Table of contents

Impurity production and transport have been studied in small Tore Supra plasmas, for which the sole source of impurities (essentially carbon) is an outboard limiter. The main diagnostic was a visible endoscope, allowing absolute intensity calibrated CCD camera images of the entire limiter to be obtained at selected wavelengths. The experimental results show that, while chemical sputtering is essential to explain the limiter images, it does not contribute much to the central impurity content (for which physical sputtering is more important). The experimental edge carbon fluxes and the core plasma carbon content were simulated by coupling the 3D Monte Carlo edge impurity code BBQ with the 1D Tore Supra core impurity transport code, thus modelling (for the first time) the global impurity production and transport.

A collisionless drift-type instability is predicted for plasma parameters pertinent to a tokamak edge with steep density gradient. A finite electron collision frequency is not essential for the instability. The growth rate and anomalous diffusivity both exhibit a favourable isotope effect (decrease with the atomic number A of hydrogen isotopes).

The HCN-interferometer/polarimeter on TEXTOR has been extended by a horizontal line of sight in the equatorial plane. The achieved signal to noise ratio and the time resolution of this polarimeter channel are sufficient to resolve the m = 1 magnetic precursor oscillation before the sawtooth collapse. The growth rate of the precursor oscillation in a sawtoothing ohmically heated discharge has been determined to be . The magnetic perturbation, determined from the Faraday rotation integral, assuming a kink-mode eigenfunction, is 13 mT just before the sawtooth crash and the `displacement' of the core is about 0.9 cm or . The amplitude of a stationary m = 1 mode observed in an additionally heated discharge with balanced neutral beam injection amounts to 26 mT.

New experiments performed at high power with radiation cooling on TEXTOR-94, upgraded for long pulses, have shown the following. (i) Improved confinement conditions at high density (for which the plasma is mainly thermal) lasting for several seconds up to the present maximum flat top time of the upgraded machine with a low and stationary central impurity content. Note that the radiation occurs near the plasma edge, mainly inside the last closed magnetic flux surface. (ii) The existence of confinement transitions during the radiatively cooled phase of the discharge: the already observed transitions from higher to lower confinement regimes have been confirmed but, remarkably, transitions from lower to higher confinement are now also present, leading to an improvement of the confinement time of up to a factor of three.

The soft x-ray diagnostic system of Alcator C-Mod, equipped with 152 detectors distributed in four arrays, is used to obtain iso-emissivity surfaces. These surfaces have been characterized by giving their elongation and relative shift from the centre of the tokamak as functions of plasma radius. Flux surfaces, provided by magnetic diagnostics, have also been described with elongation and shift. Results from the comparison of the two sets of geometric parameters obtained from magnetic and x-ray diagnostics are presented. We find that, whereas the shifts obtained from these two diagnostic methods are always in good agreement, the corresponding elongation curves show different patterns. An agreement between elongations better than 2% is only found in a range of about 2 cm in minor radius. On the other hand, the elongations can differ by 10% towards the plasma edge and the plasma centre. Error bars for the x-ray diagnostic are obtained by propagating the effect of random errors at the detector signals, and can amount to of the estimated values near the edge and the centre of the plasma. The estimated uncertainties in the determination of elongation from magnetic flux surfaces are of the order of 4%. A series of tests and simulations performed to verify the accuracy of the x-ray diagnostic system is presented. The discrepancies found could imply the existence of asymmetries in impurity concentration.

The Ohmic power in tokamak plasmas including sawtooth effects is calculated using the neoclassical and Spitzer resistivity models for a particular form of the thermal diffusivity. The radial extent of the sawtooth activity is self-consistently determined and this is dependent on the inverse aspect ratio and the safety factor at the plasma edge. Quantitative results for the neoclassical enhancement over the Spitzer Ohmic power are obtained. In addition the impact of the sawteeth on the energy confinement is calculated.

In this paper the problem of stochastic diffusion in a reversed field pinch (RFP) plasma in the presence of mode-locking has been addressed. In order to understand its effects, different spectral shapes of the perturbing magnetic field have been compared. The importance of long-wavelength perturbations in determining the stochastic diffusion process is pointed out. Moreover, we have considered the effect on energy transport and the conclusion has been reached that, due to the high parallel thermal conduction, `local' effects are not important. A comparison of the experimental values of the energy confinement time with that deduced assuming a collisionless stochastic transport model is also given.

The predictions of a reactive drift wave model for anomalous transport in tokamaks (Nordman et al 1990 Nucl. Fusion 30 983) are compared with JET profile data, using two different approaches. An interpretative approach is used to compare the radial heat fluxes under approximate steady-state conditions. For the steady states considered, taken from seven auxiliary heated JET discharges in various regimes, the predicted total heat flux tends to be over-estimated in the inner parts or under-estimated in the outer parts of the good confinement region (approximately between q = 1 and q = 2). A predictive approach is used to compare the density and temperature profiles of five L-mode discharges. These results are in good agreement with experiments (in the good confinement region the relative errors in the temperature profiles are and in the density profile approximately ). The interpretative code TRANSP is used to obtain the experimental profile data.

We obtain two alternative forms of the dispersion relation for electron Bernstein waves in a relativistic plasma, propagating at right angles to the equilibrium magnetic field. We show that its low-temperature limit leads to the well known classical result. The dispersion relations obtained could be important if electron Bernstein waves are to be used in the heating of a plasma to relativistic energies.

Time-resolved triton burn-up measurements have been performed using a new type of 14 MeV neutron detector based on scintillating fibres. The shot-integrated triton burn-up ratio was measured to be in the range 0.3 - 2.0%. Time histories of 14 MeV emission after NB turn-off have been analysed based on the classical slowing-down theory. Assuming the loss of fast tritons can be represented as a diffusivity, values increasing with increasing toroidal ripple were determined between 0.05 and 0.15 , from the modelling of the time histories of the 14 MeV emission after the NB turn-off.

A study of plasma edge turbulence carried out in the ECRH heated TJ-IU torsatron is presented. Radial profiles of ion saturation current and floating potential, together with the fluctuation levels of these have been evaluated in the plasma edge by means of Langmuir probe arrays. The existence of two different propagation modes in the proximity of the velocity shear layer has been observed. In the plasma bulk side of the limiter radius, high-frequency fluctuations are negligible and only one propagation mode stands. A detailed examination of the data shows the existence of a quasi-coherent mode probably related to the local magnetic configuration. A radial probe scan reveals an increase in the turbulent particle flux for the location of a rational surface as calculated by the VMEC free-boundary 3D equilibrium code.

We study the time evolution of the absorption coefficient for extraordinary mode electron cyclotron waves interacting with a current-carrying electron distribution, in a tokamak plasma modelled by a plasma slab. The absorption coefficient is obtained from a numerical solution of the dispersion relation, which incorporates self-consistent effects due to the quasi-linear evolution of the electron distribution function, under the action of lower hybrid and electron cyclotron waves, and collisions. Our results show that for the whole plasma slab the asymptotic quasi-linear value of the absorption coefficient for electron cyclotron waves may be a few per cent larger than the coefficient due to absorption by the lower hybrid generated electron tail at the onset of the electron cyclotron waves, for sufficiently large angle of injection of these waves relative to the normal to the magnetic field.

We present evidence that the fast-ion diffusion coefficient changes across the TFTR plasma column. Two MHD quiescent discharges are analysed: a high power D - T plasma heated with 21 MW of deuterium and tritium beams, and an ohmic plasma into which a 10 MW, 20 ms deuterium beam pulse was injected. The localized charge-exchange measurements in the ohmic plasma, and the neutron flux measurements in the D - T plasma are compared with predictions from the transport simulation code TRANSP. We have modified the code to allow modelling with arbitrary fast-ion diffusion profiles . Significant improvement in the agreement between measurement and simulation is obtained with a profile that has low values in the inner half of the plasma column and then rises rapidly towards the plasma periphery. This suggests a common underlying mechanism of enhanced beam ion transport in the two discharges, such as stochastic ripple diffusion.

The effect of non-uniform rotation with a jet profile on the drift and Kelvin - Helmholtz instabilities in a tandem mirror is investigated numerically. Certain regimes of the flow parameters are found where the drift instability is nearly suppressed and the transverse Kelvin - Helmholtz instability is not induced. The growth rate of the Kelvin - Helmholtz instability can also be reduced to almost zero by a sufficiently large rotation frequency or by a radial electric field with moderate shear. The wave - flow resonant interaction may account for the stabilizing effect of sheared flow on drift and Kelvin - Helmholtz instabilities.

The major contributions to this workshop came from TORE-SUPRA, TPX, LHD and W7-X (two tokamaks, one heliotron and a stellarator). All four devices are of similar size and designed for a similar range of parameters and, in particular, they address the same target - steady-state plasma operation. TORE-SUPRA is a circular cross section tokamak, LHD is designed with optimized continuous coils and provides a helical divertor, TPX and W7-X incorporate strongly-shaped geometries to improve confinement and stability and optimize the bootstrap current. In TPX, the system is optimized for a large bootstrap current, in W7-X for basically no bootstrap current. Also the heating systems are designed for the specific purpose of steady-state operation. TORE-SUPRA is in operation, LHD is near completion; W7-X has recently (after the workshop) been approved; TPX is still in the approval phase. Superconducting coil material is NbTi for TORE-SUPRA, LHD and W7-X and Nb₃Sn for TPX. The TF-coils of TORE-SUPRA (1.8 K, superfluid He) and the helical coil of LHD (4.2 K for the first part of experiments with 3 T) are bath-cooled. Cable-in-conduit conductors are used in the other SC coils. Largely different solutions are selected for the SC cable composition. Except for TPX, which has a double-null poloidal field divertor matched to the strong plasma shaping, most of the steady-state devices have a very flexible plasma edge configuration: islands or ergodic boundaries (W7-X, TORE-SUPRA, LHD), limiter (TORE-SUPRA), helical divertor (LHD, W7-X). The status of plasma equilibrium control, high heat flux materials and activation problems were also discussed. Power plant studies based on different steady-state confinement concepts were presented and confronted with a summary on the PULSAR study based on pulsed systems.

20--24 May 1996 12th International Conference on Plasma Surface Interactions in Controlled Fusion Devices Saint Raphael, France Contact: PSI 96 c/o Ms V Poli, Association EURATOM-CEA sur la Fusion, DRFC/SPPF, Centre d'Etudes de Cadarache, F-13108 Saint-Paul-lez-Durance Cedex, France. Tel +33-42-25-62-60, Fax +33-42-25-62-55, E-mail psi96@drfc.cad.cea.fr.

3--5 June 1996 7th International Conference on Plasma Science Boston, MA, USA Contact: M Silevitch, ECE Department, Northeastern University, Boston, MA 02115, USA.

24--28 June 1996 23rd European Physical Society Conference on Controlled Fusion and Plasma Physics Kiev, Ukraine Contact: Dr Ihor Goutych, Scientific Secretary, 23rd EPS CFPP, Bogolyubov Institute for Theoretical Physics, 252143 Kiev, Ukraine. Tel +38-044-2665362, Fax +38-044-2665998, E-mail cfpp@gluk.apc.org.

8--19 July 1996 33rd Culham Plasma Physics Summer School (UKAEA/Euratom) Oxford, UK Contact: Mrs J Bright, UKAEA Fusion, Culham, Abingdon, Oxfordshire, OX14 3DB, UK. Tel +44-1235-463773, Fax +44-1235-463474, E-mail julie.bright@ukaea.org.uk.

8--18 August 1996 International Summer School on Plasma Physics and Technology La Jolla, CA, USA Contact: Mr V Stefan, Institute for Advanced Physics Studies, PO Box 2964, La Jolla, CA 92038, USA. Tel +1-619-456-5737.

26--30 August 1996 Joint Varenna--Lausanne International Workshop on Theory of Fusion Plasmas Villa Monastero, Varenna, Italy Contact: Centro di Cultura Villa Monastero, 1 Piazza Venini, 22050 Varenna (Lecco), Italy. Tel +39-341-831261, Fax +39-341-831281. Application and abstract deadline: 15 June 1996.

2--5 September 1996 EU--US Workshop on Transport in Fusion Plasmas Villa Monastero, Varenna, Italy Further information: G Gorini, ISPP, 16 Via Celoria, I-20133 Milano, Italy. Tel +39-2-2392637, Fax +39-2-2392205, E-mail ggorini@mi.infn.it.

Administrative contact: Centro di Cultura Villa Monastero, 1 Piazza Venini, 22050 Varenna (Lecco), Italy. Tel +39-341-831261, Fax +39-341-831281. Application and abstract deadline: 15 June 1996.

9--13 September 1996 International Conference on Plasma Physics Nagoya, Japan Contact: Conference Secretariat, c/o Prof. Hiromu Momota, National Institute for Fusion Science, Nagoya 464-01, Japan. Tel +81-52-789-4260, Fax +81-52-789-1037, E-mail icpp96@nifs.ac.jp. Abstract deadline: 31 March 1996.

16--20 September 1996 19th Symposium on Fusion Technology Lisbon, Portugal Contact: Professor Carlos Varandas, Centro de Fusão Nuclear, 1096 Lisboa Codex, Portugal. Fax +351-1-8417819, E-mail cvarandas@cfn.ist.utl.pt. General information will be available via WWW with URL http://www.cfn.ist.utl.pt.

25--29 September 1996 Summer University of Plasma Physics Garching, Germany Contact: Ms Ch Stahlberg, Max-Planck-Institut für PlasmaPhysik, Boltzmannstr 2, D-85748 Garching, Germany. Tel +49-89-3299-2232, Fax +49-89-3299-1001.

2--4 October 1996 6th European Conference on Fusion Theory Utrecht, Netherlands Contact: T J Schep, FOM -- Institute for Plasma Physics, Postbus 1207, NL-3430 BE Nieuwegein, Netherlands. Tel +31-3402-312-24.

6--11 April 1997 10th Joint Workshop on Electron Cyclotron Emission and Electron Cyclotron Resonance Heating Ameland, The Netherlands Contact: J Hamers-Smit, FOM -- Instituut voor Plasmafysica 'Rijnhuizen', Postbus 1207, 3430 BE Nieuwegein. Tel +31-30-6096999, Fax +31-30-6031204, E-mail ec10@rijnh.nl. Application and abstract deadline: 17 January 1997.