Table of contents

Two antennae have been installed in JET and operated to the maximum design capability of the generators. 4.5 MW, 10 MJ have been coupled to the plasma which heated up to a maximum stored energy of 3 MJ with central temperatures of T_e0=5 keV and T_i0=4 keV without increase of the relative impurity concentration. Degradation of energy confinement is observed according to an L mode scaling. The effect of k/sub //// shaping is discussed using a quadrupole antenna. Hydrogen and helium 3 minority heating regimes give similar results.

The TFTR tokamak has reached its original machine design specifications (I_p=2.5 MA and B_T=5.2 T). Recently, the D degrees neutral beam heating power has been increased to 6.3 MW. By operating at low plasma current (I_p approximately=0.8 MA) and low density (n_e approximately=1*10¹⁹ m^-3), high ion temperatures (9+or-2 keV) and rotation speeds (7*10⁵ m/s) have been achieved during injection. At the opposite extreme, pellet injection into high current plasmas has been used to increase the line-average density to 8*10¹⁹ m^-3 and the central density to 1.6*10²⁰ m^-3. This wide range of operating conditions has enabled the authors to conduct scaling studies of the global energy confinement time in both ohmically and beam heated discharges as well as more detailed transport studies of the profile dependence.

Neutral beam (NI), ion cyclotron resonance (ICRH) and lower hybrid resonance (LHRH) heating on ASDEX are discussed with regard to their effect on plasma confinement. Comparison of NI and ICRH shows that the L and H-regimes are universal confinement modes of auxiliary-heated tokamak plasmas (i.e. independent of the heating method), and that the edge electron temperature (or a related parameter) dictates which mode prevails. In this connection it is noted that carbonization of the vessel walls impedes transition to the H-mode in the case of NI heating. Studies of energy confinement in the intermediate regime from ohmic to NI heating reveal a gradual transition from ohmic ( approximately n_e) to neutral injection L-mode ( approximately I) scaling. At the same time a remarkable invariance of electron temperature profile shape with increasing heating power is observed. Changing the NI power deposition profiles from central to off-axis leaves gross energy confinement times unchanged while central confinement is substantially improved.

WENDELSTEIN VII-A has been operated for ten years. It is a low-shear, high-aspect-ratio device. The confinement properties have been thoroughly studied for both ohmically heated and net-current free plasmas. For the latter case, NBI- and ECF-maintained plasmas were of particular importance. It was found that under optimized conditions the core of high-pressure, net-current free plasmas is mainly governed by collisional effects.

JET tokamak performance has been progressively raised, culminating in operation at the full design level of 5 MA plasma current in a toroidal field at 3.4 T. the plasma current, position, shape and line-average electron density are controlled by feed-back systems. By glow discharge cleaning in H₂/methane mixtures, the interior of the vessel has been coated with carbon. This reduces the fraction of power radiated <or=50% at intermediate densities and raises the density limit for disruption by 10%. Operation over a wide range of parameters has given data for more extensive scaling studies. With ohmic heating, the maximum global energy confinement time is 0.8+or-0.1 s and the maximum central ion-temperature approximately 3.0 keV. The highest (n tau _E T_i) product is reached with ohmic heating in a deuterium plasma.

Ion cyclotron heating on TEXTOR has now reached the Megajoule level. The heating scenario is normally mode conversion but occasionally minority heating in a D-(H) plasma. With appropriate wall conditioning by carbonization more than 1 MW of RF power has been injected for long pulse durations ( approximately 1 s). The ICRF heated plasma is characterized by a quasi-stationarity of all plasma parameters, little if no impurity increase and a loop voltage reduction resulting in the total power coupled to the plasma reaching six times the remaining ohmic power input. Evidence of the coupling of the RF power to the plasma is obtained from the increase of the thermal load on the limiters and central energy deposition is supported from analysis of the sawtooth heating rate.

Fast relaxations of the electron temperature profile during pellet injection or disruptions, and its readjustment during additional heating are described. A global picture for both phenomena is proposed.

The latest analysis of results from the Bundle Divertor and prospects for application to a tokamak reactor are presented. The upgraded version of the machine, DITE IB is described briefly. Initial results of a programme to study the causes of disruption at high density in ohmic and beam-heated discharges are also described.

JET results are presented which show a 'profile consistency' of the electron temperature under a variety of plasma conditions. This experimental fact is interpreted as due to a topology of the magnetic field lines where ordered structures such as laminar surfaces and magnetic islands coexist with ergodic domains. The development of this concept identifies different plasma regions according to the value of the safety factor and relates the observed degradation in confinement time, when additional heating is applied, to the plasma entering a multiphase state where the electron temperature gradient is limited by a critical value defined by the global plasma parameters. A preliminary scaling law for the electron temperature is proposed which indicates a strong favourable dependence on major radius and plasma current. The near future JET experimental programme is briefly described and possible ways of improving the plasma confinement are presented.

Electron cyclotron resonance heating (ECRH) provides an opportunity of adding a local power contribution to the electron plasma component. Thus, the electron temperature profile can be varied at will and the dependence of electron heat losses on the shape of the profile can be studied.

Reviews recent developments in the theory of MHD waves in connection with radio-frequency heating in the Alfven wave and ion cyclotron ranges of frequencies (AWRF, ICRF). The account focuses on the discussion of full wave solutions and the oscillation spectra in bounded, generally inhomogeneous, plasmas. Original results are presented concerning forced ICRF oscillations in a current-carrying torus. The effects on the wave structures of the equilibrium current, the size of the device, the minority concentration and the phenomenological damping are investigated. The resonant surfaces coincide with the magnetic surfaces as in the AWRF. The poloidal extension of the resonance is small in cases where a WKB approach is permitted so that there is no conflict between full wave solutions and the WKB method.

Due to the interaction of plasma with in-vessel components, serious erosion is experienced in present machines. A summary of basic processes, model experiments concerning them and their relevance to the real physical situation is reviewed. The newest trends in methods to minimize erosion and plasma contamination such as divertors, gettering and discharge cleaning techniques are also summarized.

The generation of ultra high pressure by laser beams is reviewed in the context of laser driven inertial confinement fusion. Central pressures of 200 GBar are required in an imploded pellet and are achieved by the amplification of the laser generated pressure in the implosion. The theory of laser driven ablation predicts greater efficiency with shorter laser wavelength but with the penalty of greater susceptibility to laser beam non-uniformity. A variety of experimental measurements of laser ablation is described and their results are in good agreement with applicable theories. Factors determining the overall implosion symmetry and pressure multiplication are discussed briefly.

Initial ohmic heating experiments in JT-60 were performed for a three month period of April-June 1985. A maximum plasma current of 1.6 MA was obtained for both divertor and limiter discharges. Low-q discharges of q_eff=2.5 and high density discharges of 4.8*10¹⁹ m^-3 in line-averaged density were obtained in the divertor configuration. In divertor discharges radiated loss from the main plasma can be kept at 20-30% of the ohmic input.

RF power up to 450 kW has been injected into the plasma of FT in the electron heating regime (n_e<or=5*10¹³ cm^-3) producing electron and ion temperature increases of about 1 keV and .5 keV respectively without significant enhancement of Z_eff. A density increase is observed due to an improvement of particle confinement time. An energy balance at intermediate power levels (P_RF=300 kW) is carried out for two different types of discharges, one without sawteeth the other with sawteeth. The balance shows that in order to account for the total injected power one has to assume equal electron thermal conductivity for the OH and the OH+RF phase. The energy confinement time does not vary from its ohmic value for both discharges. Finally an investigation, at n_e above the density limit, of the radial source of the fast ion tails and of the characteristics of the parametric decay instability is presented.

The basic physical problems determining the fusion prospects of open traps are considered. Main attention is paid to the possibility of a scalable modelling of the anomalous transverse transport in long solenoids (which are a necessary component of most of the types of open traps), and to the problem of the MHD stability of axisymmetric ambipolar traps. The experimental device for studying the transverse transport in a long solenoid, based on the concept of a gas-dynamic trap, is described. Various axisymmetric MHD-stabilizers for ambipolar traps-are considered, including the recently suggested stabilizer in the form of a 'fat' mirror machine. Such a stabilizer is shown to provide the suppression of large-scale flute perturbations in the framework of a 'natural' geometry of open traps.

The present status of ICF with lasers and ion beams is reviewed and the basic physics related to it is exposed (beam absorption, pressure generation, symmetry and stability). Trends and near future developments are discussed.

Studies of ion confinement in Doublet III show that the ion thermal diffusivity chi _i exceeds the neoclassical value by a factor of two to four at r=0.2a increasing to a factor of ten at r=0.7a. It appears that chi _e and chi _i are always closely equal, suggesting turbulence underlies the diffusion. Scaling data for energy confinement time dependence on heating power, density, current, and toroidal field are reviewed to constrain the formulation of a transport model based on turbulence. A model based on trapped electron turbulence is proposed which produces the correct scaling behavior of tau _E in both low and high collisionality regimes.

First ICRF experiments on ASDEX have been performed at 67 MHz, corresponding to 2 Omega _CH-heating of a hydrogen plasma at B₀=2.2 T. Despite divertor operation ICRH is accompanied by a significant increase of impurity production which can drastically be reduced by means of wall carbonisation. RF power up to 2.3 MW is routinely coupled to the plasma for pulse lengths of up to 1 sec. The RF heating is found to depend strongly on plasma preheating. In combination with neutral beam injection the ICRF heating efficiency is even higher than the one of NI. Confinement degrades with ICRH to values in between NI-L-type and OH confinement.

Recent results from the new 12-beam compression facility at Rutherford Appleton Laboratory are discussed and novel experiments are described which use thermonuclear particle spectroscopy to measure compression uniformity, fast particle dE/dx and hydro instabilities.

The present model of sawtooth oscillations does not appear to be consistent with experimental observations on JET. An alternative theory is proposed, offering possible explanations for the basic aspects of the observed behaviour.