T. Senju

First plasmas in Heliotron J

Results obtained in the initial experimental phase of Heliotron J are reported. Electron beam mapping of the magnetic surfaces at a reduced DC magnetic field has revealed that the observed surfaces are in basic agreement with the ones calculated on the basis of the measured ambient field around the device. For 53.2 GHz second harmonic ECH hydrogen plasmas, a fairly wide resonance range for breakdown and heating by the TE02 mode has been observed in Heliotron J as compared with that in Heliotron E. With ECH injection powers up to ≈ 400 kW, diamagnetic stored energies up to ≈ 0.7 kJ were obtained without optimized density control.

H-mode confinement of Heliotron J

The L–H transition in a helical-axis heliotron, Heliotron J, is investigated. For electron cyclotron heating (ECH), neutral beam injection (NBI) heating and ECH + NBI combination heating plasmas, the confinement quality of the H-mode is examined with an emphasis on its magnetic configuration dependence. The vacuum edge rotational transform, ι(a)/2π, is chosen as a label for the magnetic configuration where ι/2π is the rotational transform and a is the average plasma minor radius in metres. The experimental ι(a)/2π dependence of the enhancement factor over the L-mode confinement reveals that specific configurations exist where high-quality H-modes (1.3 < HISS95 < 1.8) are attained. is the experimental global energy confinement time and is the confinement time scaling from the international stellarator database given as . R is the plasma major radius in metres, is the line-averaged plasma density in 1019 m−3, PL is the power loss in megawatts that accounts for the time derivative of the total plasma energy content and Bt is the toroidal magnetic field strength in tesla (Stroth U. et al 1996 Nucl. Fusion 36 1063). The ι (a)/2π ranges for these configurations are near values that are slightly less than those of the major natural resonances of Heliotron J, i.e. n/m = 4/8, 4/7 and 12/22. To better understand this configuration dependence, the geometrical poloidal viscous damping rate coefficient, Cp, is calculated for different values of ι(a)/2π and compared with the experimental results. The threshold line-averaged density of the H-mode, which depends on the configuration, is in the region of 0.7–2.0 × 1019 m−3 in ECH (0.29 MW) + NBI (0.57 MW) operation. As for the edge plasma characteristics, Langmuir probe measurements have shown a reduced fluctuation-induced transport in the region that begins inside the last closed flux surface (LCFS) and extends into the scrape-off layer. In addition, a negative radial electric field Er (or Er-shear) is simultaneously formed near the LCFS at the transition.

Recent Heliotron E physics study activities and engineering developments

Recent studies of transport, magnetohydrodynamic stability, and divertor action on Heliotron E are summarized. A pellet injector and a new diagnostic system are developed. Moreover, the Heliotron groups is conducting research and development on heating and other new systems for the Large Helical Device.

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.

Gas analysis during carbonization and hydrogen recycling control by helium discharges in Heliotron-E

Abstract A gas analysis station (GAS) with a quadrupole type gas analyser has been installed on the vacuum pumping chamber of Heliotron-E. The GAS was used to observe the partial pressures of methane and hydrogen during carbonization and subsequent decarbonization of the vacuum wall. From the changes in partial pressures both the carbon deposition rate on the wall and the erosion rate of the carbon film were estimated. In plasma density control studies the GAS was used to investigate hydrogen recycling during conditioning of the carbon wall with both DC-glow and main discharges in helium. The time evolution of the partial pressure of hydrogen induced by discharges in helium indicates that three effects are involved in the recycling process: immediate desorption of hydrogen from the surface of the vacuum wall, readsorption of the desorbed hydrogen, and moderate desorption of hydrogen diffusing from the sub-surface.

Configuration Control for the Confinement Improvement in Heliotron J

Abstract In the helical-axis heliotron configuration, bumpiness of the Fourier components in Boozer coordinates is introduced to control the neoclassical transport. The bumpiness helps not only to align the mod-Bmin contours with the magnetic flux surfaces but also to control the balance of bootstrap currents due to helical and toroidal ripples. Effects of bumpiness control on the plasma performance (noninductive currents, fast-ion behavior, and global energy confinement) have been investigated in Heliotron J by selecting three configurations with different bumpiness ([curly epsilon]b = B04/B00 = 0.01, 0.06, and 0.15 at ρ = 2/3) but almost the same edge rotational transform and plasma volume. The dependence of noninductive toroidal currents is qualitatively consistent with the neoclassical prediction for the bootstrap current. The high-bumpiness configuration seems to be preferable for the confinement of fast ions. However, the longer global energy confinement time is not observed in the highest-bumpiness configuration ([curly epsilon]b = 0.15). When the dependence of the effective ripple modulation amplitude in International Stellarator Scaling 04 scaling is examined, the experimental results show that the normalized global energy confinement time seems long in the configuration with the minimum effective ripple modulation amplitude, where [curly epsilon]b is 0.06.

Study on edge plasma physics and particle control in the Heliotron-E device

The edge plasma physics and the particle control under the intrinsic magnetic limiter configuration of a helical system have been studied with the Heliotron-E device, where currentless plasmas of Te ≤ 1–2 keV, Ti ≤ 1 keV and ne ≤ 2 × 1020/m3 are produced by a combination of ECRH, NBI and/or ICRH. It is indicated that the separatrix region of the heliotron device is able to act as a divertor magnetic field. According to calculations of the magnetic field line in the edge region, the separatrix region has some different characteristics from the scrape-off layer in tokamak devices; the existence of a fine structure in the separatrix region and asymmetry of the region in toroidal and poloidal directions are observed. A localized pattern of the heat load on the first wall is experimentally observed. This agrees with the heat-load profile expected from the magnetic configuration and the distribution of the plasma in the edge region. A carbonization of the first wall is successfully applied to the Heliotron E device for reduction of metallic impurity contents. The heat load at the divertor trace decreased and that on the other part of the first wall increased in the high recycling conditions after the carbonization.

Effects of carbon wall on the behavior of Heliotron-E plasmas

Abstract Carbonization was successfully applied to Heliotron-E. Iron-inpurity radiations were strongly reduced with the carbonized wall. Main impacts of the metal reduction on plasma behaviors are sustainment of stored energy during high power, long pulse heating by NBI, achievement of a quasi-steady discharge with a low helical field and high beta, and highest electron density with pellet injection in a quasi-steady state. Hydrogen recycling was very high with the carbonized wall and low density operation was impossible. Helium glow discharge was found to be effective to control the hydrogen recycling with a carbon-tiled wall.

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.

Electron temperature measurements by TV Thomson scattering in Heliotron E

Abstract A TV Thomson scattering system, which consists of three framing cameras with image intensifiers and cooled charge coupled device cameras, has been developed on Heliotron E with a minimum spatial resolution of 1 cm. The quantum efficiency of the framing camera is 6.5% at 700 nm. The scattered light is detected as an image of 1000 × 1000 pixels. The number of spatial channels is equal to the number of divided regions of this image. The calibration methods such as the spectral sensitivity, plasma light, stray light and so on are established. The examples of the detected data and the measured T e profiles are shown.