U. Gasparino

Electron cyclotron resonance heating and current drive in toroidal fusion plasmas

A review of experiments and theory of electron cyclotron resonance heating (ECRH) and current drive (ECCD) is presented. An outline of the basic linear theory of wave propagation and absorption in the electron cyclotron range of frequencies and their harmonics is given and compared with experimental results from many devices. The experimental data base on quasilinear 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 stellarators and the results are discussed in the light of the present day theoretical understanding. The role of ECRH to improve the understanding of both particle 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 perturbative heat wave studies and the comparison with both sawtooth heat pulse propagation and the steady state power balance analysis is presented. Electron cyclotron current drive is a possible method for current profile and MHD control in tokamaks and provides means for bootstrap current compensation in stellarators. The basic theory of electron cyclotron current drive is presented and compared to experiments in both tokamaks and stellarators. Experiments on sawtooth stabilization and MHD control by ECRH or ECCD are discussed and compared 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 parameter range became possible by the recent development of sources with high power (1 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.

Experimental and neoclassical electron heat transport in the LMFP regime for the stellarators W7‐A, L‐2, and W7‐AS

The electron energy balance is analyzed for equivalent low‐density electron cyclotron resonance heated (ECRH) discharges with highly peaked central power deposition in the stellarators W7‐A [Plasma Phys. Controlled Fusion 28, 43 (1986)], L‐2 [Proceedings of the 6th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Berchtesgaden, 1976 (International Atomic Energy Agency, Vienna, 1977), Vol. 2, p. 115] and W7‐AS [Proceedings of the 9th International Conference on Plasma Physics and Controlled Nuclear Fusion Research, Baltimore, 1982 (International Atomic Energy Agency, Vienna, 1983), Vol. 3, p. 141]. Within the long mean‐free path (LMFP) collisionality regime in stellarators, the neoclassical electron heat diffusivity χe can overcome the ‘‘anomalous’’ one. The neoclassical transport coefficients are calculated by the dkes code (Drift Kinetic Equation Solver) [Phys. Fluids 29, 2951 (1986); Phys. Fluids B 1, 563 (1989)] for these configurations, and the particle and energy flu...

Transport in stellarators

The local electron and ion heat transport as well as the particle and impurity transport properties in stellarators are reviewed. In this context, neoclassical theory is used as a guideline for the comparison of the experimental results of the quite different confinement concepts. At sufficiently high temperatures depending on the specific magnetic configuration, neoclassical predictions are confirmed by experimental findings. The confinement properties in the LMFP collisionality regime are discussed with respect to the next stellarator generation, for which at higher temperatures the neoclassical transport is expected to become more important.

Current drive and bootstrap current in stellarators

A review of the experimental observation of plasma net currents in several stellarator devices and the comparison with theoretical predictions on the neoclassical bootstrap current is given. Good agreement of the experimental findings with neoclassical predictions is found with respect to the parameter dependence, the local distribution of the currents and the magnitude. The compensation of the bootstrap current by electron cyclotron current drive is experimentally demonstrated. Experiments on the ECCD-efficiency are compared with simple theoretical models.

Physics studies in W7-AS

A review of experiments with ECRH- and NBI-heated plasmas in W7-AS is given. Global results of W7-AS are summarized. Particular emphasis is put on electron cyclotron current drive, comparative analysis of electron heat transport derived from power balance and perturbative studies, and particle transport under combined ECRH- and NBI-heating conditions. The role of the plasma boundary field configuration as a necessary condition for the existence of the H-mode in W7-AS is discussed and first observations of coherent global Alfven eigenmodes and turbulent temperature fluctuations in the plasma core are reported.

Studies on electron cyclotron heating at the Wendelstein VII-A/AS stellarators

The absorption of ECF power launched nearly perpendicular to the magnetic field, ECF current drive and generation of suprathermal electrons is analysed using a 3-D ray tracing code in combination with a simplified Fokker-Planck solution for which the ECF energy source function is estimated by means of quasilinear theory. Results are compared with W VII-A experimental data of ECF current drive and electron cyclotron emission.