H. Matsumoto

Confinement and fueling studies during additional heating phase in the JFT-2M tokamak

Increments of peripheral hydrogen/deuterium neutral gas pressures (PH2/PD2) during the additional heating phase (neutral beam and RF heatings) have been observed in a D2 gas-puff fueled JFT-2M tokamak with H2-absorbed graphite limiters/divertor plates. In the beam heating phase, a large increment of PH2 raises the plasma density 2 times or more without the degrading energy confinement time. The D2 gas-puff valve is closed in this phase. This improvement is interpreted as being due to “wall fueling”, i.e., fueling by desorbed hydrogen from the graphite wall by energetic particles. The first observation of the H-mode in the INTOR-type stubby open divertor with a short divertor channel (1–8 cm) at the high density regime (4–7 × 1013 cm−3) enhanced by the wall fueling is presented. The improved energy confinement time is comparable to or higher than that of ohmically heated discharges.

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.

A new mode of improved confinement in discharges with stationary density in JFT-2M

A new mode of improved energy confinement, for which the confinement time is not worse or is sometimes even better than that for the well known H-mode and the density of which is in a quasi-stationary state, has been obtained in neutral beam heating experiments on JFT-2M. The new mode is different from the H-mode in many respects. The central electron temperature is higher in the new mode than in the H-mode. Radiation loss and density are reduced in the peripheral region but not in the central region. Therefore, the density and radiation profiles are highly peaked in the new mode, in contrast to the broad profiles in the H-mode. Particle confinement in the peripheral region seems to be worse in the new mode than in the H-mode. The new mode can be obtained in both divertor configurations and limiter discharges in JFT-2M.

H-mode phenomena during ICRF heating on JFT-2M

Significant improvement of energy confinement has been observed on JFT-2M during ICRF heating. This improvement is preceded by a sudden drop in the Hα/Dα emission and a successive increase in stored plasma energy, electron density and radiation loss. This is believed to be the same phenomenon as the H-mode transition observed in ASDEX, and in PDX divertor experiments with neutral beam injection. However, in JFT-2M, this transition is observed both in limiter discharges and in open divertor configurations.

Impurity reduction during ICRF heating in JFT-2M tokamak

Reduction of impurity line emissions associated with the ion cyclotron range of frequency (ICRF) heating is achieved by the phase control of a loop antenna array. Reduction in metal impurity emissions and radiation loss is closely correlated with the amount of power radiated from the antennae with parallel wave number near k∥ = 0. The maximum density attainable without disruption is increased over that in the Ohmic heating phase, by reduction of radiation loss.

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.

Impurity behaviour during ICRF heating in JFT-2M

Abstract The impurity behaviour during the ICRF heating phase was studied in the JFT-2M tokamak. In this experiment, impurity behaviour during the ICRF heating phase was studied with special attention to the boundary electron temperature. From these experimental results, it was concluded that carbon (limiter material) impurity was released by an ion sputtering accelerated by the sheath potential. Metal impurities such as iron and titanium released from the antenna and/or near the antenna region are caused by some additional mechanism by applying the RF during the ICRF heating phase.

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.

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.