Y. Fukagawa

Experimental study of plasma breakdown by second harmonic electron cyclotron waves in Heliotron J

Second harmonic plasma breakdown using electron cyclotron (EC) waves has been studied experimentally in the helical-heliotron device Heliotron J. The magnetic field and the injected EC beam parameters such as power, polarization and injection angle are scanned. The experimental results show that the plasma starts up around the crossing point between the injected EC beam and the resonance layer. The initial plasma moves as the crossing point is shifted. The breakdown is earliest when the beam crosses the resonant layer around the magnetic axis with the X-mode polarization. These results indicate that the single pass absorption has the dominant role on the breakdown rather than the multi-reflection absorption, although the linear absorption is quite low. The bumpiness component in the magnetic field spectrum is also scanned from negative to positive values with the resonance located on-axis. The breakdown is most delayed when the bumpiness component is zero, suggesting that the confinement of accelerated electrons is important.

Observation of H-mode operation windows for ECH plasmas in heliotron J

Abstract The H-mode transition properties of 70-GHz, 0.4-MW electron cyclotron heating (ECH) plasmas in Heliotron J have been studied with special reference to their magnetic configuration dependences, such as the edge iota dependences. Two edge iota windows for the H-mode transition were observed to be (a) 0.54 < ɩ(a)/2π < 0.56 in separatrix discharge plasmas and (b) 0.62 < ɩ(a)/2π < 0.63 in partial wall-limiter discharge plasmas if a certain threshold line-averaged electron density ([overbar]ne = 1.2-1.6 × 1019 m-3) is achieved, where ɩ(a) is the vacuum edge iota value and a is the plasma minor radius, respectively. A strong dependence of the quality of the H-mode on the edge topology conditions was revealed. The energy confinement time for the separatrix discharge plasmas was found to be enhanced beyond the normal ISS95 scaling in the transient H-mode phase, being 50% longer than that in the “before transition” phase. The window characteristics are discussed on the basis of the calculated geometrical poloidal viscous damping rate coefficient in a collisional plasma, indicating that the behavior of the viscous damping rate coefficient alone could not explain the observed characteristics. The bootstrap current properties of ECH plasmas and the relevant electron cyclotron current drive experimental results are also discussed.

Plasma Confinement Characteristics in Heliotron J—Spontaneous Change of Plasma Confinement State

Spontaneous transition of the plasma confinement mode was observed in the helical-axis heliotron device Heliotron J for three different plasma heating schemes, i.e. ECH-only, NBI-only and the combination of ECH and NBI. The transition seems to occur above a certain critical density. In addition to the confinement transition, a spontaneous shift of the hitting position of the divertor plasma flux on the wall was observed. This shift could be related with the change of the edge field topology caused by non-inductive toroidal currents.

A 70-GHz Electron Cyclotron Resonance Heating System for Heliotron J

Abstract A 70-GHz electron cyclotron resonance heating (ECRH) system has been constructed in a helical-axis heliotron device, Heliotron J, in order to realize localized heating and current drive experiments. Since the Heliotron J plasma has a three-dimensional complex shape, the ECRH system is designed to satisfy the requirement of wide steering capability in both the toroidal and poloidal directions. The low-power transmission test shows that the beam radius of the focused Gaussian beam is 22 mm at the magnetic axis, which is small enough compared to the averaged minor plasma radius (170 mm), and the launching system covers a wide toroidal steering range from perpendicular to tangential injection by replacing the steering plane mirror. Since these characteristics satisfy the condition for controlling the power localization in the three-dimensional helical-axis configuration, it is possible to explore the on- and off-axis heating over most of the plasma radius (0 < r/a < 0.7) and the electron cyclotron current drive. In the high-power transmission test, the transmission efficiency of the 20-m corrugated waveguide is 92%, and the available output power to the vacuum vessel is up to 0.4 MW. Plasma production and heating are successfully performed using this ECRH system.

Spectroscopic study of impurity behavior in heliotron-J plasmas

Abstract Behavior of intrinsic and injected impurities has been investigated in Heliotron-J plasmas by spectroscopic methods. Intrinsic impurities are identified with the vacuum ultraviolet grazing incidence spectrometer in neutral beam injection (NBI)-heated plasmas. Na-like Ni XVIII and Mg-like Ni XVIII are observed only in NBI heating phase. Helium gas is injected into electron cyclotron heating plasmas. In the density scan experiments, He II line intensities, which are normalized by the electron density, increase with decreasing electron density. For intrinsic impurities, similar dependence of line intensities on the electron density is observed. The normalized line intensity indicates the particle number of ions penetrated into the core plasma. In addition, the edge electron density is in proportion to the core electron density. These results may reflect the screening effect due to electron collisional ionization at the edge plasma. In the carbon limiter insertion, the CH radical band spectrum is observed. The carbon limiter head is formed in the hemisphere. The spatial distribution of the band emission is asymmetrical to the main axis of the limiter head. A good agreement is obtained between the spatial distribution of emissions of the band spectrum and the camera image with bandpass filter.