Table of contents

In 1990 JET operated with a number of technical improvements which include beryllium antenna screens, a prototype lower hybrid current drive system, and modification of the NI system to enable the injection of ³He and ⁴He. Continued investigation of the hot-ion H-mode produced a value of n_D(O) tau _ET_i(O)=9*10²⁰ m^-3s keV, which is near conditions required for Q_DT=1, while a new peaked density profile H-mode was developed with only slightly lower performance. Progress towards steady state operation has been made by achieving ELMy H-modes under certain operating conditions, while maintaining good tau _E values. Experimental simulation of He ash transport indicates effective removal of alpha-particles from the plasma core for both L and H mode plasmas. Detailed analyses of particle and energy transport have helped establish a firmer link between particle and energy transport, and have suggested a connection between reduced energy transport and reversed shear. Numerical and analytic studies of divertor physics carried out for the pumped divertor phase of JET have helped clarify the key parameters governing impurity retention, and an intensive model validation effort has begun. Experimental simulation of alpha-particle effects with beta _fast up to 8% have shown that the slowing down processes are classical, and have given no evidence of deleterious collective effects.

Short-wavelength laser radiation from high-power laser installations makes it possible to perform quantitative laboratory investigations of radiative heat transfer in hot, dense matter. The author concentrates on the generation and confinement of intense Planck radiation in laser-heated cavities. The results are relevant to indirect-drive inertial confinement fusion.

Examines the possibility of global climate change due to the emission of carbon dioxide and other greenhouse gases. The problem can be ameliorated by reducing fossil fuel consumption through conservation and expanded use of nuclear and solar power. In particular, major reductions can be achieved if fossil fuels are replaced in electricity generation and if electricity assumes a larger role in the overall energy economy.

A review of TFTR plasma transport studies is presented. Parallel transport and the confinement of suprathermal ions are found to be relatively well described by theory. Cross-field transport of the thermal plasma, however, is anomalous with the momentum diffusivity being comparable to the ion thermal diffusivity and larger than the electron thermal diffusivity in neutral beam heated discharges. Perturbative experiments have studied nonlinear dependencies in the transport coefficients and examined the role of possible nonlocal phenomena. The underlying turbulence has been studied using microwave scattering, beam emission spectroscopy and microwave reflectometry over a much broader range in k perpendicular to than previously possible. Results indicate the existence of large-wavelength fluctuations correlated with enhanced transport.

Thanks to the fast development of space research, in situ measurements have been performed in the various critical regions that are formed as the consequence of the interaction between the solar wind and the Earth's magnetic field. The author reviews these measurements carried out with non-destructive means of investigation and infers from them the physical processes that control this type of interaction.

Optimum confinement is realized in Wendelstein 7-AS by wall conditioning and by properly adjusting the parameters determining the magnetic field configuration. The effective heating of net current free plasmas by ECRF and neutral beam injection (NBI) involves different plasma parameters and transport regimes. Stationary plasmas are generally produced by ECRF, whereas density and impurity control is a severe problem during NBI. This has initiated different kinds of impurity and particle control scenarios. An extended parameter range with electron temperatures of 200 eV<or=T_e<or=3 keV, ion temperatures of 100 eV<or=T_i <or= 0.7 keV and electron densities of 10¹⁹ <or= n_e<or=3.10²⁰ m^-3 was accessible. The characteristics of the energy confinement and the particle and impurity transport are described and related to the specific heat and particle sources. The investigations comprise the analysis of electron and ion heat conductivity, particle transport modelling and impurity transport studies by laser blow-off-experiments. The influence of the ambipolar electric field is discussed.

Helium, as the ash of burning D-T plasma, is an unavoidable impurity component necessarily present already in near future tokamak experiments with significant alpha particle heating. Its efficient removal from the burning zone of a D-T fusion reactor plays a key role in the path towards achievement of economic fusion power production. A survey is given of the issues related to this question. Since there is as yet no experimental experience with thermonuclear plasmas significantly heated by fusion products, the review is based on results from simulation experiments of helium injection into hydrogen or deuterium tokamak plasmas, and from numerical transport code work. Both kinds of results are discussed with reference to handy ignition criteria obtained for steady D-T burning, which have been reformulated in terms relevant for the ash removal problem.

The stabilisation of plasma macroscopic instabilities by high energy particles is shown to be a consequence of the conservation of the third adiabatic invariant, i.e. the magnetic flux encircled by the precessional drift orbits. The author discusses the stabilisation mechanism, the physical processes that determine the boundaries of the stable regime, and a comparison between theory and experiment. Considerable progress on the understanding of the fast ion suppression of thermal plasma 'sawtooth' relaxation oscillations has resulted from a profitable interaction between theory and experiments.

Lower hybrid current drive (LHCD) experiments in TORE SUPRA and JET are reported. Measurements of the scattering matrices of the antennae show good agreement with theory. The current drive efficiency in TORE SUPRA is about 0.2*10²⁰ Am^-2/W with LH power alone and reaches 0.4*10²⁰ 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. 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 nonthermal ECE. In JET ICRH produced sawtooth-free periods are extended by the application of LHCD and current profile broadening has been observed consistent with off-axis fast electron populations. LH power modulation experiments performed in TORE SUPRA 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. Successful pellet fuelling of a partially LH driven plasma was obtained in TORE SUPRA.

The authors review theoretical and experimental investigations of current drive by electron cyclotron waves in tokamaks and stellarators. Advantages and disadvantages of their use in thermonuclear reactors are discussed.

During its 1990 operation, 2 large RF systems were available on JET. The ion cyclotron resonance heating (ICRH) system was equipped with new beryllium screens and with feedback matching systems. Specific impurities generated by ICRH were reduced to negligible levels even in the most stringent H-mode conditions. A maximum power of 22 MW was coupled to L-mode plasmas. A new high confinement mode was discovered. Experiments were performed with the prototype launcher of the lower hybrid current drive (LHCD) systems with coupled power up to 1.6 MW with current drive efficiencies up to <n_e> R I_CD/P=0.4*10²⁰ m^-2 A/W. The authors present the first observations of the synergistic acceleration of fast electrons by transit time magnetic pumping (TTMP) (from ICRH) and electron Landau damping (ELD) (from LHCD). The synergism generates TTMP current drive even without phasing the ICRH antennae.

The authors present a brief review of collisional (classical and neoclassical) and anomalous transport. Particular emphasis is devoted to the question of charge independence of the anomalous transport coefficients and the combined action of anomalous and collisional transport. In the light of these results the experimental facts are analysed and interpreted. It is found that impurity accumulation-characterized by peaked z_eff-profiles-is caused by the combined effects of improved confinement (i.e. reduction of anomalous transport) and peaking of the electron density profile. For the cases of pellet refuelled plasmas and counter neutral injection heating quantitative comparisons are performed which show good agreement between the experimental measurements and simulations based upon neoclassical theory.

Theoretical and experimental investigations of xenon cylindrical shell collisions with inner axial liners are presented. The implosion was driven by a current of 3.5 MA. X-ray emission with a 3 ns rise time was obtained. The Angara-5-1 experiments with superfast Z-pinches are described.