A Davaille
email: davaille@fast.u-psud.fr

B Romanowicz
email: barbara.romanowicz@gmail.com

Deflating the LLSVPs: bundles of mantle thermochemical plumes rather than thick stagnant "piles"

We use seismic tomography and fluid mechanics constraints to decipher the morphology of LLSVPs in the Earth's mantle.

• The two LLSVPs do not extend as compact, continuous and uniform piles above the core-mantle boundary.

Introduction

Already 30 years ago, the first global seismic tomographic images of the Earth's mantle revealed the presence of very long wavelength structure (degrees 2 and 3) in the lower mantle (fig. 1), correlated with similar wavelength structure in the geoid (e.g. [START_REF] Dziewonski | Large-scale heterogeneities in the lower mantle[END_REF][START_REF] Dziewonski | Mapping the Lower Mantle: Determination of Lateral Heterogeneity in P Velocity up to Degree and Order 6[END_REF]. Since then, these features have been confirmed and refined through many generations of global mantle tomographic models. They have been dubbed large low shear velocity provinces (LLSVPs), because they are best resolved in shear velocity tomography and they appear slower than the surrounding mantle. One is located beneath the Pacific Ocean and the other one beneath the Atlantic and south Africa. They cover about 25% of the surface of the core-mantle boundary (CMB), and their outside boundaries are well resolved across different tomographic models (e.g. [START_REF] Lekic | Cluster analysis of global lower mantle tomography: anew class of structure and implications for chemical heterogeneity Earth[END_REF]. The LLSVPs are separated by a ring of fast velocities that contains most of the locations of current and past subduction at least in the last 200 Myrs (e.g. [START_REF] Anderson | Hotspots, polar wander, Mesozoic covection and the geoid[END_REF][START_REF] Richards | Large-scale mantle convection and the history of subduction[END_REF][START_REF] Ricard | A geodynamic model of mantle density heterogeneity[END_REF][START_REF] Lithgow-Bertelloni | The dynamics of Cenozoic and Mesozoic plate motions[END_REF][START_REF] Dziewonski | Mantle Anchor Structure: an Argument for Bottom up Tectonics[END_REF]. On the other hand, most of the hotspots are concentrated in the areas corresponding to the surface projection of the LLSVPs (fig. 1). The hotspots with the longest tracks are located at their edges, as are the reconstructed positions of large igneous provinces (LIPs) in the last 200 Myrs (e.g. [START_REF] Torsvik | Long term stability in deep mantle structure: evidence from the 300 Ma Skagerrak-centered large igneous province (the SCLIP)[END_REF].

Because they exhibit lower seismic velocities than the surrounding mantle, LLSVPs must be hotter than average, and may well represent the primary locus of the return flow from subduction (e.g. [START_REF] Ricard | A geodynamic model of mantle density heterogeneity[END_REF], in which case their origin could be purely thermal (e.g. [START_REF] Schuberth | Tomographic filtering of highresolution mantle circulation models: Can seismic heterogeneity be explained by temperature alone?[END_REF][START_REF] Davies | Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity[END_REF]. However, there is also evidence from seismological imaging that they could be compositionally different from the surrounding mantle. In particular, forward modeling of waveforms of shear waves diffracting along the core-mantle boundary (Sdiff) indicates that their borders are sharp, at least in some regions, with a lateral transition occurring over less than 100-200 km, sharper than what would be expected from temperature variations alone [START_REF] Ni | Sharp sides to the African superplume[END_REF][START_REF] Wang | Mapping the geometry and geographic distribution of a very low velocity province at the base of the Earth's mantle[END_REF][START_REF] To | 3D effects of sharp boundaries at the borders of the African and Pacific Superplumes: observation and modeling[END_REF]. There is also evidence for anti-correlation of shear and bulk sound velocity, at least at the longest wavelengths, in the lowermost mantle [START_REF] Su | Simultaneous inversion for 3-D variations in shear and bulk velocity in the mantle[END_REF][START_REF] Kennett | Joint seismic tomography for bulk sound and shear wave speed in the Earth's mantle[END_REF], an indication for the presence of lateral compositional variations. Finally, some normal-mode based studies [START_REF] Ishii | Normal-mode and free-air gravity constraints on lateral variations in velocity and density of Earth's mantle[END_REF] and tomographic studies that include constraints from other geophysical observables such as the geoid [START_REF] Simmons | GyPSuM: A joint tomographic model of mantle density and seismic wave speeds[END_REF], have suggested that the LLSVPs may be denser than the [START_REF] French | Whole-mantle radially anisotropic shear-velocity structure from spectral-element waveform tomography[END_REF] at 2800 km depth. The surface hotspots are superimposed in green. The stars represent the hotspots with a long-lived track and/or associated with a LIP event ; Model comparison up to harmonic degree l=12 between b) SEMUCB-WM1, and c) S362ANI [START_REF] Kustowski | Anisotropic shear-wave velocity structure of the Earth's mantle: a global model[END_REF]; d) vote map from [START_REF] Lekic | Cluster analysis of global lower mantle tomography: anew class of structure and implications for chemical heterogeneity Earth[END_REF].

surrounding mantle. However, the earlier mode-based tomographic results were met with skepticism, as it was shown that models with higher or lower density than average could be equally likely, depending on parametrization and the choice of starting models [START_REF] Romanowicz | Can we resolve 3D density heterogeneity in the lower mantle?[END_REF][START_REF] Kuo | On the resolution of density anomalies in the Earth's mantle using spectral fitting of normal mode data[END_REF]. This debate continues to this day: a recent model based on the observation of Stoneley modes (modes sampling the region around the CMB) suggests that the LLSVPs are lighter than average [START_REF] Koelemeijer | Density structure of Earth's lowermost mantle from Stoneley mode splitting observations[END_REF], while another study based on a novel technique utilizing constraints from tidal data indicates that they might be denser than average [START_REF] Lau | Tidal tomography constrains Earth's deep-mantle buoyancy[END_REF]. Better constraints on the CMB topography could tell us if LLSVPs are denser (CMB deflected) or lighter (CMB uplifted) than the surrounding mantle (e.g. Deschamps et al, 2018[START_REF] Deschamps | Core-mantle boundary dynamic topography: influence of postperovskite viscosity[END_REF], but the data is not yet good enough to give a definite answer (for a recent review, see [START_REF] Koelemeijer | Towards consistent seismological models of the core-mantle boundary landscape[END_REF]. There is also some suggestion that the excess density could be concentrated in patches of ultra-low velocity zones [START_REF] Yuan | Seismic evidence for partial melting at the root of Major Hotspot Plumes[END_REF].

The nature and evolution of the LLSVPs through time, and their role in the general mantle circulation have been widely debated. If LLSVPs were dense and old enough, [START_REF] Dziewonski | Mantle Anchor Structure: an Argument for Bottom up Tectonics[END_REF] argued that they could have anchored mantle convection in the so-called "degree 2" configuration. Indeed, this configuration of the mantle longest wavelength structure is not random, but corresponds to a stable energy configuration from the point of view of the Earth's moments of inertia, with LLSVPs located antipodally at the equator, surrounded by a ring of fast velocities going through the Earth's poles of rotation, and containing the positions of these poles over at least the last 200 Myr. The fixity of LLSVPs and of their edges as plumes sources was even suggested to exist over much longer times [START_REF] Burke | Plume generation zones at the margins of large low shear velocity provinces on the core-mantle boundary[END_REF][START_REF] Torsvik | Diamonds sampled by plumes from the core-mantle boundary[END_REF]. Such fixity would be a valuable constraint to reconstruct the continents paleo-motions. However, the coincidence between plume formation and LLSVP edges is not proven statistically (e.g. [START_REF] Austermann | A statistical analysis of the correlation between large igneous provinces and lower mantle seismic structure[END_REF]. On the other hand, if LLSVPs are not so dense compared to slabs, investigations of the effect of internal mass redistribution on the position of the Earth's rotational pole show that temporal variations of the rotational axis are essentially due to changes in the subduction history in the last 410 Myr [START_REF] Rouby | Mantle dynamics, geoid, inertia and TPW since 120 Ma[END_REF][START_REF] Greff-Leftz | Sensitivity experiments on True Polar Wander[END_REF]; variations in LLSVP shape or vertical extent only modulate the signal [START_REF] Rouby | Mantle dynamics, geoid, inertia and TPW since 120 Ma[END_REF]. So the present-day number and shapes of LLSVPs could only be a manifestation of the state of present day mantle circulation [START_REF] Mcnamara | Thermochemical structures within a spherical mantle: Superplumes or piles?[END_REF]. Numerical modeling in the presence of continents by Zhong and collaborators [START_REF] Zhong | Supercontinent cycles, true polar wander, and very long-wavelength mantle convection[END_REF][START_REF] Li | Supercontinent-superplume coupling, true polar wander and plume mobility: Plate dominance in whole-mantle tectonics[END_REF][START_REF] Zhong | The long-wavelength mantle structure and dynamics and implications for large-scale tectonics and volcanism in the Phanerozoic[END_REF] suggests that the mantle convective pattern may have alternated between a "degree 1" and a "degree 2" dominated planform while the mantle evolved through geological times. On the other hand, laboratory experiments suggest the possibility of an even larger range of mantle circulation configurations [START_REF] Androvandi | At least three scales of convection in a mantle with strongly temperature-dependent viscosity[END_REF].

In the last 20 years, the interplay between compositional heterogeneities and mantle convection has been largely investigated through experimental and numerical geodynamic modeling (for recent reviews, see [START_REF] Tackley | Dynamics and evolution of the deep mantle resulting from thermal, chemical, phase and melting effects[END_REF][START_REF] Davaille | Laboratory Studies of Mantle Convection[END_REF][START_REF] Mcnamara | A review of large low shear velocity provinces and ultra low velocity zones[END_REF], and several scenarios have been proposed about the possible composition of the LLSVPs. On the one hand, LLSVPs may represent the primordial reservoir tapped by mantle plumes and providing the distinct isotopic composition of ocean island basalts (OIBs), a scenario proposed among others by [START_REF] Kellogg | Compositional stratification in the deep mantle[END_REF] and most recently addressed through a combination of trace element geochemistry, seismic tomography and geodynamic modelling [START_REF] Jackson | Evidence for a deep mantle source for EM and HIMU domains from integrated geochemical and geophysical constraints[END_REF][START_REF] Williams | Primitive Helium is Sourced from Seismically Slow regions in the lowermost mantle[END_REF]. The key issue is then how to keep this primitive reservoir isolated for Gyrs in a convective mantle. The presence of a layer of material several percent denser than the ambient mantle, maybe also more viscous, at the base of the mantle provides a possible answer (e.g. [START_REF] Davaille | How to anchor plumes in a convecting mantle? Earth Planet[END_REF][START_REF] Tackley | Strong heterogeneity caused by deep mantle layering[END_REF][START_REF] Nakagawa | The influence of MORB and harzburgite composition on thermo-chemical mantle convection in a 3-D spherical shell with self-consistently calculated mineral physics[END_REF][START_REF] Deschamps | The primitive nature of large low shear-wave velocity provinces[END_REF][START_REF] Li | Chemical complexity of hotspots caused by cycling oceanic crust through mantle reservoirs[END_REF]. Such a layer would have been swept into dense primordial "piles" [START_REF] Tackley | Strong heterogeneity caused by deep mantle layering[END_REF][START_REF] Mcnamara | Thermochemical structures within a spherical mantle: Superplumes or piles?[END_REF][START_REF] Mcnamara | Thermochemical structures beneath Africa and the Pacific Ocean[END_REF] in response to mantle convection. An alternative scenario is that the LLSVPs are made up of accumulated slab material, since basaltic crust becomes denser than the surrounding mantle in the deep mantle (e.g. [START_REF] Hirose | Phase transition and density of subducted MORB crust in the lower mantle[END_REF]. Here, the key issue is whether basaltic crust segregation has been efficient enough to build the present-day LLSVPs (e.g. [START_REF] Nakagawa | Deep mantle heat flow and thermal evolution of the Earth's core in thermochemical multiphase models of mantle convection[END_REF][START_REF] Li | The difficulty for subducted oceanic crust to accumulate at the Earth's core-mantle boundary[END_REF][START_REF] Mulyukova | Survival of LLSVPs for billions of years in a vigorously convecting mantle: replenishment and destruction of chemical anomaly[END_REF]. Recently, [START_REF] Li | Chemical complexity of hotspots caused by cycling oceanic crust through mantle reservoirs[END_REF] showed that it is possible for the LLSVPs to contain both primordial and recycled material, while [START_REF] Ballmer | Persistence of strong silica-enriched domains in the Earth's lower mantle[END_REF] proposed a model (BEAMS,"bridgmanite-enriched ancient mantle structures "), in which the LLSVPs are made of recycled oceanic crust, while the ambient mantle surrounding the LLSVPs would house primordial material which would not participate to general convection. In this case, mantle plumes rising through those regions would pick up some of the primordial signature along the way.

Whatever the origin of the compositional heterogeneities in the mantle, laboratory experiments have shown that the morphology and time evolution of hot upwellings in a heterogenous mantle critically depend on the latter's viscosity and density structure (e.g. Le [START_REF] Bars | Whole-layer convection in an heterogeneous planetary mantle[END_REF][START_REF] Davaille | Convective patterns under the Indo-Atlantic "box[END_REF][START_REF] Kumagai | On the fate of mantle thermal plumes at density interface[END_REF][START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF]. A whole zoology of shapes and behaviour could therefore develop in the mantle for density anomalies of compositional heterogeneities between 0 and 2% (e.g. [START_REF] Davaille | Simultaneous generation of hotspots and superswells by convection in a heterogeneous planetary mantle[END_REF]. As noted by [START_REF] Schubert | Superplumes or plume clusters?[END_REF], the extent of each LLSVP is much greater than the size of a single classical thermal plume, so that each LLSVP should host a bundle of plumes. Although thermochemical upwellings are predicted to be wider (Le [START_REF] Bars | Whole-layer convection in an heterogeneous planetary mantle[END_REF][START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF] than purely thermal plumes, they are still much thinner than the typical lateral extent of each LLSVP. This lead [START_REF] Davaille | Convective patterns under the Indo-Atlantic "box[END_REF] to interpret the tomographic images in the "Indo-Atlantic" LLSVP as a bundle of 9 thermochemical upwellings, each at a different stage of their evolution. This interpretation can appear at odds with the images of the tomographic voting maps (e.g. [START_REF] Cottaar | Morphology of seismically slow lower-mantle structures[END_REF] constructed based on smooth, filtered versions of tomographic models.

So, given the recent improvement in tomographic models resolution, it is time to reappraise the morphology of LLSVPs to answer several key questions: how high do LLSVPs rise above the core-mantle boundary ? are LLSVPs compact and uniform across that depth range? or are they made up of a bundle of upwellings whose long wavelength expression is the characteristic degree 2-3 pattern observed in all tomographic models [START_REF] Schubert | Superplumes or plume clusters?[END_REF][START_REF] Davaille | Convective patterns under the Indo-Atlantic "box[END_REF][START_REF] Bull | Synthetic tomography of plume clusters and thermochemical piles[END_REF][START_REF] Garnero | Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle[END_REF]?

In what follows, we first briefly review the fluid mechanics constraints on mantle convection and plume generation. We then illustrate the morphology of the LLSVPs drawing on images mainly from model SEMUCB-WM1 (French andRomanowicz, 2014, 2015). We note that many of the features considered are present in other high resolution tomographic models. We show that the LLSVPs do not extend higher than 200-300km above the CMB as continuous piles, and that their appearance of large compact piles extending to heights in excess of 1000-1500 km is due to long wavelength filtering of a bundle of distinct columnar plumes, as already proposed by [START_REF] Schubert | Superplumes or plume clusters?[END_REF]. We then provide an update of the [START_REF] Davaille | Convective patterns under the Indo-Atlantic "box[END_REF] geodynamic interpretation of tomography in the "Indo-Atlantic" domain in terms of thermochemical upwellings, and extend it to the "Pacific" domain. We limit our analysis to the lower mantle as LLSVPs spring from the CMB area.

Fluid mechanics constraints

The Earth's mantle is cooled from above by the universe, heated internally by the decay of radioactive elements, and heated from below by the core. The signature of the latter is the large temperature gradient that exists in a thin layer above the CMB (e.g. [START_REF] Stacey | The thermal boundary-layer interpretation of D" and its role as a plume source[END_REF]. In a fluid mechanics framework, this layer represents the thermal boundary layer (TBL) of the system. Hot plumes are buoyant upwellings that result from the instability of this TBL (fig. 2A; e.g. [START_REF] Loper | The dynamical and thermal structure of deep mantle plumes[END_REF][START_REF] Schubert | Superplumes or plume clusters?[END_REF]. They develop when the vigour of convection is sufficiently strong. This vigour depends on the Rayleigh number, which compares the driving thermal buoyancy force to the resisting effects of thermal diffusion and viscous dissipation:

Ra(H) = αρ∆T gH 3 κη (1)
where α is the thermal expansion, κ is the thermal diffusivity, η the viscosity, ρ the density, ∆T the temperature difference across the system, and H the height of the system. Convection starts only when buoyancy forces are strong enough, so that Ra becomes greater than a critical value Ra c ∼ 1000. For a planetary mantle, Ra typically ranges between 10 5 and 10 9 , several orders of magnitude above the critical value. For this Ra range, plumes are expected to develop. [START_REF] Howard | Convection at high Rayleigh number[END_REF] proposed that plumes were developing as soon as the local Rayleigh number Ra(δ) across the TBL of thickness δ (which is growing by thermal diffusion from the heated bottom boundary) was reaching the critical value Ra c . Then experimental and numerical studies showed that plume size and spacing were proportional to the TBL thickness:

δ = H( Ra c Ra(H) ) 1/3 (2)
In particular, the plume conduit diameter is very close to the TBL thickness (fig. 2A).

Note that plume characteristics are therefore independent of the mantle thickness, and are determined locally by the TBL (for reviews, see [START_REF] Schubert | Superplumes or plume clusters?[END_REF][START_REF] Davaille | Laboratory Studies of Mantle Convection[END_REF]. Fig. 3 shows that only rather small TBL thicknesses (between 50 and 300 km), and therefore small plumes'conduit diameters, are expected out of the bottom TBL.

Plumes spacing could reach on the order of a thousand kilometers for a viscosity of 10 22 Pa.s. Depth-dependence of material properties is not steep enough to significantly increase the plume characteristics (e.g [START_REF] Schubert | Superplumes or plume clusters?[END_REF]. When the viscosity strongly depends on temperature, convection organizes itself so that most of the viscosity variation is stored in the cold more viscous TBL at the top of the system, and the viscosity ratio in the bottom hot TBL remains small (<< 10). This produces a strong asymmetry between the two TBL and the flow pattern becomes also strongly asymmetric (e.g. fig. 2B). In the "mobile lid" regime at intermediate viscosity ratios, downwellings will form a network of cold cells each encasing a bundle of hot less viscous plumes (fig. 2B; [START_REF] Schubert | Superplumes or plume clusters?[END_REF][START_REF] Androvandi | At least three scales of convection in a mantle with strongly temperature-dependent viscosity[END_REF]. More viscous downwellings displace less viscous material, so that cold downwellings strongly influence the bottom hot thermal boundary layer [START_REF] Schaeffer | Interaction of rising and sinking mantle plumes[END_REF][START_REF] Gonnermann | Modulation of mantle plumes and heat flow at the core mantle boundary by plate-scale flow: Results from laboratory experiments[END_REF]) and the hot upwellings can fully develop only out of the zone impacted by downwellings. However, within the hot plume area, the plumes diameter and spacing are still well predicted according to (2) and fig. 3a [START_REF] Androvandi | At least three scales of convection in a mantle with strongly temperature-dependent viscosity[END_REF]. So in the purely thermal case, given the size of the mantle LLSVPs (typically 5000 to 8000 km across at CMB depth), each of them should host at least about 10 plumes (e.g. [START_REF] Schubert | Superplumes or plume clusters?[END_REF][START_REF] Davies | Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity[END_REF].

If a denser layer (of compositional origin) of thickness l b exists at the bottom of the mantle, the pattern of convection will depend on two additional parameters besides the Rayleigh number Ra and the viscosity ratio γ: the layers thicknesses ratio a = l b /H, and the buoyancy number, which compares the density contrasts of compositional origin (∆ρ X ) and of thermal origin:

B = ∆ρ X αρ∆T e (3)
where ∆T e is the averaged temperature anomaly in the bottom hot TBL. Depending on the combination of these 4 parameters, a whole zoology of convective regimes (Le [START_REF] Bars | Whole-layer convection in an heterogeneous planetary mantle[END_REF][START_REF] Davaille | Convective patterns under the Indo-Atlantic "box[END_REF] and plume shapes [START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF] appears.

The last 20 years have seen an increasing number of numerical studies to investigate these different regimes (for a recent review, see [START_REF] Mcnamara | A review of large low shear velocity provinces and ultra low velocity zones[END_REF]. Stratified convection appears when B > 1, whereby thermal plumes develop above the denser hotter layer (fig. 4Ag,E), with the same characteristics as fig. 3. When 0.5 < B < 1, the interface between the two layers becomes strongly deformed under the action of the convective motions.

If the denser layer is thin enough, it can be gathered in piles by the incoming slabs or under upwelling plumes. When B < 0.5, the whole denser layer can become unstable and invade the upper layer. The hot upwellings morphology can then be quite different from the purely thermal plumes (fig. 4). In all cases B < 1, they are "thermochemical", being constituted of material from both layers. They need to be hotter than their purely thermal counterpart since temperature-induced density anomaly has to overcome the stable compositional density contrast. The conduit diameters increase because each thermochemical plume now hosts an inner convective circulation (fig. 4). Their spacing is dictated by convection in the most viscous layer and could range between 800 km and 3500 km (Le [START_REF] Bars | Whole-layer convection in an heterogeneous planetary mantle[END_REF]. So again, given the sizes of mantle LLSVPs, each of them should host several thermochemical upwellings.

Thermochemical upwellings present a strong time-dependence, as their chemically denser component will loose their heat as they rise, and therefore eventually reach a level of neutral buoyancy, after which the denser material will fall back down (e.g. [START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF]; fig. 4B,C,D). Hence, in a thermochemical situation, all the hot material is not buoyant. It could stagnate at some level in the mantle (fig. 4D,F), or, it could even go down if its compositional density exceeds its thermal buoyancy (e.g. [START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF][START_REF] Cadio | Pacific geoid anomalies revisited in light of thermochemical oscillating domes in the lower mantle[END_REF][START_REF] Dannberg | Low-buoyancy thermochemical plumes resolve controversy of classical mantle plume concept[END_REF]. Moreover, while the chemically denser material stagnates or descends, it is still heating the surrounding mantle, which can therefore rise as a "secondary" thermal plume (fig. 4B,C,D), or even as several of them if the stagnating thermochemical upwelling has sufficiently spread laterally (fig. 4F). The timescale of a thermo-chemical plume evolution strongly depends on the lower mantle viscosity and typically ranges between 50 and 200 Ma (e.g. [START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF][START_REF] Cadio | Pacific geoid anomalies revisited in light of thermochemical oscillating domes in the lower mantle[END_REF]. 3 LLSVPs and mantle convection from global mantle seismic tomography

Model SEMUCB-WM1

In figure 1, we show the lateral variations of isotropic shear velocity (V S ) at 2800 km in model SEMUCB-WM1 [START_REF] French | Whole-mantle radially anisotropic shear-velocity structure from spectral-element waveform tomography[END_REF], and its filtered version at l max = 12 (fig. 1b), compared to that of an earlier, lower resolution, tomographic model, S362ANI [START_REF] Kustowski | Anisotropic shear-wave velocity structure of the Earth's mantle: a global model[END_REF]fig. 1c). While the filtered SEMUCB-WM1 model shows some amplitude differences with S362ANI, the overall patterns are in good agreement, as is reflected in a voting map (fig. 1d) such as that of [START_REF] Lekic | Cluster analysis of global lower mantle tomography: anew class of structure and implications for chemical heterogeneity Earth[END_REF], comparing 5 tomographic models, highlighting the agreement across the models on the extent of the two LLSVPs in the last 500 km of the mantle, and the ring of fast velocity surrounding them. While this voting map is quite remarkable, what is not always realized is that it only shows the agreement of the models at long wavelength, as these models have been filtered up to degree 16 in a spherical harmonics expansion (about 2500 km wavelength), prior to applying the voting. At these long wavelengths, all models present lower than average shear velocities within the rather well defined boundaries of the LLSVPs, and higher than average shear velocities outside of the LLSVPs, with the exception of one particular region, resolvable at that scale and named the "Perm anomaly", which exhibits lower than average V S in all five models. What needs to be emphasized, is that this does not mean necessarily that the area covered by the LLSVPs is uniformly of lower than average V S . There could be "holes" in both the LLSVPs and in the surrounding regions, which simply would be filtered out in this voting map, either because they are of too fine a scale, or because they are not consistently resolved in the same locations in all mod-els. The presence of such holes, not visible at degree 16, is evident when one inspects individual models at their nominal resolution (e.g. fig. 1a, but also other models). [START_REF] Cottaar | Morphology of seismically slow lower-mantle structures[END_REF] extended the voting map concept higher above the CMB, delineating large dome-like structures that extend, in some cases, to heights of more than 1000 km above the CMB. However, their study is again based on long-wavelength filtered versions of the 5 models considered (up to degree 18), that can miss interruptions in the domes which may not appear at this scale. Nevertheless, these maps have been very influential in conveying the idea that LLSVPs are towering "piles" or "domes" of large lateral extent and height. However, while voting maps are useful to investigate the most robust features of shear velocity models -necessarily at long wavelengths -they cannot be considered at face value as representing true mantle structure. Likewise, comparison of models in order to extract the "true" structure (e.g. [START_REF] Becker | A comparison of tomographic and geodynamic mantle models[END_REF] only works to a point, as mantle tomography techniques evolve and improve with time, so that not all models are "equal" and some important features may have been missed in earlier studies simply because of lack of resolution, while even nominally higher resolution models may vary in which regions are better resolved than others due to differences in datasets and inversion techniques. Indeed, cross-correlation between different global tomographic models shows much poorer correlation (<0.4) for degrees higher than 16 in the lower mantle (e.g. [START_REF] Meschede | Lateral heterogeneity scales in regional and global upper mantle shear velocity models[END_REF]. In model SEMUCB-WM1, while not all the small scale low velocity minima (fig. 1a) may be well resolved or stable, many of the strongest ones can be followed vertically for at least 500 km from the CMB, as illustrated in Figure 5. Here, the S-wave velocity local minima have been outlined by black circles (solid when the local minimum is lower than -1% and dashed when it is lower than -0.5%). The circles have been positioned at 2800 km depth and dropped at the same locations on all subsequent panels. The vertical continuity of the low V S areas is good, especially for the stronger minima. The typical distance between two minima is about 1000 km (fig. 6). According to section 2, these V S minima could correspond either to purely thermal TBL instabilities controlled by a mantle viscosity of 10 22 Pa.s (fig. 3), or to thermochemical instabilities. This is even more clearly seen in cross-sections, highlighting some of the strongest vertical plume-like conduits that extend vertically at least 2000 km from the CMB (fig. 78), as has been extensively discussed in [START_REF] French | Broad plumes Rooted At The Base Of The Earth's Mantle Beneath Major Hotspots[END_REF]. Figure 9 shows a comparison of cross-sections through some of these plumes in models SEMUCB-WM1(bottom) and the more recent model GLAD-M25 (Lei et al., 2020, top) showing very similar morphology in at least some of the large plume conduits in these two full-waveform-based tomographic models.

Resolution tests

We have also conducted synthetic resolution tests (fig. 10111213), in which we have designed input models representing either a compact, continuous LLSVP-like anomaly (fig. 1011) centered in the Pacific, or a narrower vertical "plume" under Hawaii (fig. 12), and passed them through the resolution matrix obtained for the last iteration of the inversion in the construction of SEMUCB-WM1, to show how these structures might get deformed or degraded by inversion. Note however, that this kind of resolution test only validates the last iteration of the tomographic inversion, which is a non-linear problem. Performing a full resolution test, which involves several iterations of forward and inverse computations would be computationally prohibitive. In fig. 10, an LLSVP-like pile with uniform V S reduction of 2% extends from the core-mantle boundary to 1800 km depth. Its lateral and height extent is well recovered in the output model, and it is not chopped up into separate columns. There is no loss of amplitude at the bottom of the mantle, and a maximum 25% loss in the mid mantle, depending on the ray coverage. In the lower mantle, a V S reduction of 2% would typically correspond to a temperature increase of ∼400 o C if there is no associated compositional anomaly (e.g. Karato, 2008;[START_REF] Deschamps | The primitive nature of large low shear-wave velocity provinces[END_REF]. Likewise, the pile shown in fig. 11, which extends only 600 km above the core mantle boundary is also well recovered both in height, lateral extent, and amplitude, without the inversion breaking it up into distinct smaller scale columns. In fig. 12, a vertical input plume of 400 km diameter and 2% velocity reduction compared to ambient mantle (located below the Hawaian hotspot) retains its shape and height in the output model, but becomes twice broader and dramatically loses amplitude (by a factor of 4). Fig. 13 shows a whole mantle depth cross-section centered on Africa, where the LLSVP is broken up into distinct columns of varying height in model SEMUCB-WM1 (fig. 13A), and the corresponding resolution test results, indicating that a thick uniform pile in this location would be well resolved (fig. 13C) as would be a series of separate narrower columns (fig. 13D). We note that forward modeling of shear wave travel times independently confirms the presence of "holes" in the African LLSVP [START_REF] Kastle | Complex Deep Structure of the African Low-Velocity zone[END_REF]. So overall, the resolution of SEMUCB-WM1 is estimated to 800 km, although with some loss of amplitude (French andRomanowicz, 2014, 2015). Any interpretation of the images in terms of plumes or thermochemical upwellings will have to keep this in mind.

How thick would be a denser pile if and when it exists ?

Fig. 14a shows a depth cross-section across the central portion of the Pacific LLSVP, far away from any known hotspot at the surface. The height of the region of significantly reduced V S across this rather uniform part of the LLSVP does not extend more than 300-400 km above the CMB, with the strongly reduced part (dlnV S < 2%) limited to an even thinner region. Such a denser pile would correspond to buoyancy numbers between 0.9 and 1.1 according to fig. 4. Higher above the CMB, this region appears as a zone of much smaller V S anomaly, which can be interpreted as either a broad and high zone of slightly elevated temperature (less than 200 o C) or a collection of weaker, poorly resolved narrower plumes. Distinguishing between these two possibilities is not possible with the current resolution of the model. However, it is clear that, if compositionally distinct pile there is, it does not extend very high above the CMB, and possibly much less than 200 km, accounting for some vertical smearing, imposed by a combination of the chosen vertical parametrization of the model, data coverage, and the long period content of the waveforms (periods > 30 s). Moreover, the base of the Pacific LLSVP is broken up into pieces with at least one big "hole" a little further east, as shown in fig. 14b.

The Indo-Atlantic LLSVP also shows the same type of discontinuous structure with some lateral gaps, for example between Cape Verde and the rest of the African slow anomaly, between Cape Verde and Iceland, or between the South Atlantic hot anomalies and the Indian Ocean ones (e.g. fig. 5, fig. 15, fig. 16a,b).

Any hot instabilities outside the LLSVPs?

Fig. 5 shows that some slower than average areas appear outside of the main LLSVPs. The "PERM" anomaly below Eurasia has been recognized for some time (e.g. [START_REF] Lekic | Cluster analysis of global lower mantle tomography: anew class of structure and implications for chemical heterogeneity Earth[END_REF], but at least 15 other slow anomalies are present in model SEMUCB-WM1, especially around the main LLSVPs. Their amplitude is often less than within the main LLSVPs. Most of them do not correspond to a hotspot at the surface. Some of them have been confirmed in detailed regional studies (e.g. [START_REF] Suzuki | Waveform inversion for 3-D S-velocity structure of D" beneath the Northern Pacific: Possible evidence for a remnant slab and a passive plume[END_REF]. Fig. 6 shows that their spacing distribution is similar to what is observed inside the main LLSVPs, with a most common spacing around 800-1000 km. So they could again be explained by instabilities from the hot TBL at the bottom of the mantle (fig. 3) and could correspond to the nascent plumes observed in the laboratory (e.g. fig. 2B), which develop out of the main plumes bundle but never reach the surface. Under the action of the cold downwellings, they either lose their heat and buoyancy rapidly, or get pushed towards the bundle within the LLSVPs and feed one of the major plumes there [START_REF] Androvandi | At least three scales of convection in a mantle with strongly temperature-dependent viscosity[END_REF][START_REF] Heyn | Stabilizing effect of compositional viscosity contrasts on thermochemical piles[END_REF].

A bundle of thermochemical upwellings in the Indo-Atlantic

That the LLSVPs were not laterally uniform but possessed lateral structures was already visible in a number of tomographic models in the early 2000's. Comparing the observations in the Indo-Atlantic domain with predictions from laboratory experiments, [START_REF] Davaille | Convective patterns under the Indo-Atlantic "box[END_REF] proposed that, in fact, the African LLSVP consisted of about 9 thermochemical instabilities at different stages of their development (fig. 15). We can now re-evaluate this analysis.

If the presently active volcanoes in Iceland, Afar, Kerguelen, Reunion and Tristan are projected radially downwards on the CMB, they are all in the vicinity of a slow anomaly. Their conduits are still difficult to trace with certainty, being only mildly slow (∼ -0.5%), with some patches of slower velocities (fig.8c,d, and fig.16a,b). This could correspond either to thermal plumes that are still too thin to be fully resolved (fig. 4Aa or g, i.e. B∼0 or B>1) or to thermochemical plumes with contorted morphologies such as fig. 4Ac,d,e (i.e. B∼0.3-0.65). The patches with the slowest velocities could be patches of compositionally denser material which would need to be hotter to rise with the plume. Note that in this case, they could still be hot but sinking, rather than rising with the plume, as was proposed for the Iceland plume [START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF]. But with informations on V S alone, we cannot discriminate between a rising or a descending patch. Fig. 16a shows that Cape Verde is a very prominent anomaly that can be traced across the whole lower mantle (fig. 8A,9).There is also a slow anomaly in the lowermost mantle below Canaries. Fig. 16a suggests that both anomalies merge around 1250 km depth. It could be because they reach there their level of neutral buoyancy (e.g. fig. 4C,D,F), which would then correspond to buoyancy numbers B∼0.45-0.7. The hotspot cluster of Cape Verde, Canaries and Azores describes a "crown" above the lower mantle slow anomalies (fig. 16a,b), as is seen in the laboratory experiments when secondary plumes are generated from the top of a spreading thermochemical instability (fig. 4F).

The lower mantle beneath Africa also shows some large features (more than 1000 km in diameter), with at least three individual locations where there is a continuous slow anomaly of less than -0.8% from the CMB to at least 1250 km (fig. 13a, fig. 16c). As already discussed, the resolution tests (fig. 13c,d) suggest that the "holes" separating the slow anomalies are real. So, instead of the long tilted slow anomaly going from South Africa to Afar suggested by some earlier studies (e.g. [START_REF] Ritsema | Complex Shear Wave Velocity Structure Imaged Beneath Africa and Iceland[END_REF], b,fig.16) shows at least 3 or 4 distincts vertical anomalies, notably one just below the East Africa volcanism (fig. 8C,16c). Given the lateral extent of the LLSVP below Africa (> 5000 km), the development of several distinct upwellings is more in line with fluid mechanics constraints, as already discussed in section 2.

A bundle of thermochemical upwellings and plumes in the Pacific

Figure 17 shows a 3D view of the Pacific LLSVP together with the active hotspots according to [START_REF] Clouard | How many Pacific hotspots are fed by deep-mantle plumes?[END_REF]. As already noticed, this LLSVP is not in one piece but hosts several individual tubular slow anomalies (numbered on fig. 17):

-To the west, there is a prominent slow anomaly (1) beneath Caroline hot spot in the lower mantle with a diameter >1000 km. It seems to spread around 1250km depth, and there is no corresponding large anomaly in the upper mantle. However, the existence of the Caroline hotspot suggests that there should exist a secondary plume between the lower mantle slow anomaly and the surface. Since the upper mantle viscosity is typically at least 30 times lower than the lower mantle's, the diameter of this secondary plume could be as thin as 100 km (fig. 3a) and therefore would not be resolved by SEMUCB-WM1. Caroline would therefore be produced by a thermochemical plume such as in fig. 4Adf, C or D, pointing towards buoyancy numbers between 0.45 and 0.7.

-The lower mantle below Hawaii (2) hosts a mild slow anomaly and some much slower patches (fig. 7b ; [START_REF] French | Broad plumes Rooted At The Base Of The Earth's Mantle Beneath Major Hotspots[END_REF]. These could correspond to the thermochemical patches seen on fig. B,C, with buoyancy numbers around 0.3-0.6. Galapagos (7), Easter (8), and Louisville (9) present the same type of patchy morphology, although their anomalies are fainter than in the Hawaiian case (fig. 18b). As already discussed for the Indo-Atlantic long-lived hot-spots, this could correspond either to plumes still too thin to be fully resolved (fig. 2Ba,b or g) or to thermochemical plumes with contorted morphologies such as fig. 2Bc,d,e. The absence of a clear slow anomaly at the root of Louisville on the CMB could suggest that Louisville has exhausted all the hot material from the TBL and is therefore a dying plume. That would fit well with its recent very weak activity (e.g. [START_REF] Clouard | How many Pacific hotspots are fed by deep-mantle plumes?[END_REF].

-The center of Pacific LLSVP hosts 3 distinct large slow anomalies continuous over the lower mantle: Samoa (4) is distinct from the large upwelling (5) north west of Tahiti, Rurutu and Rarotonga. These three hotspots could be a cluster of secondary plumes born in the upper mantle on top of the lower mantle upwelling (5) [START_REF] Davaille | Simultaneous generation of hotspots and superswells by convection in a heterogeneous planetary mantle[END_REF][START_REF] Kumagai | On the fate of mantle thermal plumes at density interface[END_REF][START_REF] Suetsugu | South Pacific mantle plumes imaged by seismic observation on islands and seafloor[END_REF], a situation already described for Cape Verde and suggesting buoyancy numbers B∼0.45-0.7. East of them, Pitcairn and McDonald appear to stem from another large upwelling (6) of the same type. This whole area from Tahiti to Pitcairn is covered by a "super swell", where the seafloor topography is abnormally high. It has been long interpreted as the signature of a large upwelling in the lower mantle (e.g. [START_REF] Mcnutt | Superswells[END_REF] ; SEMUCB-WB1 suggests that it is in fact constituted of at least two distinct thermochemical upwellings.

-North-east of Tahiti, in between Hawaii and Marquesas, (fig. 18b), there is also another strong low anomaly in the bottom lower mantle (3b) linked by a weaker one to another slower patch (3a) below the 1000 km depth horizon. At the surface, this area corresponds to a low in the geoid and in the seafloor topography, which had led [START_REF] Cadio | Pacific geoid anomalies revisited in light of thermochemical oscillating domes in the lower mantle[END_REF] to propose the existence of a hot but denser downwelling in the upper lower mantle to explain the data. SEMUCB-WM1 model see a slower patch (3a) at the same place. So this area could be another thermochemical upwelling which, after producing the Line Islands 40 Myr ago, has lost its buoyancy and is on its descent [START_REF] Cadio | Pacific geoid anomalies revisited in light of thermochemical oscillating domes in the lower mantle[END_REF] , an evolution similar to fig. 4B, corresponding to a buoyancy number around 0.45.

Two main plumes bundles and the mantle compositional structure

The previous sections suggest that the Atlantic and the Pacific LLSVPs both host at least 9 individual upwelling instabilities. On the tomographic present-day snapshots, these upwellings can present rather different morphologies. This can be due to their different compositional structure (i.e. different buoyancy numbers), but also to their different stage of development as the shape of a thermochemical plume is strongly timedependent (fig. 4). MORB enrichment), the phase (e.g. post-perovskite), and the temperature (e.g. [START_REF] Deschamps | The primitive nature of large low shear-wave velocity provinces[END_REF]. Adding informations from attenuation, or the gravity field, should help narrow down the properties of these thermochemical plumes.

Conclusion

At the present level of resolution in seismic shear velocity tomography, it is possible to assert that the LLSVPs are broken up into bundles of thermochemical plumes, some of which are well resolved, and others, likely weaker or thinner, are only suggested. The well resolved plumes have a surface expression (i.e. a hotspot) which is not always located immediately vertically above them, while some of the less well resolved, presently only "suggested" ones may not correspond to any active feature at the surface of the Earth. There is evidence for plume-like features rooted near the CMB trying to "make their way" through the regions covered by ancient slabs. This indicates that plumes are strongest and best developed outside of the ring of present and past subduction, thus forming two bundles of plumes, of which the LLSVPs represent the long wavelength expression with strong degree 2 and 3 components. Plumes are mostly where the slabs do not suppress them and their spatial distribution is governed by the distinct configuration of slabs within the ring of fast velocities passing through the Earth's rotation poles. The LLSVPs cannot be described as two compact, continuous structures of distinct composition from their surroundings, and extending high above the CMB. Instead, (1) they are each composed of several thermochemical upwellings probably enriched in denser than average material, the roots of which can even host large ULVZs as evidenced for at least four mantle plumes : Hawaii [START_REF] Cottaar | An unusually large ULVZ at the base of the mantle near Hawaii[END_REF], Samoa [START_REF] Thorne | Megaultra low velocity zone and mantle flow[END_REF], Iceland [START_REF] Yuan | Seismic evidence for partial melting at the root of Major Hotspot Plumes[END_REF] and Marquesas [START_REF] Kim | Sequencing seismograms: A panoptic view of scattering in the core-mantle boundary region[END_REF]; (2) their overall morphology is dictated by subduction position, and (3) their dynamics should strongly depend on time. This should be taken into account when developing scenarios for their origin, while awaiting further improvements in the resolution of deep mantle 3D seismic structure.

Figure 1 .

 1 Figure 1. a) Relative perturbations (in percent) in shear velocity in model SEMUCB-WM1

Figure 2 .

 2 Figure 2. Plumes development and morphology in the purely thermal case. A) Growth of TBL instabilities in sugar syrup at Ra=6.5 10 5 (run from Androvandi et al, 2011). The vertical line is a thermocouple probe. The other white lines are isotherms. a . Time t=260 s, growth of the TBL by thermal diffusion: the isotherms are flat and parallel to the heated bottom boundary; b. t=440 s, a thermal mushroom-shape plume rises from the TBL; c. t=440 s, as the plume rises, the TBL shrinks; d. t=540 s. B) Experiment in sugar syrup at high Rayleigh number and intermediate viscosity ratio (run lRa12 from Androvandi et al, 2011: Ra hot = 5.3 10 7 ; γ = 142). Vertical cross-section taken 8 cm from the front lateral wall. The bright heavy lines are isotherms. The two downwellings are part of a cold cell that surrounds a bundle of hot upwellings. The nascent plume indicated will remain trapped under the cold gravity current due to the downwelling and will eventually diffuse away. "T" indicates a thermocouple probe.

Figure 3 .

 3 Figure 3. a) Typical thickness of the hot TBL, which corresponds also to the plume conduit diameter , and b) plume spacing as a function of mantle viscosity for temperature contrasts across the TBL of 300 o C (green dashed line) and 1000 o C (red solid line).

Figure 4 .

 4 Figure 4. A) Thermochemical plumes morphology for increasing B . The orange material is denser. B-F) Development through time of the plumes for different buoyancy ratios. B) B=0.42; C) B=0.47; D) B=0.72; E) B=1.09 (A-E: adapted from Kumagai et al, 2008); F) B=2.4 (adapted from Kumagai et al, 2007).

Figure 5 .

 5 Figure 5. Slow local minima from SEMUCB-WM1 at decreasing depths: a) 2800km, b) 2650 km, c) 2500 km, d) d=2300 km. The solid black circles represent local minima lower than -1% and the dashed circles represent local minima lower than -0.5%. The surface hotspots are superimposed in green.-10-

Figure 6 .

 6 Figure 6. Histograms of the distance between two local minima from SEMUCB-WM1 : a) first neighbours, b) second neighbours, c) third neighbours, d) fourth neighbours. The blue rectangles are for local minima lower than -0.5%, and red triangles for local minima lower than -1%. The latter correspond to minima inside the LLSVPs only (if we take the long wavelength LLSVPs contours of Lekic et al (2012) vote map), while the former covers the minima population inside and outside the LLSVPs.

Figure 7 .

 7 Figure 7. Examples of depth cross-sections in the isotropic VS part of radially anisotropic model SEMUCB-WM1, expressed as relative velocity perturbations (in percent) with respect to global average at any given depth, in the Pacific. These cross-sections highlight broad plume conduits originating at the CMB and extending quasi-vertically through the lower mantle and in some cases in the upper mantle in the vicinity of prominent hotspots, with varying intensity/resolution. The locations of the cross-sections are shown in the inset maps, with hotspots indicated as green dots. Depth tickmarks are every 500 km, with the deepest one at 2500 km depth. Broken lines mark the 400, 660 and 1000 km depth horizon. In (D) the halo of the San Fernando plume, which is out of plane, is visible south of the Galapagos

Figure 8 .

 8 Figure 8. Same as Figure 7 in the Atlantic hemisphere. These cross-sections highlight broad plume conduits originating at the CMB and extending quasi-vertically up to at least 1000 km depth in the vicinity of prominent hotspots, with varying intensity/resolution. In (B), the separation between the Reunion and Marion plumes appears poorly resolved. They seem to both originate in nearby patches at the CMB. In (C), the strong plume conduit likely corresponds to the East African volcanism under Kenya, even though the triangle further South marks the projection of the location of the Comores hotspot on this cross-section.

Figure 9 .

 9 Figure 9. Comparison of two cross-sections in models GLAD-M25 (top, Lei et al., 2020) and SEMUCB-WM1 (bottom, French and Romanowicz, 2014), which shows good qualitative agreement in the morphology of fat plumes both in the Pacific and Atlantic hemispheres. We note that other authors have previously argued for the presence of fat plumes in the lower mantle based on travel times of P and S waves (e.g.[START_REF] Montelli | Finite-frequency tomo-graphy reveals a variety of plumes in the mantle[END_REF].

Figure 10 .

 10 Figure 10. Resolution test with a 1800 km-tall pile under the Pacific. The top panels show the input model, and the bottom panels show the output model, after passing through the resolution matrix computed for the last iteration of the construction of model SEMUCB-WM1.

Figure 11 .

 11 Figure 11. Same as fig.10 for a 600 km-tall pile under the Pacific.

Figure 12 .

 12 Figure 12. Resolution test for a plume rooted at the CMB at the location of the Hawaian hotspot. The input plume has a diameter of 400 km and a shear velocity reduction of 2%. The output of the resolution test (multiplication by the resolution matrix as in figs 10 and 11) shows the broadening of the plume and loss of amplitude by a factor of 4 in this location. However, the plume is not distorted and its height is well recovered.

Figure 13 .

 13 Figure 13. (A-B) Depth cross-sections (as in figures 7-8) under Africa showing at least three tubular, distinct quasi vertical conduits extending from the CMB to the mid-mantle. The black dashed contours have been drawn by hand and represent a qualitative interpretation of the tomographic images. (C-D) Resolution tests with, in each panel: (top) input model and (bottom) output model after passing through resolution matrix, showing that the inversion can resolve the difference between a fat pile (C) and a collection of individual low velocity conduits (D). The cross-section is the same as in (A). The result of the test in (D) indicates that the top part of the two left plumes contoured in (A) and (B) may be wider than considered in the test model (plume heads?).

Figure 14 .

 14 Figure 14. Depth cross-sections of shear velocity perturbations, expressed in percent relative to global average at each depth, in model SEMUCB-WM1, across a quiet region of the Pacific LLSVP. Insets show location of the cross-sections with the background map showing the model at 2800 km depth with the locations of major hotspots in green. Depth tick marks are every 500 km. In both cross sections, the zone of strongly reduced VS (< -2%) extends at most 200 km above the CMB. In (A) this zone appears continuous in this model, while in (B), it is separated into two parts with a large hole in between. The halos of the Samoa and Hawaii plumes (e.g. fig.7b) appear as diffuse zones of mildly reduced VS above the LLSVP in panel (B).

Figure 15 .

 15 Figure15. The 9 thermochemical instabilities proposed by[START_REF] Davaille | Convective patterns under the Indo-Atlantic "box[END_REF] superimposed on model SEMUCB-WM1 at 2800 km depth. 1-thermochemical plume beneath Iceland (see also[START_REF] Kumagai | Mantle plumes: Thin, fat, successful, or failing? Constraints to explain hot spot volcanism through time and space[END_REF], 2-dying plume beneath Afar, 3-plume beneath Reunion, 4-plume below Kerguelen, 5-thermochemical dome beneath a plume cluster under Marion and Crozet, 6-fading plume below Tristan, 7-"banana-shaped" thermochemical pile[START_REF] Ni | Ridge-like lower mantle structure beneath South Africa[END_REF], 8thermochemical dome beneath a plume cluster under Cape Verde, Azores, Canaries and Madeira, 9-thermochemical upwelling below central Africa. The surface hotspots are superimposed in green.

Figure 16 .

 16 Figure 16. Model SEMUCB-WM1. a) and b) 3D views of the Indo-Atlantic LLSVP between 2800 km and 1250 km. In orange the -0.8% isosurface. The four black ellipses outline the locations where there is a continuous and vertical slow anomaly of less than -0.8% from the CMB to 1250 km at least. c) 3D cross-section (20 o E-45 o E) from South Africa to Afar showing a succession of at least 3 vertical upwellings extending at least between 2800 km and 500 km.The green bars show the surface hotspots.

Figure 17 .

 17 Figure 17. 3D view of the Pacific LLSVP between 2800 km and 1250 km. In orange, the -1.0% isosurface. The names indicate the active hotspots in the Pacific according to Clouard and Bonneville (2001). The numbers indicate the different upwellings discussed in the text. The black arrows point towards the surface hotspots.

Figure 18 .

 18 Figure 18. Depth cross sections in model SEMUCB-WM1, presented as in figs 7-8, a) north of Marquesas and b) going through Louisville and Easter, which may be plumes with weak intensity, or not as well resolved as others. The numbers refer to fig.17.

According to our geodynamic interpretation of the tomographic images, the plume buoyancy numbers may range between 0.3 and 0.7, possibly even 1. So this gives a relationship between the density anomaly of compositional origin ∆ρ X and the temperature anomaly ∆T e at the base of the upwelling (equation 3). For ∆T e = 750 o C, ∆ρ X would range between 0.4% (B=0.3)and 1.5% (B=1). This suggests that the LLSVPs are also heterogeneous in composition, which could be the result of convective stirring of several components. However, we cannot go further yet in the analysis from inverting dV S alone, as dV S depends on the tomographic model resolution (i.e. on the method, the ray coverage, and the size of the anomaly), the composition (e.g. iron, SiO2, Perovskite or -24-manuscript submitted to Tectonics Acknowledgments This paper resulted from discussions that started during the Symposium on "50 Years of Plate Tectonics" at the Collège de France, Paris, in June 2018. It was improved thanks to the comments of two anonymous reviewers. BR acknowledges support from NSF CSEDI grant EAR-1464014, and AD support from LabEx PALM (ANR-10-LABX-0039-PALM, project MESOPLANET) and the Programme National de Planétologie (PNP) of CNRS/INSU, co-funded by CNES. The SEMUCB-WM1 tomography model can be downloaded at: http://seismo.berkeley.edu/wiki br/Broad plumes rooted at the base of the mantle beneath major hotspots.


P Delahaye
email: pierre.delahaye@ganil.fr.

M Dubois

L Maunoury

J Angot

O Bajeat

B Blank

J C Cam

P Chauveau

R Frigot

B Jacquot

P Jardin

P Lecomte

S Hormigos

O Kamalou

V Kuchi

B Osmond

B M Retailleau

A Savalle

T Stora

V Toivanen

J C Thomas

E Traykov

P Ujic

R Vondrasek

New exotic beams from the SPIRAL 1 upgrade

Keywords:

Since 2001, the SPIRAL 1 facility has been one of the pioneering facilities in ISOL techniques for reaccelerating radioactive ion beams: the fragmentation of the heavy ion beams of GANIL on graphite targets and subsequent ionization in the Nanogan ECR ion source has permitted to deliver beams of gaseous elements (He, N, O, F, Ne, Ar, Kr) to numerous experiments. Thanks to the CIME cyclotron, energies up to 20 AMeV could be obtained. In 2014, the facility was stopped to undertake a major upgrade, with the aim to extend the production capabilities of SPIRAL 1 to a number of new elements. This upgrade, which is presently under commissioning, consists in the integration of an ECR booster in the SPIRAL 1 beam line to charge breed the beam of different 1+ sources. A FEBIAD source (the so-called VADIS from ISOLDE) was chosen to be the future workhorse for producing many metallic ion beams. This source was coupled to the SPIRAL 1 graphite targets and tested on-line with different beams at GANIL. The charge breeder is an upgraded version of the Phoenix booster which was previously tested in ISOLDE. It was lately commissioned at LPSC and more recently in the SPIRAL 1 beam lines with stable beams. The upgrade additionally permits the use of other target material than graphite. In particular, the use of fragmentation targets will permit to produce higher intensities than from projectile fragmentation, and thin targets of high Z will be used for producing beams by fusion-evaporation. The performances of the aforementioned ingredients of the upgrade (targets, 1+ source and charge breeder) have been and are still being optimized in the frame of different European projects (EMILIE, ENSAR and ENSAR2). The upgraded SPIRAL 1 facility will provide soon its first new beams for physics and further beam development are undertaken to prepare for the next AGATA campaign. The results obtained during the on-line commissioning period permit to evaluate intensities for new beams from the upgraded facility.

Scientific motivations

During the past decades, the SPIRAL 1 facility at GANIL has been delivering radioactive ion beams of unique intensity and purity for physics experiments. SPIRAL 1 makes use of the so-called "Isotope Separation On Line" (ISOL) technique [1]. Ionized in ion sources, ISOL beams have an optical quality which is comparable to this of the stable ion beams. In particular, the energies accessible at SPIRAL, from a few keV to 20 AMeV for the lightest nuclides accelerated by CIME, allow for a unique variety of studies using common techniques of decay spectroscopy, Coulomb excitation, fusion, transfer and direct reactions. During the past decade a number of interesting physics results were achieved at SPIRAL 1 using these techniques, leading to published highlights addressing questions that can only be handled with reaccelerated radioactive beams. Despite remarkable achievements [2] and its status of first world-class facility, SPIRAL 1 has been technically limited to the production of radioactive ion beams of gaseous elements, thus limiting the physics opportunities of the facility.

A project of upgrade was started in 2011 to complement the radioactive ion beam production capabilities of the facility towards condensable elements (see Sec. 2). The scientific interest of the upgrade of SPIRAL 1 was sustained by different calls for letter of intents, which gathered a large response from the international nuclear physics community. In total, more than 100 beams were discussed during a workshop dedicated to SPIRAL 1, 2/3rd of which are using a new Target Ion Source System (TISS), which couples the SPIRAL target with a FEBIAD ion source [3]. First experiments using the new TISS should run during 2019. The upgrade opens on the one hand numerous opportunities with reaccelerated beams with innovative setups such as AGATA, MUGAST or ACTAR. On the other, it offers rich perspectives with low energy beams for weak interaction physics and beta/rare decay studies, in which DESIR [4] will play a major role in the upcoming years.

Upgrade layout

Since 2001, SPIRAL 1 makes use of thick graphite targets, on which the high power heavy ion beams of GANIL impinge. The radioactive fragments diffuse as atoms out of the hot target and effuse via a transfer passage to the ion source. Ionized to the required charge state by the Nanogan III Electron Cyclotron Resonance (ECR) ion source, the radioactive ions are then separated and accelerated in the Cyclotron d'Ions de Moyenne Energie (CIME) [1]. Fig. 1 presents the Target -Ion Source System (TISS) that was used so far at SPIRAL 1. The ionization technique prevents the ionization of condensable elements as the transfer tube and the ECR source is a cold assembly that stops the effusion of radioactive isotopes of condensable elements from the hot target to the ion source plasma. The SPIRAL upgrade project consisted in the development of a 1+ to n+ charge breeding system, permitting the use of versatile 1+ sources for extending the range of elements available for postacceleration to condensable elements.

A FEBIAD source as workhorse for the upgrade

Compared to rare gases, radioactive atoms of condensable elements produced by the ISOL method suffer from comparatively longer sticking times on surfaces that they encounter during their transport from the target to the ion source volume. As high temperature reduces the sticking time, the condensable elements are traditionally produced using hot target and ion source assemblies [5]. Typical ion sources used in this case are hot surface sources, such as surface ionization sources, Forced Electron Beam Induced Arc Discharge sources (FEBIAD sources also called "hot plasma" sources), and Resonant Ionization Laser Ion Source (RILIS). An alternative to this scenario was tested at GANIL in the past by coupling directly ECR sources to targets at the SIRa test bench [START_REF] Lecesne | Etude de la production d'ions radioactifs multichargés en ligne[END_REF][START_REF] Jardin | [END_REF]. Due to the diversity of the beams requested in the letters of intent, a FEBIAD source recently improved at ISOLDE, the so-called VADIS [8] was found to be the ion source of condensable elements to be coupled in priority with the SPIRAL targets. The FEBIAD source can deliver a number of beams in addition to those traditionally produced by surface ionization and ECR ion sources, as can be seen on Fig. 2. It is a first step towards a universal beam production that the more sophisticated, but cleaner RILIS could complement at a later stage.

New targets

The upgrade was also aiming at extending the safety authorizations for a number of combinations of primary beams and targets. The previous authorizations were limited to projectile fragmentation on graphite targets. In the frame of the upgrade, these authorizations were extended to other target materials for fragmentation and fusion evaporation reactions. The following primary beamtarget combinations are now authorized:

• 12 C (up to 95 MeV/A) to 238 U (up to 8 MeV/A) impinging on thick graphite targets, as it previously was • 12 C (2.10 13 pps) at 95 MeV/A impinging on thick targets of Z up to 41 (Nb) • 12 C (95 MeV/A@2.10 13 pps) to 238 U (8 MeV/A@Imax.) on thin targets without restriction on Z A Nb target and fusion evaporation targets are presently being developed [9,10] and should be tested in the coming years. The fusion evaporation target benefits from a collaboration with IPN Orsay [10].

ECR charge breeding

At reacceleration facilities, charge breeding is a technique consisting in the injection of a 1+ beam into the plasma of an Electron Beam Ion Source (EBIS) or Electron Cyclotron Resonance Ion Source (ECRIS) and of its subsequent multi-ionization for reaching the charge state required by the post-accelerator [11]. The charge breeding is a competitive and cost effective [12,13] and upgraded since to reach the best performances with this device.

New beams from the FEBIAD TISS

TISS development

The FEBIAD TISS couples standard SPIRAL 1 1200W C targets to the VADIS via an ohmic heated tantalum transfer tube (Fig. 3). The TISS was developed in a staged approach. The early versions were first tested online in 2011 using the SIRa test bench at modest primary beam power. They demonstrated the efficient ionization of radioactive isotopes of 8 new elements: the alkali Na, K, metallic Mg, Al, Fe, Cu, Mn and halogen Cl elements. The TISS lifetime was however limited due to various issues related to the dilatation of the transfer tube. In 2013, these issues were fixed by enabling a displacement of the ion source along the axis of the secondary ion beam extraction (Fig. 3) [14]. The modified TISS was then tested at nominal power (1200W of 36 Ar at 95 AMeV) in the SPIRAL 1 beam lines in December 2013, leading to the first scientific results obtained at SPIRAL 1 with a FEBIAD source [15,16]. These on-line tests at SPIRAL 1 gave generally even better secondary beam intensities than expected from the measurements at SIRa, showing that the yields do not scale linearly with the primary beam power. The intensities shown in Fig. 4 were published in [17]. At the very end of the tests, the anode entered in short circuit with the cathode. This failure was attributed to the slow dissociation and metallization during operation of BeO insulators supporting the anode of the FEBIAD (see Fig. 3), as Be + ions were visible in high amount (1µA), dominating the ion source spectrum. In order to increase the reliability of the target ion source, a program of test was rigorously followed on the SPIRAL 1 off-line test bench with the aim to select the best insulator, which could stand operation under high temperature. The results of these tests permitted to show that on the test bench boron nitride (BN) insulators permitted a stable operation over a period of about 3 weeks. These tests were realized with an important number of heating and cooling cycles, as the ion source was switched on in the morning and off in the evening for half of the tests period. The nominal and reproducible efficiencies for stable rare gases were determined (Sec. 3.3). At this stage, the target ion source was considered as ready for operation. 2016 and 2017 were used to provision the FEBIAD target ion sources for the startup of the upgrade and diverse characterizations detailed below. In April and May 2018, the SPIRAL 1 upgraded facility was commissioned for radioactive ion beam production. A beam of 95 AMeV of 20 Ne first impinged on the SPIRAL 1 target for the production of a 17 F beam for the E750 experiment. Unexpectedly, the anode entered again into a shortcut with the cathode very rapidly (6-8h) after the beam was tuned on target. Again, the anode insulators were identified as the probable cause for the FEBIAD breakdown, contrasting with the observations done on the test bench without beam on target. During discussions with a panel of experts from ISOLDE and TRIUMF, the possible explanations mentioned for the different behavior of the source online and off-line were the different heating /thermal conditioning of the target ion source as well as the possible deposition of direct C vapors on different parts of the source, including the insulators. In order to fix these issues, a new FEBIAD ion source was conditioned with a different configuration of heat shields permitting to cool down the insulators. A helical chicane was inserted in the transfer tube to stop the direct C vapor from the irradiated target (Fig. 5). With these modifications, the FEBIAD target ion source could be run with first a low power (100-200W) 40 Ca beam and finally a high power (>800W) 36 Ar beam, both at 95 AMeV without failure. The secondary beam intensities measured for the latest test are shown in Fig. 6. As it clearly appeared later, the diagnostic used to center the primary beam on target was malfunctioning during the commissioning tests of 2018. The wrong steering caused lower yields than expected. As it was observed later with a Nanogan TISS delivering 14 O to the E744 experiment, a factor of 50 to 100 improvement of the production yield could be obtained at the SPIRAL identification station [18] by tuning the primary beam steerers to correct for the faulty diagnostic. The wrong steering might even have been a possible cause for the early failure of the FEBIAD TISS with 20 Ne.

The FEBIAD target ion source is being presently further consolidated for final adjustment on the test bench, before being recommissioned online in the upcoming running period. In addition to the aforementioned modifications, a few adjustments are tested. In particular the material of the poles supporting the fragile insulators, originally in Mo, are replaced by Re (see inset of Fig. 3), which has a lower thermal conductivity. These Re poles will permit to further protect the insulators from the hottest parts of the ion source. With these adjustments, it is believed that the FEBIAD target ion source, will have most chances to behave very well online.

Secondary beam intensities

The 1+ beam intensities measured in 2013 and 2018 from the FEBIAD TISS at high power with a beam of 36 Ar at 95 AMeV are shown in Fig. 4 and6. The yields measured in 2018 are considered as very conservative, as it was understood later that the primary beam positioning was not controlled during the on-line commissioning, resulting in degraded production conditions.

An analysis of the data measured in 2013, briefly presented in [17], permitted to deduce from the measured intensities the typical release times and ionization efficiencies for the different elements observed with a Ge detector at the 1+ beam identification station. This analysis bases itself on the empirical release parametrization used in [19] for the ISOLDE yields. From the ionization, transport and release efficiencies deduced from this analysis, and using the EPAX V2 parametrization [20] for fragmentation cross sections, projections can be made for the isotopes of the different elements produced so far with the FEBIAD TISS. The results of these projections are shown in Fig. 7, making an optimized use of different primary beams available at GANIL. These projections were used as guideline for "day 1" beams, available for the first call for proposals of experiments with the SPIRAL 1 upgraded facility. For experiments using reaccelerated beams, these 1+ beam intensities have to be folded with breeding (~5-10%, see Sec. 4) and acceleration efficiencies (~20%), so that the resulting accelerated beam intensities are typically a factor 100 lower.

More speculative extrapolations have been done for the production of radioactive isotopes for other elements typically produced by FEBIAD sources, as for instance observed at ISOLDE. For these elements, the release efficiencies have been estimated from the parameterization of on-line data obtained at ISOLDE [19], PARNNE [21], and if not available from diffusioneffusion coefficients found in the literature, using the parametrization of diffusion and effusion described by Kirchner in [22]. The FEBIAD ionization efficiencies are extrapolated from measured data with noble gases. The results of such extrapolations are shown for the fragmentation of the different stable beams available at GANIL and using new target materials for fragmentation and fusion evaporation in Fig. 8 and9 respectively. The following fragmentation targets were considered: SiC, CaO, NiO, Nb using a 12 C beam at 95 AMeV and at the highest power available at GANIL (3.6 kW).The fusion-evaporation cross sections have been estimated using the procedure described in [23] for a number of target materialstable beam combinations. The plot shows only the best combination. The same release efficiencies as for the fragmentation targets have been assumed, which is a priori conservative as for this reaction mechanism many tricks can be used in order to speed up the transport of radioactive atoms to the ion source [9]. A list of secondary beam intensities as obtained using these different estimates are available on the GANIL -SPIRAL 2 website [24].

Ongoing investigations

During the conditioning of the various versions of the TISS, the ionization efficiencies of the FEBIAD have been repeatedly monitored on the off-line test bench with stable rare gases. These ionization efficiencies are generally reproducing the efficiencies of the traditional MK5 of ISOLDE, which are a factor of ~4 lower than the VADIS ones quoted in [8]. ISOLDE equally reports that similarly lower efficiencies are regularly obtained on-line with the VADIS. Nevertheless, it is remarkable that the efficiencies quoted in [8] could be sometimes obtained during the conditioning of the TISS at the SPIRAL 1 test bench, over periods of a few hours. The conditions for stabilizing this enhanced ionization regime have not yet been found, and are being investigated. One of the latest manifestation of this particular regime happened during the injection of a tiny leak of Xe (Fig. 10), whose flow rate enabled the control of the ionization efficiency, switching gradually from the standard low ionization regime to the enhanced ionization regime and vice-versa, for a couple of hours.

The energy profile of the 1+ beam from the FEBIAD source is of high interest for the charge breeding performances. The energy acceptance of the ECR charge breeder is defined by the width of its "V" curve; the n+ extracted ion beam current as a function of the difference of voltage between the 1+ source and ECR charge breeder. The acceptance of ECR charge breeders is typically of 5 to 10 eV FWHM for condensable elements. The energy profiles of the FEBIAD and Nanogan III could lately be measured and compared thanks to an analyzer manufactured by LPC Caen (Fig. 11). The energy dispersion of the FEBIAD ion source is of the order of Ey1.5eV, which is well within the acceptance range measured with the SPIRAL 1 charge breeder [25]. It is also remarkable that the plasma potentials of the Nanogan and FEBIAD source differ considerably. The origin for the voltage shown in Fig. 11 is somewhat arbitrary, because of the lack of calibration of our test bench HV power supplies. It corresponds to the 1+ source HV voltage within 20-30 V error bar. Latest simulations show that the FEBIAD can have plasma potentials as low as -70V, with an electron beam current as high as 300 mA. This result was experimentally corroborated by the first charge breeding of a radioactive ion performed at SPIRAL 1 as described in Sec. 4. In order to identify the most important mechanisms that governs the ionization performances of the FEBIAD, simulations combining the SIMION software for calculating the trajectory of charged particles (electrons and ions) in fields, and FEMM 4.2 for accounting for space charge effects, are being undertaken. At present, these simulations reproduce the plasma potential reasonably well, as the latter defines the energy profile of the 1+ beam (mean energy and dispersion) of the extracted beam. Possible ionization regimes by pure electron impact or in a plasma will have then to be investigated to attempt to reproduce the order of magnitudes of the measured efficiencies.

ECR charge breeder performances

Based on an early version of the Phoenix charge breeder used at ISOLDE for charge breeding studies [12,13], the SPIRAL 1 charge breeder has been upgraded in the frame of the EMILIE project [26] to benefit from the latest advances in the field [27,28]. The upgraded charge breeder is using ultra-high vacuum compliant components, including an Al plasma chamber. It is capable of 2 frequency heating, although only a single 14 GHz frequency heating was used so far. At injection, an iron plug has been symmetrized. The injection optics now include an electrostatic triplet and a mobile injection electrode. The puller is also mobile. The gas injection system is directly feeding the plasma chamber.

Fig. 12 presents the charge breeding efficiencies obtained on the off-line LPSC test bench [25] and in 2018 at SPIRAL 1 [29] for Na, K and Rb alkali ions, with He as support gas. The efficiencies are a factor 2 to 5 higher than those recorded at ISOLDE [12,13]. The charge breeding performances for Na were lately benefiting from the work carried out at LPSC [START_REF] Angot | Recherche et développement sur le Booster de charges Phoenix au LPSC[END_REF]: by fine tuning the magnetic field with iron rings and the extraction electrode position, the efficiency was improved by a factor about 2-3 compared to what was previously measured at SPIRAL 1 [29].

During the radioactive ion beam commissioning in May 2018, the charge breeding of 37 K 1+ to 37 K 9+ was successfully demonstrated. The secondary intensities of the 1+ FEBIAD TISS beams and charge bred beams were successively transported to the identification station. An efficiency of 5.3% was obtained after a very short period of beam tuning (couple of hours) by monitoring the activity of the rather short-lived (1.225s) 37 K isotopes at the SPIRAL identification station. For such short tuning time, the tuning only concerned the 1+ beam optics and voltage difference between the ECR charge breeder and FEBIAD anode (V). With an unusually high electron beam current (300 mA) emitted from the FEBIAD cathode, the optimized V was of the order of 80V. Compared to usual optimized V observed with surface ionization sources used for the off-line commissioning of the charge breeder (7-12V) [25,29], this value pinpoints a FEBIAD plasma potential of the order of 70V. Just prior to the injection of the radioactive ion beam, an efficiency of 10% was recorded for the charge breeding of a stable 40 Ar 1+ beam from the FEBIAD to 40 Ar 8+ , which is a factor of ~2 below the efficiencies measured with a properly tuned charge breeder. With optimized RF power, gas flow and magnetic confinement, it is quite likely that the 37 K charge breeding efficiency would have attained a similar value as those measured with the off-line surface ionization source at LPSC and SPIRAL 1.

Conclusions

The on-line commissioning of the SPIRAL 1 upgraded facility has started. As there was no charge breeding and subsequent acceleration of a radioactive ion beam delivered to an experiment, the full operation of the upgraded facility has not yet been fully demonstrated. Nevertheless, and despite early technical difficulties, the individual elements and combinations, which are required for the upgraded facility, are all operational. Beams have been successfully delivered from the FEBIAD TISS alone, from the FEBIAD TISS and charge breeder, from the charge breeder and CIME. The ultimate commissioning step, consisting in the successful delivery of beam to an on-line experiment, is being prepared. Final adjustments with the FEBIAD TISS are being undertaken on the off-line test bench. Charge breeding R&D is being pursued, with the aim of further improving the charge breeding efficiencies of light beams [START_REF] Angot | Recherche et développement sur le Booster de charges Phoenix au LPSC[END_REF], and investigate the critical parameters on which the charge breeding time depends. The combination of FEBIAD TISS with an ECR charge breeder is a rather unusual one for reacceleration facilities. Experience will be gained over the years with this peculiar system. As seen with the operation of Nanogan for many years, the CIME cyclotron intrinsic resolving power permits to purify cases the light (A<20) beams. For heavier masses, a casebycase study has to be done. The test of different stripping foils behind CIME to separate masses around 56 Ni delivered from the FEBIAD TISS and reaccelerated by CIME to energies of 10-12 AMeV will be undertaken in the coming months. For low energy beams, the High Resolution Separator [START_REF] Nieto | [END_REF] and the different beam purification techniques in ion traps such as PIPERADE [32], all present at DESIR, will be essential elements for the success of the associated experimental program.

Figure 1 :

 1 Figure 1: Target -ion source system used at SPIRAL 1 since 2001. It couples the Nanogan III ECR ion source with a graphite target via a cold transfer tube.

Figure 2 :

 2 Figure2: elements produced by the different sources envisaged after the upgrade. The FEBIAD source is a universal technique to produce 1+ beam of all elements that exhibit a melting point below 2000°C, and is therefore capable to ionize the same elements than the ECR ion sources and surface ionizers. Some of the elements that could be produced by fusion evaporation beyond Z=41 are shown by a star.

Figure 3 :

 3 Figure 3: FEBIAD TISS. The bottom insets is a zoom on the ion source critical parts. See text for details.

Figure 4 :

 4 Figure 4: Secondary intensities measured with the SPIRAL 1 FEBIAD TISS with 36 Ar at nominal power (~1200W) in 2013.

Figure 6 :

 6 Figure 6: Secondary intensities measured with the SPIRAL 1 FEBIAD TISS with 36 Ar at nominal power (800 -1200W) in 2018. A wrong steering of the primary beam is invoked as the reason for the lower yields as compared to 2013.

Figure 5 :

 5 Figure 5: photograph of the transfer tube and associated helical chicane.

Figure 8 :

 8 Figure 8: Extrapolation of secondary beam intensities (before post-acceleration) for the fragmentation of different stable beams available at GANIL [24].Figure 9: Projections for secondary intensities for radioactive isotopes produced by the FEBIAD TISS for the "day 1" experiments.

Figure 9 :

 9 Figure 8: Extrapolation of secondary beam intensities (before post-acceleration) for the fragmentation of different stable beams available at GANIL [24].Figure 9: Projections for secondary intensities for radioactive isotopes produced by the FEBIAD TISS for the "day 1" experiments.

Figure 7 :

 7 Figure 7: Extrapolation of secondary beam intensities (before postacceleration) for the fragmentation of different targets and fusion evaporation. See text for more explanations.

Figure 11 :

 11 Figure 11: Ionization efficiencies measured at ISOLDE and GANIL with the different FEBIAD sources and in different ionization regimes. See text for more explanations.

Figure 10 :

 10 Figure 10: energy profiles of the Nanogan and FEBIAD ion sources.

Figure 12 :

 12 Figure 12: Charge breeder efficiencies obtained at LPSC and SPIRAL 1 with the upgraded Phoenix charge breeder.

Acknowledgments

This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 654002.