Collective Thomson scattering experiments reveal the presence of high-frequency, axial electron density fluctuations at millimetric wavelengths in the Hall thruster plasma. The properties of these fluctuations are investigated experimentally and via linear kinetic theory. The relative drift of electrons and ions in the axial direction is found to be insufficient to cause excitation of the observed mode. Instead, the mode is determined to be a two-stream instability arising due to the velocity difference between singly and doubly charged ion populations in the plume.

Microturbulence has been implicated in anomalous transport at the exit of the Hall thruster, and recent simulations have shown the presence of an azimuthal wave which is believed to contribute to the electron axial mobility. In this paper, the 3D dispersion relation of this E×B electron drift instability is numerically solved. The mode is found to resemble an ion acoustic mode for low values of the magnetic field, as long as a non-vanishing component of the wave vector along the magnetic field is considered, and as long as the drift velocity is small compared to the electron thermal velocity. In these conditions, an analytical model of the dispersion relation for the instability is obtained and is shown to adequately describe the mode obtained numerically. This model is then fitted on the experimental dispersion relation obtained from the plasma of a Hall thruster by the collective light scattering diagnostic. The observed frequency-wave vector dependences are found to be similar to the dispersion relation of linear theory, and the fit provides a non-invasive measurement of the electron temperature and density.

Collective (or coherent) Thomson scattering has recently emerged as an important tool for identifying and characterizing certain instabilities in Hall thrusters. Plasma instabilities in electric thrusters are implicated in diverse phenomena, including reduced efficiency, lifetime and anomalous particle transport. This work discusses the main features of the collective scattering diagnostic PRAXIS, and recent applications of the diagnostic to study the nature of microturbulence at different thruster operating regimes. Early measurements show the presence of a small-scale azimuthal instability may be linked with regimes of unstable thruster operation.

The effect of the collective light scattering diagnostic transfer function is considered in the context of the dispersion relation of the unstable E×B mode previously reported. This transfer function is found to have a contribution to the measured frequencies and mode amplitudes which is more or less significant depending on the measurement wavenumbers and angles. After deconvolution, the experimental data are found to be possibly compatible with the idea that the mode frequency in the jet frame (after subtraction of the Doppler effect due to the plasma motion along the thruster axis) is independent of the orientation of the wave vector in the plane orthogonal to the local magnetic field.

Hall thruster performance improvement requires a good understanding of electron transport through the ion acceleration zone. In this zone, electrons are confined thanks to a permanent radial magnetic field. The electron mobility in the axial direction is higher than predicted by collisional models and more localized at the thruster exit than predicted by Bohm models. PIC simulations show small scale instabilities could be a good candidate for explaining this anomalous transport. Collective scattering measurements performed in front of a Hall thruster [3] showed the presence of small scale (mm) fluctuation modes in the azimuthal ExB drift direction as foreseen by linear models and PIC simulation [1, 2]. These observations were compared with 2D axialazimuthal PIC simulations [5, 7]. From PIC simulation result analysis, we emphasize the spatiotemporal behavior of this mode. Thruster front plasma observations with Langmuir probes show a fast large scale azimuthal mode is also present in front of the thruster [8]. The intensity of this mode also present in PIC simulations is compared to the intensity of the small scale one. A model to link small scale azimuthal instabilities to the electron axial mobility is presented [6].

Collective light scattering (CLS) has been recently extended to the observation of Hall effect thruster plasmas. A better understanding of the physics of Hall thrusters is key to improving thruster operation and lifetimes. In particular, the origins of thruster phenomena such as anomalous electron transport need to be determined. A high‐performance CLS diagnostic has therefore been developed for identifying and measuring electron density fluctuations in the thruster, at millimetric length scales and MHz frequencies. Such modes are believed to play a role in anomalous transport, and experiments so far performed have provided information on their dispersion relations, amplitude and directivity. This work describes the technical aspects of the optical bench and the range of accessible experiments (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

R. Abgrall,1 M. H. Achard, J. Adam, G. Agarici, E. Agostini, M. Airaj, F. Albajar-Vinas, L. Allegretti, J. P. Allibert, J. C. Alliez, A. Allouche,2 J. Andreoletti, J. M. Ane, P. Angelino, T. Aniel, G. Antar, N. Arcis, A. Argouarch, C. Arnas,2 G. Arnoux, R. Arslanbekov, J. F. Artaud, E. Asp, S. Assas, G.Attuel, R.Aymar,A.Azeroual, S. Balme, O. Barana, B. Bareyt, V. Basiuk, M. Basko, P. Bayetti, L. Baylor,3 B. Beaumont, R. Becherer, A. Becoulet, M. Becoulet, L. Begrambekov,4 S. Benkadda,2 F. Benoit, V. Bergeaud, G. Berger-By, S. Berio, P. Bernascolle, N. Bernier, M. Berroukeche, B. Bertrand, D. Bessette, P. Beyer,2 P. Bibet, J. Bizzaro, P. Blanchard,5 J. Blum,6 S. Boddeker, D. Boilson,7 G. Bon Mardion, P. Bonnel, X. Bonnin, J. Boscary, G. Bosia, J. M. Bottereau, F. Bottiglioni, H. Bottollier-Curtet, C. Bouchand, G. Bouligand, F. Bouquey, C. Bourdelle, R. Bregeon, F. Bremond,8 S. Bremond, C. Breton, M. Breton, C. Brosset, R. Brugnetti, J. L. Bruneau, J. Bucalossi, R. V. Budny,9 Y. Buravand, C. Bush,3 M. N. Bussac,10 A. Cambe, H. Capes, J. J. Capitain, P. Cara, J. L. Carbonnier, S. Carpentier, J. Carrasco, A. Casati, O. Chaibi, C. Chamouard, M. Chantant, P. Chappuis, D. Chatain, E. Chatelier, M. Chatelier, J. H. Chatenet,10 X. P. Chen, L. Cherigier, G. Chevet, L. Chiarazzo, D. Ciazynski, G. Ciraolo, F. Cismondi, F. Clairet, J. Clary, C. Clement, L. Colas, N. Commaux, E. Corbel, J. J. Cordier, Y. Corre, L. Costanzo, A. Cote, J. P. Coulon, L. Courtois, X. Courtois, B. Couturier, J. P. Crenn, P. Cristofani, N. Crouseilles,11 O. Czarny, P. Da Silva Rosa, C. Darbos, G. Darmet, M. Davi, R. Daviot, H. De Esch, B. De Gentile, J. C. De Haas, E. De La Cal, C. De Michelis, C. Deck, J. Decker, P. Decool, P. Degond, R. Dejarnac, E. Delchambre, E. Delmas, L. Delpech, H. Demarthe, M. Dentan, G. Depret, P. Deschamps, C. Desgranges, P. Devynck, L. Doceul, N. Dolgetta, C. Doloc, Y. Dong,12 P. Dore, D. Douai, H. Dougnac, H. W. Drawin, J. Druaux, M. Druetta,13 F. Dubois, M. Dubois, N. Dubuit, J. L. Duchateau, T. Dudok de Wit, E. Dufour, R. Dumont, G. Dunand, L. Dupas, Y. Duran,14 A. Durocher, D. Edery, A. Ekedahl, D. Elbeze, L. G. Eriksson, D. Escande,2 A. Escarguel, F. Escourbiac, T. Evans,15, F. Faisse, G. Falchetto, T. Fall, M. Farge,16 J. L. Farjon, E. Faudot,17 P. Fazilleau, N. Fedorczak, C. Fenzi-Bonizec, J. R. Ferron,15 I. Fidone, C. Figarella, E. Fleurence, I. Fleury, M. Fois, C. Forrest,15 C. A. Foster,3 S. Fouquet, C. Fourment, D. Fraboulet, P. Francois, B. Franel, D. Frigione,18 P. Froissard, G. Fubiani, V. Fuchs,14 M. Fumelli, B. Gagey, V. Galindo, D. Gambier, L. Garampon, X. Garbet, R. Garbil, J. Garcia, J. L. Gardarein, L. Gargiulo, P. Garibaldi, P. Garin, E. Gauthier, A. Geraud, T. Gerbaud, F. Gervais,19 M. Geynet, P. Ghendrih, T. Gianakon, R. Giannella, C. Gil, J. P. Girard, G. Giruzzi, L. Godbert-Mouret,2 P. Gomez, M. Goniche, A. Gordeev,4 G. Granata, V. Grandgirard, R. Gravier, B. Gravil, M. Gregoire, S. Gregoire, P. Grelot, D. Gresillon,19 C. Grisolia, G. Gros, *Other affiliations are those at the time the collaborations began. 1INRIA-CNRS, Université Sciences et Technologies, Bordeaux, France 2Physique des Interactions Ioniques et Moléculaires ~PIIM !, Université de Provence, Centre Universitaire St Jérôme, 13397 Marseille Cedex 20, France 3Oak Ridge National Laboratory, Fusion Energy Division, P.O. Box 2009, Oak Ridge, Tennessee 37831-8070, USA 4Moscow Physics and Engineering Institute ~MEPhI!, 31 Karhirskoe Sh, 115409 Moscow, Russian Federation 5Centre de Recherche en Physique des Plasmas, Association EURATOM-Confédération Suisse, Ecole Polytechnique Fédérale, PPB-Ecublens, 1015 Lausanne, Suisse 6Université Joseph Fourier, Grenoble I, B.P. 53, 38041 Grenoble Cedex 9, France 7School of Physical Sciences, Dublin City University, Glasnevin, EI-Dublin 9, Ireland 8INRIA Sophia-Antipolis, 2004 Route des Lucioles, B.P. 93, 06902 Nice-Sophia-Antipolis, France 9Princeton Plasma Physics Laboratory, James Forrestal Campus, Princeton, New Jersey 08543, USA 10Centre de Physique Théorique, Ecole Polytechnique, 91128 Palaiseau, France 11IRMA, Université Louis Pasteur, Strasbourg, France 12Southwestern Institute of Physics, Chengdu 610041, China 13Laboratoire TSI, Université Jean Monnet, 42023 St-Etienne, France 14Association EURATOM-IPP.CR, Institute of Plasma Physics AS CR, Za Slovankou 3, 182 21 Praha 8, Czech Republic 15General Atomics, P.O. Box 85608, San Diego, California 921865608, USA 16LMD, Ecole Normale Supérieure, 75 Paris, France 17LPMIA, Université Henri Poincaré, Nancy 1, B.P. 239, 54506 Vandœuvre Cedex, France 18Associazione EURATOM-ENEAsulla Fusione, C.R. Frascati, Roma, Italy 19Laboratoire de Physique et Technologie des Plasmas ~LPTP!, Ecole Polytechnique, 91128 Palaiseau, France

A new collective light scattering diagnostic was built to investigate electron density fluctuations and associated instabilities in a Hall thruster plasma. Previous numerical simulations have predicted electronic transport across magnetic field lines occurring at the scales of the electron cyclotron drift radius at certain frequencies. This paper presents the first experimental observations of the electron density fluctuations in the expected ranges of wave number and frequency, supporting the role of these instabilities in anomalous electronic diffusion. Key features such as the dispersion relations, form factor and spatial distribution of these fluctuations are discussed.

This paper presents recent efforts to better understand and quantify charged particle transport in Hall effect thrusters (HETs). Particle-in-cell (PIC) models, hybrid models, laser induced fluorescence (LIF) measurements and collective scattering (CS) experiments are combined to get a better insight into anomalous electron transport in HETs and to increase the predictive capabilities of simulation codes. PIC models have demonstrated that plasma turbulence associated with the development of a high frequency, short wavelength azimuthal instability can be responsible for anomalous transport. Scaling laws for anomalous electron mobility have not yet been derived and hybrid models, which are more practical than PIC models for parametric studies, must use empirical, adjustable transport coefficients that can be inferred from PIC results or LIF measurements of the ion velocity distribution function. CS experiments are aimed at validating the PIC model predictions of the azimuthal instability. The CS results show the first direct experimental evidence of the azimuthal instability predicted by the PIC code. The paper illustrates the synergy between experiments and models toward a complete and quantitative understanding of the physics of HETs.

Numerical models and simulations of ExB plasma discharges predict micro fluctuations at low frequencies and at the scale of electron cyclotron radius [1]. This paper describes the first experimental observation of volume fluctuation in these ranges. Optical bench and principle Volume plasma density fluctuations are diagnosed by the collective scattering of a CO2 laser beam. The optical bench PRAXIS (PRopulsion Analysis eXperiments via Infrared Scattering) uses an RF-driven laser source with a beam power of 42 Watts DC, TEM00 with M 2 =1.2. PRAXIS is shown in Figure 1, and a schematic of the observed region is shown in Figure 2.

Using a light scattering technique, we find that large density fluctuations in an air jet are clustered in time. A phase-space embedding allows us to propose a simple (S-shaped) manifold underlying the phase-space structure. In this context the temporal complexity naturally arises as stochastic fluctuations are added to the deterministic part of the model.

The information exchange dynamics between different spatial scales of a turbulent field is experimentally investigated, with particular reference to the energy cascade process. This is done by use of the collective light scattering (CLS) diagnostic, a new optical setting tuned for the observation of atomic number density fluctuations at macroscopic scales. A two-channel scattering device is described that provides simultaneously the fluctuations at two different spatial scales. The turbulent system is an axisymmetric air jet. With this system, time correlations between signal amplitudes for two different scales show a characteristic time width. This time does not depend on the observed scales: it is close to the turbulence production time. Nevertheless, signal amplitude autocorrelation shows a shorter characteristic time. The autocorrelation time scaling law behaves almost like the eddy turnover time from Kolmogorov theory.

Magnetic fluctuations (radial size approximate to 5 mm) are measured by a cross polarization scattering (CPS) diagnostic in Tore Supra. In the scenario O + (B) over tilde --> X, only the poloidal component of the magnetic fluctuations is measured, while both the radial and the poloidal component are measured in the scenario X + (B) over tilde --> O. These fluctuations are investigated quantitatively in the ohmic and low confinement regimes over a wide range of plasma currents, densities and additional heating powers. At the same time, the electron heat diffusivities expected from these fluctuations are compared with those obtained by profile analysis. Three main results are obtained: (a) The radial profile of the poloidal magnetic fluctuations in the gradient region (0.3 < r/a < 0.7) is established from these measurements. The magnetic fluctuation levels are found to increase towards the plasma edge, and this feature is compatible with that of electron heat diffusivity. (b) A strong correlation between the measured magnetic turbulence and the local temperature gradient is observed during the additional heating. (c) The local electron heat diffusivity induced by magnetic fluctuations is estimated using the non-collisional quasi-linear formula chi(e)(mag) = pi qRv(th)(delta B-r/B)(2)(,) where the radial component of the magnetic turbulence is assumed to be of the same order as the poloidal component. Both the order of magnitude and the parametric dependence of chi(e)(mag) show similarities with electron diffusivities determined by transport analysis. In particular, a threshold is observed for the dependence of fluctuation induced heat fluxes on the local temperature gradient, which is close to the critical gradient observed for the measured heat fluxes.

Significant results on steady state control of the plasma parameters have been obtained with RF systems on Tore Supra. A sustained effort is under way to improve power transmission reliability. Developments of sources and antennas are in progress for future operation on Tore Supra with purse length in tile range of several hundreds of seconds.

Specific investigations have been carried out in Tore Supra - in ohmic and additionally heated plasmas - to locally analyse electrostatic and magnetic turbulence as well as transport, and to compare their variations with those of local parameters. On each side of the electric shear layer, the frequency of the density fluctuations and the wavenumber spectra are different in ohmic as well as in additional heating regimes, showing that turbulence has not the same characteristics in the gradient and the edge regions. For the first time, radial profiles of magnetic fluctuations have been obtained, showing that the level of fluctuations increases with the minor radius; this behaviour is well correlated with the radial shape of the electron heat diffusivity. In the L mode, there is also a good correlation of fluctuations and heat diffusivity with the local temperature gradient in the region 0.4 < r/a < 0.8, with the evidence of a critical gradient threshold for magnetic fluctuations. In addition, improved confinement regimes, obtained by fast wave electron heating, show the stabilizing effect of increased magnetic shear on the density fluctuations, thus corroborating theoretical models. This shear effect can be used to explain the confinement time saturation at high density in the ohmic regime. A decrease of the density fluctuations in the vicinity of the electric shear layer is also observed. Finally, from dimensionally similar experiments and local analysis, the scaling law for the electron heat diffusivity, chi(e), is found to be gyroBohm-like, in agreement with theoretical expectations and other previous results.

Recent Tore Supra results supporting the feasibility of high-confinement. long duration discharges are reported. Two-minute discharges with improved confinement have been obtained. This progress is largely due to improvements in the operating control system of Tore Supra. which now allows real-rime feedback control of global plasma parameters. A clear correlation between improved confinement and current profile shape has been established. Transport barriers for electron heat diffusion are observed in experiments where the magnetic shear is weak or negative in the central part of the plasma. These observations strongly support development of new current drive schemes for current profile control. Considerable progress concerning particle and heat exhaust has also been achieved. A new technique allowing conditioning in the presence of a toroidal magnetic field has been implemented. A vented limiter has been tested as a means of particle exhaust through collection of neutrals, and its performance is compared with that of a "classical", ion collecting throat limiter. The knowledge gained from extensive long pulse experimentation has been used to develop a new generation of plasma facing components. which will permit further development of the long discharge capability of Tore Supra.