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The effects of divertor biasing and edge ergodization on the divertor plasma have been investigated in the JFT-2M tokamak. Experimental results show; (1) The differential divertor biasing can change the in/out asymmetry of the divertor plasma. It especially changes the density on the ion side divertor plasma. The in/out electron pressure difference has a good correlation with the biasing current. (2) The unipolar divertor biasing can change the density profile of divertor plasma. The radial electric field and shear flow are the cause for this change. (3) The electron temperature of the divertor plasma in the H-mode with frequent ELMs induced by edge ergodization is lower than that of usual H-mode. That is due to the enhancement of the radial particle flux by frequent ELMs.

The effects of plasma edge modification on confinement have been investigated on the JFT-2M tokamak since 1985. In the course of investigations, a new enhanced confinement regime, limiter H-mode, was discovered in 1986. This paper is a review of investigations into edge recycling control methods which extend the H-mode operational regime. In addition to Ti-gettering, the following methods have been shown to reduce edge neutral buildup, to extend the operational regime and to improve n τ of H-mode discharges: (1) optimization of the gas-puff programming (decreased or terminated at the start of auxiliary heating), (2) a toroidally distributed gas-puff system, (3) edge heating by ECH, and (4) particle removal with a pump limiter.

In JFT-2M, impurity behaviors during the H-mode were studied by the spectroscopic method. During the H-mode without Edge-Localized Modes (ELMs), emissions of highly ionized ion lines increased with time, although emissions of less ionized ion lines decreased just after the H-mode transition and kept constant with time. It was found from time behavior of emissions of impurity ion lines and the iron-ion density profiles that not only metal impurity but also light impurity accumulated near the plasma center during the ELM-free H-mode. During the H-mode with ELMs, the impurity accumulation was relaxed by the onset of this instability compared with the ELM-free H-mode.

The roles of the edge plasma are investigated with respect to impurity release, antenna loading, and H-mode transition. Metal impurity release is considered to be caused by an RF electric field near the antennas, and can be reduced by the out-of-phase operation. Antenna loading with H-mode plasmas is dependent on the antenna-separatrix distance, and can be increased for sufficiently small separation. Electron temperature and electron density in the edge are the key parameters for H-mode transition, and they are the same, independent of the additional heating methods, and of the impurity level.

A pump limiter is applied to the JFT-2M tokamak in order to reduce edge gas buildup. About 12% of the effective loss flux is removed by the pump limiter and the fueling efficiency is improved by 30%. In the high density regime, the gas buildup is successfully reduced and edge cooling by cold gas-puffing is prevented by the pump limiter. The pump limiter also improves the density limit and the energy confinement time. Improvement in the nτ value is about 34%. This pump limiter is shown to be compatible with the limiter H-mode.

ICRF heating of impurity-free tokamak plasmas is studied in DIVA. The frequency is fixed at 25 MHz, which is a second-harmonic cyclotron frequency of deuteron with a toroidal magnetic field of 16.4 kG. The toroidal field BT and the ratio of proton-to-deuteron density p are varied over ranges of 12 ? BT ? 19 kG and 2 p 40%, respectively. The most favourable ion heating is obtained at BT = 18 kG and p = 5 ~ 10%. Ion temperature increases from 200 to 300 eV were measured by charge-exchange neutral analysis and also by spectroscopy, where 100 kW RF net power were applied. The results are consistent with mode-conversion theory.

Neutral-beam power of up to 1.2 MW injected into the plasma has produced a volume-averaged β of up to 2.6% and a central beta β0 of up to 7%, due to the thermal components in the JFT-2 tokamak. In these beam-dominated discharges, the magnetohydrodynamic behaviour was studied. Four types of internal oscillations were observed: i) enhanced sawtooth oscillations with long repetition time and large sawtooth amplitude; ii) round sawtooth oscillations and/or reduced sawtooth oscillations with short repetition time and small sawtooth amplitude; iii) high-frequency oscillations without sawtooth oscillations, and iv) high-frequency oscillations with sawtooth oscillations. The measured beta values are compared with the critical ones as found from high-n ballooning-mode analysis, and the relationship between MHD behaviour and beta values is also investigated.

Diagnostic instruments for measuring the JT-60 plasma are fully developed and ready to operate. Each instrument and its fundamental characteristics are described as well as some aspects of the plasma parameters. The new features in JT-60 diagnostics are that the diagnostics system which has its own data processing system separate from the machine control computer, and the utilities for each diagnostics instrument (vacuum, electricity, water, air, liquid nitrogen) constitute one system in the total diagnostics system.

Electron cyclotron heating (ECH) and pre-ionization using a 28 GHz 200 kW gyrotron were investigated in the JFT-2 tokamak. Three modes of the wave were launched from different types of launch system, i.e. the extraordinary mode from the high field side of the torus, the ordinary mode from the low field side, and TE 02 mode from the top to examine their heating characteristics. The oblique launch of the extraordinary mode gave the largest central heating in consistent with the theoretical prediction. It was shown that the polarization of the launched wave as well as the effect of the mode conversion at the upper hybrid resonance layer contribute to the heating mechanism.

Transitions of L-mode plasmas to the H-mode have been induced by an electron cyclotron heating (ECH) pulse. The transitions occur when ECH is applied to plasmas preheated either by a neutral beam or by waves in the ion cyclotron range of frequency with power levels well below their own threshold power for the H-mode transition. The position of the electron cyclotron resonance layer has been scanned and it has been shown that edge heating rather than central heating is effective in inducing the transition to the H-mode.

The H-mode transition is accompanied by the development of an electron temperature pedestal of about 0.3 keV and a dramatic steepening of the density gradient inside the separatrix. A change of density profile from peaked to flat is obtained for both H- mode and ohmically heated plasmas, after an internal disruption. Hollow ne profiels are found in low Bt discharges (low q). These phenomena indicate a reduction of the electron energy transport at the edge of the plateau region.

A mode conversion theory of the fast magnetosonic wave is compared with the experimental results in a deuterium plasma with minority hydrogen. The direct measurements of toroidal wave length, toroidal damping length, and the increase of ion temperature show that parameter dependences of ion heating are well explained by the mode conversion theory including tunneling, reflection, and absorption of the fast wave at a two-ion hybrid resonance layer.

Numerical studies on the stability of kink and resistive tearing modes in a current-carrying linear stellarator are presented for various current profiles and helical fields. In the case of an l = 2 helical field, the magnetic shear vanishes and the stability diagram is given by the straight lines with ισ + ιδ = const., where ισ is the rotational transform due to the plasma current and ιδ is that due to the helical field. For the l = 2 stellarator with , the magnetohydrodynamic stability against kink and tearing modes is improved compared to that in tokamaks. When an l = 3 helical component exists, magnetic shear plays an important role in the stability properties. The stability diagrams become fairly complex; they are, however, understandable from properties of the Euler equation. It should be noted that the internal kink modes are made more unstable than in a tokamak, because of the l = 3 helical field.

Deuterium and hydrogen energy spectra during ICRF heating as well as nH/nD 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.

ICRF heating experiments were carried out in the JFT-2 tokamak with pure high-field-side excitation. Almost pure electron and ion heating regimes were made possible in the experiment so that the metal impurities were reduced. Power balance of the plasma heated by ICRF is revealed by a transport analysis of the experimental results. RF power deposition profiles of each heating regime are obtained. The results of this analysis may serve as an experimental basis for refining the present theoretical treatment of ICRF heating.

It is observed that fast waves launched into an electron cyclotron heated plasma from a phased four-strip-line antenna array couple effectively with core electrons. This coupling produces enhanced electron cyclotron emission and a drop in the loop voltage. These effects depend on the phase difference between adjacent strip lines and on the electron density. An absorption efficiency of 20% is deduced under the conditions {ital v}{sub {ital ph}}/v{sub e0}{similar to}3 and {ital f}{sub 0}/f{sub LH}(0){similar to}0.38. The experimental results agree well with predictions based on electron Landau damping of fast waves.

Summary Form only given, as follows. The MTX program has focused on ohmic operation of the tokamak (Alcator-C), construction of the free-electron laser (FEL) microwave source and quasi-optical transport system, and preparation for the first measurements of absorption. The first microwave pulses from the FEL to the tokamak were planned for the spring of 1989. Measurements of the absorption of single, high-peak-power pulses (140 GHz, 2 GW, 20 ns) will follow. Thereafter the FEL will be upgraded to generate pulses at 5 kHz for 10 ms, providing an average power 0.5-1.0 MW, and then further upgraded to 250 GHz and 2 MW average power. Several diagnostics have been prepared for the initial microwave absorption experiments. These include microwave measurements, fast X-ray and cyclotron emission from the heated electrons, Thomson scattering from the tail, and fast magnetic probe measurements. Modeling of absorption and wave refraction have been done, and a computer database has been established for rapid analysis of the experimental results and comparison with theoretical predictions. Following the upgrade to multiple-pulse operation and subsequent absorption measurements, experiments on heating and transport will be carried out. Attention will be directed to heating in peaked density profiles generated by pellet injection and to the effects of varying the power deposition profile. >

The role of runaway electrons in the formation of arcing on the electrically isolated target in the scrape-off layer of tokamaks is discussed. Runaway electrons offer the sufficient sheath potential to produce an arc spot and supply the circulating current to sustain an arc in the current rising phase and in the disruptive phase of tokamak discharge.