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

The effect of resistive shell modes (RSMs) on reversed-field pinch (RFP) operation has been studied on the EXTRAP T2 device. The T2 shell has a magnetic penetration time of 1.5ms and pulses are sustained for about ten shell times. Magnetic diagnostics have been used to measure the mode dynamics. Toroidally localised magnetic perturbations that penetrate the shell are observed. They grow and decay on the shell time-scale but have very small rotation frequencies and are essentially stationary toroidally. Fourier analysis of the mode structure shows that the localised perturbations consist of internally resonant modes with poloidal mode number m = 1 and toroidal mode numbers in the range n = 10-16. The perturbations can cause a degradation of the pulse primarily through impurity release due to localised plasma wall interaction. In addition to this stationary RSM, m = 0 perturbations, associated with the dynamo activity, are observed. These modes rotate in the electron diamagnetic drift direction with velocities of 20-30kms(-1). Measurements of the characteristic parameters modelling the parallel current profile indicate that n operates close to the marginal linear stability limit for an ideal shell RFP. The operation parameters are in the same range as those measured for conducting shell RFPs, despite the fact that the configuration has a resistive shell and RSMs are observed.

Anomalous heating of ions has been observed in the EXTRAP-T2 reversed-field-pinch (RFP) plasma. Ions are heated primarily in the parallel direction (with respect to the magnetic field), resulting in an appreciable anisotropy of the ion temperature. This observation suggests that the magnetohydrodynamic fluctuations are dissipated primarily by the ion viscosity.

An infrared (IR) camera has been installed on TdeV (Tokamak de Varennes) to view both the inner and the outer divertor plates. With a finite element code (PDE Protran), the energy deposited on the plates is calculated from the surface temperature profile measured by the IR camera. The experimental results have revealed that the divertor plate temperature decreases with an increase of the main plasma density, suggesting that the plasma detaches from the divertor plates at higher plasma densities (above 4.0×1019 m−3). During divertor biasing, the heat flux increases on the plates of the `active' divertor as a result of the E×B flow. However, energy deposition can be reduced by increasing the plasma density. The neutral gas pressure in the divertor region increases strongly at higher plasma density, and appears to be correlated with the reduction in deposited energy.

Preliminary results concerning the influence of negative biasing of the divertor plates on particle recycling and on power loss distribution in single null discharges of TdeV during lower hybrid (LH) current drive and heating experiments are presented. The beneficial effects of negative biasing of the divertor plates, such as the ability to control power and particle fluxes in the SOL, remain effective in the presence of auxiliary heating and current drive. Up to 0.7 MW of auxiliary power were injected in these experiments. With a negative biasing of 150 V, and the E ⊗ b flow vector pointing towards the outer divertor chamber, a roughly 2 fold increase in the divertor pressure and the radiation from the divertor region is observed.

Measurements in TdeV in the double null configuration show strong asymmetries between the upper and lower divertors. The divertor to which the ion ∇ B drift is directed has a higher density, which varies little with the central line-average density. The electron temperature also exhibits a poloidal variation which depends on the direction of the toroidal magnetic field. The divertor plasma shows a double peak structure which evolves as a function of the central density, with the outer peak varying more rapidly. We have measured flow reversal near the separatrix in the bottom divertor when the ion ∇ B drift is directed away from the X -point. Divertor plate biasing allows us to control the flux of plasma in the SOL; under negative biasing there is a strong increase of the electron density in the active divertor, the one favoured by the E × B flow. Calculations of the energy deposition derived by flush-mounted probes agree well with measurements of the heat load derived from the temperature increase of the divertor tiles.

Effective control of the particle and power flux distributions between the upper and lower diverters has been demonstrated on TdeV, using biasing of the divertor plates, in ohmic discharges with a double null magnetic configuration. Negative biasing of the divertor plates with respect to the walls significantly increases the pressure and the emitted power from the plasma in the active divertor into which E x B drift is directed, but it has little influence on the opposite divertor. This enhanced power dissipation is the result of improved particle retention in the active divertor. The incident power on the plates in the active divertor also increases with biasing, but at a slower rate. The Franck-Condon and charge exchange neutrals contribute significantly to the power loss in the diverters of TdeV in the present operating mode. Positive biasing of the divertor plates has little influence on the total losses from the divertor regions.

The isotope ratio RI=D/(H+D) was measured during ohmic discharges in the magnetically diverted tokamak TdeV, using spectroscopy (Halpha and Dalpha ), gas analysis (H2, HD and D2), mass analysis of charge exchange neutrals, and detection of H and D in a collector probe, in the course of four different experimental phases: (I) 'dynamical' isotope switchover of the fuelling gas during a discharge, (II) shot-to-shot evolution with pure D2 fuelling, (III) same with walls conditioned by D2 glow discharge, and (IV) 'dynamical' switchover combined with divertor biasing and pumping. The different values obtained for RI are compared and analysed in terms of the particle recycling properties of the device. The dynamical scenario (phase I) revealed a lack of equilibrium between the edge and core isotopic composition that is an indication of the degree to which the particle flux is amplified by recycling in the vicinity of surfaces. The phase II results were fitted to a model which yielded an estimate of the magnitude of the wall inventory available for recycling. In phase III, the effectiveness of conditioning was assessed. Phase IV revealed an even more pronounced disequilibrium than phase I, and demonstrated the power of divertor biasing and pumping in exhausting the wall inventory and controlling the isotope ratio.

Electrically insulated divertor plates are used on TdeV (Tokamak de Varennes) [18th EPS Conference on Controlled Fusion and Plasma Physics Berlin (European Physical Society, Petit-Lancy, 1991), Vol. 15C, Part I, pp. 1–141] to produce various biasing configurations, which can be decomposed into two basic modes. Plasma biasing, with a radial electric field Er in the scrape-off layer (SOL), is most promising for divertor applications. The Er field is produced with a particular divertor plate geometry, causing a nonambipolar radial current and a particle flow in the Er×BT direction, toward one of the divertors (the active divertor). The pressure and impurity retention in the active divertor are shown, in the Ohmic regime, to be strongly increased by biasing. He exhaust through this divertor is increased by a factor of almost 3 with modest biasing voltages and currents scalable to larger devices. Biasing also modifies the power repartition between the divertors, with the active divertor also receiving a larger fraction of the power.

Recent Lower Hybrid Current Drive (LHCD) experiments in TORE SUPRA and JET are reported. Large multijunction launchers have allowed the coupling of 5 MW to the plasma for several seconds with a maximum of 3.8 kw/cm2. Measurements of the scattering matrices of the antennae show good agreement with theory. The current drive efficiency in TORE SUPRA is about 0.2 x 10(20) Am-2/W with LH power alone and reaches 0.4 x 10(20) Am-2/W in JET thanks to a high volume-averaged electron temperature (1.9 keV) and also to a synergy between Lower Hybrid and Fast Magnetosonic Waves. At N(e)BAR = 1.5 x 10(19) m-3 in TORE SUPRA, sawteeth are suppressed and m = 1 MHD oscillations the frequency of which clearly depends on the amount of LH power are observed on soft x-rays, and also on non-thermal ECE. In JET ICRH produced sawtooth-free periods are extended by the application of LHCD (2.9 s. with 4 MW ICRH) and current profile broadening has been clearly observed consistent with off-axis fast electron populations. LH power modulation experiments performed in TORE SUPRA at N(e)BAR = 4 x 10(19) m-3 show a delayed central electron heating despite the off-axis creation of suprathermal electrons, thus ruling out the possibility of a direct heating through central wave absorption. A possible explanation in terms of anomlous fast electron transport and classical slowing down would yield a diffusion coefficient of the order of 10 m2/s for the fast electrons. Other interpretations such as an anomalous heat pinch or a central confinement enhancement cannot be excluded. Finally, successful pellet fuelling of a partially LH driven plasma was obtained in TORE SUPRA, 28 successive pellets allowing the density to rise to N(e)BAR = 4 x 10(19) m-3. This could be achieved by switching the LH power off for 90 ms before each pellet injection, i.e. without modifying significantly the current density profile.

Preliminary experiments in the TORE SUPRA tokamak using the ergodic divertor configuration have shown a strong effect on plasma impurities. The result is a decontamination of the central plasma which is due to a decreased carbon content. This is the consequence of a screening effect of the peripheral ergodic layer which is due to an important increase of the recycling impurity flux, as well as of a modification of the impurity source terms.

The present ergodic divertor experiments in Tore Supra have been devoted to benchmarking the operational regimes of the apparatus. Two major effects are reported: on the one hand, strong changes occur in the ergodized boundary layer (up to 20% of the minor radius), and on the other hand, the central plasma and especially the confinement is not directly affected, i.e. the observed modifications are induced by edge effects. The basic trends, which are recorded are a decrease of both the edge electronic temperature and the edge density gradient while the radiated power is increased at the very edge of the ergodic region. The latter feature is in agreement with the impurity line emission characterized by an increase of the peripheral lines with a strong decrease of the central lines.

TORE SUPRA is a large superconducting tokamak designed for sustaining long inductive pulses (t approximately 30 s). In particular, all the first wall components have been designed for steady-state heat and particle exhaust, particle injection, and additional heating. In addition to these technological assets, a strict control of the plasma-wall interactions is required. This has been done at low power: experiments with ohmic heating have been mainly devoted to the pump limiter, ergodic divertor and pellet injection experiments. Some specific problems arising in large tokamaks are encountered; the pump limiter and the ergodic divertor yield the expected effects on the plasma edge. The effects on the bulk are discussed.