Table of contents

The paper presents a new method of RF plasma heating by an extraordinary wave in toroidal devices. The method combines localization of absorption at the centre of the plasma column with a marked decrease in pumping frequency compared with cyclotron frequency. A stationary distribution of the electromagnetic radiation as a result of multiple reflections from the chamber walls is found. It is shown that, compared with linear systems, the threshold for the parametric instability decreases because of the return of oscillations to the pumping region after a circumference of the torus. The effective collision frequency, describing pump-wave conversion into plasma waves, is determined in the framework of the weak turbulence theory. Heating estimates for L-2, T-10, Alcator C and T-15 show the efficiency of the proposed method.

Digital signal processing techniques were used to study low-frequency fluctuations (f < 100 kHz) in the Ohmically heated plasma (_e ≈ 2 × 10¹¹ cm⁻³, T_e ≈ 10 eV) of the Proto-Cleo torsatron. Coherent density fluctuations show a mode structure with a helicity matching that of the vacuum field. The poloidal mode numbers vary from m = 4 at 20 kHz to m = 16 at 100 kHz. The m = 4 mode has the largest amplitude, ñ/n ≈ 15%, and the fluctuation amplitudes decrease roughly linearly with frequency. Fluctuations in plasma current, H_β emission, and X-ray flux show spectra similar to and coherent with that of the density fluctuations. Comparison with theory and previous experimental results indicates that rippling modes may be responsible for these fluctuations. These results may be relevant to the physics of edge plasmas in larger devices.

A model for the effect of an electron beam on the drift cyclotron loss cone (DCLC) instabilities in mirror machines is presented. It is shown that because of the ponderomotive force of the beamgenerated plasmons, the growth rates of the DCLC instability are modified substantially. The model explains some of the observations on the Constance mirror machine where the DCLC instability was found to be quenched by the injection of an electron beam.

Electron temperature data are presented from Thomson scattering measurements performed on field-reversed configuration plasmas generated in the FRX-C experiment. Two experimental conditions have been investigated, corresponding to initial deuterium fill pressures of 5 and 20 mtorr. At 5 mtorr, T_e values of (175±25) eV are observed, which are nearly independent of time and radial position inside the separatrix. An electron power loss of approximately 80 MW is inferred. At 20 mtorr, T_e values of 70–120 eV are measured and good agreement with radial pressure balance is obtained. This agreement indicates that radiation from low-Z impurity ions does not dominate the observed energy confinement.

Observation of the intensity of the recycling particle flux at the main plasma edge for various limiter and divertor discharges indicates that the gross energy confinement of beam-heated discharges is closely related to the intensity of the edge particle flux. In limiter discharges, the global particle confinement time and the energy confinement time τ_E show many similarities: 1) linear I_p dependence at I_p < 600 kA, 2) no B_T dependence, and 3) deterioration against injection power. Improvement of τ_E by increasing I_p, for example, is associated with high temperatures at the plasma edge region accompanied by reduced particle recycling. – Divertor discharges with low particle recycling around the main plasma show better energy confinement than limiter discharges at high plasma densities. The improvement of τ_E is primarily originated in the reduction of heat transport at the main plasma edge region, which is associated with the reduction of recycling particle flux at the main plasma edge. Under certain operation condition, for example, excessive cold-gas puffing, the discharge shows relatively high scrape-off plasma density and strong particle recycling between the main plasma and the limiter. The energy confinement time of these discharges degrades somewhat or reduces completely to that of the limiter discharge. – In low-recycling divertor discharges, the central electron and ion temperature is proportional to the injection power, and the plasma stored energy is proportional to _eP_abs (scales as INTOR scaling). With ≈ 4 MW beam injection, high-temperature and high-density plasmas were obtained (stored energy up to 280 kJ, T_e(0) ≈ T_i(0) ≈ 2.5–3.0 keV at _e ≈ (6–7) × 10¹³ cm⁻³, τ_E* ≈ 70 ms).

The stable lifetimes of many field-reversed configuration (FRC) experiments have been observed to be limited by the onset of the m = 2 rotational instability. The origin of the rotation which drives this instability is investigated with the aid of hybrid simulation methods. The loss of particles with guiding centres on open field lines is found to lead to localized rotation near the separatrix and this effect is found to be enhanced by end-shorting of the radial electric field outside the separatrix. In simulations allowing azimuthal variation the velocity shear is eventually relaxed so that the bulk of the plasma begins to rotate. These simulations also show that the m = 2 instability does not appear as long as rotation is localized near the separatrix. It is found that by increasing the separatrix radius it is possible to significantly delay the transfer of rotation from the separatrix, hence delaying the onset of the m = 2 instability.

Deuterium and hydrogen energy spectra during ICRF heating as well as n_H/n_D were measured with the E||B type mass-separated neutral particle energy analyser. High-energy tails of deuterium and hydrogen were observed in the dilute-minority case. From Fokker-Planck calculations of the charge-exchange spectrum and power-balance analysis it is shown that second-harmonic cyclotron damping by the majority deuterium is the dominant mechanism of ion heating in the dilute-minority case.

Realistic density profiles in the very periphery of tokamak discharges have been considered in the study of lower hybrid wave coupling by a waveguide array (grill). The density profile is assumed to be exponential in the scrape-off region up to the limiter radius and then linear. The effect of a sharp density gradient or of a density discontinuity at the limiter radius is discussed. It is shown that these effects lead to a strong modification of the transmission coefficient which can be optimized by adjusting the grill mouth position.

A theoretical analysis of ICRF wave heating in a plasma with neutral-beam injection (NBI) is performed. Wave propagation and absorption are examined kinetically in the presence of a high-energy ion beam component. The following findings are reported: 1) the wave coupling efficiency does not deteriorate by the beam; 2) co-operative heating is to be expected, and 3) selective heating of high-energy ions is possible.

The basic requirements for the electrostatic enhancement of magnetic mirrors, as in tandem-mirror and thermal-barrier designs, are examined. This discussion leads to a simple, intuitively convincing picture of the operation of such systems and resolves the question of the self-consistency of simultaneous electrostatic confinement of electron and ion populations.

The 11th European Conference on Controlled Fusion and Plasma Physics was held from 5 to 9 September 1983 in the Kármán Auditorium of the University of Aachen, FRG. The Conference, organized by the Nuclear Research Centre Jülich, was attended by nearly 500 participants from over 30 countries, among them guest delegations from several non-European countries. The scientific material was presented in 24 invited papers, 36 short oral contributions, and about 250 posters spread over five poster sessions.

The present review is necessarily limited and cannot give proper credit to each work presented at the Conference. According to the Conference programme, this report is divided into ten sections with emphasis on magnetic confinement and related problems and topics.