C Ruhrmann

M Westermeier

A Bergner

J F Luijks

P Awakowicz

J Mentel

Investigating the gas phase emitter effect of caesium and cerium in ceramic metal halide lamps in dependence on the operating frequency

The work function and with it the temperature of tungsten electrodes in HID lamps can be lowered and the lifetime of lamps increased by the gas phase emitter effect. A determination of the emitter effect of Cs and Ce is performed by phase resolved measurements of the electrode tip temperature T tip (ϕ), plasma temperature T pl (ϕ) and particle densities N (ϕ) by means of pyrometric, optical emission and broadband absorption spectroscopy in dependence on the operating frequency. The investigated HID lamps are ceramic metal halide lamps with transparent discharge vessels made of YAG, filled with a buffer gas consisting of Ar, Kr and predominantly Hg and seeded with CsI or CeI 3 . In the YAG lamp seeded with CsI and CeI 3 as well as in a YAG lamp seeded with DyI 3 (corresponding results can be found in a preceding paper) a gas phase emitter effect is observed in the cathodic phase due to a Cs, Ce or Dy ion current. In the YAG lamp seeded with CsI the phase averaged coverage of the electrode surface with emitter atoms decreases and the electrode temperature rises with increasing frequency, whereas the emitter effect of Ce and Dy is extended to the anodic phase, which leads to a decreased average temperature T tip (ϕ) with increasing frequency. This different behaviour of the averaged values of T tip (ϕ) for increasing frequency is caused by the differing adsorption energies E a of the respective emitter materials. In spite of the influence of E a on the coverage of the electrode with emitter atoms, the cathodic gas phase emitter effect produces in the YAG lamps seeded with CsI, CeI 3 and DyI 3 a general reduction of the electrode tip temperature T tip (ϕ) in comparison to a YAG lamp with Hg filling only.

Introduction

Ceramic metal halide lamps offer presently the cheapest option to realize a point shaped light source with an output of more than one k-lumen satisfying the requirements of an efficacy of more than 100 lm/W, a life time of more than 10000 h and a colour rendering index higher than 90 cri. But the high thermal load of the electrodes, resulting in a high electrode temperature, is a quite serious problem with this special version of HID lamps.

The electrode temperature, both of the cathode and of the anode, is mainly determined by the work function of the electrode material [1,2,3,4,5,6,7,8]. If pure tungsten electrodes are used the temperature of the electrode tip is raised up to its melting point, T m = 3695 K, to supply a sufficiently high thermionic electron emission for a diffuse attachment of the lamp arc on the electrodes [9,10,11,12,13,14,15]. But the work function and with it the electrode temperature can be substantially reduced if a dipole layer consisting of a monolayer of atoms being electropositive with respect to tungsten is deposited on the tungsten surface. It lowers the potential barrier at the electrode surface for electrons leaving or entering the electrode.

Several techniques are applied to establish this so called emitter effect in HID lamps by a monolayer of electropositive atoms [START_REF] Waymouth | Electric Discharge Lamps[END_REF]. Doping tungsten with T hO 2 is the most traditional method to cover the electrode surface with a dipole layer of thorium atoms dated from Langmuir [START_REF] Langmuir | [END_REF]. According to the experience in vacuum electronics it is assumed that the monolayer is formed by the diffusion of thorium from the interior of the electrode, after dissociation of T hO 2 into T h metal and oxygen [18]. But this simple view is doubtful in the case of HID lamp electrodes since detailed investigations on thoriated electrodes have revealed that only HID cathodes show an emitter effect [2,19] but not HID anodes [6,7]. Another method to reduce the work function is realised by depositing emitter material within the interspaces of a closely wounded tungsten coil ending up in some distance from the electrode tip. When the electrode is heated up the emitter material, e.g. barium, diffuses along the electrode surface to the tip [20,[START_REF] Westermeier | Phase resolved Dy-density and plasma temperature measurements by absorption-and emision-spectroscopy of Dy spectral lines[END_REF].

The most versatile approach is the gas phase emitter effect, in which case the monolayer is deposited from the gas phase. The number of pure metals, which are appropriate to realize a gas phase emitter effect in HID lamps, is rather low. But the number of elements can be considerably extended, if the high vapour pressure of metal iodides is employed, e.g. that of rare earth iodides, to implement a gas phase emitter effect. In some cases a gas phase emitter effect is clearly realized, both at the cathode and at the anode, e.g. in high pressure sodium lamps, if appropriate lamp parameters are chosen [3,8]. In other cases it may be confined to the cathode, at least if it is operated with a dc current. This situation seems to be present in a YAG lamp being seeded with DyI 3 and different other additives as NaI and T lI [START_REF] Luijks | [END_REF]. If HID lamps, which are seeded with T hI 4 [23], DyI 3 [24,25] or other metal iodides are operated with an ac or switched-dc current, it could not be decided definitely, whether the emitter effect is limited to the cathodic phase or also being effective within the anodic phase. To clarify this point, the gas phase emitter effect of Dy is investigated in a preceding paper [26] by spectroscopic measurements in front of the electrode and pyrometric measurements along the electrode rod in dependence on the operating frequency. These measurements have shown that the reduction of the electrode temperature is correlated to a high Dy ion density in front of the electrode within the cathodic phase, suggesting that the Dy monolayer is essentially sustained by a Dy ion current towards the cathode. But at higher operating frequencies a reduction of the work function is also effective within the anodic phase, as is shown by the reduction of the electrode temperature with increasing operating frequency. It may be attributed to the finite life time of the layer reflecting the adsorption energy of Dy on the tungsten surface of E a = 4.97 eV.

If the hypothesis is valid that the atomic monolayer is mainly generated by an ion current towards the cathode then the gas phase emitter effect does not only depend on the reduction of the work function ∆φ and the adsorption energy E a of the atoms within the dipole layer but also on the ionisation energy E i of the emitter material and the mobility µ i ∝ M -1 i of the ions, on the electrode polarity and with it on the operating frequency. The reduction of the work function of tungsten φ W may vary between ∆φ = 0 and ∆φ = φ W -φ emitter + δφ where φ emitter is the work function of a surface consisting of many layers of emitter material and δφ represents a reduction of the work function in the order of several hundred meV if the coverage Θ of the tungsten surface is incomplete (0.4 < Θ < 1) [27].

To clarify the role of the ion current the emitter effect of the alkali metal caesium and of the rare earth metal cerium are compared by measurements in YAG lamps performed at operating frequencies being varied from 1 Hz to 1 kHz. The buffer gas of the YAG lamps consisting of Ar, Kr and predominantly Hg is seeded either with CsI or CeI 3 . Caesium is characterized by a work function φ Cs = 1.8 eV and an ionisation energy E i = 3.89 eV, which is very favourable for the gas phase emitter effect, but also by a quite low adsorption energy E a = 3.37 eV [28] [28,29,30] is characterized by φ Ce = 2.7 eV, E i = 5.54 eV, values, which are less favourable than those of Cs but comparable with those of Dy, and E a = 6.84 eV [29,30], which is nearly by a factor of two higher than the adsorption energy of Cs and also higher than the adsorption energy of Dy. Since the relative masses of caesium (A r = 132.9) and cerium (A r = 140.12) are not very different, it may be assumed, that the mobility of the two ions is quite similar. Both a cerium ion current and a caesium ion current may generate within the cathodic phase a distinct emitter effect, but as a consequence of the different adsorption energies the life time of the Ce monolayer may be longer and therefore the dependence of the gas phase emitter effect of cerium on the operating frequency more pronounced than that of caesium. The data of φ, E i , E a and A r of Cs, Ce and of Dy, T l, by which the lamp was seeded in the preceding paper [26], are summarized in table 1.

To check this speculation the electrode temperature within the YAG lamps and the densities of the emitter materials in front of the YAG lamp electrodes are investigated in dependence on the operating frequency. In the case of the YAG lamp seeded with CsI the same measuring methods as in [26] are applied. The electrode temperature as well as the plasma temperature and the Cs atom density are determined by emission spectroscopy. The Cs ion density can not be measured since an appropriate Cs ion line is not available. But it is calculated taking into account that LTE is realized. In the case of the YAG lamp seeded with CeI 3 for the electrode temperature as well as for the Ce atom density new measuring methods are applied, which give more reliable results than the previous ones. The 2D distribution of the electrode temperature is recorded by a photograph with a CCD camera, which is calibrated for the infrared wavelength of a narrowband interference filter in front of the camera [START_REF] Luijks | [END_REF]. The Ce atom density is determined by absorption spectroscopy. For this purpose the measuring system for emission spectroscopy is supplemented by an intense back light, realized by an UHP lamp [20,[START_REF] Westermeier | Phase resolved Dy-density and plasma temperature measurements by absorption-and emision-spectroscopy of Dy spectral lines[END_REF][START_REF] Westermeier | Investigation of the emitter effect in HID-lamps by absorption-and emissionspectroscopy[END_REF].

Experimental set up

YAG lamp

The lamps under investigation are special research HID lamps with simple rod-type pure tungsten electrodes with a diameter of 0.36 mm, a length of 5 mm, an inter electrode spacing of 7 mm and an ovoid-shaped discharge vessel made of yttrium alumina garnet (Y 6 Al 5 O 12 ) shortened YAG [START_REF] Luijks | [END_REF]. YAG is a transparent ceramic and therefore suitable for optical observations. The YAG vessel is placed inside a sealed and evacuated outer bulb, made of fused silica doped with lead, for protection from UV light, mechanical protection and thermal insulation of the inner bulb. The discharge vessel is filled with a buffer gas, which consists of an argon-krypton-mixture (30 kPa), and with 6 mg mercury to generate a background pressure of 2 MPa during lamp operation being required for a sufficiently high power input into the arc. 1 mg of the respective emitter element (CsI, CeI 3 , DyI 3 , T lI) is added to the lamp filling to generate a gas phase emitter effect. A lamp without any emitter material is used as reference. The results which are obtained by the lamp seeded with DyI 3 and the reference lamp are already published in [26] (figure 3 -5 and figure 12).

A sketch of the inner tube of a YAG lamp is shown in figure 1 while the filling properties of all YAG lamps presented in this work and in the preceding paper consulted for comparison [26] The YAG lamps, built for a nominal input power of 70 W, are operated in vertical position. Since the accumulation of the liquid salt at the bottom of the discharge vessel hampers optical observations, all optical measurements are performed at the upper electrode. The investigated lamps are operated with switched-dc current at 0.8 A and frequencies of 1, 10, 100, 500 Hz and 1 kHz by a voltage controlled current source (Feucht pxpx Figure 2: Schematic drawing of the complete optical set up Elektronik DCU/I 2250-28, max. voltage: 400 V, max. current: 28 A, max. power: 2250 VA) which is controlled by a voltage from a signal generator (Agilent 33120A).

Measuring system for phase resolved emission and absorption spectroscopy

A schematic drawing of the optical set up is shown in figure 2. To determine the axial electrode temperature profile within the YAG lamp seeded with CeI 3 the upper electrode of the YAG lamp is focused on a CCD camera (PCO Sensicam Fastshutter) by means of a zoom objective (Navitar Zoom 6000). The record is controlled by an electromechanical shutter (Uniblitz VS14) in front to avoid the undesirable smear effect [START_REF] Pco | [END_REF]. A narrow bandpass filter at 890 nm (10 nm FWHM) between the CCD camera system and the YAG lamp is used for recording a two dimensional image at the particular wavelength. By calibration with a tungsten ribbon lamp (Wi17g3) from OSRAM absolute electrode temperatures T el (z) are deduced from the camera picture. Furthermore, the shape of the arc attachment can be extracted from the images. This 2D-method was presented in [START_REF] Luijks | [END_REF] for the first time. By replacing the infrared filter in front of the CCD camera by a neutral density filter (Newport) it is also possible to record more detailed images of the arc attachment.

Optical emission spectroscopy is realised by a 1/4 m spectrograph (Chromex 250is, grating: 1200 lines/mm) and a second CCD camera (PCO Sensicam QE). Two lenses (focus length of 310 mm, respectively) project a 1:1 image of the lamp on the entrance slit of the spectrograph. A rotatable so-called Dove prism between the two lenses is used for turning the image around the optical axis.

To measure the axially resolved electrode temperature T el (z) spectroscopically, the upper electrode within the YAG lamp seeded with CsI has to be adjusted by means of the prism parallel to the entrance slit of the spectrograph (slit width: 100 µm). An edge filter at 600 nm is positioned in front of the entrance slit to suppress radiation from the second order spectrum produced by the grating at λ meas /2. Measurements of T el (z) are performed at a single suitable wavelength in the infrared region where at best no plasma radiation is present. After a calibration with the tungsten ribbon lamp, absolute temperature values are received.

To determine the plasma temperature T pl and particle densities of atoms N a and ions N i in front of the electrode, the emission of a discharge cross section at a fixed distance from the electrode tip has to be captured by the entrance slit of the spectrograph (slit width: 25 µm). For this purpose the image of the upper lamp electrode is adjusted perpendicular to the slit by turning the Dove prism through 45 • around the optical axis. The intensities, recorded by the CCD camera at wavelengths from approximately 350 nm to 950 nm, are calibrated in absolute units by means of the tungsten ribbon lamp. Radial resolved measurements of T pl , N a and N i are performed in this work 125 µm in front of the upper electrode. They are accomplished by an evaluation of radial distributions of line emission coefficients from measured lateral line intensity distributions by an inverse Abel transformation.

A more detailed description of the measuring system for emission spectroscopy can be found in [14,24,26,[START_REF] Reinelt | Experimental investigation on electrodes of HID lamps at low and high operating frequencies[END_REF].

To measure particle densities by absorption spectroscopy the set up for emission spectroscopy is extended by a back light source being positioned behind the YAG lamp on the elongation of the optical axis [20,[START_REF] Westermeier | Phase resolved Dy-density and plasma temperature measurements by absorption-and emision-spectroscopy of Dy spectral lines[END_REF][START_REF] Westermeier | Investigation of the emitter effect in HID-lamps by absorption-and emissionspectroscopy[END_REF]. A powerful broadband light source with a high radiance within the visible spectral range and a temporarily very stable emission is realized with an ultra high pressure (UHP) lamp [START_REF] Derra | [END_REF]. The UHP lamp (Philips 228 L4), originally designed for video projection systems, is electrically driven by the electronic circuit from a video projector. It is operated with a 90 Hz switched-dc current. Its amplitude amounts to 2.2 A and the electrical power input to 120 W. A specially designed reflector is already equipped around the UHP lamp so that a maximal light power is emitted as a quasi-parallel light beam by the lamp. To minimize the influence of the irradiation by the powerful UHP lamp on the operational conditions of the YAG lamp an optical bandpass filter (Andover Corporation 50 mm) is installed between both lamps. The filtered UHP light is focused by an achromatic lens (focus length of 80 mm) to an area with a diameter of d = 10 mm around the measuring point. It provides a largely homogeneous radiance perpendicular to axis of the YAG lamp. The beam emanating from the UHP lamp is not strictly parallel within the tube of the YAG lamp as it is indicated in figure 2. But the angle of beam spread is so small and the optical path length within the discharge so short that the beam divergence can be neglected. A calibration of the optical signals with the tungsten ribbon lamp is unnecessary due to the special analysis of the measuring data presented in section 4.2. By the same reason the phase modulation of the light emission of the UHP lamp which is on the order of 2 % has not to be considered.

All quantities are measured with some phase resolution. It corresponds to temporal resolution as long as the lamp operation is really periodic. It is accomplished by trigger signals for the cameras, which are shifted with an adjustable time delay with respect to a well defined reference phase angle. A reproducible reference phase angle can be deduced from zero crossing of the lamp voltage as it was shown by Langenscheidt [13].

To trigger the cameras only the rising edge of the lamp voltage is used. The uncertainty of the trigger pulse does not exceed 2 µs. A pulse delay generator (Stanford Research Systems DG 535) is used to shift the trigger signal from voltage zero crossing. A block diagram of the trigger set up can be found in [14]. [26] for electrode temperature T el , plasma temperature T pl and particle density measurements of atoms N a and ions N i of the respective emitter material Cs, Ce and Dy

Electrode

In table 3 the different measuring methods used in this and in the reference work [26] are assigned to the lamp types given in table 2.

Pyrometric electrode temperature measurement (1λ-method)

For accurate pyrometric temperature measurements a segment of the spectrum emitted by the electrode has to be chosen which is as far as possible in the infrared region but still in the sensitivity range of the CCD camera. In addition the plasma radiation emitted by the YAG lamp in the selected spectral range should be insignificant compared to the emission of the electrode surface. In the case of the YAG lamp seeded with CsI the emission of the electrode in the infrared is superimposed by an intense multi line emission of Cs [35]. Therefore a pyrometric measurement with a spectrograph is an appropriate choice. An inspection of the spectrum showed that an applicable wavelength for this kind of temperature measurement is 745 nm. But in the case of a lamp seeded with CeI 3 both requirements are met within a spectral range around 890 nm as was checked spectroscopically. Therefore the 2D-method with an interference filter at 890 nm is the more favourable pyrometric measuring procedure.

The spectral radiance along the upper electrode I el (λ, z) at λ = 745 nm or λ = 890 nm is recorded by a single measurement at a selected phase angle ϕ. It is assumed that the spectral radiance emitted by the tungsten electrodes is represented by that of a grey body radiator I gb (λ, T el ), which can be calculated in dependence on the temperature T el and wavelength λ from Planck's law and the spectral emissivity of tungsten (λ, T el ) < 1. Reliable spectral emissivities of tungsten are available in literature [36]. Axial temperature profiles T el (z, ϕ) are obtained by comparing I el (λ, z, ϕ) and I gb (λ, T el ).

The surface roughness is enhanced by the arc attachment in the vicinity of the electrode tip and with it the emissivity is exaggerated in an undefined manner compared to the values given in literature. As a consequence the temperature measurement at the electrode tip becomes doubtful. In previous investigations of HID electrodes operated with a dc-current the axial electrode temperature distribution was additionally simulated by integrating the 1D heat balance of the electrode and matching it to the measured distribution within a region of confidence, in which the values of were trustable. The temperature of the electrode tip T tip was then determined by an extrapolation of the integrated 1D heat balance. Reliable results were obtained with this method for electrodes operated with a dc-current [1,6,11]. But if the electrodes are operated with an ac-current the modulation of the electrode temperature is limited to a small area behind the electrode tip [15]. Therefore also the tip region with its exaggerated values of has to be taken into account when the integral of the heat balance is matched to the measured temperature distribution. A more realistic temperature profile can be used for the matching procedure if instead of a one dimensional a two dimensional temperature distribution is available. In this case it is easier to identify and remove locally excessive temperatures. A detailed discussion of the limited measuring accuracy of T tip is presented in [15,26]. It was shown that the phase resolved experimental determination of T tip is afflicted with an error of the order of ∆T tip ≈ 50 K.

Spectroscopic plasma temperature and particle density measurements

Emission spectroscopy

The spectral radiances I ν (y) of optical thin lines are measured in a fixed distance d from the electrode tip along a y-axis perpendicular to the optical axis x and the discharge axis z. They are calibrated in absolute units and integrated over line profiles, providing lateral distributions of line intensities I L (y). By an inverse Abel transformation of I L (y) radial emission coefficients L (r) are obtained if the discharge is rotational symmetric.

λ 1 = 576.96 nm λ 2 = 579.07 nm A 1 = 1.709 • 10 8 s -1 A 2 = 2.039 • 10 8 s -1 g 1 = 5 g 2 = 5 E 1 = 8.853 eV E 2 = 8.845 eV
Table 4: Spectroscopic data for the calculation of the plasma temperature T pl from Hg-line intensities [START_REF] Kurucz | KURUCZ CD-ROM NO. 23 -Atomic Line List[END_REF] At first the plasma temperature T pl inside a YAG lamp is determined using two overlapping mercury lines at λ 1 = 576.96 nm and λ 2 = 579.07 nm [15,24,[START_REF] Schoepp | Elektrische und strahlungsphysikalische Eigenschaften von Hochdruckentladungen -insbesondere Quecksilberdampfentladungen -für die Beleuchtungstechnik[END_REF]. The corresponding spectroscopic data is given in table 4 with the transition probabilities A 1 and A 2 , the statistical weights g 1 and g 2 and the excitation energies of the upper line levels E 1 and E 2 . By assuming local thermal equilibrium, the common temperature of all particles, in this paper always named as plasma temperature T pl , is calculated from the sum of the respective line emission coefficients postulating a Boltzmann population of the upper emitting states and using the ideal gas law.

ul = hc 0 4π p 0 k b T pl 1 Q Hg (T pl ) g 1 A 1 λ 1 exp - E 1 k b T pl + g 2 A 2 λ 2 exp - E 2 k b T pl (1)
A constant mercury pressure of p 0 = 2 MPa is assumed. A plasma temperature of T pl ≤ 7000 K with an error margin of about 4 % was found within YAG lamps with similar parameters using the two Hg-lines [24,26].

The Cs atom density N a,Cs within the YAG lamp seeded with CsI and the Ce ion density N i,Ce within the lamp seeded with CeI 3 are evaluated by means of L (r) of the respective atomic or ionic line, assuming a Boltzmann population of the emitting states defined by T pl (r). In both cases two overlapping lines are used. The sum of their emission coefficients is given by a formula as that for the two mercury lines (equation 1) in which the factor p 0 /k b T pl is exchanged by N a,Cs or N i,Ce . The corresponding spectroscopic data for a determination of N a,Cs is presented in table 5 while the data for the evaluation of N i,Ce is given in table 6.

The complete measuring and data processing procedure is described in detail in [24,26,[START_REF] Westermeier | Investigation of the emitter effect in HID-lamps by absorption-and emissionspectroscopy[END_REF][START_REF] Reinelt | Experimental investigation on electrodes of HID lamps at low and high operating frequencies[END_REF][START_REF] Langenscheidt | Elektroden für HID-Lampen -Diagnostik und Simulation[END_REF].

The determination of the densities N a,Cs of Cs atoms and N i,Ce of Ce ions by emission spectroscopy is afflicted mainly with three different error sources: the

λ 1 = 697.33 nm λ 2 = 698.35 nm A 1 = 7.514 • 10 6 s -1 A 2 = 1.296 • 10 6 s -1 g 1 = 6 g 2 = 4 E 1 = 3.232 eV E 2 = 3.230 eV
Table 5: Spectroscopic data for the calculation of Cs atom density N a,Cs [START_REF] Kurucz | KURUCZ CD-ROM NO. 23 -Atomic Line List[END_REF] λ 1 = 457.23 nm λ 2 = 457.28 nm

A 1 = 2.663 • 10 7 s -1 A 2 = 1.515 • 10 7 s -1 g 1 = 12 g 2 = 10 E 1 = 3.395 eV E 2 = 4.192 eV
Table 6: Spectroscopic data for the calculation of Ce ion density N i,Ce [START_REF] Kurucz | KURUCZ CD-ROM NO. 23 -Atomic Line List[END_REF] measuring inaccuracy of the emission coefficient ul , which amounts at least 20 %, the uncertainty of the transition probabilities of at least 10 % and the measuring error of the plasma temperature T pl of 4 %, which is multiplied by a factor E u /k b T pl being on the order of 5 -7. The error sources are discussed in more detail in [26]. The resulting error margins are quite high, they are at least on the order of 50 %. They show that the particle densities obtained by emission spectroscopy represent only an order of magnitude, but the variations in dependence on phase, ∂N a /∂ϕ and ∂N i /∂ϕ, provide a quite reliable information.

Absorption spectroscopy

The

By an integration of the absorption coefficient α(ν) with respect to the frequency ν a so called line absorption coefficient α L is defined:

α L = ∆ν α(ν)dν = N l e 2 4π 0 π m e c 0 f lu . (4)
N l is the population density of the absorbing atomic state and f lu the absorption oscillator strength of the specific optical transition, the other symbols have the usual meaning. α L can be related to the measured absorption by an integration of the optical depth τ over a frequency interval ∆ν, which comprehends the absorption dip. By an inverse Abel transformation of

∆ν τ (ν, y)dν = ∆λ τ (λ, y) c 0 λ 2 dλ = x=+R/2 x=-R/2 α L (x, y)dx (5)
the radial line absorption coefficient α L (r) is obtained and with it the radial density distribution N l (r) of the absorbing atomic state if the discharge is rotational symmetric.

If N l is the ground state of an atomic resonance line the total atomic density N a (r) can be calculated from:

N a (r) = N l (r) Q a (T pl ) g l = α L (r) f lu 4π 0 e 2 m e c 0 π Q a (T pl ) g l . ( 6
)
g l is the statistical weight of the atomic ground state and Q a the corresponding atomic partition function. λ = 577.36 nm f = 5.492 • 10 -3 g 0 = 9

Table 7: Spectroscopic data of the Ce resonance line [START_REF] Kurucz | KURUCZ CD-ROM NO. 23 -Atomic Line List[END_REF] for the calculation of Ce atom density N a,Ce

The Ce resonance line at λ = 577.36 nm is chosen to determine the Ce atom density N a,Ce (r) within the YAG lamp seeded with CeI 3 . An interference filter with a centre wavelength of 577 nm and a bandwidth of 10 nm FWHM is used to avoid a distracting irradiation of the YAG lamp by the UHP lamp. The atom density N a,Ce (r) is evaluated from α L taking into account the spectroscopic data of table 7 and the partition function Q a (T pl ) of the Ce atoms. It is calculated using the atomic data given in [START_REF] Kurucz | KURUCZ CD-ROM NO. 23 -Atomic Line List[END_REF]. The absorption by the wings of the mercury lines at 576.96 nm and 579.07 nm is eliminated by using the extrapolated value of I UHP ν as a reference.

As an example N a,Ce (r) obtained by a measurement at a switched-dc current of 0.8 A at 10 Hz within the cathodic phase at phase angle ϕ = 7π/5 is presented in figure 4. The logarithmic derivative of equation 6 is used to estimate the error margins of N a,Ce :

∆N a,Ce N a,Ce = ∆α L α L + ∆f lu f lu + ∂lnQ a,Ce ∂lnT pl ∆T pl T pl . (7)
The line absorption coefficient α L may be afflicted with an error of the order of 20 % mainly by reason of the inverse Abel transformation, which reacts, especially on the axis, quite sensitive on deviations from symmetry of the lateral distribution of the integrated optical depth. The knowledge of the oscillator strength f lu is not better than 10 %. The error margin of the plasma temperature, which is multiplied in equation 7 by ∂lnQ a,Ce /∂lnT pl ≈ 1, is of the order of 4 %. The resulting error margin is of the order of |∆N a,Ce /N a,Ce | • 100 ≈ 35 %, which is much lower than the error margins of the particle densities determined by emission spectroscopy. This means that the absorption spectroscopy provides, different to the emission spectroscopy, quantitative information on particle densities within the YAG lamp.

A first discussion of the broadband absorption spectroscopy with an UHP lamp is given in [20,[START_REF] Westermeier | Investigation of the emitter effect in HID-lamps by absorption-and emissionspectroscopy[END_REF] while first results by means of this method are presented for YAG lamps seeded with DyI 3 in [START_REF] Westermeier | Phase resolved Dy-density and plasma temperature measurements by absorption-and emision-spectroscopy of Dy spectral lines[END_REF][START_REF] Westermeier | Investigation of the emitter effect in HID-lamps by absorption-and emissionspectroscopy[END_REF] and with CeI 3 in [START_REF] Westermeier | Investigation of the emitter effect in HID-lamps by absorption-and emissionspectroscopy[END_REF][START_REF] Ruhrmann | Determination of the ground state density of Ce in front of the electrode of a YAG lamp by absorption spectroscopy[END_REF].

Estimation of the Cs ion density by SAHA equation

A spectroscopic measurement of the Cs ion density N i,Cs within the YAG lamp seeded with CsI is nearly impossible, since the energy gap between the ion ground state and the lowest excited state, which emits lines in the visible spectral range (λ 1,2,3,4 = 492.57, 522.70, 527.40, 537.10 nm [START_REF] Striganov | Tables of spectral lines of neutral and ionized atoms (IFI/PlenumDataCorporation[END_REF]), is very large. It amounts to E i,ex = 15.68 eV, so that its population and with it the intensity of the ion lines is extremely low for plasma temperatures of the order of 7000 K in the lamp. Alternatively N i,Cs may be evaluated by inserting the Cs atom density N a,Cs and the plasma temperature T pl into the SAHA equation for caesium S Cs (T pl ), using the fact that LTE is established at the measuring point within the lamp plasma:

S(T ) = N i,Cs N e N a,Cs = Q i,Cs (T pl ) Q a,Cs (T pl ) 2(2πm e k b T pl ) 3/2 h 3 exp - E i,Cs k b T pl . ( 8
)
But this raises the question whether the electron density N e is equal to the Cs ion density N i,Cs . The ionisation energy of caesium (E i,Cs = 3.893 eV) is much lower than the ionisation energy of iodine (E i,I = 10.45 eV) and of mercury (E i,Hg = 10.43 eV). It is clear that the iodine ion density N i,I can be neglected compared to the Cs ion density N i,Cs , if is taken into account that the corresponding atom densities within the ratio of the SAHA equations for iodine and caesium are of the same order of magnitude:

N i,I N i,Cs = N a,I N a,Cs Q i,I Q a,Cs Q a,I Q i,Cs exp - E i,I -E i,Cs k b T pl . ( 9
)
But the density of Hg atoms, N a,Hg , is three orders of magnitude higher than the density of I atoms so that the density of the Hg ions N i,Hg may be comparable to that of the Cs ions N i,Cs , as is shown by a relation similar to equation 9, in spite of a much lower ionisation degree of mercury compared to caesium.

Due to quasi neutrality the electron density N e is determined in the YAG lamp filled with Hg and CsI by

N e ≈ N i,Cs + N i,Hg . (10)
Combining equation 10 and the SAHA equations for caesium S Cs (T pl ) (equation 8) and that for mercury S Hg (T pl ), the ion densities N i,Cs and N i,Hg can be expressed by the corresponding atom densities according to:

N i,Cs = S Cs (T pl )N a,Cs [S Hg (T pl )N a,Hg + S Cs (T pl )N a,Cs ] 1/2
(11)

N i,Hg = S Hg (T pl )N a,Hg [S Cs (T pl )N a,Cs + S Hg (T pl )N a,Hg ] 1/2 . ( 12
)
Results for N i,Cs and N i,Hg will be given in chapter 6.1. Equation 11shows that the Cs ion density N i,Cs is reduced by the competing ionisation of Hg. According to equation 11 the error margins of N i,Cs may be still higher than those of the Cs atom density N a,Cs :

∆N i,Cs N i,Cs ≥ ∆N a,Cs N a,Cs . ( 13
)

Results and discussion of measurements at the YAG lamps

Measurements are performed at YAG lamps which are operated with switched-dc current of 0.8 A and operating frequencies of 1, 10, 100, 500 Hz and 1 kHz. At most, measurements at 10 equidistant phase angles over one period are accomplished. For operating frequencies greater than 100 Hz it was often not possible to carry out more than one measurement per half period. In the subsequent figures, which present the phase resolved measuring results, the first half period (ϕ = 0 -π) is the anodic and the second half period (ϕ = π -2π) the cathodic phase. Phase resolved results of the electrode tip temperature T tip (ϕ), plasma temperature T pl (ϕ) and density of atoms N a (ϕ) and ions N i (ϕ) are presented. The pyrometric measurements are performed at the upper electrode, all spectroscopic measurements at a distance of 125 µm in front of it. The reported values of T pl (ϕ), N a (ϕ) and N i (ϕ) are averages over a rotational symmetric cross section with the same diameter as the electrode.

YAG lamp seeded with CsI

Figure 5 shows the electrode tip temperature T tip (ϕ) of the upper electrode for different frequencies. The maximum values of T tip (ϕ) are at least 290 K below the melting point of tungsten of 3695 K. The courses of T tip (ϕ) demonstrate for f < 100 Hz a heating of the electrode within the anodic phase while in the cathodic phase a reduction of its heating is visible. The modulation diminishes with increasing frequency, but the resulting average value of T tip (ϕ) increases with frequency for frequencies of f ≥ 500 Hz. It is obvious that a reduction of T tip (ϕ) of at least 200 K is induced by the Cs atoms and Cs ions within the discharge compared to a YAG lamp with Hg filling only being shown in figure 1(a) of the reference paper [26]. It amounts even 400 K for f = 1 kHz. But the increase of T tip (ϕ) with increasing frequency is a striking difference to the decrease of T tip (ϕ) in YAG lamps seeded with rare earth iodides as is shown in [26] figure 5(a) for DyI 3 and in section 6.2 for CeI 3 . Figure 6(a) presents the plasma temperature T pl (ϕ) measured in front of the upper electrode for different frequencies. T pl (ϕ) is at least 700 K higher in the anodic phase than in the cathodic phase. T pl (ϕ) decreases in the anodic phase for f ≥ 500 Hz. The considerably reduced plasma temperature in front of the cathode indicates an enhancement of the Cs density within the cathodic phase by cataphoresis. It is generated by the electric force on the Cs + ions and a counteracting pressure gradient of the Cs ion density. The density of the Cs ions and Cs atoms is coupled by charge exchange, ionisation and recombination processes. A flow of Cs atoms, which is propelled by a decline of Cs atoms in front of the cathode, compensates the Cs ion current towards the cathode. By reason of the low ionisation energy E i = 3.89 eV of Cs only a low plasma temperature is required to generate a sufficiently high electron density N e in front of the cathode. Similar courses of T pl (ϕ) were measured by Langenscheidt et al at a YAG lamp seeded with NaI, T lI and DyI 3 [24]. The plasma temperature in front of the upper electrode T pl (ϕ) of a YAG lamp without any salt filling ( [26] figure 2) or seeded with CeI 3 (figure 8(b)) shows a completely different behaviour for the same current amplitude and waveform. T pl (ϕ) is generally higher in these cases, moreover it is higher in the cathodic phase than in the anodic phase. In addition with increasing frequency a decrease of T pl (ϕ) independent of the phase is visible. Hence it is obvious that the ionisation energy E i of the respective metal iodide has a distinct influence on T pl (ϕ).

The Hg ion density N i,Hg (ϕ), estimated with equation 12, is shown for different frequencies in figure 6(b). The course of N i,Hg (ϕ) is similar to the course of T pl (ϕ) (figure 6(a)): N i,Hg (ϕ) is up to 18 times higher within the anodic phase than within the cathodic phase where N i,Hg (ϕ) varies between 2 • 10 20 m -3 and 4.5 • 10 21 m -3 . N i,Hg (ϕ) decreases simultaneously with T pl (ϕ) in the anodic phase for f ≥ 500 Hz. The course of N i,Hg (ϕ) reflects the influence of the mercury ionisation on N e particularly in the anodic phase. Hence N i,Hg (ϕ) has to be taken into account referred to equation 10 when the Cs ion density N i,Cs (ϕ) is determined with SAHA equation.

Figure 7(a) shows the Cs atom density N a,Cs (ϕ) determined in front of the upper electrode for different frequencies. N a,Cs (ϕ) is lower by one order of magnitude in the anodic phase than in the cathodic phase. N a,Cs (ϕ) increases weakly in the anodic phase with increasing frequency and decreases by one order of magnitude in the cathodic phase. For higher operating frequencies (f ≥ 500 Hz) the density is nearly constant over time.

The Cs ion density N i,Cs (ϕ) being estimated with equation 11 is shown for different frequencies in figure 7(b). The values of N i,Cs (ϕ) are higher than the Cs atom density N a,Cs (ϕ) which is caused by the low ionisation energy E i of Cs, but the trend is nearly the same. N i,Cs (ϕ) is according to N a,Cs (ϕ) in the anodic phase by one order of magnitude lower than in the cathodic phase. N i,Cs (ϕ) decreases in the cathodic phase and increases in the anodic phase with increasing frequency, except for f = 1 kHz. Also an assimilation of N i,Cs (ϕ) takes place with increasing frequency within both phases.

The increase of N a,Cs (ϕ) (figure 7(a)) and N i,Cs (ϕ) (figure 7(b)) in the cathodic phase demonstrates the existence of a Cs ion current towards the cathode. It may not only substitute in part the thermionic electron emission of the cathode but above all produce a monolayer of Cs atoms on the surface of the cathode. This layer may reduce the work function and thereby the temperature of the electrode. According to numerical calculations given in [4] the reduction of T tip (ϕ) is only marginally induced by a lowering of the plasma temperature T pl (ϕ) caused by the Cs vapour in front of the cathode, but mainly by a reduction of the work function φ by a Cs monolayer. Its formation becomes obvious by comparing the electrode tip temperatures T tip (ϕ) in figure 5 and figure 1(a) of [26]. Furthermore, the missing formation of an emitter spot within the cathodic phase is an indication for a moderate Cs ion current density in front of the cathode [20,24]. The decrease of N a,Cs (ϕ) and N i,Cs (ϕ) in the anodic phase is caused by the reversal of the Cs ion current. But this takes place with a finite time constant, as is indicated by the increase of the Cs atom and Cs ion density in front of the anode with increasing frequency. Nevertheless figure 5 reveals an increase of the average electrode tip temperature with increasing frequency. This may result from a decreasing phase averaged coverage of the electrode surface with emitter atoms for increasing frequencies due to the low adsorption energy E a of Cs of 3.37 eV (table 1). Therefore an expansion of the emitter effect to the anodic phase with increasing frequency, which may effect a reduction of the average work function, does not take place. 5) a heating of the electrode in the anodic phase and a reduction of its heating in the cathodic phase takes place. This change with polarity diminishes with increasing frequency. Additionally the maximal values of T tip (ϕ) given in figure 8(a) are at least 390 K below the melting point of tungsten. A comparison with T tip (ϕ) of a YAG lamp with Hg filling only (given in figure 1(a) of reference [26]) confirms a general reduction of T tip (ϕ) of at least 300 K effected by the presence of Ce atoms and Ce ions in the arc plasma. By comparing the courses of T tip (ϕ) of figure 5 and 8(a) it is visible that for 1, 10 and 100 Hz the temperature values of the YAG lamp seeded with CsI are below T tip (ϕ) of the lamp seeded with CeI 3 , only for 1 Hz between ϕ = π/2 and ϕ = π a more pronounced temperature increase is observed for a lamp seeded with CsI. But the most important difference to figure 5 is a decrease of T tip (ϕ) with increasing frequency. For f = 1 kHz a reduction of ∆T tip (ϕ) = 600 K is induced by a seeding with CeI 3 compared to a YAG lamp operated with a pure mercury vapour atmosphere.

YAG lamp seeded with

T pl (ϕ), measured in front of the upper electrode, is presented in figure 8(b) for different frequencies. The values of T pl (ϕ) decrease slightly with increasing frequency. T pl (ϕ) is higher in the cathodic phase than in the anodic phase. Similar courses of T pl (ϕ) were measured by Langenscheidt et al at a YAG lamp seeded only with DyI 3 , beeing shown in figure 12 and figure 13 of reference [24]. It reflects a higher power requirement to generate Ce ions instead of Cs ions by reason of the higher ionisation energy of Ce. At 1 kHz the courses of T pl (ϕ) in the anodic and cathodic phase converge. T pl (ϕ) is at most 150 K higher in comparison to a YAG lamp with Hg filling only ( [26] figure 2) while the course of T pl (ϕ) is similar. The difference between the plasma temperatures in both lamps is within the error margin of approximately 300 K. represents courses of the Ce ion density N i,Ce (ϕ) in front of the upper electrode for different frequencies. The Ce atom density as well as the Ce ion density is higher in the cathodic than in the anodic phase. But the difference of the Ce atom density within the cathodic and anodic phase is distinctly smaller than in YAG lamps seeded with CsI (figure 7(a)) or with DyI 3 (as is shown in figure 3 and figure 4 of reference [26]). Moreover the Ce atom density N a,Ce (ϕ) and ion density N i,Ce (ϕ) are higher than the corresponding Cs densities. Especially they are of the order of ten higher than the Dy atom and ion densities (being shown in figure 3 of reference [26]). Both Ce densities decrease with increasing frequency in the cathodic phase. But in the anodic phase a general trend is not visible for the Ce atom density N a,Ce (ϕ) as well as for the Ce ion density N i,Ce (ϕ). N i,Ce (ϕ) increases within the cathodic phase with a time constant on the order of 10 ms and N a,Ce (ϕ) with an extended time constant of 20 ms. Accordingly at operating frequencies above several ten Hertz the maximum values of N a,Ce (ϕ) and N i,Ce (ϕ) are lowered with increasing frequency. The variation of N i,Ce (ϕ) and the delayed variation of N a,Ce (ϕ) in dependence on phase and on operating frequency demonstrates that Ce is transported cataphoreticly in front of the electrode. A Ce atom monolayer is deposited on the electrode surface by a Ce ion current from the plasma to the cathode. It reduces the work function and therefore T tip (ϕ) which is proved by comparing figure 8(a) with figure 1(a) of [26]. The reduction of N a,Ce (ϕ) and N i,Ce (ϕ) in the anodic phase is produced by a reversal of the Ce ion current. N a,Ce (ϕ) and N i,Ce (ϕ) are nearly constant over time for higher frequencies indicating that the Ce ions are too immobile to follow the frequency above 500 Hz. They remain directly in front of the electrode, recombine and form a layer of Ce atoms on the electrode surface even during the anodic phase. Thus the emitter effect remains active also within the anodic phase and because of this T tip (ϕ) decreases with increasing frequency (figure 8(a)).

Conclusion

In the YAG lamp seeded with CsI a cathodic gas phase emitter effect due to a Cs ion current is observed at low frequencies (f < 500 Hz), but the emitter effect does not overlap onto the anodic phase with increasing frequency. This is caused by an insufficient coverage of the electrode surface with Cs atoms within the anodic phase due to the low adsorption energy E a of Cs (E a = 3.37 eV) within the anodic phase. It leads to an increase of the averaged electrode tip temperature T tip (ϕ) with increasing frequency. A direct indication that the layer of Cs atoms is removed within the anodic phase is the increase of T tip (ϕ) at the end of the anodic phase for f = 1 Hz up to 3400 K, which exceeds the otherwise higher electrode tip temperature T tip (ϕ) of a YAG lamp seeded with CeI 3 by 100 K. Though, the cathodic gas phase emitter effect reduces the work function and temperature of the electrode and increases the lifetime of the lamp.

In the YAG lamp seeded with CeI 3 a gas phase emitter effect is produced at low frequencies (f < 500 Hz) mainly by a Ce ion current in the cathodic phase whereas it disappears with a time constant of some milliseconds within the anodic phase. The reduction of the electrode tip temperature T tip (ϕ) is at low frequencies less pronounced than within a YAG lamp seeded with CsI but more pronounced than within a lamp seeded with DyI 3 . The difference between the Ce and Cs lamp can be attributed to the lower work function φ of the Cs monolayer compared to that of Ce, but the difference between the Ce and Dy lamp can be attributed, at least in part, to the higher adsorption energy E a of Ce compared to Dy on tungsten. The higher adsorption energy E a of Ce may also be the reason for a higher Ce atom and ion density in front of the electrode and a weaker modulation of the Ce particle density in dependence on polarity compared to Dy. Moreover it has to be taken into account that the spectroscopic measurements at a distance of 125 µm from the electrode tip reflect only partially the density variation immediately in front of the electrode.

At higher operating frequencies the reduction of the electrode tip temperature T tip (ϕ) is most pronounced within the Ce lamp, less distinct within the Dy lamp and even reversed within the Cs lamp. It reflects the decrease of the adsorption energy E a from Ce to Dy and Cs which corresponds to a successive reduction of the life time of the Ce, Dy and Cs monolayer. The emitter effect and with it the reduction of the electrode tip temperature overlaps to the anodic phase at higher frequencies with increasing life time of the monolayer, e.g. for lamps seeded with CeI 3 or DyI 3 but not for lamps seeded with CsI in spite of an increase of the Cs ion density in front of the anode with increasing frequency.

The findings are another test of the hypothesis that in HID lamps emitter material can be deposited on the surface of hot tungsten electrodes by an ion current from the arc plasma to the cathode. Appropriate materials are able to reduce the work function of tungsten by the formation of an atomic monolayer and with it the electrode temperature.

It supplements the investigation of the gas phase emitter effect of Dy given in a preceding paper [26]. In this paper not only a correlation between an enhanced Dy ion density in front of the electrode and a reduced electrode temperature was demonstrated. It was also confirmed by an additional seeding of the YAG lamp with T lI that not the substitution of thermionic electron emission by an ion current but a reduced work function is responsible for the temperature reduction of the electrode. The vapour pressure of T lI is much higher than that of DyI 3 . The metallic T l exhibits a much higher work function than metallic Dy, but the ionisation energy of T l vapour (E i = 6.11 eV) is nearly the same as that of Dy vapour. Therefore the current transferred by metal ions in front of the cathode within a lamp additionally seeded with T lI cannot be smaller than in a lamp seeded only with DyI 3 . In spite of that the emitter effect is suppressed by the thallium additive in accordance with its higher work function.

2 αFigure 3 :

 23 Figure 3: Example for the absorbed I abs,U HP ν

  px

Figure 4 :

 4 Figure 4: Radial Ce atom density N a,Ce (r) measured 125 µm in front of the upper electrode. Parameter: CeI 3 , electrode diameter d E = 0.36 mm, electrode length l E = 5 mm, i = 0.8 A switched-dc, 10 Hz, ϕ = 7π/5 (cathodic phase)

Figure 5 :

 5 Figure 5: Electrode tip temperature T tip (ϕ) measured at the upper electrode over one period for various frequencies. Parameter: CsI, electrode diameter d E = 0.36 mm, electrode length l E = 5 mm, i = 0.8 A switched-dc

CeI 3 Figure 8

 38 Figure8(a) shows T tip (ϕ) of the upper electrode for different frequencies. Similar to the course of T tip (ϕ) of a YAG lamp seeded with CsI (figure5) a heating of the electrode in the anodic phase and a reduction of its heating in the cathodic phase takes place. This change with polarity diminishes with increasing frequency. Additionally the maximal values of T tip (ϕ) given in figure8(a) are at least 390 K below the melting point of tungsten. A comparison with T tip (ϕ) of a YAG lamp with Hg filling only (given in figure1(a) of reference[26]) confirms a general reduction of T tip (ϕ) of at least 300 K effected by the presence of Ce atoms and Ce ions in the arc plasma. By comparing the courses of T tip (ϕ) of figure5and 8(a) it is visible that for 1, 10 and 100 Hz the temperature values of the YAG lamp seeded with CsI are below T tip (ϕ) of the lamp seeded with CeI 3 , only for 1 Hz between ϕ = π/2 and ϕ = π a more pronounced temperature increase is observed for a lamp seeded with CsI. But the most important difference to figure5is a decrease of T tip (ϕ) with increasing frequency. For f = 1 kHz a reduction of ∆T tip (ϕ) = 600 K is induced by a seeding with CeI 3 compared to a YAG lamp operated with a pure mercury vapour atmosphere.

Figure 9 (

 9 Figure 9(a) shows courses of the Ce atom density N a,Ce (ϕ) while figure 9(b) represents courses of the Ce ion density N i,Ce (ϕ) in front of the upper electrode for different frequencies. The Ce atom density as well as the Ce ion density is higher in the cathodic than in the anodic phase. But the difference of the Ce atom density

  px

  Hg ions (SAHA equation)

Figure 6 :

 6 Figure 6: Plasma temperature T pl (ϕ) and mercury ion density N i,Hg (ϕ) at a distance of 125 µm in front of the upper electrode over one period for various frequencies. Parameter: CsI, electrode diameter d E = 0.36 mm, electrode length l E = 5 mm, i = 0.8 A switched-dc

Figure 7 :

 7 Figure 7: Caesium atom density N a,Cs (ϕ) and estimated ion density N i,Cs (ϕ) over one period for various frequencies at a distance of 125 µm in front of the upper electrode. Parameter: CsI, electrode diameter d E = 0.36 mm, electrode length l E = 5 mm, i = 0.8 A switched-dc

Figure 8 :

 8 Figure 8: Electrode tip temperature T tip (ϕ) measured at the upper electrode over one period for various frequencies and plasma temperature T pl (ϕ) measured over one period for various frequencies at a distance of 125 µm in front of the upper electrode. Parameter: CeI 3 , electrode diameter d E = 0.36 mm, electrode length l E = 5 mm, i = 0.8 A switcheddc

Figure 9 :

 9 Figure 9: Cerium atom density N a,Ce (ϕ) and ion density N i,Ce (ϕ) over one period for various frequencies at a distance of 125 µm in front of the upper electrode. Parameter: CeI 3 , electrode diameter d E = 0.36 mm, electrode length l E = 5 mm, i = 0.8 A switcheddc

Table 1 :

 1

	, which is counterproductive. Cerium

Values for work function φ, ionisation energy E i , adsorption energy E a on a tungsten surface and relative mass A r for Cs, Ce, Dy and T l

Table 2 :

 2 are listed in table 2. Properties of the filling of YAG lamps presented in this work and in the preceding paper consulted for comparison[26]

	Figure 1: Schematic drawing of the inner tube of a YAG lamp
					Metal halide salt
	Lamp Hg Total salt	CsI	CeI 3	DyI 3	T lI
	type (mg)	(mg)	(mol%) (mol%) (mol%) (mol%)
	Hg	6	0	0	0	0	0
	Cs	6	1	100	0	0	0
	Ce	6	1	0	100	0	0
	Dy	6	1	0	0	100	0
	Tl/Dy	6	2	0	0	50	50

Table 3 :

 3 Measuring methods used in this work and in

			Plasma	Atom	Ion
	Lamp temperature temperature	density	density
	type	T el	T pl	N a	N i
	Hg	emission spectroscopy spectroscopy emission	-	-
	Cs	emission spectroscopy spectroscopy spectroscopy emission emission	Saha equation
	Ce	2D-method	emission spectroscopy spectroscopy spectroscopy absorption emission
	Dy	emission spectroscopy spectroscopy spectroscopy spectroscopy emission emission emission
	Tl/Dy	emission spectroscopy spectroscopy spectroscopy spectroscopy emission emission emission

Acknowledgements

The authors thank Philips Lighting, NL, for scientific and financial support. Financial support from the Research School of the Ruhr-University Bochum is also gratefully acknowledged by the authors.


Ramasamy Pothiraja
email: ramasamy.pothiraja@rub.de

Nikita Bibinov
email: nikita.bibinov@rub.de

Peter Awakowicz
email: awakowicz@aept.rub.de

Amorphous carbon film deposition on inner surface of tubes using atmospheric pressure pulsed filamentary plasma source

published or not. The documents may come

Amorphous carbon film deposition on the inner surface of tubes using atmospheric pressure pulsed filamentary plasma source

Ramasamy Pothiraja, Nikita Bibinov, Peter Awakowicz

Introduction

Uniform thin film deposition on the inner surface of curved three-dimensional objects is one of the challenging processes in modern plasma technology. Several research groups have reported a systematic development of such film deposition processes like coating of inner surface of tubes and bottles [1][2][3][4][5][6][7][8][9][10][11]. Mostly microwave or RF driven ICP, CCP and jet based microplasma or magnetron plasma sources were used for their studies. Generally, in several methods used for coating films on inner surface of tubes, precursors are decomposed in the confined electrode region where the plasma is active, chemically active species are transported inside a tube and film coating is carried out by gas flow. In this case, because of polymerization and recombination reactions during the transport process, the nature of chemically active species (constituent of polymer film) is different at different places along the axis of the tube. This phenomenon could reduce the film uniformity along the axis of the tube. In our method of film coating on inner surface of tubes, a long filament of plasma is generated inside the tube in a gas containing a precursor as an admixture [12]. The plasma filament, which is thinner than the diameter of the tube to be coated, is active for long distance in the region of film coating. This plasma filament can ionize and/or dissociate precursor molecules. Using this method, chemically active species can be generated everywhere along the axis of the tube within close vicinity of inner surface of the tube. In this way, the differences in the nature of depositing chemically active species at different places along the axis of the tube can be reduced and films with better uniformity can be deposited. In addition to this, film deposition is supported by ion fluxes, which results in the formation of high quality film.

In the previous publication [12], we have reported our plasma source characterization and the model used for plasma characterization. In the present studies, the plasma source has been modified in order to increase the distance between the electrodes as well as to reduce the region which is covered by the grounded electrode. Characterization of deposited film, determination of plasma parameters, and simulation of chemical kinetics are carried out for this modified plasma source. In this paper, we report the following, 1. configuration of our modified plasma source, 2. film deposition with two different positions of driven electrode, 3. characterization of the film deposited on the inner surface of the tube, 4. plasma parameters, 5. simulation of chemical kinetics on the basis of determined plasma parameters, 6. correlation of film properties with the plasma parameters and chemical kinetics, and 7. mechanism of film deposition processes.

Experimental setup and simulations

Experimental setup

Experimental setup is similar to one reported previously [12], except the dimension and the location of the grounded electrode (figure 1). A copper tube (length, 10 mm) is used as the grounded electrode. For all the experiments reported in this article, the grounded electrode is fixed at 140 mm apart from the spike of the driven electrode. We performed two sets of experiments. First set of experiments are carried out in argon and methane mixture (Ar, 2.4±0.024 slm; CH 4 , 3±0.03 sccm) to deposit carbon based film on inner surface of a quartz tube. The discharge runs at low dynamic pressure, 1.8 Pa, because of low gas flow rate. It is carried out at two different positions of the driven electrode. Films deposited at this condition are characterized using various surface analysis techniques. Second set of experiments are carried out with argon, methane and nitrogen gas mixture (Ar, 2.4±0.024 slm; CH 4 , 1.0±0.01 sccm; N 2, 2.5±0.025 sccm) for plasma characterization. Emissions of nitrogen molecules and nitrogen molecular ions are used for the determination of plasma parameters. Plasma parameters determined at this condition are considered to be the same as the plasma parameters during the film deposition process in first set of experiments. The effect of absence of nitrogen on plasma parameters is balanced by increasing the methane quantity. This fact (balancing of plasma parameters) is confirmed from the similar simulated EVDFs for both sets of experiments (with/without nitrogen). It is also confirmed by measuring argon emission spectra at various places along the axis of the tube in both cases. Argon emission intensities are very close to each other in both cases and also have the similar trend along the axis of the tube. Analytical instruments used for the plasma characterization are the same as those used for previous characterization [12], except that the spatial resolution of CCD camera is about 10 µm for an objective used for the plasma volume measurement in this study. Since the film is coated on the inner surface of the tube, we do not have the possibility of recording FTIR-ATR, XPS and XRD spectra for this film. Hence, the film is coated also on quartz plates as follows. Quartz plates of dimensions 0.1-0.23 mm × 2-3 mm × 25 mm and /or 1 mm × 2-3 mm × 25 mm are placed inside the quartz tube at different places in the region between the powered and the grounded electrodes. Film deposition is carried out on these quartz plates. The film coated plates are used for FTIR-ATR, XRD and XPS spectral measurements. A FTIR spectrometer (BRUKER VECTOR 33) with a resolution of 0.3-1 cm -1 is used. Colour 3D laser scanning microscope (LSM) (Keyence, pitch resolution, 100 nm; Z measurement distance, 13 µm; XY calibration, 46 nm/pixel; Z calibration, 1 nm/digit) is used for surface profile measurement. LSM and scanning electron microscope (SEM) (LEO Gemini SEM 1530 electron microscope) are used to obtain the cross sectional images of film. Film thickness is determined from the cross-sectional image of the film in the tube [13]. These values obtained from the images of LSM and SEM are in accordance with each other. A few atomic layer of gold film is coated on the samples used for SEM measurements for the electrical conductivity. Information about amorphous nature of the film is obtained using X-ray diffraction analysis (Bruker D8 Advanced AXS diffractometer; Cu-K radiation (1.5481 Å); position sensitive detector (PSD); films are analyzed in the -2 geometry and measured in Bragg-Brentano geometry at room temperature; constant divergence slit is 5 mm; Ni-filter is used; step 0.02°; 90 seconds per point). Elements present in the film are determined using EDX spectra. EDX spectra are obtained from the spectrometer attached to the SEM instrument. Chemical nature of the elements is analyzed using XPS spectra (Physical Electronics, PHI 5000 VersaProbe, Al-K radiation). For a survey spectrum, it is scanned in the binding energy range of -2 to 1198 eV with the pass energy, 187.85 eV; the energy step, 0.5 eV; number of cycles, 5. For the core level line measurement, the pass energy is kept as 23.5 eV with the energy step of 0.05 eV. Temperature of the tube is measured using digital thermometer attached with K type thermocouple (Greisinger electronic, model GTH 1150). For this purpose, plasma is ignited for 10 minutes, and then it is switched off. Immediately after switching off the plasma, temperature is measured by keeping the thermocouple in contact with the tube. The error in measurement due to the response time of thermocouple is corrected with the temperature decay profile. Absorption spectra of deposited films are obtained using a deuterium lamp as a light source. Ocean optics emission spectrometer is used for transmitted light measurements.

Determination of gas temperature, EVDF, excitation rate constants and electron density

Optical emission spectroscopy (OES), voltage-current measurement, microphotography and numerical simulations are used for the determination of gas temperature in the filament and the plasma parameters. The details of applied diagnostic methods and parameters used from the literature are described in our previous publication [12]. Therefore, it is described only shortly as follows. Gas temperature in active plasma volume is one of the most important parameters, because of its influence on gas density in plasma and on the rate constants of chemical reactions. It is determined from the rotational temperature of diatomic molecules namely, N 2 and CN, from the emissions of N 2 (C-B, 0-0) and CN(B-X, 0-0).

The relative intensity of N 2 (C-B, 0-0) with respect to the intensity of N 2 + (B-X, 0-0) is used for the determination of EVDF. For this purpose, the relative intensity of N 2 (C-B, 0-0) with respect to the intensity of N 2 + (B-X, 0-0) is simulated for various EVDFs for our experimental conditions by numerical solution of the Boltzmann equation in local approximation and varied electric field applying the program code "EEDF" developed by Napartovich et al [14]. Finally, by comparing the experimentally determined relative intensities ( ) with the simulated relative intensities ( ) for various EVDFs, the actual EVDF is determined.

Using the normalized EVDF and the known collisional cross section σ exc (m 2 ) for electron impact excitation [15], we calculate the rate constants k (m 3 s -1 ) for electron impact excitation of N 2 , Ar and CH 4 using the equation ( 1):

! d , (1)
where, m e and e are the mass and charge of electron, and E is the kinetic energy of electrons (eV).

(in eV -3/2 ) is EVDF, and it is normalized to fulfil, d "#

The electron density (n e , m -3 ) is determined using the equation ( 2) from the measured absolute intensity of N 2 (C-B, 0-0) emission ( , phot•m -3 •s -1 ), the nitrogen density ([N 2 ], m -3 ), the electron impact excitation rate constant for N 2 (C-B, 0-0) emission ( , m 3 •s -1 ), the contribution of excitation of N 2 (C) by collision with argon metastables (% &' ( ) ) [12], the contribution of the quenching of N 2 (C) by argon (* ), the plasma volume (V p , m 3 ), the value of fraction of time in which plasma is active (t f ), and the geometrical conditions during the measurement of emission spectrum (g f ).

+ ,N 2 - . % &' ( ) * / 0 1 2 g 2

Temporal and spatial distribution of the gas temperature as well as fluxes of chemically active species

In order to determine the temporal and spatial distribution of gas temperature as well as fluxes of chemically active species from the plasma filament towards the surface of the tube, we numerically solve the equations for thermal conductivity (3) and diffusion (4) for a cylindrical symmetry [16,17]:

34

Results and Discussion

The For the determination of opt in order to have both absorbanc following figure (figure 3, right) different experimental conditions axis of the tube at the radial cen are stochastically distributed in th electrode region. It results the m 5 ge is ignited [12] in the mixture of argon and me argon [START_REF] Veldhuizen E M V | Int. Symp. HAKONE VIII Pühajärve[END_REF]. This long filament of plasma genera diameter of about 200 µm. The duration of the dis ofile and the position of filaments in the tube chan (profile and position of filaments are stationary f out 2000 filaments in series have the same pro haracterized using surface analysis techniques. F which differ in the position of driven electrode) ce morphology are the same. However, variation details of film analysis are discussed below.

ed film the films deposited on quartz plates show some images of the film deposited on quartz tube are images, film thickness is determined at 65 mm d e, film thickness at various places along the axis m on the basis of Beer-Lambert law. From th eposition, the film growth rate is determined. UV e entire region between the electrodes show simi dicating the uniform nature of the deposited ma ugation [START_REF] Cooper | Spectroscopic Techniques for Organic Chemists[END_REF].

ross-section (left) and LSM image of the film surface tical density with space resolution (~1 mm), wave ce as well as transmittance for the films in all ra shows the optical density as well as growth rate s. In the first experimental condition, the driven e ntre. In this case, the filaments ignited from the he tube cross section and touch the tube surface o maximum film growth rate at the middle regio ethane. This discharge is ated along the axis of the scharge is around 160 ns. nge with the frequency of for about 100 ms) and the ofile and position. Films For the films deposited in , nature of the elements, n of film thickness along e roughness (Rz = 8 nm) obtained using LSM and distant from the spike. By of the tube is determined he known values of film V-Vis absorption spectra ilar profiles of absorption aterials, composed of sp 2 e (right) at middle region in elength 350 nm is chosen ange of thicknesses. The of films deposited in two electrode is placed on the spike of driven electrode only within the grounded on in between electrodes (figure 3, right, --). In the second experimental condition, the driven electrode with spike is shifted 0.3 mm from the radial centre of the tube towards the surface of the tube. In this case, all filaments approach the tube surface at the same point (at about 20 mm from the spike) and pass on tube surface for a distance of about 20 mm. From this point till the grounded region, the filaments are stochastically distributed around the axis of the tube. This causes increase in the deposition rate by a factor of about three, in the region, where the filament touches the surface of tube (figure 3, right, --). Film thickness is almost constant from about 40 to 120 mm from the spike. In both experimental conditions, the deposition rate is very low shortly before the grounded area. The deposition rate within the grounded electrode region is almost two orders of magnitude higher than the average deposition rate in the area in between the electrodes. The increase in film growth rate near the spike, caused by the contact of plasma filament on the surface of tube, is important for the deposition of film on inner surface of tubes of different radii, and will be investigated in future. Also, position of the filaments in the tube, and therefore deposition rate, can be controlled by placing a grounded conductor (e.g. wire) on or near the outer surface of the tube. FTIR spectra are measured in ATR mode with a diamond crystal for the films deposited on quartz plates. Before measurement of the sample, background measurement in atmospheric air is carried out in the ATR mode. After this background measurement, FTIR spectra in the ATR mode are measured for the film coated quartz plates. Background corrected spectrum of film coated plate is considered as a spectrum of coated film [START_REF] Murugavel | [END_REF]21]. FTIR-ATR spectrum of the film on a quartz plate is shown in figure 4. There is a pseudo-high transmittance peak in the background corrected FTIR-ATR spectrum of film because of the presence of CO 2 in background air and its absence in the film. Hence, the peak at 2350 cm -1 is not shown in the figure 4 for clarity. Analysis of FTIR-ATR spectra of the deposited films indicates the presence of aliphatic C-H groups in the film through their characteristic absorption. All the spectra have the peaks at 2954, 2926 and 2870 cm -1 corresponding to CH 3 asymmetric stretch, CH 2 asymmetric stretch and CH 2 symmetric stretch, respectively [22][23][24][25], indicating the presence of hydrogenated sp 3 carbon in the film (figure 4). Presence of strong peak at around 1650 cm -1 shows the presence of double bonded sp 2 carbon on the surface of the film [26,27]. Absence of peak for stretching of (sp 2 )C-H above 3000 cm -1 indicates that sp 2 carbons are not hydrogenated. Hydrogen in the film is mainly attached to the sp 3 hybridized carbon. The relative intensity of "(sp 3 )C-H" absorption with respect to intensity of "(sp 2 )C=C(or O, N)" absorption varies slightly for the films on the quartz plates, which are placed at different places inside the tube between the electrodes during the film deposition process. Sharp peaks at 1455 cm -1 and 1375 cm -1 are due to the bending vibrations of CH 2 and CH 3 groups respectively. . FTIR-ATR spectrum of the film deposited on the quartz plate placed in the middle region in between the electrodes during the pulsed filamentary discharge (Ar, 99.87%; CH 4 , 0.13%; total gas flow rate, 2.4 slm). The region below 1300 cm -1 is not shown because of strong overlapping of absorption of quartz plate. Pseudo-high transmittance at around 2350 cm -1 , due to the presence of CO 2 in air during the background measurement, is removed for clarity.

X-ray diffraction analysis is carried out in order to find out the crystallinity of the film. It does not show any characteristic diffraction for graphene or diamond form of carbon. It only shows diffraction pattern characteristic for quartz substrate as shown in figure 5. It clearly indicates that the film is highly amorphous [28]. The nature of the elements present in the film at various places along the axis of the tube is determined using EDX measurement. It shows that film is composed of mostly carbon with traces of O and N elements (also contains hydrogen, on the basis of FTIR-ATR spectra). Oxygen, water and nitrogen molecules are the impurities in the supplied gas mixture, Ar/CH 4 . Presence of trace of W element in addition to the traces of O and N elements has been observed for the film deposited near the spike. In order to investigate the chemical nature of these elements and their composition, XPS measurements are carried out. It also indicates that the film is composed of mostly carbon with traces of O and N. Chemical nature of the carbon is analyzed using curve fitting procedure (figure 6, left). It shows that film is mostly composed of sp 2 carbon bonded to other sp 2 carbon with less quantity of sp 3 carbon bonded to other sp 3 carbon [29]. It also shows that some quantity of carbon is attached to O as well as to N. This fact is also supported by the presence of characteristic peaks for O and N in the XPS spectrum.

The film has two phases as shown in figure 6, right. One phase in the film has large area with smooth surface, as also shown in figure 2, right. The secondary phase in the film has hemispherical top faces, which are occupied in a small area of total film surface. Preliminary chemical inertness test shows that the deposited film is stable against brushing with water, iso-propanal and soap solution.

Characterization of plasma conditions

To study the mechanism of film deposition, we characterize plasma conditions (gas temperature in plasma, electron density, EVDF, etc.), calculate production rates of atoms and excited molecules, simulate fluxes of excited chemical species to the inner surface of the tube, and simulate the chemical kinetics. OES, microphotography and current-voltage measurements are used for the characterization of plasma conditions. In the following section, gas temperature, reduced electric field and electron density in the plasma are discussed. Because of the high neutral gas temperature in pulsed plasma filament, the tube is also heated. Since the temperature of tube will influence the film growth process and film properties, tube temperature is also determined. It is discussed along with the gas temperature in plasma and its fluxes towards the surface of the tube.

The gas temperature in the plasma filament is about 1000 K. Although it is high (figure 7a), duration of the discharge pulses is short of about 160 ns. Therefore, the actual (stationary) temperature of tube will be much lower than the gas temperature in the plasma filament. The actual temperature of the tube during the discharge is measured using a thermocouple. It shows that tube temperature is less (about 330 K) close to the spike and it reaches about 400 K close to the grounded electrode (figure 7b). Gas flow could be one of the reasons for this trend in tube temperature, since the gas mixture at room temperature is entering the spike region and relatively hot (or warm) gas mixture is entering the region close to the grounded electrode. With these data, equation for thermal conductivity is numerically solved to simulate the gas temperature in afterglow phase with temporal and spatial resolution. The results of this simulation presented in figure 8 show that the steady state conditions in the tube will be reached after 500 pulses. It is to be noted that, as mentioned above, about 2000 filaments in series have the same profile and position. The reduced electric field determined (section 2.2) during the discharge along the axis of the tube is shown in figure 9. It reveals that the reduced electric field is higher near the spike of the powered electrode than near the grounded electrode. Standard deviation in the reduced electric field is estimated by considering the fluctuation in the intensity of emissions of N 2 (C-B, 0-0) and N 2 + (B-X, 0-0). The electron density is almost constant (1.7×10 18 m -3 to 2.8×10 18 m -3 ) in most part of the region in between the electrodes (figure 10). The filament propagates far outside of the grounded region during the discharge with very less intensity of emission compared to the region in between the electrodes. Investigation of this process is in progress.

Chemical kinetics

The rate constants for electron impact methane dissociation, argon ionization and argon metastables formation are determined (figure 11) using the equation ( 1), from the known values of cross-section for the corresponding process [30][31][32][33] and the determined EVDFs. Electron impact dissociation rate constant of methane molecules is higher than the electron impact ionization rate constant of argon atoms (figure 11). Despite this fact, because of the higher concentration of argon than methane in the supplied gas mixture, electron impact argon ionization rate is much higher than the electron impact methane dissociation rate. Since the charge exchange reactions of argon ions with hydrocarbon molecules (methane) have big cross sections [34], mainly these reactions are involved in the ionization and dissociative ionization of hydrocarbon species. Hence, argon ion plays dominant role in the methane ionization, dissociation and consequent film deposition processes. This is in contrast to the low pressure plasma chemical kinetics, where electron impact plays dominant role in methane ionization, dissociation and consequent film deposition processes [35][36][37].

Since the exact density of various chemically active species and their diffusion coefficients are not known, methane diffusion in argon is simulated in order to have some hints about the diffusion of various chemically active species. Simulated results for methane diffusion in the figure 12 show that it takes about 10 to 20 ms for a chemically active species generated in the filament, which is approximately placed at the middle of the tube, to reach the tube surface. During this transport process, it may undergo several chemical reactions. Gas velocity is 1.4 m•s -1 for the gas flow rate of 2.4 slm. Hence, in the duration of 10 to 20 ms, neutral radicals will be moved to a distance of 1.5 to 3 cm along the axis of the tube. On the basis of densities of argon, methane and electron, as well as rate constants of various processes determined from the plasma parameters [38][39][40][41][42][43][44], most probable reactions among others are calculated using the equation (5),

D E F ,G- F $ H
where, [M] is the density of argon or methane or electron (m -3 ), P is the probability and k is the rate constant (m 3 s -1 ). The possible important chemical reactions with the inverse of their probability ( , life time of active species concerning the corresponding reaction) are shown in the following scheme (figure 13). The high probable reactions among others are shown in thick arrows in the scheme. Because of high electric field in active plasma volume, argon ion formation is more probable than argon metastable formation. At atmospheric pressure conditions, Ar + ion produces molecular ion, Ar 2 + through a three particles reaction. This is in contrast to the low pressure plasma, where the probability for three particles reactions is very low [35][36][37][45][46][47]. Life time of this molecular ion in argon-methane mixture is very short because of effective charge transfer reaction with methane [37]. This reaction produces CH 4 + ion.

This ion undergoes further reaction with CH 4 to produce CH 5 + ion and CH 3 radical. These chemically active species on further reaction with electrons present in the afterglow phase leads to the formation of CH 3 radical. Though this methyl radical is mostly produced through the above stated mechanism, it is also produced by direct reaction of argon metastables with methane as well as by electron impact dissociation of methane. Electron impact dissociation of methyl radical ultimately leads to the formation of carbon atom through CH 2 and CH species. The presence of C and CH are confirmed through their characteristic emissions (figure 14) [48][49][50] measured using echelle spectrometer. In addition to the production of molecular argon ion (Ar 2

+

) through termolecular reaction, argon ion (Ar + ) also produces CH 3 + ion through a high probable reaction with methane. The further reaction of CH 3 + ion with methane and electron leads to the formation of diatomic molecular carbon C 2 , which has been observed through emission spectrum.

The C 2 molecule on further reaction with electron leads to the formation of atomic carbon and its presence has also been observed through its emission. Since the electron density in afterglow phase is not known, probability for the electron impact reactions in afterglow phase could not be calculated. To determine life time of positive ions in afterglow phase of the discharge, the measured current is compared with the displacement current (I disp. ), which is calculated as I disp. = dV/dt and normalized to the measured current immediately after the filamentary discharge (figure 15). The decay time of charged particles in the tube in afterglow phases of the discharge is about 30 µs. Since the measured current is higher than the calculated displacement current in first positive and first negative phases of voltage following the ignition of discharge, it is concluded that some additional active discharge is ignited in the tube after the discharge. Microphotographical studies of this phenomenon show that it is a glow discharge, because the "negative glow" covers the surface of electrode when it is cathode. Electric field and photoemission in this discharge phase are low, and hence it does not influence considerably on the plasma parameters determined using OES for the filamentary discharge.

Mechanism of film deposition

On the basis of i) plasma parameters determined applying optical emission spectroscopy and simulation, ii) rate constants calculated using plasma parameters, iii) voltage-current measurements and iv) characterization of the film deposited inside the tube between the spike and the grounded area, it is concluded that the hydrocarbon ions produced in the charge exchange reactions of argon ions with hydrocarbon molecules play a dominant role in the film deposition process. At steady state conditions, methane, which is a part of gas mixture flows through the tube, is fully ionized and dissociated in the tube at the distance of approximately 20 mm from the spike. However, the film deposition rate has sharp maxima under the grounded area, which is at 140 mm away from the spike. The film deposited in the area between the spike and the grounded electrode has different thickness at different places, but with similar properties and components. This film is dense and amorphous; has smooth surface and low content of hydrogen. We suppose that this film is deposited during ion fluxes [45] on the wall after ambipolar diffusion and drift. In the frame of this assumption, we can explain different observed facts such as i) increase of deposition rate in the region where filament passes on or near the wall because of small distance to the wall, ii) higher deposition rate in the region under grounded electrode because of higher drift velocity in comparison to the velocity of ambipolar diffusion, and iii) low deposition rate near the grounded area because of high axial component of electric field in this region. The hydrocarbon film deposited in the region beyond the grounded area is differed strongly from the film deposited before and under the grounded area. This film is soft, rough and low dense. This film may be deposited by flux of neutral hydrocarbon species which are produced by collisions of hydrocarbon ions with the surface of the tube under the grounded area. The kinetic energy of these ions is high enough for their partial dissociation. The neutral hydrocarbon species formed in this process flow with gas along the tube and deposit on the wall after diffusion.

The mechanism of ions transport from the spike area to the grounded area is under investigation. The steady state conditions of this filamentary discharge depend on the distance between the electrodes as well as the amplitude and the frequency of applied voltage. For longer distance between electrodes, lower voltage frequency requires higher applied voltage for steady state discharge. We suppose that the residual charged species produce the electric field near the propagated head of the filament, and stimulate the propagation to a long distance. As mentioned above, the life time of the ions in the tube is about 30 µs, which is comparable with the pulse duration (45 µs). During this period, the charged species diffuse from the plasma channel and also undergo electron-ion recombination reaction; hence density of these species is reduced. The average density of these charged species during the initiation of next discharge is about 10 15 m -3 at the steady state conditions. On the other hand, very high density of charged species also hampers the ignition of the filamentary discharge, as discussed in the previous publication [12]. The facts discussed above show that plasma conditions in our experiment are different along the plasma channel. To make plasma conditions more uniform along the axis of the tube, the applied voltage with frequency of several tens of kHz can be modulated with sufficiently low frequency (pulse package modulation frequency) for filling of all tube with fresh gas mixture between plasma treatment phases. These experimental conditions will be used and studied in our forthcoming investigations.

Summary

The pulsed filamentary plasma source has been constructed to produce a long filament of plasma inside a tube for film deposition on inner surface of tubes. FTIR-ATR, XRD, SEM, LSM and XPS analyses give the conclusion that deposited film is amorphous composed of non-hydrogenated sp 2 carbon and hydrogenated sp 3 carbon with traces of O and N. UV-Vis absorption spectra of the film deposited on inner surface of the tube also confirm the presence of doubly bonded sp 2 carbon, which are shortly conjugated. Optical emission spectroscopy, voltage-current measurement, microphotography and numerical simulations are used for determination of the plasma parameters and the gas temperature. With these data, equation for thermal conductivity is numerically solved to simulate the gas temperature in afterglow phase with temporal and spatial resolution. The rate constants for argon ionization and methane dissociation are almost constant in the entire region in between electrodes although it is slightly higher close to the spike. Based on the analysis of kinetics of deposition processes, it is concluded that ions play important role in the deposition of amorphous carbon film.

Figure 1 .

 1 Figure 1. Schematic view of the experimental setup.

  ) -thermal diffusivity, λ conduction -thermal conductivity, ρ -density, C p -specific heat capacity, D -diffusion coefficient.

  pulsed filamentary discharg similar to a corona discharge in tube during this discharge has a d During plasma operation, the pro several Hz. Because of this fact ( pulse frequency is 22 kHz, abo deposited at this condition are ch both experimental conditions (w chemical composition and surfac the axis of tube is different. The d 3.1. Characterization of deposite LSM images of the surface of t (figure 2, right). Cross sectional SEM (figure 2, left). From these keeping this value as a reference using optical density of the film thickness and duration of film d (figure 3, left) of the films in the with λ max at around 220 nm ind carbon with a short range of conj

Figure 2 .

 2 Figure 2. SEM image of the film cr between electrodes.

Figure 3 .

 3 Figure 3. Left: UV-Vis absorption spectrum of the film; Right: Optical density of the films along the axis of the tube at 350 nm (--, electrode at the radial centre of the tube; --, electrode at 0.3 mm below the radial centre of the tube; ∇, value in the grounded electrode region is divided by 100).

Figure 4

 4 Figure 4. FTIR-ATR spectrum of the film deposited on the quartz plate placed in the middle region in between the electrodes during the pulsed filamentary discharge (Ar, 99.87%; CH 4 , 0.13%; total gas flow rate, 2.4 slm). The region below 1300 cm -1 is not shown because of strong overlapping of absorption of quartz plate. Pseudo-high transmittance at around 2350 cm -1 , due to the presence of CO 2 in air during the background measurement, is removed for clarity.

Figure 5 .

 5 Figure 5. XRD of deposited amorphous carbon film on quartz plate.

Figure 6 .

 6 Figure 6. XPS of carbon (left) and SEM image (right) of the film at middle region in between electrodes.

Figure 7 .

 7 Figure 7.Gas temperature in the plasma filament obtained using CN (B-X, 0-0) and N 2 (C-B, 0-0) emissions (a), and tube temperature (b) along the axis of the tube during the pulsed filamentary discharge (Ar, 99.85%; N 2 , 0.11%; CH 4 , 0.04%; total gas flow rate, 2.4 slm).

Figure 8 .

 8 Figure 8. Spatial distribution (in the radial direction) of the gas temperature from the plasma filament towards the surface of the tube. The filament diameter, 200 µm, is used for the simulations on the basis of micro-photographic images of the filaments.

Figure 9 .

 9 Figure9. Variation of the reduced electric field along the axis of the tube during the pulsed filamentary discharge (Ar, 99.85%; N 2 , 0.11%; CH 4 , 0.04%; total gas flow rate, 2.4 slm).

Figure 10 .

 10 Figure10. Electron density profile along the axis of the tube during the pulsed filamentary discharge (Ar, 99.85%; N 2 , 0.11%; CH 4 , 0.4%; total gas flow rate, 2.4 slm).

Figure 11 .

 11 Figure11. Variation in the rate constants for electron impact dissociation of methane, argon ionization and argon metastables formation along the axis of the tube during the pulsed filamentary discharge (Ar, 99.87%; CH 4 , 0.13%; total gas flow rate, 2.4 slm).

Figure 12 .

 12 Figure 12. Spatial distribution of chemically active species generated in the filament towards the surface of the tube. Since several chemically active species are generated in the filament, CH 4 diffusion in Ar is simulated.

Figure 13 .Figure 14 .

 1314 Figure 13. Chemical reactions involved in the film growth process. Thick arrow indicates high probable reaction. Emissions of active species in thick frames are observed in the measured emission spectrum.

Figure 15 .

 15 Figure 15.Voltage and current profiles during the pulsed filamentary discharge (Ar, 99.87%; CH 4 , 0.13%; total gas flow rate, 2.4 slm). The pulse frequency is 22 kHz. The voltage frequency of each pulse sequence is around 200 kHz. Displacement current is calculated as I disp. = dV/dt and normalized to the current measured immediately after the discharge.

Acknowledgement

This work is supported by the 'Deutsche Forschungsgemeinschaft' (DFG) within the frame of the research group 'FOR1123 -Physics of Microplasmas'. We thank Dr. Kirill Yusenko for XRD spectra.