Teruyuki Sato

Plasma production by helicon waves

High density plasma production using m=+1 and m=-1 helicon waves is studied. Characteristics of cylindrical helicon waves including effects of a vacuum gap between the plasma and the conducting wall and of a non-uniform density profile are presented. The m=+1 and m=-1 helicon modes are separately excited by a helical antenna, and the dependences of plasma density and antenna loading resistance on RF power are different for these modes.

Radio-frequency plugging of a high density plasma

Mirror end loss can be suppressed by applying an rf field at the mirror throat through low impedance coils. This method is verified to be effective for a high density plasma up to 1014 cm−3. Experimental results show that the rf field strength required for the plugging is dependent on the plasma density. The dependence differs according to the geometry of the coil. The mechanism of giving the density dependence is theoretically clarified for each coil. Particularly, it is shown that the electric field induced in the direction of the static magnetic field is intrinsic for the result of type‐III coil where its efficiency for rf plugging is insensitive to the plasma density.

Theory and experiment on radio‐frequency plugging of magnetically confined plasma in an open‐ended system

The theory of the rf plugging of a magnetically confined plasma in an open‐ended system is presented; it takes explicit account of the collective effect brought about by the dynamic screening action of the plasma. The theory predicts a maximum efficiency of the rf plugging at an optimum frequency corresponding to a zero of the longitudinal dielectric constant. The plugging efficiency is calculated as a function of the plasma density, the strength of the rf field, and the intensity of the magnetic field. These theoretical predictions are compared with experimetal observations, demonstrating satisfactory agreement in over‐all features. The results provide a useful scaling law associated with the concept of the rf confinement. The theory also offers an explanation for the discrepancy between the experimental values of the optimum frequency and those predicted by a conventional theory based on an individual‐particle model.

Dynamic Control of a Cusped Plasma near the Ion Cyclotron Frequency

An r f electric field near the ion cyclotron frequency is applied perpendicularly to the magnetic field of cusp regions in order to plug up cusp ends and to heat ions. A helium plasma is steadily fed from one of point cusps into the containment region whose density is varied from 1×10 8 to 1×10 11 cm -3 . For the line cusp loss good plugging is satisfied up to 1×10 11 cm -3 , but for the point cusp loss it is not obtained for the density above 1×10 10 cm -3 . Under the resonance condition the ion energies consist of two components both of which display Maxwellian-like distribution of T i of about 40eV and 150∼200eV respectively when E r f =100V/cm.

PLASMA PRODUCTION WITH ROTATING ION CYCLOTRON WAVES EXCITED BY NAGOYA TYPE-III ANTENNAS IN RFC-XX

A method of plasma production by ion cyclotron wave heating has been developed in RFC-XX. Nagoya Type-III antennas were used for wave excitation, and gas was supplied through a gas box. The effect of the rotating field excitation on plasma production and heating was investigated. In the m = − 1 rotational mode (rotation in the direction of ion cyclotron motion), the plasma density profile is flat within the gas box bore with a line integrated density nl = 3 × 1013cm−2, and the ion temperature is Ti ≈ 150 eV. For the m = +1 mode, a high density plasma was obtained with a different profile having a peak at the centre. In this mode, the line integrated density is nl = 3 × 1014cm−2, with the peak density n(0) = 7 × 1013cm−3.

A slow-wave heating experiment on RFC-XX using an array of phased antennas

Experimental data for ion cyclotron resonance heating in the RFC-XX machine in IPP-Nagoya are presented. The achieved ion temperature is as high as 100 eV at n = 1013 cm−3 and 1 keV at n = 1012 cm−3. The ion energy confinement becomes worse by the application of a longer pulse, which is found to be due to the enhanced charge-exchange loss and/or electron drag. Axially and azimuthally arrayed antennas are used in the heating, and the importance of the phasing is demonstrated. A simple model of the multiple-antenna problem is also given and used to interpret the experimental data.

Effects of Electrode Geometry on Breakdown Voltage of a Single-Gap Pseudospark Discharge

The breakdown voltage of a single-gap pseudospark discharge device is measured for a wide range of nitrogen gas pressure, p, and various geometrical dimensions of the electrodes, i.e., anode-cathode gap distance, L, diameter of electrode hole, d, thickness of the electrode, t, and depth of the hollow cathode cavity, h. The breakdown voltage Vs is expressed as an empirical scaling relation, Vs=(197/p3.40L3.41){t(L/d)3.25/[t2(L/d)2+50]1/2}. The breakdown voltage characteristic can be qualitatively explained by the geometrical dependence of the penetrating electric field through electrode holes. The real discharge path length in the pseudospark discharge is estimated.

Experiment on nonadiabatic rf plugging in a cusped field

Under nonadiabatic conditions, rf plugging is accompanied by effective ion heating, which in turn causes increased particle losses from a cusp configuration. An empirical scaling relation for rf plugging of line cusp loss is obtained among the required rf electric field, the static magnetic field, and the plasma density, and is compared with the relation obtained by a linear theory.

Measurements of Atomic Hydrogen-Density Profiles in the RFC-XX-M Machine Using Laser Fluorescence Spectroscopy at the H? Transition

To assist in the study of the power balance in a linear confinement machine, atomic hydrogen-density profiles in plasmas of the RFC-XX-M device were measured using laser fluorescence spectroscopy tuned to the Balmer alpha line. Measured profiles showed marked decreases of neutral densities in the centre of the plasma, which are explained by charge-exchange and electron ionization pumping by the plasma. It is important to note that the neutrals are well burned out in the central cell, Nl/Ne \lesssim0.01. Further, charge-exchange losses are quantitatively evaluated and discussed from the neutral-density profiles and parameters of the plasma.

Some Experiments on Plasma Alfven Waves

Transverse hydromagnetic waves (Alfven waves) were generated in a plasma of ionized air in a linear glass tube with weak magnetic fields, and their phase velocity and attenuation were measured. The velocity and amplitude of the waves are plotted vs. the axial magnetic field, gas pressure, and pre-ionization currents. (D. L.C.)