Toshihide Ogawa

Compact toroid injection experiment in JFT-2M

Compact toroid (CT) injection experiments with Hxa0mode plasmas were carried out for the first time in the JFT-2M tokamak. The soft Xxa0ray emission profile shows deep CT penetration into the core region of Hxa0mode plasmas heated by 1.2xa0MW NBI as well as in OH plasmas, with a toroidal magnetic field of 0.8xa0T. The line averaged electron density rapidly increased by Δe ≈ 0.2 × 1019xa0m-3 at a rate of 4 × 1021xa0m-3·s-1 in Hxa0mode and the fuelling efficiency was roughly 20%. An asymmetric radial profile in the soft Xxa0ray emission was produced for ~50xa0μs by deep penetration of the CT.

Measurement of peripheral electron temperature by electron cyclotron emission during the H-mode transition in JFT-2M tokamak

Time evolution and profile of peripheral electron temperature during the H-mode like transition in a tokamak plasma is measured using the second and third harmonic of electron cyclotron emission (ECE). The so called “ H-mode ” state which has good particle/energy confinement is characterized by sudden decrease in the spectral line intensity of deuterium molecule. Such a sudden decrease in the line intensity of D α with good energy confinement is found not only in divertor discharges, but also in limiter dischargs in JFT-2M tokamak. It is found by the measurement of ECE that the peripheral electron temperature suddenly increases in both of such phases. The relalion between H-transition and the peripheral electron temperature or its profile is investigated.

Spectroscopic method to directly measure electric field distribution in tokamak plasma edge

A supersonic helium beam source operated in pulse mode was constructed for direct measurement of electric field distribution in the tokamak plasma edge region with the aid of laser-induced fluorescence (LIF) technique. In this technique only the polarization has to be observed of a LIF resulting from a laser-excited forbidden transition due to the Stark effect and the electric quadrupole to determine the electric field strength. No calibration is needed of the absolute intensity of LIF and tunable laser used. The helium atom beam density was obtained (about 1020 He atoms cm−2u2009s−1) at a distance of 7 cm from the pulsed nozzle. A model-type experiment to make clear the influence of a magnetic field on the LIF is reported. Design study was also made to install the supersonic beam and spectroscopic measurement system on a medium size tokamak.

Enhancement of Wall Fueling and Improvement in nτ of a Beam-Heated H-Mode Discharge by Electron Cyclotron Wave in the JFT-2M Tokamak

The additional increases both in the density n and confinement time τ of the beam heated H-mode discharges are observed ( n τ value is improved by about 40%) when an electron cyclotron wave (ECW) resonating near the plasma boundary is applied. The ECW is shown to enhance wall fueling, to decrease the minimum heating power for switching the H-mode on or to extend the operational regime on n -τ diagram of the H-mode discharges.

Numerical investigation of a neutral helium beam as a diagnostic probe in the plasma edges

Abstract We have made a simulation study of the interaction of a He neutral beam and the JFT-2M tokamak plasma by using a set of coupled rate equations. Our objective is to develop a spectroscopic diagnostic based on the He neutral beam to study plasma parameters in the edge region of the tokamak plasma. One of the important issues is the spatial evolution of the metastable helium. Numerical results indicated that the He metastable density was large enough for the measurements in the interaction region, and that the several lower level populations (2 1 P, 3 1 D, etc.) were almost linearly proportional to the plasma density. Hence it is possible to measure the plasma density by observing the emission line originating from the levels. We have also studied the forbidden excitation by a tunable laser (Stark effect) to measure the electric field induced in the plasma edge region.

Impurity Behavior during the H-Mode in JFT-2M

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.

Hollow Density Profile on Electron Cyclotron Resonance Heating JFT-2M Plasma

The first hollow electron density profile in the central region on the JAERI Fusion Torus-2M (JFT-2M) is measured during electron cyclotron resonance heating (ECRH) with a TV Thomson scattering system (TVTS). The peripheral region is not hollow but is accumulated due to pump-out from the central region. The hollowness increases with time but is saturated at ~40 ms and maintains a constant hollow ratio. The hollowness is strongly related to the steep temperature gradient of the heated zone.

Supersonic helium beam for measurement of electric field in torus plasma edges

Abstract Numerical investigations on the basis of a collisional-radiative model have been made on the measurement of the electric field in torus plasma edges using a supersonic helium beam with aid of laser excitation. In this simulation, the laser-induced fluorescence intensity of the He I 492.2 nm line associated with the forbidden excitation due to the Stark effect is estimated as a function of electric field strength. The results show that an electric field as low as 50 V cm −1 can be measured by the supersonic beam with sufficient beam density (about 10 12 cm −3 ). A numerical discussion is also given on the measurement of the electron temperature and density.

Ion Bernstein Wave Experiment on JFT-2M Tokamak

The ion Bernstein wave (IBW) is launched into the JFT-2M tokamak plasma in order to investigate the antenna loading resistance and the heating effect. Antenna loading resistance increases as the scrape-off electron density increases, in the same manner as that of fast wave antennas. Although the hydrogen ion temperature increases due to the IBW, most of the wave power is absorbed by the peripheral electrons. This is mainly due to the antenna location, which is at an oblique angle to the midplane. Peripheral electron heating induces the enhanced radiation loss, and IBW power of only about 120 kW can be launched because of the radiation limit.

OBSERVATION OF DIVERTOR PLASMA DURING H- AND L-MODE OPERATION IN JFT-2M TOKAMAK

The density and temperature profiles in front of the divertor plates were measured using Langmuir probe array installed in the divertor plates in JFT-2M tokamak. The experimental results showed that the direction of toroidal field affected on this electron temperature profile. For the upper single null plasma with CCW B T (B T is counter clock wise direction from the top), the electron temperature at the outer side throat is about two times larger than that at inner side one. This observation is consistent with the simulation results using 2-D transport code that takes account of the thermal flow due to the B x ∇T i term in neoclassical theory.