Table of contents

The mechanism generating the toroidal sheet current after the sawtooth crash (observation on TEXTOR tokamak) is discussed. A possible hypothesis is presented. The crash of central pressure causes a steep gradient near the boundary between the flattened region and the unperturbed region. This sharp pressure gradient gives rise to a strong secondary current (transient Pfirsch - Schlüter current). The location, polarity, magnitude and time evolution of the current sheet are analysed. Comparison with present experimental data is made, and future necessary measurements are discussed.

A first estimate of the bootstrap current in the ETE (Experimento Tokamak Esférico) small aspect ratio tokamak using the Hirshman single-ion collisionless model shows that we can expect a ratio of 15 to 30% of total bootstrap current in relation to the total equilibrium current depending on the optimization level of the plasma profile parameters. Bootstrap current levels basically depend on the values which must be kept under a critical level due to stability conditions and current alignment requirements. Preliminary studies of Shaing's predictions regarding bootstrap current calculations in collisional plasmas are briefly described and different methods for the trapped particle fraction calculation are also illustrated.

An explanation for the L - H transition through the electromagnetic mechanism (suppression of drift wave turbulence by the skin effect) is offered. In dimensional space, the bifurcation is attributed to the involvement of two parameters in L - H transition physics: and (the latter scales as in physical parameters). The maximum of the diffusion coefficient, corresponding to the L - H transition, is reached when the collisional skin depth ( is the drift frequency) equals the characteristic radial displacement of the drift turbulence. The same criterion can also be presented as: - the condition for equal rates of the plasma diffusion into the magnetic field and the diffusion of the magnetic field into the plasma. The analysis yields the combination (scales as in dimensionless parameters) as a critical parameter for the L - H transition, for the case of the scaling for the wavevector of the drift turbulence. This threshold parameter should be applied near the separatrix position. At low densities, the requirement that the collisionless skin depth must be smaller than the radial displacement of the drift fluctuations in the L-mode, which is necessary for turbulence suppression, determines the threshold for the L - H transition.

The proposed mechanism for the L - H transition clarifies the R dependence of the H-mode power threshold: scaling is predicted, with for in ITER. The critical parameter for the L - H transition, together with dimensionless parameters characterizing pressure gradient and resistivity, create the set of similarity parameters describing ELM behaviour. The scaling for the separatrix density, normalized to the Greenwald density limit with the machine size and toroidal field which ensures `similar' ELM behaviour, can thus be obtained. For the fixed similarity parameters, the analysis yields weak but favourable dependence of on the major radius. In recent experiments on JET and other machines, the degradation in the edge confinement associated with increased ELM frequency was found to be responsible for the density limit in high-power H-modes. Owing to the approximately dependence, an excess over the Greenwald limit, , by about 30% higher in ITER compared with JET for `similar' conditions (q, , separatrix and the ratio, wall conditions, the use of pellets etc) in ELMy H-modes is predicted. This is with the provision that a limit on the central density, related to mechanisms in the plasma core, is not encountered.

The use of a permanent magnetic field in superconducting magnetic fusion devices impedes conditioning by glow discharges as actually applied in most pulsed machines. An alternative has been studied in Tore Supra for this purpose with a 3.8 T permanent field: a discharge produced by ion cyclotron range of frequency wave injection (ICRF). Helium ICRF discharge conditioning (ICRF-DC) has already been shown to desaturate the deuterium-loaded carbon first wall efficiently. In this paper, we describe how ICRF-DC can be applied to clean the wall or change its hydrogen isotopic ratio. This is achieved by pumping wall-desorbed molecules induced by particle bombardment from the ICRF plasma. The conditioning efficiency is optimized as a function of two input parameters: the gas pressure and the applied power. A plasma characterization is also given as a function of these parameters and a global model has been used to compare the experimental results and to interpret the main processes to which the neutral and charged species are subjected. The key to optimizing ICRF-DC is the wave ion heating to produce energetic particle bombardment of the first wall, while still achieving a low electron density, low-temperature plasma , to avoid reionization and redeposition of wall-desorbed neutrals before they can be evacuated by the torus pumping system.

A linear analysis of MHD instabilities driven by the combined effects of unfavourable curvature and the pressure gradient is performed in the scrape-off layer of a limiter configuration. The dispersion relation of fast growing (electromagnetic) modes is derived by means of both analytical and numerical techniques. The growth rate dependence on the sheath resistivity and the magnetic shear is investigated. Perturbations are not completely stabilized by conducting end walls due to finite sheath conductivity. Modes develop into ideal ballooning modes as the plasma beta increases. The comparison of their behaviour to resistive ballooning modes shows that in most cases of interest the plasma stability is determined by the electromagnetic modes. As a consequence, a new dependence of the instability growth rate on the plasma parameters is found, where the key parameter is the density close to the end plates. Results given here can be applied to a single-null divertor configuration.

The fraction of the coupled lower hybrid (LH) power absorbed in divertor plasmas in JET has been determined experimentally with a method utilizing the time derivative of the total stored energy (plasma and magnetic). This method can account for the power absorbed inside a normalized flux coordinate . The experimental LH absorption coefficient reaches 100% at low plasma densities, , and decreases to 25% at . The LH wave accessibility to the plasma core has been found to play an important role in determining the power absorption and the radial deposition profile. The decreasing absorption is correlated with a gradual shift of the LH power deposition profile, as determined by the hard x-ray profiles, towards the plasma periphery. Similar behaviour is found in ray-tracing + Fokker - Planck code calculations. The frequency spectrum of the LH pump wave, as determined by a probe outside the tokamak vessel, broadens strongly as the wave accessibility is reduced and the absorption drops.

The flow of heat in the scrape-off layer (SOL) region of ASDEX Upgrade is investigated and compared with simple modelling. Parallel heat transport is found to be consistent with electron heat conduction based on Spitzer - Härm conductivity. Cross-field heat transport is characterized using radial e-folding distances for power, temperature and plasma pressure, which are all found to vary weakly over a wide range of discharge conditions. Type I ELMs, also characterized, introduce a discreteness to the power flow into the SOL and carry approximately half of the power exhaust from the discharge. The divertor plates are effectively screened from the ELM energy, even in low-radiation discharges, suggesting enhanced radiation rates during ELMs.

The reconstruction of ion temperature profiles from energy-resolved neutral flux measurements in the energy range from 20 eV to 20 keV reveals an uncertainty in positioning the steep gradient region at the plasma edge, even if the measured energy range of the neutral fluxes includes ion temperature values of the gradient region. This empirical observation is studied analytically with a simple model for the neutral particles. Using the line integral of the electron density one finds that translations of the whole ion temperature spectrum with respect to this coordinate cannot be resolved below a certain value. It turns out that the lowest energy for which fluxes can be measured as well as the accuracy of the flux measurements determine the amount of uncertainty, which is in our case . To reduce this uncertainty, the total neutral flux which is determined mainly by neutrals with energies of a few eV or less, has been fitted simultaneously with the absolutely calibrated energy-resolved flux spectra. The total neutral flux can be measured with an ion gauge and is closely related to the emissivity. The fit of the total neutral flux or the signal requires simulation of the ionization zone in the scrape-off layer, i.e. measured electron temperatures must be combined with an accurate model for the dissociation and ionization of molecules and atoms.

27 August - 2 September 1997 International Conference on Superstrong Fields in Plasmas Villa Monastero, Varenna (Lc), Italy Contact: Mrs D Pifferetti, Secretariat Villa Monastero, 1 Piazza Venini, 22050 Varenna, Italy. Tel +39-341-831261, Fax +39-341-831281, E-mail ccvm@ccvm.cil.lc.it (abstracts should be sent to Dr M Lontano, Istituto di Fisica del Plasma, Consiglio Nazionale delle Ricerche, Via Cozzi 53, 20125 Milano, Italy. Tel +39-2-66173260, Fax +39-2-66173239, E-mail lontano@ifp.mi.cnr.it). Application and abstract deadline: 13 June 1997

4 - 12 September 1997 International Workshop on Diagnostics for Experimental Fusion Reactors Villa Monastero, Varenna, Italy Contact: D Pifferetti, Centro di Cultura `Villa Monastero', 1 Piazza Venini, 22050 Varenna (Lc) Italy. Tel +39-341-831261, Fax +39-341-831281, E-mail ccvm@ccvm.cil.lc.it. Application and abstract deadline: 13 June 1997.

8 - 12 September 1997 12th International Conference on Gas Discharges and their Applications Greifswald, Germany Contact: Dr G Babucke, Inst. f. Niedertemperatur-Plasmaphysik, Robert-Blum-Str. 8 - 10, 17489 Greifswald, Germany. Tel +49-3834-554411, Fax +49-3834-554301, E-mail gd97@public.inp.uni-greifswald.de.

8 - 19 September 1997 3rd Carolus Magnus Summer School on Plasma Physics Spa, Belgium Contact: Dr G van Oost, c/o Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany. Tel +49-2461-61-5626, Fax +49-2461-61-3331, E-mail vanoost@fusion.rma.ac.be. Application deadline: 15 May 1997, abstract deadline: 15 July 1997.

8 - 10 October 1997 7th European Fusion Theory Conference Jülich, Germany Contact: A L Rogister, Institut für Plasmaphysik, Forschungszentrum Jülich GmbH, EURATOM-FZJ Association, D-52425 Jülich, Germany. Tel +49-2461-61-5440, Fax +49-2461-61-2970, E-mail d.petschner@kfa-juelich.de.

13 October - 7 November 1997 Autumn College on Plasma Physics Miramare-Trieste, Italy Contact: S M Mahajan, International Centre for Theoretical Physics, Autumn College on Plasma Physics, PO Box 586, I-34100 Trieste, Italy. Tel +39-40-2240357, Fax +39-40-224163, E-mail smr1012@ictp.trieste.it. Application deadline: 30 April 1997.

10 - 14 November 1997 Topical Workshop on Plasma Physics: New Perspectives of Collective Effects Miramare-Trieste, Italy Contact: P K Shukla, International Centre for Theoretical Physics, PO Box 586, I-34100 Trieste, Italy. Tel +39-40-2240357, Fax +39-40-224163, E-mail smr1014@ictp.trieste.it. Application deadline: 31 March 1997.