Motoyasu Sato

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.

Studies of particle behaviour in Heliotron E by means of balmer-alpha laser fluorescence spectroscopy

Measurements of atomic hydrogen density profiles in Heliotron E plasmas were performed, where attention was focussed on the anisotropy of the profiles in the poloidal cross section. Obtained penetration profiles of atomic hydrogen agreed well with the Monte Carlo code calculations of the penetration of Franck-Condon neutrals when averaged electron densities bar n e are larger than 10 19 m -3 . For bar n e <10 19 m -3 , however, experimental profiles decreased steeper than calculated ones. Degree of the anisotropy of the neutral profiles in the poloidal cross section was also found to change with the value of bar n e . Global particle confinement time τ p was evaluated from the measured atomic hydrogen profile. The results showed that τ p is arourud 20 ms for B =1 .9 T. In case of NBI plasma, τ p increases as increasing the magnetic field strength, while it decreases as increasing the absorbed power.

Spectrum Identification of Heliotron E Plasma Impurities by a Time-Resolving Grazing Incidence Spectrometer with a Multichannel Detector

A flat field vacuum ultraviolet spectrometer with a multichannel detector has been constructed to identify impurity spectral lines over the wavelength from 100 A to 400 A in Heliotron E plasmas. The resonance emissions of iron, titanium, chromium and nickel ions were identified in heated plasma by ion cyclotron range of frequency (ICRF) pulse. The lines were enhanced as ICRF power increased. The possible sources of these impurities are discussed briefly.

106-GHz electron cyclotron heating system for heliotron-E

A new high-power electron cyclotron heating system has been installed for the Heliotron-E helical device. This system is designed to operate at 106-GHz frequency with a half-megawatt output power. The system consists of a pulse gyrotron with TE[sub 12,2] whispering gallery mode (WGM) output, conversion system of the WGM into the Gaussian-like beam, transmission line for HE[sub 11] mode, and launching system. From measurement of radiation patterns, it was confirmed that the WGM was effectively converted into the Gaussian-like beam, and the emergent radiation profile from the tubular oversized corrugated waveguide was close to a circular Gaussian one even when the beam coupled to the HE[sub 11] mode had the side lobes before the transmission. This indicates that the oversized corrugated waveguides act as a mode filter. The launching system effectively focuses the Gaussian beam in the free space to a 2-cm (poloidal) [times] 3-cm (toroidal) e-folding power spot size. These are small enough compared with the plasma minor radius ([approximately]15 cm). It is expected that the power deposition can be well localized in the plasma central region. 21 refs., 10 figs.

Improvement of Magnetohydrodynamic Stability and Beta Limit by Toroidal Plasma Current in Heliotron DR

The effect of toroidal plasma currents on magnetohydrodynamlc stability has been studied for the currentless ECH plasmas (e=(2–3)×1019 m-3, Teo=300–400 eV, =0.35–0.5%) in Heliotron DR (R=0.9 m, ap~0.07 m, B=0.5–0.6 T). It is shown that small plasma currents (gtrsim500 A) which reduce vacuum rotational transform improve the MHD stability. The experimental result is explained theoretically by the change of the magnetic configuration due to the plasma current (position of =1 surface, shear, well).

Neutral Density Measurements by Charge Exchange Analysis on the Heliotron E Plasma

Measurements of neutral density in ECRH, Ohmically heated and neutral beam heated plasmas on the Heliotron E device are described. Profiles of neutral density determined by the vertically scanned charge exchange neutral flux are compared with the results of the Monte Carlo simulation. In Ohmic and ECRH plasmas, the obtained profiles agree well with the simulation results using Franck-Condon neutral (∼3 eV) as incident particles. In NBI case, the recycling particle with higher energy (typically ∼30 eV) dominates the particle source. The dependence of the neutral density on the average electron density is represented by the parameter I p . Although the neutral density in Ohmic and ECRH plasmas decreases as bar N e increases, NBI plasma does not have such dependence. The global particle confinement time is estimated, and τ p decreases as I p increases.

Thermonuclear fusion neutrons under “Currentless plasmas” in Heliotron E

Thermonuclear fusion neutron emissions of up to 3 × 109 neutrons/s have been observed in Heliotron E deuterium plasmas. “Currentless plasmas” were produced by electron cyclotron resonance (53 GHz, 150 kW) and further heated by hydrogen neutral beam injection (2.2 MW, H0→D+). It is found that all observed neutron emissions have thermonuclear origin due to the absence of hard X-ray background. Agreement between neutron and charge exchange ion temperature measurements (500 eV < Ti(0) × 900 eV) have been found at intermediate densities (1.5 × ne × 3 × 1013cm−3). The neutrons reported in this paper are the first observations of pure thermonuclear fusion neutrons in a helical heliotron plasma confinement device.

Ray Tracing Analyses of Current-Free ECRH Plasmas in Heliotron E

Ray tracing calculation of both the ordinary (O) mode and the extraordinary (X) mode using the cold plasma dispersion relation were carried out for the fundamental resonance (ω=ω ce ) and the second harmonic resonance (ω=2ω ce ) in a straight-heliotron configuration, where ω ce denotes electron cyclotron frequency. Absorption coefficients are evaluated by the optical thickness along the ray trajectories without relativistic effects. In Heliotron E, dominant contribution to the heating by electromagnetic waves launched from the low field side is the O-mode for ω=ω ce and the X-mode for ω=2ω ce , when single path absorption is significant and mode mixing at wall reflection is negligible. Good heating efficiency requires adjustment of the cylotron resonance at the magnetic axis, which is consistent with the experimental data.

Investigation of Electron Heating Due to the Kinetic Shear Alfven Wave in the Toroidal Plasma

Shear Alfven wave heating experiments are reported in which a bulk electron heating of the toroidal plasma for a high initial temperature plasma ( T e >50 eV) and a local electron heating for a lower temperature plasma were observed. The results indicate the excitation of the kinetic shear Alfven wave and are consistent with the theoretical calculation of the damping length of the wave.

The Sources of Fe Impurity in Heliotron E Plasma during NBI

The sources of iron impurity in the Heliotron E plasma during NBI are investigated. When ne is smaller than 1×1019 m-3, the main mechanism of iron impurity production is sputtering of the beam dump by a high energy oxygen beam which is included in the neutral beam. The iron impurity is reduced by half after Ti-gettering at the ion source of the NB injector. When ne is larger than 3×1019 m-3, the main impurity source is the plasma-wall interaction, and this impurity is not reduced by the Ti-gettering.