H. Aikawa

Observation of very dense and cold divertor plasma in the beam-heated doublet III Tokamak with single-null poloidal divertor

A Langmuir probe array in the divertor plate has been used to investigate the dense, cold divertor plasma associated with remote radiative cooling in neutral-beam-heated, single-null open-divertor discharges in Doublet-Ill. With injected powers of up to 1.2 MW, the divertor plasma becomes denser and colder as the main plasma line-averaged density e increases, reaching ned= 2.8 X 1014 cm−3. Since the electron temperature drops to Ted = 3.5 eV under these conditions, this cold, dense plasma can provide a solution to the problem of wall erosion.

Effects of ergodization on plasma confinement in JFT-2M

A steady-state H-mode plasma with frequent ELMs (edge localized modes) was obtained by applying magnetic fields with high poloidal mode numbers resonant in the plasma edge. The induced ELMs limit the density and impurity accumulation normally observed during ELM-free H-mode. An EML coil with numerous resonant modes ( m 10) in the plasma edge appears the most effective. This method of control is extended to higher auxiliary heating power by increasing the magnitude of the magnetic perturbations.

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.

Head load reduction of the divertor plate by remote radiative cooling in D-III beam-heated divertor discharges

Abstract The heat load on the divertor plate has been measured by a 28 thermocouple array and an infrared camera both measuring the divertor area. The sum of the radiative power from the main plasma and the divertor along with the power to the divertor plate accounts for ∼ 80% of the input power. The radiative power from the divertor plasma increases as the density of the main plasma increases, which results in the reduction of the heat load on the divertor plate by 50%. This result is obtained with a neutral beam injection power of 2 MW.

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.

Divertor plasma modification by divertor biasing and edge ergodization in JFT-2M

Abstract 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.

Edge plasma characteristics during ICRF heating on JFT-2M Tokamak

Abstract 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.

Pump limiter experiment on the JFT-2M tokamak

Abstract 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.

Limiter damage due to over-heat load in doublet III tokamak

Abstract Two tiles of the primary limiter, which are composed of TiC coated Poco graphite, are heavily cracked during neutral beam heating experiment (beam power of 2 MW). These damages are considered to be caused by the thermal stress due to the over-heat load (2.3 kW/cm2). The major disruption occurs after a rapid increase of the impurity influx following to the tile cracks. A base temperature of the limiter tile is over 300°C at the time of this accident. These kind of accidents are avoidable with a careful monitoring of the limiter temperature.

Langmuir probe measurements in beam-heated divertor discharges in D-III

Abstract A Langmuir probe array and a thermocouple array installed in the divertor plate were used to measure the edge properties of 4 MW beam-heated divertor plasmas in D-III. Particle recycling near the divertor plate in high-recycling (poor confinement) discharges was approximately twice that of low-recycling (good confinement) discharges. Some low-recycling discharges exhibit ∼ 30% higher edge temperatures than high-recycling discharges. The strong particle shielding of the divertor, however, is capable of making the divertor plasma dense and cold (Te≲ 20 eV, ne ∼ 1014cm−3) while keeping the recycling near the main plasma low enough to maintain good energy confinement.