Table of contents

In the JT-60U tokamak a pre-formed reversed magnetic shear configuration was sustained non-inductively by means of lower hybrid current drive (LHCD). It was formed by neutral beam heating at current build-up and had a reversed shear region extending to 65% of the minor radius. The configuration was sustained for 7.5 s, and the extent of the reversed shear region was kept as large as 55% of the minor radius until the very end. The plasma was stable against MHD activities. It is also demonstrated that LHCD could modify reversed magnetic shear keeping plasma parameters, such as the plasma current and the toroidal magnetic field, the same.

This review paper addresses the physics of stochastic boundaries. Although it is focused on the tokamak configuration many features are common to the stochastic boundaries of stellarators. The stochastic properties of magnetic field lines are recalled and related to the spectrum of the radial magnetic perturbation. The stochastic region, referred to as the divertor volume, is shown to be bounded to the edge plasma. Furthermore, the stochastic features discriminate two regions. On short scales, the stochasticity is not effective and parallel transport dominates, this defines the laminar region. On the long scales one recovers the proper stochastic features which characterize the ergodic regime. Theoretical predictions for the transport of energy, current and particles in the divertor volume are analysed for both the laminar and ergodic regimes. A strong increase in electron transport is expected which should lead to a strong increase in the heat diffusivity, a strong increase in the resistivity in the toroidal direction and generally a decrease in the free electron lifetime in the divertor volume. Ambipolarity of particle transport is ensured by a radial electric field. The ion transport, i.e. particle transport, is then more difficult to analyse since one has to consider the strong coupling to the electron temperature field and to the electric potential field. The perturbation level is such that the particle transport induced by the stochasticity remains comparable to the anomalous transport. The experimental data show good agreement with the predictions on electron transport. This translates into a flattening of the edge temperature gradient, a narrowing of the current channel, which probably governs the observed stabilization of MHD activity, and a strong decrease in the lifetime of the runaway electrons. Regarding particle transport, the response is larger than expected and stochastic boundaries are characterized by significant screening properties compared to limiter shots. This property is shown to be a signature of a pumping capability combined with a change of transport properties. Indeed the transport of neutrals is changed since the ratio of the ionization scales to the distance between the recycling surfaces and the separatrix is reduced. Furthermore, a decreased lifetime of the ions at the very edge of the plasma is expected. Screening effects are thus observed for species exhibiting large wall pumping capability, while He and Ne are weakly affected by the stochastic boundary. The plasma properties in the ergodic volume, namely a reduced edge temperature, an increased impurity radiation, an efficient particle screening and the stabilization of MHD activity, have opened the way to radiating layer investigations on Tore Supra. Stable operation has been achieved with 80% of radiated power and radio frequency heating up to 6 MW.

Investigations of the temperatures of the in situ impurity ions , , , and have been performed at the Extrap-T1 reversed-field pinch (RFP) via measurements of the Doppler broadening of emission lines in the near-UV wavelength region. Ion temperatures of the high ionization stages are found to be of the order of the electron temperature.

This means that ion heating can be connected with RFP dynamo activity giving rise to the non-Spitzer part of the power input. Power input to the ions of the order of the non-Spitzer part of the total power input is required to explain the high ion temperatures. MHD effects could be responsible for this heating. We present scaling of the observed ion temperatures with plasma current , electron density , pinch parameter and magnetic fluctuations. We also present an empirical scaling law in which the ion temperature can be expressed as . Keeping constant, this gives a constant ion beta poloidal .

The method of self-sustained turbulence is applied to the tokamak plasma, incorporating the effect of an inhomogeneous radial electric field. The transport coefficient is derived, making a bridge between L- and H-phase plasmas. It is possible to construct a unified transport model of the L- and H-mode phases. The anomalous transport coefficients are obtained in a unified and explicit form in terms of profile parameters such as the plasma pressure gradient, the magnetic shear, the shear and curvature of the radial electric field. Strong reductions of the thermal conductivity, , the electron and ion viscosities, and , and the turbulent level in the H-phase plasma are explained. Furthermore, an additional stability window due to is discovered in the higher pressure-gradient regime. The anomalous ion viscosity determines , the typical scale length or . Self-consistent solutions of and are discussed.

Using a two-dimensional hydrocode, the collapse of the mid-section of a cylindrical shell target is investigated. The suitability of the resulting compressed plasma for the `hole boring' and `ignition' stages of the fast igniter scheme for inertial confinement fusion is considered. We estimate that for the implosion model considered, the cylindrical targets investigated offer less gain for a given amount of input energy than conventional spherical targets. From our simplified model we find that it is better to bore radially rather than along the axis.

The energy distributions and radial density profiles of the fast confined trapped alpha particles in DT experiments on TFTR are being measured in the energy range 0.5 - 3.5 MeV using the pellet charge exchange (PCX) diagnostic. A brief description of the measurement technique which involves active neutral particle analysis using the ablation cloud surrounding an injected impurity pellet as the neutralizer is presented. This paper focuses on alpha and triton measurements in the core of MHD quiescent TFTR discharges where the expected classical slowing-down and pitch angle scattering effects are not complicated by stochastic ripple diffusion and sawtooth activity. In particular, the first measurement of the alpha slowing-down distribution up to the birth energy, obtained using boron pellet injection, is presented. The measurements are compared with predictions using either the TRANSP Monte Carlo code and/or a Fokker - Planck Post-TRANSP processor code, which assumes that the alphas and tritons are well confined and slow down classically. Both the shape of the measured alpha and triton energy distributions and their density ratios are in good agreement with the code calculations. We can conclude that the PCX measurements are consistent with classical thermalization of the fusion-generated alphas and tritons.

Extremely fast vertical displacement events (VDEs) induced by a strong collapse were found in a vertically elongated , high tokamak with a resistive shell through computer simulations using the tokamak simulation code. Although the plasma current quench which has been shown to be the prime cause of VDEs in a relatively low tokamak (Nakamura Y et al 1996 Nucl. Fusion 36 643), was not observed during the VDE evolution, the observed growth rate of VDEs was almost five times faster than the growth rate of the usual positional instability . The essential mechanism of the -collapse-induced VDE was clarified to be the intense enhancement of positional instability due to a large and sudden degradation of the magnetic field decay n-index in addition to the significant destabilization due to a reduction in the stability index . The radial shift of the magnetic axis caused by the collapse induces eddy currents on the resistive shell, and these eddy currents produce a large degradation of the n-index.

It is pointed out that the shell geometry characterizes the VDE dynamics, and that the VDE rate depends strongly both on the magnitude of the collapse and the n-index of the equilibria just before the collapse occurs. The JT-60U vacuum vessel is shown to possess the capability of preventing -collapse-induced VDEs.

A new scheme for making combined interferometric and polarimetric measurements on tokamak plasmas with high temporal resolution is presented. The method, which can be regarded as a generalization of techniques suggested by other workers, can achieve bandwidths in excess of 100 kHz using an optically pumped far-infrared triple laser source. A bandwidth of 10 kHz with an accuracy of better than has already been demonstrated. Calibration measurements as well as first plasma measurements obtained with the new approach are presented and evaluated.

The equilibrium of a cylindrical plasma with purely poloidal mass flow and cross section of arbitrary shape is investigated within the framework of the ideal MHD theory. For the system under consideration it is shown that only incompressible flows are possible and, consequently, the general two-dimensional flow equilibrium equations reduce to a single second-order quasilinear partial differential equation for the poloidal magnetic flux function , in which four profile functionals of appear. Apart from a singularity occurring when the modulus of Mach number associated with the Alfvén velocity for the poloidal magnetic field is unity, this equation is always elliptic and permits the construction of several classes of analytic solutions. Specific exact equilibria for a plasma confined within a perfectly conducting circular cylindrical boundary and having (i) a flat current density and (ii) a peaked current density are obtained and studied.

Dissipated power in the spectral gap is taken into account in deriving analytically the lower hybrid current drive efficiency. The efficiency is determined by a minimum down-shifted phase velocity and the quasi-linear velocity diffusion coefficient at the spectral gap in addition to the original wave spectrum. To explain present experimental results in both JT-60 and ASDEX, must be close to the Landau damping limit and must be the same order of magnitude as the collisional friction at . With the suggested values of these two parameters from experimental results, the driven current is mainly determined by the launched wave spectrum, while most of the wave power is dissipated at the spectral gap. This characteristic can explain both the temperature and the dependence of the current drive efficiency.

Theoretical and computational analyses of bootstrapped tokamaks with oscillating field current drive (OFCD) have been developed using a nonlinear magnetic helicity model. Assuming a rigid current profile, the conditions for optimal tokamak steady-state operation are derived and are shown to agree with the results of computer calculations. Generalized limit formulae for the toroidal plasma beta and bootstrap current fraction are also obtained. The results of the analyses indicate that steady-state bootstrapped tokamak operation with high toroidal plasma beta can best be achieved in tight aspect ratio reactors with OFCD supplying as much as a quarter of the total toroidal plasma current.

11 - 15 November 1996 38th Annual Meeting of the Division of Plasma Physics, APS Denver, CO, USA Contact: Dr Richard Hazeltine, University of Texas, Institute for Fusion Studies, RLM 11.314, Austin, TX 78712. Tel +1-512-471-1322, E-mail stewart@hagar.ph.utexas.edu.

17 - 18 February 1997 Plasma '97: 21st Australian Institute of Nuclear Science and Engineering Plasma Science and Technology Conference Sydney, Australia Contact: Margaret Lanigan, Conference Manager, PMB 1, MENAI NSW 2234, Australia. Fax +61-(0)2-439-6561, E-mail ainse@ansto.gov.au.

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.

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.