S. Hidekuma

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.

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.

Impurity pellet injection into current driven plasmas of the JIPP T-IIU tokamak

For interaction studies, impurity pellets of stainless steel and plastic carbon with a diameter of 0.5 mm and a velocity of 400 ± 100 m·s−1 have been injected into plasmas driven by fast wave current, with a sustained plasma current of 35-50 kA and an electron density of (2-5) × 1012 cm−3. The density rise is (6-8) × 1012 cm−3 for stainless steel pellets and 4 × 1012 cm−3 for plastic carbon pellets. At pellet injection, the current driven plasmas show no disruption, whereas all of the Ohmic discharges are disruptive. These phenomena are interpreted by a difference in the collision time with ablated pellets between thermal and non-thermal electrons. From measurements of the temporal evolution of the soft X-ray emission, the decay time of the injected impurity is estimated to be 25 ms. The effective charge states of the material of the injected pellets are calculated from the density rise and it is found that they are in the range of 0.8-1.5.

Azimuthal non-uniformities induced by ICH and ECH in the RFC-XX mirror plasma

The loss flux to an azimuthally segmented limiter in the central cell of the cusp end cell stabilized, axisymmetric mirror machine, RFC-XX, shows that non-axisymmetric illumination by either ICH or ECH can induce azimuthal non-uniformities in the plasma. Measurements of the floating potential in the vicinity of the limiter and by the limiter itself suggest that a large stationary azimuthal electric field is induced which can convectively transport plasma across magnetic field lines. The local radial component of the electric drift velocity is as large as 7 × 104 cms−1. Examinatio n of the plasma end loss versus azimuth and probe measurements deep in the plasma show that the non-uniformities extend into the plasma and are not localized near the outer boundary. The data suggest that the RF fields are driving an anomalous radial loss.

Radio frequency heating of multi-ion species plasma

Experimental results of ion heating by the use of an electrostatic ion cyclotron wave are presented. When a plasma is composed of several ion species, it is possible to heat the desired one by selecting the frequency of the externally applied r.f. field. A theory of the r.f. plasma heating including the multiple ion species is also presented. A computation carried out for the appropriate experimental parameters agree with the experimental results.

Stability studies of a hollow plasma in the double cusp experiment

Axisymmetric cusp end cells have application for the stabilization and plugging of a tandem‐mirror‐type reactor. Experiments have been performed on the RFC‐XX double cusp in Nagoya, Japan to measure the stability of the hollow plasma in the adiabatically confined region of the device. Experiments were performed with both a uniform field central section and with a mirror central section connecting the two cusps. The plasma is produced by an rf discharge of gas puffed into the vessel. Plasma fluctuations were measured by Langmuir probes and magnetic probes. Stable regions of operation with a hollow plasma in the cusp are found for both magnetic configurations. Various types of instabilities are observed: electrostatic drift type waves with m≥1, lower‐frequency waves with a magnetic perturbation associated with the density perturbation, an m=0 ion‐acoustic mode, and a large amplitude relaxation oscillation.

ICRF heating experiments on JIPP T-II

Data of JIPP T-II ICRF heating experiments are presented. The experiment covers three typical cases: the low-concentration hydrogen minority case, the high-concentration hydrogen minority case, and the 3He-minority case. The best heating efficiency is obtained for the 3He-minority case. It is shown through power balance analysis that the two H-minority cases are different in the wave energy deposition profile. The difference is explained by the presence of a local cavity mode in the high-concentration minority case. The ion temperature stops rising at a power density level of 0.65 W-cm−3 for the hydrogen minority experiments. No such deterioration is found in the case of the 3He minority experiment up to a power density level of 0.45 kW-cm−3. This is the maximum possible to attain within the maximum power injection (180 kW) up to which the experiment is conducted. An analytic solution of the Fokker-Planck equation is derived in order to interpret the deterioration of the heating efficiency.

Scaling Studies on RF Plugging Potential in the Cusp-Anchored Mirror Device, RFC–XX–M

A scaling relation for the RF plugging potential is investigated in the line cusp plasma of a cusp-anchored mirror device, RFC-XX-M. The RF plugging potential is measured by two independent methods: One is measured by multi-grid energy analyzers and the other is deduced from the measured ion density under RF electrodes and the ion energy distribution in terms of the Boltzmann relation. These values agree well each other. The scaling relations of the RF plugging potential for an applied RF field and an ion density are investigated. These are compared with theoretical calculations. The RF plugging potential is formed under RF electrodes by the enhanced RF electric field associated with an eigenmode of the ion Bernstein wave with a perpendicular mode number, n =1 and is in accordance with the ponderomotive potential expression.

EXCITATION OF ICRF WAVES IN RFC-XX DEVICE

Fast wave excitation is studied in the frequency range ωci < ω < 6 ωci. Perpendicular charge exchange analysis shows a high energy ion tail Ti tail ~ 20 Ti bulk. n nThe effective ion tail temperature peaks just below each cyclotron harmonic as expected theoretically. Electron temperature shows a four fold increase.