Y Kurimoto

Impurity behavior in boronized Heliotron E

Abstract Changes of impurity behavior and plasma performances by a boronization were investigated in Heliotron E. A 2.45 GHz ECH boronization with helium and decaborane at B=0.046 T was developed. The concentration of H/B atomic ratio in the deposited boron layer was much smaller than that of conventional dc glow discharges and the typical value was 0.1. The light impurity, oxygen and carbon, decreased drastically, but the reduction of the metallic impurity, iron and chromium, was not observed. A small H/B atomic ratio and low levels in the light impurity provided high ion temperature and low density plasma by neutral beam injectin heating.

Behavior of pellet injected Li into Heliotron E plasmas

Abstract Li pellet injection has provided a complex plasma with a large fraction of Li ions, which is characterized by intense emissions from Li I and III. The spatial profiles of the fully ionized Li 3+ ions are measured by charge exchange recombination spectroscopy with a resolution of 13 mm and the local decay time of the injected Li ion has been estimated. The spectral profile of the charge exchange recombination line of Li III from n = 5 to n = 4 shows a complicated structure, which depends on Li 3+ density. The effects on other intrinsic impurities and recycled Li are also discussed.

Formation and termination of high ion temperature mode in heliotron/torsatron plasmas

The physics of the formation and termination of high ion temperature mode (high mode) is studied by controlling density profiles and radial electric field. High ion temperature mode is observed for neutral beam heated plasmas in heliotron/torsatron plasmas (Heliotron-E). This high mode plasma is characterized by a peaked ion temperature profile and is associated with a peaked electron density profile produced by neutral beam fuelling with low wall recycling. This high mode is terminated by flattening of the electron density caused either by gas puffing or by second-harmonic ECH (core density `pump out).

Dipole moment of the Pfirsch-Schluter current in a finite beta stellarator plasma in Heliotron E

A magnetic determination of the dipole moment of the Pfirsch-Schluter current in a finite beta stellarator/heliotron plasma is described. The rotating cosine saddle diagnostic coils and the poloidal flux loops are used to identify the dipole current in a finite beta toroidal plasma. A typical dipole moment of the toroidal current in the standard Heliotron E configuration is 0.6 kA m, when the volume averaged beta is 0.50% at a magnetic field of 0.94 T. The measured equilibrium dipole current is practically proportional to the diamagnetic volume average beta available up to 0.95%. The observed dipole moment of the Pfirsch-Schluter current is compared with a magnetohydrodynamic (MHD) calculation for a finite beta stellarator plasma

Experiments on Li pellet injection into Heliotron E

Li pellets of large size were injected into electron cyclotron resonance (ECR) heated plasmas and neutral beam injection (NBI) heated plasmas of Heliotron E. The discharge behaviour, pellet ablation and wall conditioning were studied. The electron pressure is doubled after injection into the NBI plasma and remains unchanged in the case of ECR heating. This may be due to the energy exchange between the electrons and thermal ions with the fast ions from the neutral beam. The observed discrepancy between the experimental and modelled ablation rates may be caused by both the plasma cooling due to pellet ablatant and the ablation stimulated by the fast ions in the NBI-heated regime and by the fast electrons in the ECR-heated regime. In preliminary experiments on wall conditioning by Li pellet injection, no improvement of plasma performance after Li pellet injection was observed in the divertor or limiter configuration, with the limiter radii .

ECE diagnostic using multi-channel radiometer in Heliotron-E

Abstract Electron cyclotron emission (ECE) is measured with a multi-channel radiometer system in Heliotron-E. The system detects the second harmonic emission from 67 to 115 GHz at the magnetic field B = 1.9 T, covering the plasma confinement region up to the last closed flux surface. The radial and temporal evolution of the electron temperature has been obtained for electron cyclotron heated (ECH) and neutral beam injected (NBI) plasmas. Nonthermal electrons are observed at the plasma breakdown, and the nonthermal electron energy is estimated from the frequency downshift. We also discuss the effect of the magnetic shear on the second harmonic electron cyclotron wave propagating through the plasma with a strong magnetic shear such as heliotron/torsatron configurations.

Transition phenomena observed during edge electron cyclotron heating in the Heliotron-E helical device

Abstract An edge electron cyclotron heating (ECH) experiment has been performed on the Heliotron-E helical device using 35 GHz and 53 GHz gyrotrons. Transition phenomena have been observed during the 53 GHz edge ECH. The Hα emission and the outflux to the wall dropped, and then the core electron density increased by 20%–60%. The electron stored energy seems to be improved up to 20% after the transition. This phenomenon is found to depend strongly on the edge heating power, the target plasma density and the limiter position. A possible mechanism is discussed in terms of high energy electrons produced by the edge ECH.

Study of the Fast Ion Slowing–Down Process and ECRH Effects in Heliotron E

In the Heliotron E device, combining a neutral beam injection (NBI) with an electron cyclotron resonance heating (ECRH) system, new physical issues are applied to the investigation of fast ion behavior. Especially, the slowing-down (deceleration and parallel diffusion) and the loss processes are studied experimentally through measurements of the temporal change of charge-exchange neutral particle energy spectra. As results, first, in the longer slowing-down time (τ s ∼200 ms) plasma realized by ECRH, discrepancies from the theoretical values are detected in the deceleration and the parallel diffusion processes, whose factors are 4∼6 for the former, 6∼13 for the latter. Second, the fast ion loss cone is reduced by the ECRH on the NBI plasma, of which cause is discussed associated with the modification of the radial electric field, E r .

A new approach for vacuum surface mapping in Helitron E

Abstract Wall boronization was applied in Heliotron E with special reference to the study of impurity and recycling control in the helical system. However this technique was also found to work as an advantageous tool to enhance the diagnostic sensitivity of the beam impedance method for measuring the edge and divertor field line structure. In this paper we describe a new approach to the beam impedance method that utilizes the wall boronization.

DD Fusion neutron measurements in the beam-heated stellarator deuterium plasmas on Heliotron E

Abstract This paper describes the measurement of D-D fusion neutrons in the toroidal stellarator plasmas on Heliotron E. We used three moderated 3He counters and a moderated BF3 counter. Calibration of neutron detectors was previously done with a 252Cf spontaneous fission neutron source. We describe the measured fusion neutron emissions with several discharge conditions with hydrogen neutral beam injection heating to the deuterium plasmas. The Heliotron E plasmas are usually initiated by electron cyclotron resonance heating at a magnetic field of 1.9 T without an ohmic heating plasma current. We have tried to deduce the central deuterium ion temperature from measured neutron flux and line-averaged electron density. We have compared the central ion temperatures by neutron flux, with the passive charge exchanged particle analyses and diamagnetic stored energy. We discuss the possible error sources for the central neutron deduced deuterium temperature.