K. Ohasa

Transition from the L-mode to the H-mode by electron cyclotron heating of a tokamak edge plasma

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.

Carbon wall experiment in diva

Abstract The sputtering characteristics of various types of carbon limiter surfaces, e.g. pyrolitic graphite (PG), pulverized carbon film and carbon film produced by methane discharges, are tested. Arcing is only observed on the pulverized carbon surface in a stable discharge or the other surfaces in an unstable discharge. Ion sputtering is shown to be the dominant process of carbon efflux from the normal surfaces. Employing PG limiters and a carbon wall coated by rf-sputtering method, very low q plasmas with a good confinement characteristic are obtained. The coated carbon surface is pure and still not contaminated after 600 or more discharges.

Ion sputtering, evaporation and arcing in DIVA

Abstract An experimental study of the metal impurity origin in DIVA is described. Three processes for the release of the metal impurities, that is, ion sputtering, evaporation and arcing have been identified. Among these processes, ion sputtering is the dominant process in the quiet phase of the discharge, that is the phase characterized by no spikes in the loop voltage and no heat flux to a specific part of the first wall. Moreover it is demonstrated that a honeycomb structure can decrease the release of the metal impurity. This paper is being published separate [1].

Magnetohydrodynamic activity in the JFT-2 tokamak with high-power neutral-beam-injection heating

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.

Divertor experiment for impurity control in DIVA

Abstract Divertor actions for controlling the impurity origin and penetration into the main plasma were investigated and demonstrated in DIVA.

Absorption of fast waves excited by a phased four-loop antenna array in the JFT-2M tokamak

The absorption characteristics of fast waves excited by a phased four-loop antenna array have been investigated experimentally in JFT-2M. The frequency of the fast waves is 200 MHz, which corresponds to approximately the tenth harmonic of the ion cyclotron frequency of hydrogen. The fast wave power is absorbed mainly by the bulk thermal electrons. It is shown that the absorption efficiency of the excited fast waves is improved with increasing density and temperature, and with decreasing phase velocity of the fast wave. The results are consistent with the theoretical predictions obtained from ray-tracing calculations. The power deposition profile is obtained through synchronous detection of the electron cyclotron emission modulated by a periodic heat source. In this modulation experiment with a limiter plasma on JFT-2M the electron thermal diffusivity is 2-3 m2s−1 and the convection velocity is 20-40 ms−1 at e = 2 × 1019 m−3 and Ip = 230 kA. The resultant power deposition profile has a peak at the plasma centre and agrees well with that calculated with the ray-tracing code. The absorption efficiency calculated from the power deposition profile is 0.3-0.4 at e = 2 × 1019 m−3 and Te0 = 0.7 keV, which agrees roughly with that estimated from the initial rise of the plasma stored energy. The electron heating efficiency estimated from the absorption efficiency is (4–5) × 1019 eVm−3kW−1 and the incremental confinement time is 8-10 ms, which is slightly longer than that in L-mode plasmas heated by neutral beam injection and/or ion cyclotron heating in JFT-2M.

Coupling of fast waves launched into the JFT-2M tokamak by a phased four-loop antenna array

The coupling characteristics of the fast wave excited by a phased four-loop antenna array are described. Fast waves in the lower hybrid frequency range have the potential of generating plasma current in hot and dense plasmas. Fast waves are excited at the plasma edge by an oscillating magnetic field parallel to the toroidal direction. The parallel wavenumber of the excited fast waves is determined by the relative phase of the RF current on each antenna. The loading resistance of the antenna increases with density, but at densities above 2 × 1019 m−3 it starts to decrease with density, because of the steepening of the density gradient. The loading resistance is strongly dependent on the antenna phasing. The maximum loading due to surface wave excitation is obtained at Δ = 0 and the minimum at Δ = π. Substantial absorption of the excited fast waves is observed at the plasma centre when the antenna phasing is Δ = π. The absorption efficiency rises with decreasing phase velocity of the excited waves. The experimental results obtained in the coupling experiment are consistent with theoretical predictions.

Electron Cyclotron Heating and Pre-Ionization in the JFT-2 Tokamak

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.

High-efficiency ICRF heating in DIVA

High-efficiency ICRF heating in DIVA is studied. At least 80% of the RF power coupled to the plasma is deposited with the bulk ions in the narrow central region, and about 70% of the deposited power is absorbed by the majority deuterium. The ion temperature increases up to three times the base temperature and thus exceeds the electron temperature.

Characteristics of the edge localized mode in the JFT-2M H-mode

The characteristics of the edge localized model (ELM), also referred to as edge relaxation phenomenon, were investigated in H-mode plasmas of JFT-2M. It was found that the ELM is mainly a density fluctuation phenomenon at the edge and that the electron temperature at the edge, except near the separatrix, is not very strongly perturbed. Some experimental conditions controlling the ELM intensity are plasma density, plasma ion species, heating power and plasma current ramping. ELMs found in low density deuterium discharges are suppressed by raising the density. ELMs are more strongly pronounced in hydrogen plasmas than in deuterium plasmas. ELMs seen in hydrogen plasmas or in near marginal H-mode conditions are suppressed by increasing the heating power. ELMs are found to be suppressed by plasma current ramp-down whereas they are enhanced by current ramp-up. The MHD aspects of the ELM were investigated. Although no consistent MHD features of the ELMs were found, they sometimes trigger bursts of the m = 2 mode. The enhanced broadband magnetic fluctuation suggests a temporary return to the L-mode during the ELM event.