ho & Nosseir [START_REF] Ho | Dynamics of an impinging jet. part 1. The feedback phenomenon[END_REF] and Nosseir & Ho [START_REF] Nosseir | Dynamics of an impinging jet. part 2. The noise generation[END_REF] . Round supersonic jets impinging on a flat plate normally have been investigated experimentally by Henderson & Powell [START_REF] Henderson | Experiments concerning tones produced by an axisymmetric choked jet impinging on flat plates[END_REF] , Krothapalli et al. [START_REF] Krothapalli | Flow field and noise characteristics of a supersonic impinging jet[END_REF] and Henderson et al. [START_REF] Henderson | An experimental study of the oscillatory flow structure of tone-producing supersonic impinging jets[END_REF] In some cases, a feedback mechanism is observed as in subsonic jets. This is very often the case when the jet is ideally expanded, but this happens only for some nozzle-to-plate distances when the jet is imperfectly expanded. Henderson & Powell [START_REF] Henderson | Experiments concerning tones produced by an axisymmetric choked jet impinging on flat plates[END_REF] suggested that, in the latter case, the feedback loop establishes only when a Mach disk forms just upstream from the plate. Recirculation zones were also observed between this Mach disk and the flat plate for some nozzle-to-plate distances by Krothapalli et al. [START_REF] Krothapalli | Flow field and noise characteristics of a supersonic impinging jet[END_REF] . More recently, for underexpanded impinging jets, Risbord & Soria 8 explored the instability modes of the jets using ultra-high-speed Schlieren and shadowgraphy techniques. Axial and helical modes were visualised and the Mach disk located just upstream from the plate was found to oscillate. For similar jets, Buchmann et al. [START_REF] Buchmann | High spatial resolution imaging of a supersonic underexpanded jet impinging on a flat plate[END_REF] pointed out the periodic formation of large-scale structures in the jet shear layers using a high spatial resolution Schlieren imaging. The complete feedback mechanism was visible, namely large-scale structures in the shear layers propagating downstream from the nozzle to the plate and acoustic waves propagating upstream from the plate to the nozzle. Mitchell et al. [START_REF] Mitchell | The visualization of the acoustic feedback loop in impinging underexpanded supersonic jet flows using ultra-high frame rate schlieren[END_REF] studied the periodic oscillation of the shear layer of underexpanded impinging jets using time-resolved Schlieren image sequences. Finally, Davis et al. [START_REF] Davis | Investigation of impinging jet resonant modes using unsteady pressure-sensitive paint measurements[END_REF] studied the wall pressure oscillations in ideally expanded impinging jets using a fast-response Pressure-Sensitive Paint on the plate. They identified tone frequencies associated with axisymmetrical and helical oscillation modes of the jets thanks to phase-conditioned Schlieren images. For such modes, they presented the phase-averaged distributions of the fluctuating pressure on the flat plate. The turbulent structures organized axisymmetrically or helically in the jet shear layers were shown to persist after the impact, as they propagate radially in the wall jets, even several diameters away from the jet axis.

In the present study, the azimuthal organisation of turbulent structures in the jet shear-layers and on the flat plate, in the wall jet created after the impact, are studied from data provided by large eddy simulations. The jets are defined by a Nozzle Pressure Ratio of 4.03, a fully expanded Mach number of 1.56, an exit Mach number of 1, and a Reynolds number of 6 × 10 4 . Their aerodynamics and acoustic properties have been analysed previously in detail in Gojon et al. [START_REF] Gojon | Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle[END_REF] The near pressure and density fields of the jets are explored in the present paper using Fast Fourier Transform. In particular, the spatial organisation and the convection velocity of the turbulent structures located in the wall jet created on the plate after the impact are examined. The results are compared with the experimental phase-averaged distributions of the fluctuating pressure of Davis et al. [START_REF] Davis | Investigation of impinging jet resonant modes using unsteady pressure-sensitive paint measurements[END_REF] for ideally expanded supersonic impinging round jets. For the present underexpanded jets, the presence of shock-cell structures is expected to affect the radial propagation of the structures on the plate. The paper is organised as follows. The numerical methods and the jet properties are presented in section II. An analysis of the turbulent structures in the jet shear-layers and in the wall jets is performed in section III by plotting the amplitude and phase fields of the pressure and density fields at the tone frequencies. Concluding remarks are finally given in section IV.

II. Parameters

II.A. Numerical parameters

In the LES, the unsteady compressible Navier-Stokes equations are solved using cylindrical coordinates (r, θ, z) with an explicit six-stage Runge-Kutta algorithm for time integration, and low-dissipation and low-dispersion explicit eleven-point finite differences for spatial derivation [START_REF] Bogey | A family of low dispersive and low dissipative explicit schemes for flow and noise computations[END_REF][START_REF] Berland | High-order, low dispersive and low dissipative explicit schemes for multiple-scale and boundary problems[END_REF] . At the end of each time step, a high-order filtering is applied to the flow variables in order to remove grid-to-grid oscillations and to dissipate subgridscale turbulent energy [START_REF] Bogey | Large eddy simulations of transitional round jets: influence of the Reynolds number on flow development and energy dissipation[END_REF][START_REF] Bogey | Turbulence and energy budget in a self-preserving round jet: direct evaluation using large eddy simulation[END_REF] . The radiation conditions of Tam & Dong 17 are implemented at the boundaries of the computational domain, in combination with a sponge zone at the outflow boundaries combining grid stretching and Laplacian filtering to damp turbulent fluctuations before they reach the boundaries. Adiabatic conditions are imposed at the nozzle walls and at the flat plate. Finally, a shock-capturing filtering is applied in order to avoid Gibbs oscillations near shocks [START_REF] Bogey | A shock-capturing methodology based on adaptative spatial filtering for high-order non-linear computations[END_REF] . The axis singularity is treated with the method proposed by Mohseni & Colonius [START_REF] Mohseni | Numerical treatment of polar coordinate singularities[END_REF] . A reduction of the effective resolution near the origin of the polar coordinates is also implemented [START_REF] Bogey | Finite differences for coarse azimuthal discretization and for reduction of effective resolution near origin of cylindrical flow equations[END_REF] . Finally, a forcing 21 is added in the boundary layer in the nozzle in order to generate velocity fluctuations at the nozzle exit.

The simulations are carried out using an OpenMP-based in-house solver, and a total of 180, 000 iterations are performed in each case after the transient period. The simulation time is thus equal to 1000r 0 /u j . The cylindrical meshes contain between 171 and 217 million points, as reported in Table 1. They have been designed in order to allow acoustic waves with Strouhal numbers up to St = f D j /u j = 6.4 to be well propagated.

II.B. Jets properties

In this section, the main jet properties are provided. More informations can be found in Gojon et al. [START_REF] Gojon | Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle[END_REF] The jets originate from a pipe nozzle of radius r 0 , whose lip is 0.1r 0 thick. The nozzle-to-plate distances L are respectively equal to 4.16r 0 , 5.6r 0 , 7.3r 0 and 9.32r 0 , as shown in table 2. The jets are thus referred to as JetL4, JetL5, JetL7 and JetL9. They have an ideally expanded Mach number of M j = u j /a j = 1.56, where u j and a j are the exit velocity and the speed of sound in the ideally expanded equivalent jet. Their Reynolds number is Re j = u j D j /ν = 6 × 10

III. Results

III.A. Snapshots

In order to illustrate the flow fields of the present impinging jets, isosurfaces of density obtained for JetL5 and JetL7 are displayed in figure 1. Shock-cell structures can be seen in the two jets between the nozzle exit and the flat plate. The jet mixing layers and the wall jets developing on the flat plate in the radial direction are well visible. Moreover, both large-scale structures and small-scale vortical structures appear in the mixing layers, in agreement with the Reynolds number Re j of 6 × 10 4 . The pressure fields in the plane θ = 0 are also represented in figure 1 in order to show the near acoustic field of the jets. The acoustic waves seem to come mainly from the region of jet impact on the flat plate.

III.B. Tone frequencies

The sound pressure levels obtained in the vicinity of the nozzle at r = 2r 0 and z = 0 are plotted in figure 2 as functions of the Strouhal number St = f D j /u j . They reveal several tone frequencies. The Strouhal numbers of the tones whose levels are 5 dB higher than the broadband noise are reported in table 3. As detailed in Gojon et al. [START_REF] Gojon | Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle[END_REF] , these tones are generated by an aeroacoustic feedback mechanism occurring between the nozzle and the plate, consisting of two steps. First, in the jet shear layers, a turbulent structure is convected in the downstream direction from the nozzle exit to the flat plate. The structure impinges on the plate, and generates an acoustic wave propagating upstream. This wave is reflected back by the nozzle lip, which excites the shear layer, and leads to the formation of a new turbulent structure. The time period of this loop can thus be determined from the convection speed of the shear-layer structures, the speed of the upstream-propagating acoustic waves and the nozzle-to-plate distance. Ho & Nosseir [START_REF] Ho | Dynamics of an impinging jet. part 1. The feedback phenomenon[END_REF] proposed, for instance:

f = L < u c > + L a 0 (1)
where < u c > is the mean convection velocity of the turbulent structures in the shear layers between the nozzle and the plate, a 0 is the speed of sound in the ambient medium, and N is the mode number indicating the number of times the feedback mechanism occurs during the fundamental period. A very good agreement has been found between the tone frequencies of the present simulated jet, those found in the experimental study of Henderson et al. [START_REF] Henderson | An experimental study of the oscillatory flow structure of tone-producing supersonic impinging jets[END_REF] , and the frequencies predicted by the relation (1). More details can be found in Gojon et al. [START_REF] Gojon | Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle[END_REF] .

III.C. Properties of the near pressure and density fields

In the LES, pressure has been recorded every 50 th time step in the planes z = 0, z = L and θ = 0. Moreover, density has also been stored every 50 th time step in the plane z = L. For each plane, the results are arranged in M × N matrices:

P all =      P 1 1 P 2 1 . P N 1 P 1 2 P 2 2 . P N 2 . . . . P 1 M P 2 M . P N M      (2)
where N is the number of samplings, and M is the total number of points in the plane. The pressure or density field obtained at a given time is thus provided by one column of the corresponding matrix. A Fast Fourier Transform is applied to each row of the matrices P all . In this way, for a given frequency, the amplitude fields and the phase fields can be displayed. Informations on the oscillation modes of the jet are given by the phase field, and informations on the sound sources are provided by the amplitude and phase fields.

For JetL4, the properties obtained at the two main tone frequencies at St 1 = 0.375 and St 2 = 0.505 are examined in figures 3 and 4, respectively. For St 1 = 0.375, the amplitude and phase fields of the fluctuating pressure at z = 0 are given in figures 3(a,e). The amplitude field do not exhibit a clear pattern. On the contrary, the phase field shows two opposite regions out of phase near the jet axis followed by a spiral shape of isophase surfaces. This indicates an helical organisation of the acoustic waves. In the plane θ = 0, in the phase field in figure 3(f), a 180-degree phase shift is visible with respect to the jet axis, suggesting a sinuous or helical oscillation mode of the jet. More precisely, using a Fourier decomposition of the fluctuating pressure on 32 sensors regularly spaced in the azimuthal direction at z = 0 and r = 2r 0 , the mode is found to be helical. Therefore, the feedback mechanism is organised helically at the frequency St 1 = 0.375. The amplitude and phase fields of the fluctuating pressure obtained on the plate, at z = 4.16r 0 are reported in figures 3(c,g). The amplitude field exhibits a region of high intensity for r < 2.6r 0 , in the region located inside the jet, under the Mach disk and the annular oblique shock. The phase field, in figure 3(g), shows a spiral shape which extends over the entire domain. At z = 4.16r 0 , the amplitude and phase fields of the fluctuating density on the plate are presented in figures 3(d,h). They exhibit the same behaviour as that of the pressure fluctuations. The turbulent structures organized helically in the jet shear layers at the tone frequency St 1 = 0.375 impinge on the plate and this organisation seems to persist as they propagate radially on the plate.

For St 2 = 0.505, the amplitude and phase fields of the fluctuating pressure and fluctuating density are represented in figure 4. The amplitude and phase fields of the fluctuating pressure obtained on the plane z = 0 are given in figures 4(a,e). The acoustic waves appear to be organized in a axisymmetric manner.

The amplitude field in the plane θ = 0 of figure 4(b) reveals a cell structure between the nozzle and the plate, containing three cells. This structure has been described in Gojon et al. [START_REF] Gojon | Large-eddy simulation of supersonic planar jets impinging on a flat plate at an angle of 60 to 90 degrees[END_REF] . It is due to the generation of an hydrodynamic-acoustic standing wave by the aeroacoustic feedback mechanism. The number of cells correspond to the mode number of the feedback mechanism in the model of Ho & Nosseir 3 . The phase field, in figure 4(b), exhibits a symmetric organisation with respect to the jet axis, corresponding to an axisymmetric oscillation mode of the jet. Thus, the feedback mechanism is organised axisymmetrically at the frequency St 2 = 0.505. In figures 4(g) and 4(h), in the phase fields of the fluctuating pressure and of the fluctuating density on the plate, concentric rings are observed. These rings may be associated with the radial propagation, on the plate, of the coherent structures organised axisymmetrically in the jet shear layers at St 2 = 0.505.

In order to confirm the two hypotheses above, phase profiles are represented in figure 5. The first profile considered is the phase profile obtained in the θ = 0 plane, along the black line visible in the phase field of figure 4(f). It is represented in figure 5(a) as a function of the distance l impact from the point located on the wall at z = 3r 0 . This point was chosen as it corresponds to the location of the source of the acoustic component radiating in the far field, see in Gojon et al. [START_REF] Gojon | Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle[END_REF] The maxima in the phase profile in figure 5(a) are located at l impact = 1.0r 0 , 4.25r 0 and 7.6r 0 , giving wavelengths of 3.25r 0 and 3.35r 0 . The corresponding phase speeds are 327 m.s -1 and 338 m.s -1 , respectively. They correspond to the ambient sound speed, as expected for acoustic waves. In figure 5(b), the phase profile obtained in the z = L plane along the black line shown in the phase field of figure 4(g) is plotted as a function of the radial coordinate. The wavelength of the concentric rings visible in figures 4(g,h) can thus be measured. The wavelength between the first and the second maxima is 1.75r 0 and the wavelength between the second and the third maxima is 2.15r 0 . The corresponding phase speeds are equal to 0.40u j and 0.49u j , respectively. Given those values, the oscillations obtained at the wall cannot be acoustic waves, but are associated with the radial convection of turbulent structures in the wall jet. For JetL5, the amplitude and phase fields obtained for the two main tone frequencies at St 1 = 0.335 and St 2 = 0.415 are showed in figures 6 and 7, respectively. Given the pattern of the phase fields at z = 0 and θ = 0 in figures 6(e,f) and 7(e,f), these two tones appear to be associated with helical oscillation modes. Moreover, in the phase fields of figures 6(g,h) calculated on the plate for St 1 = 0.335 and in figures 7(g,h) for St 1 = 0.415, spiral shapes are visible over the entire domain. As for the tone frequency St 1 = 0.375 of JetL4, these shapes are due to the radial propagation on the plate of the turbulent structures organized helically in the jet shear layers.

For JetL7, the amplitude and phase fields obtained for the main tone frequency at St 1 = 0.345 are given in figure 8. It can be seen from the phase fields in the planes z = 0 and θ = 0 in figures 8(e,f) that this tone is linked to an helical jet oscillation mode. The results on the plane z = L show spurious noise because the amplitude of the tone is low compared to the level of tones reported for JetL4 and JetL5 in figure 2. Nevertheless, a spiral shape can be seen in the two phase fields in figures 8(g,h), especially in the phase field of the density field. This shape results from the radial propagation on the plate of the turbulent structures

Figure 1 .Figure 2 .

 12 Figure 1. Isosurfaces of density for (a) JetL5 and (b) JetL7. The blue and red isosurfaces are obtained for the values of 0.8 and 2.5 kg.m -3 , respectively. The isosurfaces of 1.25 kg.m -3 are also represented, colored by the Mach number. The pressure field in the plane θ = 0 is shown. The nozzle and the plate are in grey.

N

Figure 5 .

 5 Figure 5. Profiles obtained for JetL4 at St2 = 0.505, (a) in the plane θ = 0 along the black line represented in the phase field of figure 4(f ), and (b) in the plane z = L along the black line represented in the phase field of figure 4(g).

Figure 6 .

 6 Figure 6. Amplitude (top) and phase (bottom) fields obtained for JetL5 at the tone frequency St1 = 0.335; from pressure (a,e) at z = 0, (b,f ) at θ = 0 and (c,g) at z = L; from density (d,h) at z = L.

Figure 7 .

 7 Figure 7. Amplitude (top) and phase (bottom) fields obtained for JetL5 at the dominant tone frequency St2 = 0.415; from pressure (a,e) at z = 0, (b,f ) at θ = 0 and (c,g) at z = L; from density (d,h) at z = L.

Table 1 .

 1 Mesh parameters: number of points nr, nθ and nz in the radial, azimuthal and axial direction, and total number of points.

	n r	n θ	n z number of points
	JetL4 500 512 668	171 × 10 6
	JetL5 500 512 764	195 × 10 6
	JetL7 500 512 780	200 × 10 6
	JetL9 500 512 847	217 × 10 6

Table 2 .

 2 [START_REF] Nosseir | Dynamics of an impinging jet. part 2. The noise generation[END_REF] , where D j is the nozzle diameter of the ideally expanded equivalent jet and ν is the kinematic molecular viscosity. The exit Mach number is M e = u e /a e = 1, where u e and a e are the exit velocity and the speed of sound in the jet. At the nozzle exit, a Blasius mean velocity profile is imposed with a boundary-layer thickness of 0.15r 0 . The ejection conditions of the jets and the nozzle-to-plate distances are similar to those in the experimental study of Henderson et al.[START_REF] Henderson | An experimental study of the oscillatory flow structure of tone-producing supersonic impinging jets[END_REF] × 10 4 4.16r 0 JetL5 1.56 6 × 10 4 5.6r 0 JetL7 1.56 6 × 10 4 7.3r 0 JetL9 1.56 6 × 10 4 9.32r 0 Jet parameters: ideally expanded Mach number Mj , Reynolds number Rej , and nozzle-to-plate distance L.

	M j	Re j	L
	JetL4 1.56 6

Table 3 .

 3 Strouhal numbers emerging in the pressure spectra of figure2. The Strouhal numbers of the dominant tones are in bold.

		St 1	St 2	St 3
	JetL4 0.375 0.505 1.01
	JetL5 0.335 0.415	-
	JetL7 0.345	0.42	-
	JetL9	0.27	0.34 0.42

Figure 4. Amplitude (top) and phase (bottom) fields obtained for JetL4 at the dominant tone frequency St2 = 0.505; from pressure (a,e) at z = 0, (b,f ) at θ = 0 and (c,g) at z = L; from density (d,h) at z = L.

Acknowledgments

This work was performed using HPC resources of P2CHPD (Pôle de Calcul Hautes Performances Dédiés) and IDRIS (Institut du Développement et des Ressources en Informatique Scientifique) under the allocation 2015-2a0204 made by GENCI (Grand Equipement National de Calcul Intensif). This work was performed within the framework of the Labex CeLyA of Université de Lyon, within the program "Investissements d'Avenir" (ANR-10-LABX-0060/ ANR-11-IDEX-0007) operated by the French National Research Agency (ANR).

organized helically in the jet shear layers. For JetL9, the amplitude and phase fields for the main tone frequency at St 2 = 0.34 are represented in figure 9. Given the phase fields in the planes z = 0 and θ = 0 in figures 9(e,f), the tone appears to be associated with an helical oscillation mode of the jet. On the plane z = L, spiral shapes are visible in the phase fields, in figures 9(g,h). This indicates a radial propagation on the plate of the turbulent structures organized helically in the jet shear layers.

In order to give further insight into the convection velocity of the structures organised helically in the wall jets for all the jets, the density phase fields on the plate are investigated in the same way as for JetL4 in figure 5. First, let us consider two regions of interest. The first on is near the jet axis over 0 < r < 3r 0 , and the second one is several diameter away over 3r 0 < r < 6r 0 . The mean wavelengths are extracted from the phase fields in each of the two regions. The results for the tone frequencies of JetL4, JetL5 and JetL9 are given in table 4. However, the phase fields of the density fields for the dominant tone frequency of JetL7 at St 1 = 0.345 and for the tone frequency of JetL5 at St 1 = 0.335 are too noisy, and do not provide converged results. The results found in figure 5 for the dominant tone frequencies of JetL4 at St 2 = 0.505 are added.

Jet

St u c for 0 < r < 3r 0 u c for 3r 0 < r < 6r 0 JetL4 St 1 = 0.375

. Mean convection velocities of the turbulent structures in the wall jets for different main tone frequencies.

Over 3r 0 < r < 6r 0 , the convection velocity of the turbulent structures on the wall varies between 0.40u j and 0.49u j for the present jets. Those results are in agreement with the experimental measurements of Davis et al. [START_REF] Davis | Investigation of impinging jet resonant modes using unsteady pressure-sensitive paint measurements[END_REF] for impinging supersonic ideally expanded round jets. In the phase averaged distributions of the fluctuating pressure, these authors found a convection velocity of the turbulent structures on the wall equals to 0.47u j several diameters away from the jet axis by using Pressure-Sensitive Paint on the flat plate. The results are also consistent with the mean convection velocity found in Gojon et al. [START_REF] Gojon | Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle[END_REF] for the turbulent structures in the jet shear layer. Indeed, the velocity < u c > was found to be between 0.54u j and 0.59u j for the present jets. Over 0 < r < 3r 0 , the convection velocity of the turbulent structures on the wall varies between 0.27u j and 0.40u j in the four jets. These values are lower than the value 0.56u j found experimentally by Davis et al. 11 in this region of the plate. This difference can be due to the fact that the present jets are non-ideally expanded, leading to the formation of a Mach disk or to the presence of oblique shocks upstream from the plate, see in Gojon et al. [START_REF] Gojon | Large-eddy simulation of underexpanded round jets impinging on a flat plate 4 to 9 radii downstream from the nozzle[END_REF] Thus, the jet shear layers are deviated and impinge on the plate at r ≃ 2r 0 . Consequently, the mean convection velocities computed near the jet axis do not correspond to the convection velocity of the turbulent structures, as the motion of these structures is not only radial in this region. On the contrary, in an ideally expanded jet, the turbulent structures in the jet shear layers impinge near the jet axis as observed by Krothapalli et al. [START_REF] Krothapalli | Flow field and noise characteristics of a supersonic impinging jet[END_REF] and Davis et al. [START_REF] Davis | Investigation of impinging jet resonant modes using unsteady pressure-sensitive paint measurements[END_REF] Finally, for JetL4, in the region 3r 0 < r < 6r 0 , the convection velocity of the turbulent structures organised axisymmetrically at the tone frequency St 2 = 0.505 is 18% higher than the the convection velocity of the turbulent structures organised helically at St 1 = 0.375.

The convection velocity in the wall jets is now computed from radial velocity cross-correlations at one mesh upstream from the plate. The results are represented in figure 10(a) as a function of the radial position between r = 0 and r = 6r 0 . The maximal mean radial velocity in the wall jets is also provided in figure 10(b). In figure 10(a), the mean convection velocity for r ≥ 3r 0 is of about 0.45u j in the four jets. This result is in good agreement with the convection velocities found from the phase fields for the structures organized axisymmetrically or helically, which are given in table 4. In the region r ≤ 3r 0 , for JetL4, JetL5 and JetL7, a region of negative convection velocity appears around r = r 0 . In this region, the turbulent structures travel toward the jet axis. This is due to the presence of a recirculation zone near the region of impact, observed experimentally for similar jets by Krothapalli et al. [START_REF] Krothapalli | Flow field and noise characteristics of a supersonic impinging jet[END_REF] This recirculation zone is visible in figure 10(b) with a negative mean radial velocity for r < r 0 . It is responsible for the difference in the region 0 < r < 3r 0 between the convection velocities found from the phase fields of the present underexpanded jets and those measured in the phase averaged distributions of the fluctuating pressure of the ideally expanded jets of Davis et al.

IV. Conclusion

In this paper, the azimuthal organisation of the turbulent structures in underexpanded impinging round jets has been studied using compressible large eddy simulation. This organisation is studied in the jet shearlayers but also on the flat plate, in the wall jet created after the impact. The jets are characterized by a Nozzle Pressure Ratio of 4.03, a fully expanded Mach number of 1.56, and a Reynolds number of 6 × 10 4 . The distance between the nozzle and the plate varies from 4.16r 0 to 9.32r 0 . Shock cell structures and acoustic waves coming from the region of jet impact on the flat plate are observed in the flow snapshots. The spectra in the near pressure fields revealed several tone frequencies due to a feedback mechanism occurring between the nozzle lips and the flat plate. The near pressure and density fields of the jets are then analysed using Fast Fourier Transform on the nozzle exit plane, the plate plane and an azimuthal plane. It has been found found that the helical or axisymmetric organisation of the turbulent structures in the jet shear layers, specific to each tone frequency, persists after the jet impact on the plate. The radial propagation of these structures in the wall jets leads to a spiral shape or to concentric rings in the phase fields, respectively. In particular, for one of the jets, a spiral shape and concentric rings associated with two tone frequencies generated simultaneously have been observed. Finally, the convection velocity of the turbulent structures in the wall jets was evaluated from the phase fields and cross-correlations of radial velocity. Over 3r 0 < r < 6r 0 , the convection velocity of the structures on the wall varies between 0.40u j and 0.49u j in the present jets. These results are in agreement with measurements for ideally expanded impinging supersonic round jets performed using Pressure-Sensitive Paint on the flat plate. Near the jet axis, differences are observed. They are due to the fact that the present jets are non-ideally expanded, leading to the formation of a Mach disk or to the presence of oblique shocks upstream from the plate.


Pauline Lefebvre

Serge X Cohen

Cécile Callou

Chloé Martin

Frédérique Chlous

Archaeology research databases: how are the user communities articulated?

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Archeology research databases: how are the user communities articulated ?

Pauline Lefebvre 1,3 , Serge Cohen 2 , Cécile Callou 3,4 , Chloé Martin 3 , Frédérique Chlous 1

1 UMR 208 PALOC (CNRS/IRD/MNHN), 2 USR 3461 IPANEMA (CNRS/ Ministère de la culture/ UVSQ), 3 UMS 3468 BBEES (CNRS/MNHN), 4 UMR 7209 AASPE (CNRS/MNHN) Contact : pauline.lefebvre@edu.mnhn.fr

Context and objectives

As open science grows, the management of research data is not standardized. The archaeological community tends to take over more and more the relational databases and is now collecting natively digital data. Using the methods of socio-anthropology, the purpose of this study is to understand the obstacles to the sharing of scientific information in the use of databases among researchers and with archeologists. For this purpose, the structuring of research communities is analyzed at different levels, individual, collaborative and institutional, in terms of disciplines and professionals, designer and user. These interactions generate friction (Vinck, 2009), translated into negotiations, thus allowing the production of new knowledge.

Research questions

How does the database object articulate the different communities? What are the nature and the consequences of the frictions thus created?

How the representions around the object are in conflict? About what and how are the actors negociating? Is the production of knowledge thanks to the database effective?

Acknowledgments DIM BBEES

Bibliography

Discussion and conclusion

There is no simple reproducibility when creating a search database. It is therefore difficult to predict its acceptability. Financing remains, however, the crucial point for ensuring the sustainability and development of a database. The design process is still not yet recognized fully as an scientific activity.

Fields and Methods

Structuring of communities on the chaîne opératoire of databases

Results

At this point, we found that interdisciplinary collaborations were less important after the creation of the database. If the database delimits the action of its users, the sharing of data depends on individual wishes. We observed that the dialogue between engineers and researchers is considered a key element and that the transmission of skills is partial. The ethnographic study focuses on the design of four database projects on ancient materials (Fig. 2).

Interviews and observations are conducted in the different communities, and grapho-numeric objects (Fig. 3), understood as intermediate objects, are also collected.

  Fig. 3: A screenshot of a database interface, printed and corrected

Fig. 2 :

 2 Fig. 2: Schéma logique de la base de données « Kharman»


MD Hamed Bourenane

MD Vincent Galand

MD Dominique Boulmier

MD, PhD Christophe Leclercq

MD Guillaume Leurent

MD Marc Bedossa

MD Nathalie Behar

MD Camille Pichard

MD Baptiste Polin

MD Jacques Tomasi

MD, PhD Jean-Philippe Verhoye

MSc Sam Sharobeem

MSc Marielle Le Guellec

MD, PhD Erwan Donal

MD Dominique Pavin

MD, PhD Raphael P Martins

MD Hervé Le Breton

MD, PhD Vincent Auffret
email: vincent.auffret@chu-rennes.fr

Electrophysiological Study-Guided Permanent Pacemaker Implantation In Patients With Conduction Disturbances Following Transcatheter Aortic Valve Implantation

Keywords: Transcatheter aortic valve replacement; Electrophysiological study, bundle branch block, atrioventricular block, pacemaker

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

A series of large prospective randomized trials established the superiority of transcatheter aortic valve implantation (TAVI) over surgical aortic valve replacement in treating patients with severe aortic stenosis with intermediate to high surgical risk [START_REF] Siontis | Transcatheter aortic valve implantation vs. surgical aortic valve replacement for treatment of severe aortic stenosis: a meta-analysis of randomized trials[END_REF] and more recently its shortterm non-inferiority among low surgical risk patients [START_REF] Mack | Transcatheter Aortic-Valve Replacement with a Balloon-Expandable Valve in Low-Risk Patients[END_REF][START_REF] Popma | Transcatheter Aortic-Valve Replacement with a Self-Expanding Valve in Low-Risk Patients[END_REF] . Unlike other periprocedural complications, conduction disturbances (i.e. mainly high-degree atrioventricular block

[HAVB] requiring permanent pacemaker [PPM] implantation and new-onset left bundlebranch block [LBBB]) have remained stable with recent reports even suggesting an increased risk associated with the newer-generation transcatheter heart valves [START_REF] Faroux | Clinical impact of conduction disturbances in transcatheter aortic valve replacement recipients: a systematic review and meta-analysis[END_REF][START_REF] Vincent | Conduction Disturbances After Transcatheter Aortic Valve Replacement[END_REF] . While HAVB represents an unequivocal PPM indication in international guidelines [START_REF] Brignole | ESC Guidelines on cardiac pacing and cardiac resynchronization therapy: the Task Force on cardiac pacing and resynchronization therapy of the European Society of Cardiology (ESC). Developed in collaboration with the European Heart Rhythm Association (EHRA)[END_REF] , the management of milder conduction disturbances such as worsening or new-onset first-degree atrioventricular block (FDAVB) or LBBB remains controversial. Experts have suggested the use of electrophysiological studies (EPS) may help stratify patients with post-TAVI new-onset or worsening conduction disturbances who would require PPM implantation [START_REF] Vincent | Conduction Disturbances After Transcatheter Aortic Valve Replacement[END_REF][START_REF] Rodés-Cabau | Management of Conduction Disturbances Associated With Transcatheter Aortic Valve Replacement: JACC Scientific Expert Panel[END_REF] . However, this suggestion relies on small retrospective studies, and, considering the evolving nature of post-TAVI conduction disturbances, the overall mid-term diagnostic performances of an inhospital EPS-guided PPM implantation strategy remain unclear. The objective of this study was to evaluate the safety and diagnostic performances of such a strategy.

METHODS

The full version of the methods can be find in the supplemental appendix. A retrospective study of patients who underwent TAVI at a single center was performed.

Patients with post-TAVI persistent new-onset or worsening conduction disturbances not representing absolute PPM indications who underwent EPS during the index hospitalization were included in the present study. All patients gave written informed consent for the procedures and anonymous collection of their data, which were prospectively gathered in an electronic database as part of the FRANCE TAVI registry [START_REF] Auffret | Temporal Trends in Transcatheter Aortic Valve Replacement in France: FRANCE 2 to FRANCE TAVI[END_REF] . Details regarding the preprocedural workup were previously described [START_REF] Auffret | Predictors of 6-month poor clinical outcomes after transcatheter aortic valve implantation[END_REF] . Post-procedural care consisted of heart rhythm monitoring for at least 48 hours after intervention, laboratory tests and a 12-lead electrocardiogram (ECG) on a daily basis. Clinical outcomes were defined according to Valve Academic Research Consortium 2 criteria [START_REF] Kappetein | Updated standardized endpoint definitions for transcatheter aortic valve implantation: the Valve Academic Research Consortium-2 consensus document[END_REF] .

The surface ECGs before the TAVI procedure and at the time of the EPS, were employed to record rhythm, heart rate, PR interval, and QRS duration. Left anterior and posterior fascicular hemiblocks, LBBB, and right bundle branch block (RBBB) were defined according to the American Heart Association, American College of Cardiology Foundation, and Heart Rhythm Society recommendations for standardization and interpretation of the electrocardiogram [START_REF] Brignole | ESC Guidelines on cardiac pacing and cardiac resynchronization therapy: the Task Force on cardiac pacing and resynchronization therapy of the European Society of Cardiology (ESC). Developed in collaboration with the European Heart Rhythm Association (EHRA)[END_REF] . HAVB was defined as type 2 second degree AVB or third degree AVB.

Patients with persistent HAVB post-TAVI underwent PPM implantation in accordance with current European guidelines [START_REF] Brignole | ESC Guidelines on cardiac pacing and cardiac resynchronization therapy: the Task Force on cardiac pacing and resynchronization therapy of the European Society of Cardiology (ESC). Developed in collaboration with the European Heart Rhythm Association (EHRA)[END_REF] . For patients with milder new-onset or worsening conduction disturbances persisting without improvement upon serial ECG at least 24h after the procedure, a local EPS-guided PPM implantation strategy was implemented in our center in June 2017. A senior electrophysiologist performed the EPS in the local electrophysiology laboratory, and all measurements were carried out on a General Electric electrophysiology workstation. PPM implantation was recommended for an infra-hisian significant conduction delay defined as HV interval > 70ms, or second-or third-degree His-Purkinje block demonstrated during incremental atrial pacing with a rate ≤ 100 bpm. Final decision for PPM was at the discretion of the electrophysiologist taking into account the EPS findings but also the in-hospital ECG evolution and clinical data.

Follow-up was performed by on-site clinical visit and/or telephone at 1-month post-TAVI, at 1 year and yearly thereafter. Records from referring cardiologists, general practitioners, and other hospitals were retrieved for further information. Ventricular pacing (VP) percentage was recorded among patients who underwent PPM implantation.

The primary objective of the study was to evaluate the diagnostic performances (sensitivity, specificity, positive and negative predictive value) of the local EPS-guided PPM implantation strategy. False negative outcomes were defined as patients discharged without PPM implantation who required PPM for HAVB within 6 months of the initial EPS procedure. False positive outcomes were defined as patients discharged with a PPM with a ventricular pacing (VP) percentage < 1% at follow-up. Patients who received a pacemaker due to mechanically-induced HAVB during EPS (bump of the right bundle branch in patients with an LBBB) were not included in the calculation of these diagnostic performances as they do not represent patients among whom conduction disturbances were solely TAVI-related.

Continuous variables were presented as mean ± standard deviation or median (interquartile range) depending on their distribution, which was assessed using the Kolmogorov-Smirnov test, and were compared using t-tests or the Mann-Whitney U test as appropriate. Categorical variables were summarized as numbers (percentages), and compared using chi-square tests or the Fisher exact test. A multivariable backward stepwise logistic regression was performed to identify prolonged HV predictors. Variables with a p-value < 0.1 in univariable analysis were included in the model. Results were reported as adjusted oddsratio (OR) with the corresponding 95% confidence interval (CI). All tests were 2-sided at the 0.05 significance level. Statistical analyses were conducted using the Statistical Package for Social Sciences version 25 (SPSS Inc., IBM, Armonk, New York).

RESULTS

Figure 1 provides the flow-chart of the study population. Baseline characteristics of the population are detailed in Table 1. There was no statistically significant difference between the PPM and no PPM groups except for a higher body mass index and a more frequent history of pulmonary edema in the former group. Table 2 summarizes ECG findings before and after TAVI. There was no significant difference in procedural characteristics and in-hospital complications between the two groups (Table 3).

Post-TAVI ECG changes leading to the performance of the EPS are detailed in Figure 2. EPS was performed at a median of 3 days (2-5) following TAVI. There was no difference in EPS indication or timing between the PPM and no PPM groups (p= 0.62 and 0.57, respectively). Median HV interval was 65ms (56-79) in the total population, 80ms (73-92) in the PPM group, and 60 ms (50-65) in the no PPM group (p<0.001). Thirty patients had a HV interval > 70ms. Six mechanically induced AVB by direct injury to the right bundle branch in patients with LBBB occurred and led to PPM implantation. No other complication of the EPS occurred. Multivariable predictors of a prolonged HV interval (>70ms) were the pre-TAVI PR interval (OR = 1.02, 95%CI: 1.003-1.03; p=0.02), and the post-TAVI aortic valve area (OR = 0.09, 95%CI: 0.02-0.49; p=0.005).

PPM implantation was deemed necessary by the operator and performed in 35 patients following EPS. Among them, HV interval > 70 ms was the sole indication for PPM for 26 patients. Mechanical HAVB during EPS led to PPM implantation in 6 patients, among whom 3 had a HV interval ≤ 70 ms. In one of these patients, right bundle branch block injury precluded any HV interval measurement and required prompt PPM implantation. PPM implantation was decided by the operator despite HV intervals ≤ 70 ms in 3 cases: one for a long antegrade effective refractory period of the atrioventricular node (700ms), one for a 160ms RBBB with supra-hisian AV block, and one for junctional rhythm and sinus node dysfunction associated with a 160ms LBBB and a HV interval of 70ms. On the contrary, 2 patients with a HV interval > 70ms did not receive PPM: one with a HV interval of 78ms who demonstrated a complete resolution of his conduction disturbances after the EPS, and one with a borderline HV of 71ms and a supra-hisian block with an antegrade effective refractory period of the atrioventricular node of 470ms. PPM brand were Microport CRM-Sorin in 12 patients (33%), St-Jude Medical in 9 patients (25%), Medtronic in 8 patients (22%), Biotronik in 4 patients (11%), and Boston scientific in 3 patients (8%). Programmed modes were VVI/VVIR in 17 patients (47%), DDD/DDDR in 13 (36%), and DDD-ADIR, AAI-DDD, or AAI-SafeR in 6 (17%). Among patients with a VVI/VVIR pacemaker, the basal pacing rate or hysteresis was ≤ 45 bpm in 7, ≤ 55 bpm in 7 and 60/min in the remaining 3 patients.

The median follow-up was 147 (71-465) days. There was no significant difference between the two groups regarding all-cause death, heart failure hospitalizations or new-onset AF. No SCD occurred (Table 3). Among patients without PPM at discharge, one patient reported episodes of faintness, which were not associated with high-degree conduction disturbances. Two patients received a PPM. The first received CRT at a follow-up of 386 days for heart failure associated with reduced left ventricular ejection fraction and a newonset persistent LBBB of 180ms (160ms at discharge), the second received CRT for faintness, type 1 second-degree AVB, LBBB and reduced left ventricular ejection fraction at a followup of 617 days. Among the PPM group, one lead fracture requiring PPM extraction without reimplantation in a patient with a VP percentage <1% was reported. One upgrading to CRT was required 527 days after PPM implantation for heart failure and left ventricular dysfunction in a patient with a VP percentage of 90%.

Figure 3 illustrates the repartition of VP percentage among PPM patients at a median follow-up of 137 days (68-406 days). VP percentage was retrieved for 33 out of 35 PPM patients (94.3%). One patient died from a non-cardiovascular cause before the first device assessment. For the second patient, we were unable to obtain data regarding VP percentage despite completing the clinical follow-up. After exclusion of the 6 patients implanted for a mechanically-induced AVB, the diagnostic performances of the EPS-guided PPM implantation strategy were evaluated among 27 PPM recipients and 43 of their no-PPM counterparts. The rate of false negative outcomes was 0%. False positive outcomes occurred in 5 (18.5%) patients without differences between patients with a VVI/VVIR pacemaker and patients with other modes (3 vs. 2 patients, p=0.20). The underlying rhythm pathologies requiring pacing in these subjects are summarized in supplemental table 1. The sensitivity, specificity, positive and negative predictive values of the EPS-guided PPM implantation strategy were 100%, 89.6%, 81.5%, and 100%, respectively. Of note, among the 6 PPM patients with a mechanical AVB, 3 had a VP percentage between 1 and 25% while 3 had a VP percentage > 50%.

DISCUSSION

The natural history of post-TAVI conduction disturbances and their most appropriate management remain unclear due to the scarcity of prospective studies addressing this issue.

The multicenter prospective LBBB-TAVI study focusing on post-TAVI new-onset LBBB will hopefully shed some light on this area [START_REF] Massoullié | Prognosis assessment of persistent left bundle branch block after TAVI by an electrophysiological and remote monitoring risk-adapted algorithm: rationale and design of the multicentre LBBB-TAVI Study[END_REF] . The Ambulatory Electrocardiographic Monitoring for the Detection of High-Degree Atrio-Ventricular Block in Patients With Newonset PeRsistent LEft Bundle Branch Block After Transcatheter Aortic Valve Implantation (MARE) study elegantly provided insights into the natural history of TAVI-induced newonset LBBB by implanting a cardiac monitor in 103 TAVI recipients [START_REF] Rodés-Cabau | Arrhythmic Burden as Determined by Ambulatory Continuous Cardiac Monitoring in Patients With New-Onset Persistent Left Bundle Branch Block Following Transcatheter Aortic Valve Replacement: The MARE Study[END_REF] . PPM implantation was required for HAVB or severe bradycardia in 10% of these patients at a mean delay of 42 days following TAVI. Therefore, allowing such patients to be discharged without PPM implantation based solely on routine ECG and continuous rhythm monitoring data may subject a significant proportion of them to the risk of a life-threatening bradyarrhythmia. On the other hand, a recent meta-analysis suggested an increased risk of long-term mortality and heart failure hospitalization associated with PPM implantation post-TAVI [START_REF] Faroux | Clinical impact of conduction disturbances in transcatheter aortic valve replacement recipients: a systematic review and meta-analysis[END_REF] . EPS appears as a potential candidate to help select patients who will benefit from PPM implantation or conversely who can be safely discharged without PPM implantation. An expert panel recently suggested that EPS may play a significant role in selecting PPM implantation candidates in two specific groups of patients: patients with ECG changes (increase in PR or QRS duration ≥20 ms) in the presence of pre-TAVI conduction disturbances, and patients with new-onset LBBB 7 .

To date, studies assessing the role of EPS in the post-TAVI setting have been scarce and each included less than 100 patients [START_REF] Tovia-Brodie | Use of Electrophysiological Studies in Transcatheter Aortic Valve Implantation[END_REF] . In four studies assessing the value of a post-TAVI EPS in predicting PPM implantation, with a HV interval cut-off ranging from 65 to 100ms, and reported a long-term follow-up and the need for PPM, no SCD occurred and no patients needed a PPM during at least 1 year of follow-up [START_REF] Tovia-Brodie | Use of Electrophysiological Studies in Transcatheter Aortic Valve Implantation[END_REF] . Only two prior studies, including 121 patients undergoing EPS, truly reported the results of predefined EPS-guided strategies [START_REF] Tovia-Brodie | Use of Electrophysiological Studies in Transcatheter Aortic Valve Implantation[END_REF] . Tovia-Brodie et al. presented the results of 81 patients with conduction abnormalities after TAVI, showing an improved event-free survival (mortality or PPM implantation) among the 26 EPS-assessed patients with a 75ms HV interval cut-off prompting PPM implantation (9% vs. 0%, p = 0.04) [START_REF] Tovia-Brodie | The value of electrophysiologic study in decision-making regarding the need for pacemaker implantation after TAVI[END_REF] . In another study including 95 patients who developed conduction disturbances without an absolute indication for PPM, Rogers et al. showed the safety of their EPS-guided strategy regarding the risk of PPM implantation without significant mortality difference at 1-year of follow-up between patients who received a PPM and those who did not based on the EPS findings [START_REF] Rogers | Utility of Invasive Electrophysiology Studies in Patients With Severe Aortic Stenosis Undergoing Transcatheter Aortic Valve Implantation[END_REF] . However, the HV interval threshold prompting PPM implantation was 100ms with a pharmacological test involving IV procainamide, which is not available worldwide, limiting the reproducibility of the study. In addition to this two previous studies, the present analysis provides valuable data regarding the mid-term diagnostic performances of an EPS-guided strategy by evaluating the ability of a post-TAVI EPS to predict PPM dependency at follow-up. We demonstrated an acceptable 18.5% rate of false positive patients, with VP percentage <1% at follow-up, using a single, widely accepted, 70ms HV interval cut-off.

A theoretical limitation of the use of EPS in the post-TAVI conduction disturbances setting is the "screenshot" characteristic of EPS, which captures AV conduction properties at a specific time. Indeed, it is well described that post-TAVI conduction disturbances may worsen or improve over time [START_REF] Vincent | Conduction Disturbances After Transcatheter Aortic Valve Replacement[END_REF] . Therefore, the prognostic value of an EPS performed during the index hospitalization regarding the prediction of late HAVB or PPM dependency remains largely elusive. To circumvent this limitation, experts recommend that EPS is realized at least 24h after TAVI and after stabilization of ECG findings. Using a strategy essentially aligned with this recommendation, with a HV interval cut-off of 70ms prompting PPM implantation, which falls in the range proposed by Tovia-Brodie et al [START_REF] Tovia-Brodie | Use of Electrophysiological Studies in Transcatheter Aortic Valve Implantation[END_REF] , we demonstrated a reassuring 100% NPV regarding PPM requirement within the first 6 months of follow-up among patients with a negative EPS.

Importantly, the EPS-guided PPM implantation strategy was associated with a significant number of HAVB induced by a direct injury to the right bundle branch as 7.6% of patients were implanted because of a mechanical HAVB. None of these patients were considered PPM independent at follow-up. Nonetheless, this observation stresses the need to reduce EPS-related implantation to further increase the safety of EPS-guided PPM implantation strategies.

In the three EPS-guided PPM implantation studies, which assessed PPM dependency post-TAVI, one used a dependency definition of VP >50% while the others defined PPM dependency as underlying permanent AV block [START_REF] Makki | Conduction disturbances after TAVR: Electrophysiological studies and pacemaker dependency[END_REF][START_REF] Badenco | Respective role of surface electrocardiogram and His bundle recordings to assess the risk of atrioventricular block after transcatheter aortic valve replacement[END_REF][START_REF] Kostopoulou | Permanent pacing after transcatheter aortic valve implantation of a CoreValve prosthesis as determined by electrocardiographic and electrophysiological predictors: a single-centre experience[END_REF] . In the present study, we aimed at "penalizing" the negative predictive value estimation to increase the confidence regarding the long-term safety of our strategy. Therefore, we chose a conservative definition of VP>1% for PM dependency, since even rare paroxysmal HAVB, which would translate in a low VP percentage, may cause SCD or serious bradyarrhythmia-related events.

Several limitations of the present analysis should be considered. First, this was a single-center study of limited size, which may limit the generalizability of our results. Second, most patients received a balloon-expandable valve, the magnitude and timing of effect of which upon the conduction system may differ from other transcatheter heart valve designs.

However, in a recent study including 2804 patients, 82% and 94% of conduction disturbances occurred within 48h and 4 days of TAVI, respectively, without difference between valve type [START_REF] Kooistra | Late onset of new conduction disturbances requiring permanent pacemaker implantation following TAVI[END_REF] . The MARE study demonstrated no difference in the timing of bradyarrhythmic events post-TAVI according to valve type, and self-expandable valves were not a significant predictor of delayed HAVB in a study by Toggweiler et al. including 1064 TAVI recipients [START_REF] Toggweiler | The Electrocardiogram After Transcatheter Aortic Valve Replacement Determines the Risk for Post-Procedural High-Degree AV Block and the Need for Telemetry Monitoring[END_REF] .

On the contrary, a small study encompassing 133 patients demonstrated a delayed occurrence of PR prolongation among self-expandable valve recipients compared with their balloonexpandable counterparts [START_REF] Coeman | Different dynamics of new-onset electrocardiographic changes after balloon-and self-expandable transcatheter aortic valve replacement: Implications for prolonged heart rhythm monitoring[END_REF] . However, PR prolongation occurred within 48h of the procedure after self-expandable valves' implantation. Thus, it is unlikely that the indications and diagnostic performances of our strategy would be vastly different in a population including a greater proportion of self-expandable valves given the 3 days median delay between TAVI and EPS in the present study. Finally, the choice of pacemaker brand, programmed modes and frequencies were not protocoled but left to the operator discretion. Thus, although programs preserving spontaneous conduction were systematically preferred, the interpretation of pacing data from device interrogations may be challenging.

FIGURES LEGENDS

Figure 1 -

 1 Figure 1-Flow chart of the study population

Figure 2 -

 2 Figure 2 -Indications for the electrophysiological study

Figure 3 -

 3 Figure 3 -Ventricular pacing percentage at follow-up

lecture fees from Edwards Lifesciences. No extra-mural funding supported this work

CRediT Author statement

Declaration of interests

☐ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒ The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Dr Vincent Auffret received lecture fees from Edwards Lifescience and Medtronic. Pr Hervé Le Breton received lecture fees from Edwards Lifesciences. The other authors have nothing to disclose. Values are median (interquartile range). n (%). or mean ± standard deviation AF = atrial fibrillation ; CAVB = complete atrioventricular block ; PPM = Permanent pacemaker. † All vascular complications were minor.


Gonadotropin-releasing hormone (GnRH) neurons are hypothalamic neurons that represent the final output cells of the neural network regulating fertility in all mammalian species [START_REF] Herbison | Physiology of the adult gonadotropin-releasing hormone neuronal network[END_REF][START_REF] Boehm | Expert consensus document: European Consensus Statement on congenital hypogonadotropic hypogonadism--pathogenesis, diagnosis and treatment[END_REF]. GnRH neurons migrate from the nose to the brain during early embryogenesis and remain scattered throughout the basal forebrain [START_REF] Casoni | Development of the neurons controlling fertility in humans: new insights from 3D imaging and transparent fetal brains[END_REF]. Most of them send long projections (over 1mm) to the median eminence (ME) at the base of the brain, from where they release GnRH into the pituitary portal system in a pulsatile fashion. One interesting feature of GnRH projections in mice is that they can both receive synaptic inputs and initiate action potentials, which they propagate to the synaptic terminal in the ME. Since they share the properties of both dendrites and axons, mouse GnRH projections have been termed "dendrons" [START_REF] Herbison | Physiology of the adult gonadotropin-releasing hormone neuronal network[END_REF].

The pulsatile release of GnRH into the portal circulation is critical for the pulsatile, periodic and cyclic secretion of gonadotropins from the anterior pituitary (i.e. luteinizing and follicle stimulating hormones LH and FSH, respectively), that promote gonadal growth and gametogenesis [START_REF] Herbison | Physiology of the adult gonadotropin-releasing hormone neuronal network[END_REF][START_REF] Boehm | Expert consensus document: European Consensus Statement on congenital hypogonadotropic hypogonadism--pathogenesis, diagnosis and treatment[END_REF][START_REF] Ojeda | Puberty in the rat[END_REF]. The mechanisms behind this exquisite control of the GnRH release are being actively investigated. In mice, a neuronal population located in the arcuate nucleus of the hypothalamus that expresses kisspeptin, neurokinin B and dynorphin (i.e. KNDy neurons) was pinpointed as the initiator of the GnRH pulsatile release [START_REF] Herbison | Physiology of the adult gonadotropin-releasing hormone neuronal network[END_REF]. Both in vivo and ex vivo studies in mice confirm that kisspeptin directly stimulates GnRH neuron distal dendrons and terminals, and generates GnRH secretion from the median eminence [START_REF] Herbison | Physiology of the adult gonadotropin-releasing hormone neuronal network[END_REF]. Until now, GnRH dendrons had only been characterized in mice. In other mammalian species like the Rat, which relies less on olfactory cues to control their reproductive physiology, KDNy neurons are organized differently and send projections to the external zone of the median eminence [START_REF] Ciofi | Sexual dimorphism in the organization of the rat hypothalamic infundibular area[END_REF]. Whether GnRH dendrons exist in in such species remained unknown.

In the current issue of Endocrinology, Hoong Yip et al. from Alan E. Herbinson laboratory describe for the first time a GnRH-cre transgenic rat [START_REF] Yip | Innervation of GnRH neuron distal projections and activation by kisspeptin in a new GnRH-Cre rat model[END_REF]. The model was generated using a zinc finger nuclease (ZFN) technology, by inserting an Internal ribosomal entry site (IRES) sequence that drives the independent translation of the Cre recombinase, downstream from the translational stop site of the Gnrh1 open reading frame. Thus, this rat expresses the Cre recombinase under the endogenous transcriptional control of the GnRH1 gene without altering its physiological expression or regulation. In their study, Yip and colleagues present a thorough characterization of the model, showing that GnRH-cre allele is expressed in 94% of the GnRH neuronal population and absent elsewhere. When injecting an adenoassociated virus (AAV) expressing a floxed mCherry sequence into the rostral preoptic area, they further demonstrate that the GnRH-cre allele drives not only specific but efficient recombination of floxed sequences, opening a wide range of possibilities for the study of the GnRH system.

The mCherry membrane labelling of the GnRH neurons also allowed a thorough study of the the GnRH projections in GnRH-cre rats. Their morphology is similar to what was already described for mice and congruent with previous descriptions in rats [START_REF] Ciofi | Sexual dimorphism in the organization of the rat hypothalamic infundibular area[END_REF][START_REF] Kato | Characterization of voltage-gated calcium currents in gonadotropin-releasing hormone neurons tagged with green fluorescent protein in rats[END_REF]. However, the authors demonstrate for the first time the existence of dendron-like structures in the rat GnRH neurons, similar to their mouse counterpart. First, combining mCherry immunofluorescence with immunolabelling, they show that GnRH axon terminals are apposed to synaptophysin-labelled nerve boutons in the external zone of the ME and speculate that the distal projections of GnRH neurons receive synaptic inputs, as suggested previously by electron microscopic studies demonstrating direct contacts between GnRH and KNDy axon terminals in the rat ME [START_REF] Ciofi | Sexual dimorphism in the organization of the rat hypothalamic infundibular area[END_REF]. Then, using an AAV expressing a floxed calcium reporter (AVV-Flox-GCamP6), they show ex vivo that kisspentin puffs triggers calcium waves in the distal projections of the neurons, recapitulating the findings in mouse GnRH neurons.

There is no doubt that this novel GnRH-cre rat model will be a powerful genetic tool for the study of the molecular pathways involved in the control of GnRH neuronal activity, in a much-awaited nonmouse genetic model. Rat models may finally take their revenge on scientific history: indeed, over the last 20 years there was a switch from rats to mice as the model organism in biomedical science. Although rat models provided invaluable knowledge in the hypothalamic-pituitary-gonadal axis (4), the possibility of genetic manipulations offered by mice soon outmoded them. Fortunately, recent advances on rat transgenesis will increase the number of rat models available, and prompt discoveries, especially in neuroscience.

Rats are less dependent on pheromones to regulate their fertility and social interactions, and they are incomparably more valuable than mice for the study of cognition and behavior, making them physiologically more similar to humans. Moreover, the bigger size of rat brains will facilitate in vivo imaging (e.g. in vivo Calcium imaging through implantation of a miniscope), electrophysiology or microdialysis to study the neural circuits regulated by GnRH. Brain surgery will not only be eased, but also more accurate and less damaging, resulting in smaller regions affected. In addition, stereotactic drug or virus administrations will target more specific anatomical areas. The tighter control of all these techniques is particularly important for preclinical studies in the central nervous system.

Altogether, the GnRH-cre rat model opens up a whole new world of possibilities to improve our knowledge on the hypothalamic physiology. Combining this tool with cre-dependent AAV will not only allow tracking the GnRH neurons, but also silencing or overexpressing genes in these neurons. Crossing the existing GnRH::GFP rat model [START_REF] Kato | Characterization of voltage-gated calcium currents in gonadotropin-releasing hormone neurons tagged with green fluorescent protein in rats[END_REF] with the new GnRH-cre model should also facilitate FACS sorting to study gene expression in genetically modified GnRH neurons. Alternatively, GFP can be used as a reporter of the GnRH promoter activity. Using cre-activated Sh-RNA treatments to silence the expression of transcription factors or miRNAs, one may identify the ones able to regulate GnRH reporter activity [START_REF] Messina | A microRNA switch regulates the rise in hypothalamic GnRH production before puberty[END_REF]. Eventually this model will reach its maximum potential when new rat inducible genetic models will become available. Until then, let's explore all the possibilities that this new model brings us. New exciting findings on GnRH system are awaiting! Stay tuned! Authorship: I.M-C. and V.P. wrote the manuscript.

Acknowledgments: This work has been supported by the European Research Council COST action BM1105 for the study of GnRH deficiency coordinated by the Prof. Nelly Pitteloud (CHUV, Lausanne).


The circadian clock is widely known to shape patterns in physiology, metabolism and behavior. Disruptions of the circadian rhythm, such as disrupted light/dark cycles during shiftwork, are known to have adverse health effects, including on reproductive function. In women, this can take the form of menstrual irregularities, reduced fertility and an increased risk of miscarriage [START_REF] Sen | The Circadian Timing System and Environmental Circadian Disruption: From Follicles to Fertility[END_REF]. Despite these reports, the effect of chronodisruptive conditions on reproductive parameters in preclinical animal models remains largely unexplored.

Reproductive life in female mammals is characterized by the presence of regular cycles, i.e. the estrous cycle in rodents and the menstrual cycle in women. These cycles are tightly controlled by the hypothalamic-pituitary-gonadal (HPG) axis, a complex and tightly regulated feedback loop between hypothalamic gonadotropin-releasing hormone (GnRH) neurons, pituitary gonadotropins (luteinizing hormone and follicle stimulating hormone, LH and FSH) and ovarian sex steroids. Together, these different components orchestrate the maturation of oocytes, which may be divided into two phases. The first phase, comprising metestrus-diestrus in rodents and the follicular phase in women, is marked by gradually increasing levels of FSH and the recruitment and development of ovarian follicles, leading up to a transient LH surge that triggers ovulation within a few hours in rodents and within 9-23 hours in women. This preovulatory surge in LH secretion marks the start of the second phase, comprising proestrus-estrus in rodents and the luteal phase in women, and takes place approximately every 4-5 days in rodents and every 28 days in women. This surge in LH, which occurs specifically around the transition from night to day or vice versa depending on species, suggests the involvement of the circadian clock, a possibility confirmed by the presence of severe impairments in estrous cyclicity and fertility after ablation of the suprachiasmatic nucleus (SCN), the master circadian clock, or by mutations in clock genes (see for review [START_REF] Sen | The Circadian Timing System and Environmental Circadian Disruption: From Follicles to Fertility[END_REF][START_REF] Simonneaux | A Kiss to drive rhythms in reproduction[END_REF]).

A report in the current issue of Endocrinology [START_REF] Bahougne | Impact of Circadian Disruption on Female Mice Reproductive Function[END_REF] by the Simonneaux lab demonstrates that chronodisruptive conditions have marked reproductive effects in female mice, in accordance with previous observations in women. By acutely shifting the light/dark cycle by 10h, the authors slightly increased the length of the estrous cycle and altered the occurrence and timing of the preovulatory LH surge almost immediately (as early as the first post-phase-shift cycle). This effect appeared to be transient and was attenuated rapidly by the third proestrus after the phase shift. Similar to the acute phase-shift protocol, the effect of a chronic phase-shift protocol (3 days on a 10h phase advance/4 days on a 10h phase delay over 9 months) on the length and regularity of the estrous cycle was limited. Nonetheless, chronic disruption severely affected the occurrence and timing of the preovulatory LH surge during the entire exposure period. In line with the observed alterations in the LH surge, female mice exposed to a chronic disruption of the light/dark cycle were less fertile, i.e. showed fewer successful gestations and fewer pups per litter when compared to control mice. One aspect that was not addressed in the study, however, is that of ovarian histology. Do chronodisrupted mice become hypogonadal? Do they develop polycystic ovary syndrome (PCOS)-like traits? This would indeed be an intriguing finding since, in addition to altered fertility, circadian misalignments cause adverse metabolic effects [START_REF] Potter | Circadian Rhythm and Sleep Disruption: Causes, Metabolic Consequences, and Countermeasures[END_REF] that at least partially overlap those seen in women with PCOS [START_REF] Escobar-Morreale | Polycystic ovary syndrome: definition, aetiology, diagnosis and treatment[END_REF].

The findings of this study raise several new questions, notably concerning the mechanisms by which the circadian clock could regulate reproductive parameters. The current view is that SCN signals act either directly on GnRH neurons or indirectly through preoptic region kisspeptin neurons that drive the preovulatory surge in LH secretion via vasoactive intestinal peptide (VIP) and vasopressin, respectively [START_REF] Simonneaux | A Kiss to drive rhythms in reproduction[END_REF]. It would be interesting to explore whether disruptions of the circadian rhythm alter VIP and vasopressin release in the rostral preoptic region and possibly alter the electrophysiological properties of GnRH and preoptic kisspeptin neurons. Equally interesting is the putative role of the ovarian circadian clock, which has also been suggested to alter the timing of ovulation [START_REF] Yoshikawa | Timing of the ovarian circadian clock is regulated by gonadotropins[END_REF], in this process. Could alterations in the preovulatory LH surge be attributed in their entirety to changes in the master SCN clock, or is there some interplay with peripheral circadian clocks?

It is also worth mentioning that chronodisruptive conditions could lead to accelerated ageing overall, including the premature ageing of the reproductive axis. A recent study suggests that premenopausal women working on rotating night shifts are at an increased risk of precocious menopause onset [START_REF] Stock | Rotating night shift work and menopausal age[END_REF]. In rodents, it has been reported that the timing and amplitude of the preovulatory LH surge are impaired starting from 12 months of age [START_REF] Bahougne | Individual evaluation of luteinizing hormone in aged C57BL/6 J female mice[END_REF]. In this context, it would be interesting to examine whether the observed impairments in the LH surge and fertility result from ageing-independent alterations (e.g. an imbalance in VIP and vasopressin, the peptides secreted by SCN neurons) or whether exposure to a chronic shift protocol induces the accelerated ageing of the reproductive axis, which indirectly leads to these conditions.

A last point of interest lies in the long-term effects of chronodisruptive conditions. The authors observe that a chronic disruption of the light/dark cycle induces dramatic alterations in the profile of the preovulatory LH surge as early as the beginning of the phase shift protocol and lasting until just after it ceases [START_REF] Bahougne | Impact of Circadian Disruption on Female Mice Reproductive Function[END_REF]. Reduced reproductive success is observed immediately after a 4-week disruption of the light/dark cycle. Notwithstanding the significant impact of these results, it would have been fascinating to see whether the alterations in reproductive parameters could be (partially) reversed following the experimental period by normalizing light/dark cycles, or whether these observations constitute lifelong reproductive alterations. This information would be particularly interesting for women working in long-term rotating shifts. It appears from this study that a brief disruption of the light/dark cycle, e.g. an overseas journey, would not have a negative impact on women's fertility. However, reproductive success appears to be adversely affected in women continuously exposed to circadian clock disruption, such as employees working rotating shifts, frequent travelers and flight attendants. According to what has been described in the literature, the impact on reproductive capacity in the latter group of women can present as menstrual irregularities, reduced fertility and increased risk of miscarriage [START_REF] Sen | The Circadian Timing System and Environmental Circadian Disruption: From Follicles to Fertility[END_REF]. Yet, the fundamental question whether these alterations are completely reversible by following a normal light/dark cycle remains unanswered.

In conclusion, the study by Bahougne and colleagues demonstrating that exposure to chronic chronodisruptive conditions severely alters the preovulatory LH surge improves our understanding of how women working in shifts are more prone to reproductive dysfunction. Like many other studies focusing on relatively unexplored topics, these findings raise more questions than they answer regarding the mechanisms by which the circadian clock affects the reproductive axis and fertility. The way is clear for new and exciting studies that could help to further elucidate these mechanisms, and to increase the overall well-being of women working in long-term rotating shift work. Authorship: V.L. and V.P. wrote the manuscript.


J Dejeu

R Salut

M Spajer

F Membrey

A Foissy

D Charraut

Self-assembled film thickness determination by focused ion beam

The self-assembled polyelectrolytes films are obtained by successive and alternate adsorption of two polyelectrolytes of opposite charge. This procedure was previously described by Decher in the early 90s [1]. Recent reviews on the subject show that many theoretical and practical aspects of the interaction between the two polymers along with the film growth remain ambiguous and debated [2]. The multilayer film formed layer by layer is an efficient method for controlling the shell permeability [START_REF] Decher | Multilayer Thin Films, Sequential Assembly of Nanocomposite Materials[END_REF] or the release of the active matter in electronics [START_REF] Cheung | [END_REF], electrocatalysis [5] and many other areas [START_REF] Lvov | Handbook for Surfaces and Interfaces[END_REF]. Controlled release was performed with an appropriate choice of the thickness and porosity of the shell, the polyelectrolytes properties (hydrophobic, hydrophilic) and the incorporation of counterions [START_REF] Gupta | [END_REF]. Two types of multilayer growth process were reported: films with thickness that increases linearly with the number of deposition steps [1] and other systems whose thickness growth is nonlinear and usually called exponential [8]. Linear growth is due to the fact that during the build-up process the polyelectrolyte from the solution interact only with a polyelectrolyte of opposite charge forming the outer layer of the film [1].For the ''exponential'' growth, on the other hand, it was stated that it results from the diffusion ''in'' and ''out'' of the whole film of at least one of the polyelectrolytes which constitute the multilayer [9]. However, this view appears oversimplified since the recent observation that the exponential growth regime only holds in fact over a limited number of deposition steps, before the crossover to a linear growth regime [10]. The thickness increment is usually much smaller in the linear regime than in the exponential one (typically a few nanometers versus hundreds of nanometers). For the couple of polymers studied here, the two types of growth process are found: linear [1] or exponential [11], depending on the pH of construction condition.

The thickness of the multilayer film is usually measured by ellipsometry [12], AFM [13], confocal spectroscopy [14], X-ray [15], laser reflectometry [16], and optical waveguide lightmode spectroscopy (OWLS) [17]. All these techniques can be used with any autoassembled films but have specific advantages: the two first methods require a sample with a small roughness, the confocal a transparent sample and the last one a thickness film inferior to near 200 nm [18]. In this study, the deposition of multilayers is

The thickness evolution of multilayer film is investigated by focused ion beam (FIB) in the domain of polymer multilayers. This method, currently used in the modification and the characterization of integrated circuits, proves it is possible to determine the polymer film thickness. Sample cutting and its observation of the cross-section are performed in the FIB without leaving the vacuum chamber. Two main conclusions can be drawn: (1) the roughness of the film increases with the number of layer deposit, (2) the film growth changes from nonlinear (called exponential) to linear beyond 300 nm (70 layers).

realized on silica substrate. The roughness obtained for a great number of layers forbids the traditional techniques. They are replaced by FIB milling [START_REF] Orloff | High Resolution Focused Ion Beams[END_REF]. The focused ion beam (FIB) is an important tool in failure analysis, defect characterization, design modification, and process control in a variety of industries. In this work, it is used to cut the film and measure its thickness. Scanning ion microscopy (SIM) offers a spatial resolution of 5 nm, with different contrast mechanisms than those of electron microscopy. With this technique, we have determined the thickness of different thick auto-assembled films (up to 320 layers) and then deduced the type of growth.

Experimental procedures

Materials and chemicals

The sodium salt of Poly(4-styrenesulfonate) (PSS) with an average molecular weight 70 kDa was purchased from Alfa Aesar. PSS is a strong polyacid that is totally ionized in the whole range of pH used in this study. The Poly(allylamine hydrochloride) (PAH) from Sigma-Aldrich had a similar average molecular weight. The silicon/silica wafers substrates used for the preparation of films were purchased from ACM (Villiers St. Frederic 78640, France). They were 150 mm in diameter, with a silicon (orientation 100) thickness 625 AE 15 mm and thermo-oxide layer of approximately 100 nm or 300 nm. They were cut in 1 cm bands to be adapted to the reflectometer.

Layer by layer deposit on the silica wafer

The wafers were cleaned by immersion in a Piranha solution (2 parts of sulphuric acid 98% to 1 part of hydrogen peroxide 33%), rinsed and finally stored in Milli Q water. The deposit on the wafers was done by circulating the solutions (100 mg/L, NaCl 10 À3 M, pH 9) in an impinging jet cell similar to that used for reflectometric measurements [START_REF] Roques-Carmes | [END_REF]. No water washing was performed between the deposit steps. The deposit time, 5 min for each layer, was sufficient to obtain the plateau of adsorption (no variation in the reflectometric signal after 2 min). In this cell, the flow was perpendicular to the silica wafer. The FIB analyses were done at the stagnation point at which molecules were transported onto the surface by diffusion only.

Characterization of polymer films

The polymer film deposit on the wafer was covered with a chromium layer by cathodic sputtering. The aim of this layer is to increase the picture contrast and to avoid charge accumulation on the deposit which would reduce the stability of the ion beam. The cathodic sputtering was done on a Plassys apparatus with a pressure of 7 Â 10 À3 mbar, a continuous current of 1 A, deposition rate of 120 nm/min during 1 min and 15 s.

The 30 keV focused Ga + ions (FIB Orsay Physics Canion 31) was used to etch polymer films of various thickness and their Si/SiO 2 substrate. After FIB irradiation, scanning ion microscopy (SIM) was used to observe the surface. The wafer was tilted and viewed at an incidence angle of 408 or 358: the same ion beam, with a current limited to 30 pA, was used as a scanning probe to image the crosssection of the film.

Different layers of materials can be clearly identified due to good material contrast in the ion-induced secondary electron image. The thickness of SiO 2 and polymer film can be measured directly from the cross-section view by compensating the tilt angle, the apparent width of the layers being divided by sinus of the tilt angle.

Results and discussion

Wafer with no adsorbed polymer film

A silicon wafer recovered by 300 nm of silica by thermooxidation was etched on a rectangle of 2 mm  5 mm. On the image of the wafer (Fig. 1), the different layers, silica and chromium, are clearly differentiated.

The thickness of the layers deduced after angular correction from the scale on Fig. 1 is, respectively, of 150 and 330 nm for chromium and silica. The discrepancy with the measurements given by ellipsometry remains within the uncertainty. This reference experiment gives the thickness of the chromium layer deposited on the multilayer films.

Wafer with adsorbed polymer film

Different numbers of polyelectrolyte layer (from 5 to 160 bilayers) are deposited on the wafers. A cross-section is realised on each deposit and their images are obtained by SIM (Fig. 2).

Fig. 2 shows an important change of the morphology of the deposits with the increase of the number of bilayers. For a low number of bilayers, the upper surface of the deposit has a smooth structure with some bumps. When the number of layers increases, the surface becomes more and more granular to reach a typical ''cauliflower'' structure. Above 80 layers, the observed deposit presents holes and cracks inside the adsorbed polymer layer. We are entitled to wonder if these defects do not result from sample drying and\or from the vaccum of the FIB chamber. A previous study has shown that the drying step do not change the morphology of the deposit beyond 5 bilayers [11]. However, this structure can be obtained by the conformation of the polymer during the adsorption. Indeed, in a previous paper [11], we have shown that the polyelectrolytes form PAH/PSS aggregates on the silica surface and that PAH can adsorb on the top of the aggregates and so coalesce to aggregates by the summits. In this process, cavities form between two aggregates and can be observed by FIB.

Polymer film thickness

In Fig. 2, it is impossible to differentiate the chromium layer from the polymers. This chromium layer is however present and must be subtracted from the total thickness. For 30 layers of polyelectrolytes, the thickness of total film (chromium + polymers) on the silica substrate is 270 AE 50 nm. As the measured chromium layer deposited on polymer film was 150 nm, the polymer film is 270 À 150 = 120 AE 50 nm. The same calculation can be done for the other films. For the thin polymer film, the thickness can be determined by laser reflectometry [11], and the Fig. 3 compares the thickness found by the two methods. The uncertainty of the measurements comes from the granular texture and the surface roughness which are visible on the cross-section.

In a previous work, we have shown that the thickness exhibits a peculiar trend. It increases strongly during the first layers (near 60 nm after 4 layers) and increases slowly after. This quasi plateau is attributed to the polymer film concentration. After the formation of coacervates between PSS and PAH on the substrate in the first layer, there are densification of the aggregates by diffusion of the PSS into the aggregates [11]. Similar results were found by Buron et al. [21] for MAQUAT/PAA.

A good concordance is found between the two methods. So the focused ion beam can be used to determine the thickness of the polymer film when it is impossible by the other methods, because of the laser diffusion at the polymer film surface. The diffusion of the laser depends on the polymer couple and on the adsorption condition (pH, ionic strength, etc.). In our case, it is observed beyond 30 layers. The thickness of the polymer film are summarized in Fig. 4.

A change of the multilayer growth is observed the Fig. 3.I na first approximation, nonlinear to linear transition takes place near 300 nm (70 layers). This value is close to that found with the other couples of polymers [22] where the transition is in the range of 150-250 nm. This transition is still not fully understood. Exponential growth results from the diffusion of at least one of the two polyelectrolytes in the multilayer film formed by aggregates [23].

It has been proposed that the linear growth after an exponential growth results from a gradual restructuration of the multilayer consecutive to the diffusion process, which progressively hinders the diffusion of polyelectrolytes into the restructured zone, which becomes inaccessible by diffusion. This forbidden zone then grows with the number of deposition steps and the outer zone of the film, which is still concerned by diffusion keeps a constant thickness and moves upward as the total film thickness increases. This process was proposed by Hubsch et al. [24] and more recently by Salomaki et al. [10] for, respectively (PGA x -PSS 1Àx /PAH) and (PDADMAC/PSS).

With a total thickness of the polymer film about 2.1 mm and an assumed refractive index of the adsorbed layer of 1.47, the total polyelectrolyte adsorbed amount is evaluated, by the De Feijter relationship [25], at 1.7 AE 0.35 g m À2 .

Conclusion

The thickness of multilayer film can be evaluated by focused ion beam when the other methods are affected by roughness, mainly for thick polymer film. FIB milling creates a cross-section in the polymer film. The thickness of the film is observed with the same device in scanning microscopy mode after tilting the sample. The thickness determined for a thin film (30 layers) is in agreement with the value found by laser reflectometry. Beyond a thickness of 300 nm, the growth multilayer changes from exponential to linear. The roughness of the multilayer film increases with the number of deposited layers.

The study of the thick polyelectrolyte auto-assembled film allows to better understand the mechanism of the multilayers growth. A further paper will demonstrate how the present characterization method allows (1) to develop the reflectometry model in order to convert the reflectometry signal to adsorbed amount of polymer, and (2) to analyse the film composition.

Fig. 1 .

 1 Fig. 1. SIM image of the silica wafer covered of chromium with a tilt of 408 to study the cross-section. All the scales must be divided by sin (408) to give the real thickness.

Fig. 2 .

 2 Fig. 2. SIM images of film multilayer deposit on silica wafer and covered by chromium layer with a tilt of 408 (images b-f) or 308 (image a and g) to study the cross-section. (a) 5 bilayers, (b) 10 bilayers, (c) 15 bilayers, (d) 20 bilayers, (e) 40 bilayers, (f) 80 bilayers, (g) 160 bilayers. All the scales must be divided by sin 408 (image b-f) or sin 358 (image a and g) to give the real thickness.

Fig. 3 .

 3 Fig. 3. Thickness variation vs. the number of layers obtained by FIB (grey) and laser reflectometry (black).

Fig. 4 .

 4 Fig. 4. Thickness variation vs. the number of layers.

Acknowledgments

This work was supported by the EU under contract NMP4-CT-2003-001428: Nano-capsules for Targeted Controlled Delivery of Chemicals and by a CNRS grant.


K Ghoumid
email: kamal.ghoumid@univ-fcomte.fr

R Ferriere

B.-E Benkelfat

G Ulliac

R Salut

J.-Y Rauch

T Gharbi

Effect of depth etching on Bragg reflectors realized by Focused Ion Beam in Ti:LiNbO3 waveguide

Keywords: Bragg grating, Lithium niobate, Transmittance, Focused Ions Beam

In this paper we have studied effect of depth etching on the Bragg gratings (BGs) realized by Focused Ions Beam. This technique has the advantage to induce a direct waveguide structuring without intermediate media, comparing to traditional methods. A reflectivity of 96% within a window centred at 1550 nm is obtained. The effect of the depth etching on the transmittance and the bandwidth at half maximum is demonstrated.

INTRODUCTION

A Bragg grating is a periodic or aperiodic perturbation of the effective refractive index in the waveguide or in the core of an optical fiber. This leads to the reflection of light in a narrow/wide range of wavelengths, for which a Bragg condition is satisfied. This basically means that the wavenumber of the grating matches the difference of the wavenumbers of the incident and reflected waves, In other words, the complex amplitudes corresponding to reflected field contributions from different parts of the grating are all in phase so that can add up constructively; this is a kind of phase matching. Other wavelengths are nearly not affected by the BG.

The reflection bandwidth of a BG depends on the length, the depth and the strength of the refractive index modulation.

The narrowest bandwidth values, as are desirable e.g. for the construction of single-frequency fiber lasers or for certain optical filters, are obtained for long gratings with weak index modulation. Large bandwidths may be achieved with short and strong gratings, but also with aperiodic designs. One of the key parameters to increase the reflectivity is the depth of modulation index.

Bragg grating inscription is commonly used in integrated optics applications for the realization of wavelength-dependent resonators, distributed Bragg reflectors (DBR) and fiber lasers [START_REF] Yariv | Periodic structure for integrated optics[END_REF], and there continues to be significant interest in studying convenient and reproducible fabrication technologies. Fabrication methods are usually based on the photorefractive [START_REF] Das | Integrated optical distributed feedback laser with Ti:Fe:LiNbO 3 waveguide[END_REF], effect and dry etching [START_REF] Söchtig | Ti:LiNbO 3 stripe waveguide Bragg reflector gratings[END_REF], and ultraviolet (UV) induced surface-relief gratings on LiNbO 3 channel waveguides have also been reported [START_REF] Wu | UV-induced surface-relief gratings on LiNbO3 channel waveguides[END_REF][START_REF] Sidorin | Integration of Bragg gratings on LiNbO3 channel waveguides using laser ablation[END_REF], a photorefractive-index difference induced by femtosecond laser pulses has been employed to realize gratings in oxides [START_REF] Takeshima | Fabrication of a periodic structure wit a high refractiveindex difference by femtosecond laser pulses[END_REF].

In this paper, we propose an efficacy method for the design of BGs in a Ti-indiffused waveguide by Focused Ions Beam [START_REF] Chaganti | Sub-micron grating fabrication on hafnium oxide thin-film waveguides with focused ion beam milling[END_REF][START_REF] Lacour | Nanostructuring lithium niobate substrates by focused ion beam milling[END_REF]. Because of the sputtering capability, the FIB is used as a micro-machining tool, to modify or machine materials at the micro-and nanoscale, FIB micro machining has become a broad field of its own. The milling of samples using FIB is different from the other "traditional" techniques involving lithography and subsequent etching. The essential difference comes from the fact that it uses a direct etching without other additional technological steps. The ion beam processing on a solid target induces a significant transformation of its structure and its physical properties. Thus, using this technique, we study the effect of the depth etching of corrugation on the reflectivity and bandwidth. where N is number of periods).

THEORY AND SIMULATIONS

( ) ( ) ( ) ( ) L sh j L ch L sh δ β δ δ δ κ λ Δ + = ℜ (2) 2 2 . 1 . ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ - = Δ π κ λ λ L L n eff (3)
The transmittance is defined by

) ( 1 ) ( λ λ R T - = (losses are neglected)
Any increase in the depth l of the modulation index, an increased coupling coefficient κ , therefore an increase in reflectivity ℜ (T decrease), and the bandwidth at half maximum λ Δ .

CONCLUSION

We have demonstrated that using a FIB to etch Bragg gratings in a titanium-diffused optical waveguide in lithium niobate allows to achieve very high reflectance ($96\%$). We have studied the influence of the depth etching l on the transmittance and the bandwidth at half maximum. Thus, any increase in the depth of the index modulation, leads to a decrease in transmittance and increased bandwidth at half maximum.

This method of machining lithium niobate opened the voice on several applications.

Fig. 1

 1 Fig.1shows the basic structure considered, consisting of a waveguide of index n 2 indiffused in a substrate of index n 1 and the whole structure is embedded in air with index n 3 = 1. A grating is formed as a result of periodic index modulations

Fig 1 : 2 =

 12 Fig 1: Basic scheme of a Bragg grating etched in a Ti:LiNbO 3

  The curves of simulation of the transmittance versus wavelength for various values etching depth are given in figure2. In these simulations, n eff = 2.2184, m = 5 (fifth order) and the relationship between the etching depth, the reflectivity and bandwidth; if l increases, then ℜ and λ Δ also increase (T decrease).

Fig 2 :

 2 Fig 2: Transmittance versus wavelength for various values of depth etching.

Fig 4 :

 4 Fig 4: Measurement setup to characterize Bragg grating transmittance.

Fig 5 :

 5 Fig 5: Transmittance versus wavelength for various etching depths.

FABRICATION PROCESS AND RESULTS

The single-mode optical waveguides in the 1550 nm window were fabricated by standard indiffusion of 80 nm thick and 7 μm wide titanium layers at 1020°C during 10 h, on a sample of lithium niobate of type X-cut, Z-propagation and TM polarization. The next step, is etching BG. The ion beam processing on a solid target induces a significant transformation of its structure and its physical properties. However, for a sufficient amount of ions, it produces a sputtering of the target material. For the experiments, we use a dual beam Orsay Physics Canion 31 / LEO 4400 FIB, the command of the beam deflection is realized by a Raith Elphy Quantum 4.0 driver. The current and ions energy values used for the LiNbO 3 sputtering are respectively 300 pA and 30 keV.

We get then a BG one whose parameters are: period Λ = 1.8 μm, m = 5, N = 80, it is given in Figure 3. The experimental setup for visualization of the transmittance is given in Figure 4. It consists of a continuous spectrum source in the wavelength range of {850 -1750} nm; focusing lenses; P is a Glan polarizer to selectively probe TM mode and an optical spectrum analyser (OSA).

Experimental results of the transmittance and the bandwidth versus wavelength for various etching depths are given in figures 5, and compared to the reference curve for which l = 0. We can see that the reflectivity increases with the etching depth, indeed reflectivity greater than 70%, 90% and 96% are achieved for l = 1.7 μm, 2 μm and 2.8 μm respectively. At the same time, the bandwidth at half maximum Dn decreases from 60 to 28 nm.

The realization of the BG based on Ti:LiNbO 3 optical waveguide has been demonstrated and the experimental results confirm well the theoretical results.