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E-Verhandlungen 2001
Programm und Abstracts der Sitzung P 8

Experiments on influencing the turbulence in the plasma boundary of Wendelstein 7-AS stellarator with active Langmuir probes

•Henning Thomsen, Martin Schubert und Michael Endler
Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald, Wendelsteinstra\ße 1, 17491 Greifswald

The fluctuations in the edge plasmas of magnetic fusion experiments are thought to play an important role in terms of anomalous energy and particle transport [1]. The possibility of changing turbulence properties by externally applying electrical signals to probes was already shown in smaller devices [2,3,4] One approach is to lock the turbulence on a mode by the application of external electric field and current patterns. Another method is to feed signals from passive Langmuir probes back into the plasma boundary with appropriate phase shifts. Therefore, experiments were carried out at the Wendelstein 7-AS stellarator to investigate the influence of these methods on the turbulence in the plasma boundary of a larger toroidal magnetic confinement device. Two movable poloidal probe arrays, one located on the inboard side of the vessel and the other on the outboard side, were used for the experiments. Those probe tips, that were not used for feeding-in signals, collected data from the fluctuations in the plasma boundary. Therefore, the influence of the active probes on the plasma turbulence could be studied in poloidal direction as well as toroidally. [1] B. Richards, Phys. Plasmas 1(5), 1994 ; T. Uckan, Nuc. Fus. 35(4), 1995 [2] Chr. Schröder, Th. Klinger, to be published

Conditional averaging of fluctuations in the SOL of the Stellarator W7-AS

•O. Grulke^1,2, T. Klinger³, M. Endler² und A. Piel^1
1IEAP,Christian-Albrechts-Universität, 24098 Kiel
²Max-Planck-Institut für Plasmaphysik, EURATOM Association, 85748 Garching
³Institut für Physik, Ernst-Moritz-Arndt-Universität, 17489 Greifswald

The results of an experimental study of the dynamics of large-scale density and potential fluctuations in the SOL of the stellarator Wendelstein-7AS are presented. The fluctuations are measured by electrostatic probes. To extract large-scale coherent fluctuation structures from the noisy uncorrelated background, data is analyzed with conditional averaging [1]. It turns out that monopole-like potential structures with typical lifetimes of 20ms and spatial extension of 5mm-10mm propagate poloidally in ion diamagnetic drift direction. The cross-parameter study between potential and density fluctuations shows that the fluctuations are poloidally phase-shifted by -p/2. A toroidal phase shift is not observed. The results strongly support that the formation of the large-scale fluctuation structures is based on the interchange instability [2].

[1] Johnsen et al., Phys. Fluids 30(7),1987

[2] Endler et al., Nucl. Fusion 35(11),1995

Fast swept probe measurements at W7AS

•Martin Schubert
Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstr.2, D-85748 Garching

The aim of fast swept Langmuir probe measurements at W7AS is to measure poloidal electrostatic fluctuations, which may contribute significantly to the anomalous radial energy transport at the boundary of fusion plasmas. Probe bias voltage sweep allows one to make time-resolved measurements of the electron temperature, which is particularly useful for determination of the correct plasma potential from floating potential. The voltage sweep, however, must be faster than the bandwidth of the fluctuations of several 100 kHz. This paper reports the current status (setup, electronics, DAQ system) of fast swept probe measurements in the stellarator Wendelstein-7AS. Probe array measurements at the high and the low field side close ( < 1cm ) to the separatrix are also presented. Probe voltage and current signals have been recorded for different plasma confinement conditions. The obtained probe characteristics provide the parameters V_float, T_e and I_sat, in principle, but challenges in data handling ( ~ 100 MB per shot ), separation of single characteristics and fitting have to be coped with.

Investigation of MHD-instabilities in W7-AS

•S. Klose, A. Weller und the W7-AS team
Max-Planck-Institut fuer Plasmaphysik, Bereich E3, Boltzmannstrasse 2, D-85748 Garching b. Muenchen

The spatio-temporal structure of the soft X-ray emission ( < 10 keV) of a plasma cross-section can be reconstructed with the soft X-ray multi-camera-tomography-system at W7-AS [1, 2]. The X-ray emissivity depends mainly on the plasma pressure which is nearly constant on the magnetic flux surfaces. Hence, the spatio-temporal structure of the magnetic flux surfaces, in particular displaced due to instabilties (e.g. MHD-modes), can be observed with this diagnostic, and in most cases a strong correlation with Mirnov-probe data is found. Recent results from the last experimental program (including the new divertor) will be presented.

[1] Goerner, C. et al., Stellarator News, No. 55, 1 (1998)

[2] Weller, A. et al., Rev. Sci. Instr., 70 (1999) 484

The Power Balance in W7-AS Plasmas bounded with Divertor Modules by means of Calorimetry and Bolometry

•Franz Gadelmeier, Peter Grigull und Louis Giannone
Max-Planck-Institut für Plasmaphysik, Boltzmannstraße 2, D-85748 Garching

W7-AS is a 5-period modular stellarator. In the summer of 2000, W7-AS resumed operation with ten newly installed divertor modules, five up and five down. The time-integral thermal loading of individual target tiles, 17 CFC-tiles per module, is registered via thermocouples embedded in the tile backside (calorimetry). The radiated power outside the divertor module region is measured by means of a 32-channel bolometer array. During the first divertor-experiments emphasis was placed upon the power balance and to assess up-down and toroidal asymmetries.

The calorimetric measurements indicate i-dependent asymmetries. The accountability (thermal load on divertor targets and radiative losses) amounts up to 50% of the input power. Beyond this, the accountability does not decrease with increasing i in contrast to former limiter-bounded plasmas. Thus, in high i discharges the present divertor modules warrants a better coverage of the plasma edge compared to the former limiter configuration.

Divertor Retention and Penetration Probabilities for Tungsten at ASDEX Upgrade

•Alexander Geier, Rudolf Neu, Hans Maier, Karl Krieger und ASDEX Upgrade Team
Assoziation

The applicability of tungsten as plasma facing material was investigated at ASDEX Upgrade by coating of tiles at the lower central column and in the divertor. Thereby no severe adverse effects on the plasma performance could be observed. Prior to an extrapolation to ITER, however, more physical understanding is necessary. Essential parameters for the central tungsten concentration are the divertor retention potential for tungsten and its penetration probability p through the Scrape-Off-Layer into the region of closed fieldlines. At ASDEX Upgrade this probability was measured by means of the Laser-Blow-Off system comparing injecton from the high-field and th low-field side. Using the sublimation probe [1], the value for p could be compared between different poloidal positions. The divertor retention potential could be measured using the sublimation probe in the divertor along with a deposition probe in the main chamber. Therefore, however, a significant increase of the detection sensitivity of the deposition probe was necessary. Additionally, the spectroscopic detection limit for tungsen in the main plasma could be reduced using a focussing X-Ray spectrometer.

[1] A. Geier et al., Rev. Sci. Instr. 70, (1), 1999

Detection of Neutral Beam Generated Suprathermal Electrons by Thomson Scattering on ASDEX Upgrade

•Bernd Kurzan, Heindieter Murmann, Martin Jakobi, Denis Bolshukhin, Wolfgang Suttrop und ASDEX Upgrade Team
Max-Planck-Institut für Plasmaphysik, EURATOM Association, Boltzmannstr. 2, 85748 Garching

On the ASDEX Upgrade tokamak a Thomson scattering apparatus with 50^o scattering, and laser beams running tangetially to the toroidal direction, is now in operation. This diagnostic measures spatially resolved the electron energy distribution nearly parallel to the toroidal magnetic field. This is a supplement to the already existing Thomson scattering apparatus with 90^o scattering where the laser beams run vertically in the poloidal plain, and which measures spatially resolved the electron energy distribution perpendicular to the magnetic field. During neutral beam injection with a power P ³ 5 MW in low density plasmas, n_e £ 4*10¹⁹ m^-3, the Thomson scattering diagnostics measure in the plasma center an increase of the mean parallel electron energy to about 20 keV, while their mean perpendicular energy is in the range of 4-5 keV. The values of the perpendicular and parallel energies are confirmed by electron cyclotron resonance emission measurements and spectra of the soft x-ray radiation respectively. The experimental results are discussed and compared to model calculations.

Microwave interferometer for investigating the plasma parameter at TJ-K

•J. Stoeber¹, C. Lechte¹, T. Bindemann², H. Hartfu\ß² und U. Stroth^1
1Institut für Experimentelle und Angewandte Physik, CAU Kiel
²Max-Planck-Institut für Plasmaphysik, EURATOM Ass., Garching

The poster describes the setup of a 1-channel microwave interferometer at TJ-K. Using a heterodyne system we will be able to measure the plasma density unambiguously in a range from 10¹⁶m^-3 to 10¹⁹m^-3. The measurements will be compared with results from a Langmuir probe diagnostic and with theoretical estimates from a particle and power balance analysis. The latter yields upper values for the achievable plasma temperature and density depending on the machine parameters, which are natural gas pressure, heating power, magnetic field strength and different working gases (H, He and Ar).

Microscopic Structure of Turbulence in the Torsatron TJ-K

•Carsten Lechte¹, Nils Krause¹, Sven Niedner² und Ulrich Stroth^1
1Institut für Exp. und Ang. Physik, CAU Kiel, D-24098 Kiel
²MPI für Plasmaphysik, EURATOM Ass., Garching

In magnetically confined fusion plasmas, turbulence leads to enhanced radial transport, which degrades confinement. The TJ-K torsatron is operated with rf discharges. The microscopic structure of the resulting turbulence is measured with spatio-temporal resolution.

Apart from [B\dot] probes and microwave interferometry, the main diagnostics are Langmuir probe arrays. Due to the relaxed plasma parameters in TJ-K, they can be employed in the whole discharge volume. The Langmuir data are processed using correlation analysis and conditional averaging, yielding structure sizes and time scales of the turbulence. Together with the statistical properties of the fluctuations of density, plasma potential and transport, these quantities are compared to our numerical simulations.

Special interest lies in the scaling of these turbulence properties with the plasma parameters and the ion mass, so that the relevance of the TJ-K results for fusion plasmas can be assessed.

Flußflächenmessungen für das Torsatron TJ-K

•R. Westphal, U. Stroth, S. Niedner, N. Krause und C. Lechte
Institut für Experimentelle und Angewandte Physik, CAU Kiel

Geschlossene magnetische Flächen sind die Voraussetzung für guten Plasmaeinschluß. In dieser Arbeit werden die magnetischen Flußflächen des Stellarators TJ-K mit Hilfe eines Elektronenstrahls gemessen. Der Elektronenstrahl wird mittels einer Wolframglühwendel und Lochanode erzeugt. Durch ein sich senkrecht zur toroidalen Achse bewegender Stab, der oberflächlich mit aktiviertem bzw. fluoroeszierendem Zinkoxid behaftet ist, wird der mehrfach umlaufende Elektronenstrahl optisch sichtbar (Leuchtpunkte). Durch Variation der Anodenspannung bzw. Elektronenenergie und des Einschußwinkels, werden die Elektronendriftbewegungen untersucht, und durch Variation des Abstandes von der toroidalen Achse, die verschiedenen ineinander geschachtelten Flußflächen gemessen. Die Magnetfeldkonfiguration wird durch Änderung des Quotienten IV/IH, wobei IV der Strom durch die Vertikalfeldspule und IH der Strom durch die Helikalfeldspule ist, eingestellt. Die gemessenen Ergebnisse werden mit den simulierten, theoretischen Ergebnissen, die mithilfe eines von Gourdon geschriebenen Fortranprogramms erhalten werden, verglichen.

Measurements of impurity density profiles using a diagnostic hydrogen beam in TEXTOR-94

•A. Kreter¹, R. Jaspers², B. Schweer¹ und B. Unterberg^1
1Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, EURATOM Association, 52428 Jülich, Germany^*
2FOM Instituut voor Plasmafysica, EURATOM Association, Postbus 1207, 3430 BE Nieuwegein, The Netherlands^*

Measurements of the impurity density profiles in TEXTOR-94 were performed by means of Charge-eXchange Recombination Spectroscopy (CXRS) using the diagnostic hydrogen beam RUDI. RUDI is based on a rf-discharge ion source and provides an equivalent neutral current of 1 A with an extracting voltage of 50 keV, an angular divergence of ±0.5^° and a pulse duration of 4 s, modulated with 500 Hz.

The density profiles of C⁶⁺, C⁵⁺, Ne¹⁰⁺, Ne⁹⁺, Ne⁸⁺ and O⁸⁺ were measured in discharges with pure ohmic and additional heating. The accuracy of the measurements is mainly determined by the accuracy of the CX-rate coefficients and amounts to 30-40 % for the fully ionized impurity ions. The influence of the excited beam particles on the CX-signals was taken into account.

The measured impurity densities were compared with calculations of the selfconsistent 1D impurity transport code RITM. There is a good qualitative agreement between the radial shapes of the measured and calculated impurity density profiles. Quantitatively there are discrepancies of up to 50 %, which in the case of the not fully ionized particles can be partly explained by uncertainties in CX-rate coefficients.
^*Partners in the Trilateral Euregio Cluster

Measurement of magnetic fluctuations by means of a multiple Hall probe in TEXTOR-94

•G. Mank¹, I. Duran², K.H. Finken¹ und G. van Oost^3
2Institute of Plasma Physics, Ass. EURATOM-IPP.CR, 18221 Praha 8, Czech Republic
³Ghent University, Belgium
¹Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, EURATOM Ass., 52425 Jülich, Germany

A multiple Hall probe diagnostic has been tested in the tokamak TEXTOR-94. Up to 9 Hall probes for multidimensional measurements can be mounted on a probe head which is inserted into the plasma edge. The Hall probe set up is air cooled and shielded by a boron nitride head. The absolute sensitivity is 10e-4 allowing the measurement of perturbation fields of less than 0.1 mT for frequencies in the range from several 100 Hz up to 80 kHz. The advantages of such a system are the compactness and plainness. Frequency spectra have been measured during strong mode activity as e.g. during disruptions. The magnetic turbulence level for the radial and poloidal component has been investigated using fast Fourier analysis. The frequency spectra show the magnetic perturbation due to the precursors of the sawtooth activity. The change of the frequency spectra during neon injection into radiatively improved mode discharges is clearly observable.

Analysis of the thermal energy quench period during disruptions in the tokamak TEXTOR-94

•A. Savtchkov, G. Mank und K.H. Finken
Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, EURATOM Ass., 52425 Jülich, Germany

The thermal energy and current decay phase of disruptions at the tokamak TEXTOR-94 have been observed by the means of several diagnostics. Besides plasma current, loop voltage, and spectroscopic observations a fast measurement of heat pulses during disruptions is possible with an infra-red (IR) camera system. The 12 bit IR observation allows a time resolution of about 100 ms for one dimension. These measurements are combined with an IR diode observation, allowing to observe heat pulses with 4 ms time resolution. The toroidal belt limiter ALT-II is equipped for current measurement and allows an estimate of the toroidal extension of the flux hitting the plasma facing components. Besides the determination of the modes numbers with Rogowski coils and ECE diagnostic the magnitude of the field perturbation can be determined by the use of Hall probes, which can be inserted into the scrape off layer.

Heating scenario in a linear magnetized ECR mirror device

•Ulrich Wolters, Dirk Meyer und Klaus Wiesemann
Experimentalphysik inbes. Gaselektronik, Ruhr-Universität Bochum, D-44780 Bochum

In the past different authors have studied intensely the necessity of phase randomization during ECR heating in a magnetic mirror within the single particle approximation. The excitation of parametric instabilities introduces another system of wave particle interactions. Collective motions imply a different mechanism of phase mixing with time scales close to the inverse growth rate of a modulational instability, the characteristic time being shorter than the electron bounce frequency in the mirror trap. Measurements on ECR heating show the presence of both spectral broadening of the pump wave as well as low frequency noise close to the lower hybrid frequency. The occurrence of these effects comes together with a density increase in the discharge centre, changing the wave dispersion of several modes, thus leading to a change of the heating scenario of the entire plasma. The aspects of this scenario are discussed.

First approaches to density profile calculations of an ECR-discharge

•Michael Küll, Ulrich Wolters und Klaus Wiesemann
Experimentalphysik insbes. Gaselektronik, Ruhr-Universitaet Bochum, D-44780 Bochum

The plasma density distribution of a low-pressure magnetic mirror device (ECR-discharge) is of interest for two reasons. On the one hand the heating scenario changes with the spatial position of the upper hybrid resonance zones, which depends on the plasma density distribution. On the other hand we utilize line-of-sight integrating diagnostics such as microwave interferometry, where the density profile is needed for the evaluation of the plasma parameters. Therefore we developed a two dimensional model for the density distribution within the ECR-discharge based on the first two moments of a Boltzmann type equation. The model separates the different loss processes parallel and perpendicular to the vacuum magnetic field, namely classical-/Bohm diffusion and ionic free fall with electrically conducting walls (Simon short circuit). For the simulation we implemented an ADI-method, alternatively we used a simplification leading to an eigenvalue problem. We will discuss first results and difficulties.

Das Plasmapotentialprofil eines einfach magnetierten Torus

•Mirko Ramisch, Franko Greiner, Olaf Grulke und Alexander Piel
Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität, 24098 Kiel

Obwohl in einem einfachen magnetisierten Torus mit rein toroidalen Magnetfeld ohne Rotationstransformation kein MHD-Gleichgewicht existiert, läßt sich darin ein Quasigleichgewicht etablieren. Die Teilcheneinschlußzeiten liegen dabei weit über den durch die Toroidaldriften erwarteten Werten von etwa 1ms. Im Kieler Plasmatorus TEDDI können neben der klassischen Plasmaerzeugung mit Hilfe eines geheizten Filaments auch Plasmen mittels einer Helikonantenne erzeugt werden [1]. Dieser Beitrag stellt Untersuchungen vor, die zeigen, daß das Plasmapotentialprofil eines einfach magnetisierten Torus empfindlich vom Verhältnis der Ionen- und Elektronenflüsse abhängt. Zusätzliche Teilchenflüsse durch die Plasmaerzeugung können deshalb dazu führen, daß das Potentialprofil invertiert wird. Eine Trennung des Einflusses der Quelle von den Effekten, die durch die Magnetfeldkonfiguration verursacht werden, ist durch den Einsatz verschiedener Plasmaquellen möglich.

[1] F. Greiner et al. Posterbeitrag auf dieser Tagung

Helikonplasma in einem einfach magnetierten Torus

•Franko Greiner, Olaf Grulke und Alexander Piel
Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität, 24098 Kiel

Im TEDDI kann Plasma mittels einer Helikonantenne erzeugt werden. Im Vergleich mit der Filamentquelle zeigt sich, daß die Plasmaerzeugung sowohl auf die Gleichgewichtsprofile als auch die statistischen Eigenschaften der Turbulenz erheblichen Einfluss hat.

An antenna for Electron Bernstein heating of the Pegasus spherical tokamak

•Anja Mück¹, Hartmut Zohm¹, Walter Kasparek² und Cary Forest^3
1Max-Planck-Institut für Plasmaphysik, EURATOM Assoziation, Boltzmannstr. 2, 85748 Garching
²Institut für Plasmaforschung, Universität Stuttgart, Pfaffenwaldring 31, 70569 Stuttgart
³University of Wisconsin - Madison, 1150 University Ave, Madison, WI 53706, USA

In a spherical tokamak, due to the low cut-off density n_e = 10¹⁷\frac1m³, direct electron cyclotron heating is not possible. However, the conversion from ordinary to extraordinary mode at the plasma cut off layer and from extraordinary to Bernstein mode at the upper hybrid frequency is supposed to make the access of electron cyclotron waves to the plasma center possible. The antenna system is a combination of a rotatable elliptical reflector and a feed antenna. To optimize the conversion process quasi circular polarization is needed which determines the helical shape of the feed antenna. The antenna design for the spherical tokamak Pegasus, Madison, WI, is presented. The construction offers the possibility to verify experimentally the occurrence of the O-X-B mode conversion process and its angular dependence at a frequency of f = 2.9 GHz.

Plasma treatment of vapor grown carbon fibers

•Ekkehard Pasch, Moritz Heintze und Karsten Schröder
INP-Greifswald

Vapor-grown carbon fibers (VGCF) are produced catalytically on metallic surfaces from gaseous hydrocarbons. VGCF have excellent values of tensile strength and modulus. Therfore, there has been a recent interest in investigating the potential of VGCF as a reinforcement for composite materials. To improve the covalent linking of the VGCF to the polymer matrix, some surface treatment of the VGCF is needed. Methods normally used are oxidation in air or more recently by plasma treatment. The latter has the advantage of a controlled chemical influence without changing the fiber structure. The type and density of functional groups depend on the plasma process conditions. Commonly oxygen or nitrogen plasmas are used.

Chemical surface analysis results of VGCF after treatment in oxygen or nitrogen containing plasmas are presented. For treating larger amounts of fibers a low-pressure fluidized bed reactor was developed.

Technische Realisation und Anwendungsgebiete eines Infrarot-Bildwandlers auf der Basis eines Halbleiter-Gasentladungs-Systems

•S. Matern¹, Yu.A. Astrov², V. Marchenko¹, L. Portsel² und H.-G. Purwins^1
1Institut für Angewandte Physik, Westfälische Wilhelms-Universität Münster, Corrensstrasse 2-4, 48149 Münster
²A. F. Ioffe Physico-Technical Institute, Russian Academy of Science, St. Petersburg 194021, Russia

Ein planares, dünnschichtiges Halbleiter-Gasentladungs-System, desssen Kathode aus einem hochohmigen Si-Halbleiter besteht, wandelt 2-dimensionale infrarote Strahlungsfelder im Bereich von 1 mm bis 11 mm in sichtbares Licht. Bei einer effektiven Pixelzahl von 256 ×256 erreicht es eine zeitliche Auflösung von bis zu 50 ns. Das System wird entweder mit flüssigem Stickstoff oder mit einem kompakten Stirling Kühler gekühlt. Koppelt man den Bildwandler mit im sichtbaren Licht arbeitenden, schnellen Kameras, kann erstmalig eine zeitlich und räumlich hochaufgelöste Analyse von infraroten Strahlungsfeldern erfolgen. Anwendungsfelder für das System sind: Lasermodenanalyse, Überwachung von Laserschweissprozessen und Verbrennungsvorgängen.

BSCCO ceramics sputtering in hollow cathode discharge

•Valentin Mihailov, Renna Djulgerova, Vesselina Gencheva, Angelina Ivanova und Ginka Mihova
Institute of Solid State Physics - Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee blvd., 1784 Sofia, Bulgaria

The use of the hollow cathode discharge for technological purposes, namely for deposition and etching, recently turns out to be a perspective application of this spectral and plasma source. In this connection, the study of entering of different elements, especially from many component materials, into the hollow cathode plasma is a important task. The purpose of the present work is to study the sputtering of the different elements from Bi_1.8 Pb_0.4 Sr₂ Ca_2.2 Cu_3.2 O_10+y ceramics in the hollow cathode glow discharge plasma and correlation of sample composition with plasma composition. The used hollow cathode design is suitable for probe measurement of the quantity of sputtered substance put in the hollow cathode as a powder. A probe having the plasma potential is axially inserted into the hollow cathode plasma and the sputtered substance condenses on it. The mass of the deposits of different elements from BSCCO ceramics under studying is measured by quantitative spectral analyses performed by ICP-OES (27 MHz ICP Jobin-Ivon system equipped with 1m Czerny-Turnur monochromator). The quantity relations of the sputtered components into the hollow cathode plasma are studied as a function of discharge conditions. The results obtained by this study are important for hollow cathode discharge applications in fabrication of high temperature superconductor layers.

Study of (Ne+H₂) plasma in a hollow cathode and in a positive column glow discharges

•Vesselina Gencheva¹, Valentin Mihailov¹, Renna Djulgerova¹, Karol Musiol², Stefan Labus² und Lukas Bratasz^2
1Institute of Solid State Physics - Bulgarian Academy of Sciences 72 Tzarigradsko Chaussee blvd., 1784 Sofia, Bulgaria
²Institute of Physics of Jagellonian University 4 Reymonta, 30-039 Krakow, Poland

Mixtures of H₂ and other gases are used when glow discharges are applied in material processing technologies. There the negative ions play an important role and the knowledge of their formation and destruction processes is necessary for working condition optimization. We have made theoretical description of Ne+H₂ plasma in both hollow cathode and positive column glow discharges. As a result the concentration of H^-, Ne⁺, H, H⁺, H₂⁺, the population of excited Ne levels and excited vibrational H₂ states are obtained. One can conclude from these models that the main processes for H^- formation in the glow discharges under studying are the dissociative and electron attachment. The main processes for H^- destruction are the Ne⁺ and H^- recombination and the collisions between H^- and H₂ in hollow cathode discharge and also the H^- diffusion in the positive column discharge. The obtained theoretical results for H^- concentration as a function of H₂ concentration in (Ne+H₂) mixtures are compared with the measured by us values using photodetachment method.

In situ Measurements to Determine the Effective Work Function

•Lars Dabringhausen, Daniel Nandelstädt und Jürgen Mentel
Ruhr University of Bochum, Fundamentals of Electrical Engineering Group, Universitätsstr. 150, 44780 Bochum, Germany

The main task of a cathode in high intensity discharge (HID-)lamps is to emit electrons thermionically. The thermionic emission process is strongly affected by the cathode work function, which can vary during lamp operation. To determine the effective work function, precise measurements have been carried out on a special model lamp.

In particular, spatially resolved measurements of the cathode surface temperature have been performed using a single-wavelength pyrometer. Essential terms of the cathode power balance and the ion current fraction can be obtained using these measurements. In combination with the estimated size of the current transfer zone, the work function can be evaluated by applying Richardson's equation to cathodes of different temperatures. Reasonable results were found. The enclosed iterative calculations show robust convergence for arbitrary initial guess values.

Financial support supplied by the German Federal Ministry of Education and Research, Ref.-No. 13N7763

Diagnosis of a benzene discharge with a mass-selective spectroscopic technique

•Felix Güthe, Thomas Pino, Hongbin Ding und John P. Maier
Institute for Physical Chemistry, University of Basel,Klingelbergstrasse 80, CH-4056 Basel, Switzerland

The products formed in a benzene discharge have been probed by resonant two photon ionisation spectroscopy. The molecules formed are cooled in a supersonic expansion and mass-analysis combined with spectroscopic information is used to identify the species. By this means styrene, phenylacetylene, methylstyrene, indene, fluorene and tolane are recognized. No 6-ring polycyclic aromatic hydrocarbon could be detected. With this method the plasma is sampled at an early stage where the ethynylated and cyclopentafused polycyclic aromatic hydrocarbons are more abundant than the more stable 6-ring ones. The coupling of a discharge source to a REMPI detection system enables electronic spectra of neutral molecules to be to obtained which can be used to study their role in combustion processes, plasmas as well as in astrophysics.

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