T. Tetsuka

Observation of lower-hybrid-current drive in the JIPP T-II torus

Current driven by injecting lower hybrid waves has been observed in low-density plasmas in the JIPP T-II. It is confirmed that RF-driven current is generated by momentum transfer from lower hybrid waves to suprathermal electrons with an energy of 8–25 keV. The driving efficiencies in tokamak and stellarator configurations are 1 kA·kW−1 and 0.3 kA·kW−1, respectively, at a power level of 40 kW. Enhanced electron cyclotron emission due to pitch-angle scattering of RF-driven suprathermal electrons is observed, and spikes in loop voltage and X-ray bursts appear coincidentally. In a long RF-pulse, these rapid changes advance to relaxation oscillations. It is concluded that the pulsating changes originate in the instantaneous scattering of RF-driven suprathermal electrons by the unstable waves excited at an anomalous Doppler resonance.

Wafer-Voltage Measurement in Plasma Processes by Means of a New Probe Method and an Impedance Monitor

A new probe method, which can measure wafer voltage directly from the wafer back-side without fluctuating the plasma, was developed. This method clarified the effect of the bias power, gas pressure and gas component on wafer Vpp, Vdc, and Vdc/Vpp. In addition, current, voltage, and phase shift were measured with an impedance monitor (IM) installed between the matching network and the bias electrode. Furthermore, wafer Vpp was calculated from IM data according to a devised equivalent circuit model. It was found that calculated wafer Vpp agrees with actual wafer Vpp within 3.4% when bias power is ranged from 200 W to 500 W.

HCN laser scattering on the JIPP T-IIU tokamak

AbstractA 4-channel HCN laser scattering system has been developed and applied to the JIPP T-IIU tokamak. Main aim of this system is to study microturbulence excited in the plasma, especially in the high power ICRF heating experiment. The observed density fluctuations are in a frequency range of the electron diamagnetic drift wave and have broad frequency spectra. The main part of the wavenumber is found to satisfy the condition of

Impurity origin during ICRF heating in the JIPP T-IIU tokamak

k_ \bot \rho _i \lesssim 1

Measurement of Small-Scale Density Fluctuation in JIPP T-II Plasma by Millimeter and Sub-Millineter Wave Scattering

. In the heating experiments, deterioration of the energy confinement time is observed, and the density fluctuation level increases with the increase of the heating power.

Observation of asymmetric power spectra of density fluctuations in the JIPP T-IIU plasma by far-infrared laser scattering

The paper describes the characteristic behaviour of lower hybrid current startup in JIPP T-IIU. The current startup is carried out by the injection of 800 MHz lower hybrid waves into cold and low density plasmas (Te = 10 − 20 eV, e = (1−2) × 1012 cm−3 produced by electron cyclotron resonance or lower hybrid waves (LHW) only. The plasma current rises with a characteristic rise-time of τr ( 30-50 ms) and approaches a quasi-steady state value, Ipm (= 5-20 kA), whereupon 10-50 kW LHW power is injected into the torus, controlling the vertical field. The rise-time is inversely proportional to the bulk electron density, ne, and is comparable to the collision time of current carrying high energy electrons with the bulk plasmas. On the other hand, the current drive efficiency in the quasi-steady state is almost independent of e, i.e. Ipm/PLH = 0.4−0.7 AW−1 for e = (0.8−4) × 1012 cm−3. The conversion efficiency of RF energy injected into the torus is typically 5% during the current rise phase and 10% in the most efficienct case. The effects of the initial injection of ECH power and the observed parametric instabilities on the current startup are investigated from the viewpoint of seed current generation. During the rapid current rise when an appreciably negative loop voltage is observed, the bulk electrons are heated up to 150 eV. Various heating mechanisms responsible for the bulk electron heating are discussed.

ICRF heating experiments on JIPP T-II

Replacing stainless steel limiters by graphite limiters, we found that radiations from iron and titanium ions were significantly reduced. Total radiation and loop voltage also decreased. This indicates that the limiters are the major impurity sources both in the ohmic and RF heating phases. Although titanium radiations increased with RF power injected by an antenna with a titanium Faraday shield, the maximum intensity was much smaller than in the experiment where the titanium-flashed stainless steel limiters were used. Thus it has been found that the Faraday shield is less important as an impurity source than limiters. Toroidal asymmetry observed for O II radiation suggest that the energetic charge-exchange neutrals play a role in releasing oxygen from the wall and that those energetic particles are relatively abundant in the toroidal sections near the antenna. The Hα + Dα radiation decreases during the RF pulse around the limiter, which may be due to the change in hydrogen/ deuterium recycling at the limiter. The reduction of Hα + Dα is greater with graphite limiters than with stainless steel limiters. The relation between recycling and impurity release is briefly discussed.

Observation of Mode-Converted Ion Bernstein Wave by an HCN Laser Scattering

Fundamental characteristics of a GaAs Schottky barrier diode mixer have been measured for the purpose of using it as the detector of a twin 118.8 µm CH3OH laser interferometer. The mixer sensitivity for beat oscillations of the twin lasers was measured in a frequency region from 1 MHz to 7 MHz. A signal to noise ratio of I.F. signal over 45 dB is obtained for the frequency up to 6 MHz. The dependence of the mixer sensitivity on local beam power was also measured. The I.F. signal level is approximately proportional to the local beam power in power level from 0.1 mW to 1.2 mW.