A review of experiments and theory of electron cyclotron resonance heating (ECRH) and current drive (ECCD) is presented. An outline ofthe basic linear theory of wave propagation and absnrption in the electron cyclotron range of frequencies and their harmonics is &en and compared with experimentd results from many devices. The experimental data base on quasilinem and nonlinear physics as well as on parametric wave decay is reviewed and compared to theory. Experiments and theory on doppler shifted absorption either by bulk or tail electrons (which can be created by other means) are discussed. ECRH provides means for controlled plasma breakdown and current ramp up in tokamaks and plays a key role in net current-free stellarator research. Start-up was investigated in many tokamaks and stelhalors and the results are discussed in the light of the present day theoretical understanding. The role of ECRH to improve the understanding of both parlicle and energy confinement is described and special heating correlated features, such as 'density pump out' during ECRH are discussed. The application of modulated ECRH for pertdative heat wave studies and the comparison with both sawtooth heat pulse propagation and the steady state power balance analysis is presented.Electron cyclotron eurrent drive is a possible method for current profile and MHD control in tokamaks and provides means for bootstrap current wmpensation in stellarators. The basic theory of electron cyclotron current drive is presented and compared to experiments in both tokamaks and steilarators. Experiments on sawtooth stabilization and MHD control by ECRH or ECCD are discussed and wmpared to theory. An increasing number of fusion devices is equipped with ECRH for bulk heating and sophisticated plasma physics investigations. A remarkable extension of the accessible plasma panmeter range became possible by the recent development of sources with high power (I MW) and frequency (110-160 GHz). Particular emphasis is given to new experiments and the refinement of theory incorporating plasma phenomena and the mutual impact on the Wave physics.

The theoretical and experimental development of stellarators has removed some of the specific deficiencies of this configuration, viz., the limitations in β, the high neoclassical transport, and the low collisionless confinement of α particles. These optimized stellarators can best be realized with a modular coil system. The W7-AS experiment [Plasma Phys. Controlled Fusion 31, 1579 (1989)] has successfully demonstrated two aspects of advanced stellarators, the improved equilibrium and the modular coil concept. Stellarator optimization will much more viably be demonstrated by W7-X [Plasma Physics and Controlled Fusion Research, Proceedings of the 12th International Conference, Nice, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 369], the successor experiment presently under design. Optimized stellarators seem to offer an independent reactor option. In addition, they supplement, in a unique form, the toroidal confinement fusion program, e.g., energy transport is anomalous in stellarators too, but possibly more easily understandable in the frame of existing theoretical concepts than in tokamaks.

The neoclassical prediction of the “electron root,” i.e., a strongly positive radial electric field, Er (being the solution of the ambipolarity condition of the particle fluxes), is analyzed for low-density discharges in Wendelstein-7-AS [G. Grieger, W. Lotz, P. Merkel, et al., Phys. Fluids B 4, 2081 (1992)]. In these electron cyclotron resonance heated (ECRH) discharges with highly localized central power deposition, peaked Te profiles [with Te(0) up to 6 keV and with Ti≪Te] and strongly positive Er in the central region are measured. It is shown that this “electron root” feature at W7-AS is driven by ripple-trapped suprathermal electrons generated by the ECRH. The fraction of ripple-trapped particles in the ECRH launching plane, which can be varied at W7-AS, is found to be the most important. After switching off the heating the “electron root” feature disappears nearly immediately, i.e., two different time scales for the electron temperature decay in the central region are observed. Monte Carlo simulations in five-dimensional phase space are presented, clearly indicating that the additional “convective” electron fluxes driven by the ECRH are of the same order as the ambipolar neoclassical prediction for the “ion root” at much lower Er. For the predicted “electron root,” the ion fluxes calculated based on the traditional neoclassical ordering are much too small; shortcomings of the usual approach are indentified and a new ordering scheme is proposed.

Kinetic effects are important in low-density high-power ECRH discharges, and the electron distribution function can significantly deviate from Maxwellian. The ECRH power deposition is analysed for perpendicular on-axis heating in W7-AS, with different magnetic configurations characterized by either a minimum or a maximum of B on the plasma axis in the RF injection plane. The different heating scenarios are modelled by means of a new bounceaveraged Fokker-Planck code, well suited for the magnetic field geometry close to the plasma axis of W7-AS.The power deposition profile is estimated from the analysis of heat wave propagation stimulated by ECRH power modulation. In general, peaked deposition profiles as predicted from a ray-tracing code are obtained, but with an additional much broader contribution.The broadening of the thermal power deposition profile is assumed to be related to the radial transport by the ∇B drift of locally trapped suprathermal electrons. This is simulated by means of a simple convective Fokker-Planck model. The theoretical predictions are shown to be consistent with the experimental findings.Kinetic effects on the determination of the temperature both by Thomson scattering and by ECE diagnostics are briefly discussed.