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The preliminary experimental results of ac plasma discharges in the CT-6B tokamak have been achieved by an active plasma current feedback system and the keys to a smooth transition through current zero and some phenomena related to the current reversal phase are also presented.

A full current ac-modulating tokamak discharges have been achieved in the CT-6B device. Obvious modulating changes are observed in plasma current, loop voltage, soft x-ray radiations, electron density, Hα line emission, and hard x-ray radiations when the modulating pulse is added. The continuous tokamak operation with many cycles of low frequency modulating plasma current indicates the possibility of steady state tokamak operation.

Obvious heating effects have been observed in RF heating experiments on CT-6B tokamak, in which microwave beams of frequencies 34.34 GHz and 20.1 GHz generated by two gyrotron systems were launched from top and outside of the device, respectively. It is shown that these heating effects arise becouse of the different heating mechanisms. The former is due mainly to the resonance heating of O mode. The latter belongs to down shift-over dense heating. Its heating mechanism may be the nonlinear phenomena occuring near the high density cut off layer of X mode. A kind of nonlinear heating phenomena (secondary heating) has been observed in two experiments, which may come from the different physical mechanisms.

An electron cyclotron resonance heating experiment has been performed by using a gyrotron system for the fundamental and second harmonic resonance heating on CT-6B tokamak. Obvious changes are observed in soft x-ray radiations, electron cyclotron emission, displacement of plasma column, impurity line radiations, and bremsstrahlung radiation when heating pulse is injected, indicating an increase of the plasma electron temperature.

A spectrographic survey of the impurity lines was made on the CT-6B Tokamak with I metre grazing incidence spectrograph. The observed wavelength ranges were from 1580 to 550 ? and from 460 to 43 ?, using 300 lines/mm and 1200 lines/mm gratings respecrively. The survey revealed that the main impurities in CT-6B Tokamak plasma are Mo for high Z impurities and C, N, O for low Z impurities. There are a large number of OV and OVI lines with intense emission. We also observed MoVIIl resonance lines 234.314 ?, 235.510 ? and 237.215 ?. The shortest observed wavelengths are the second order of OVII lines 21.6020 ? and 21804 ?. The first order of these two lines situate beyond the film and can not be recorded. According tc the features of the spectra and the ionization potentials of OV, OVI as well as MoVII, the average electron temperature for CT-6B Tokamak is estimated to be about 140 eV. Oxygen is the main impurity in CT-6B Tokamak, and its lines emission is the main portion of energy loss in CT-6B Tokamak. Lines of the main components of the vacuum-vessel material such as Fe, Cr, Ni and Ti have not been found.

The time evolution of electron temperature in plasma current rising phase on CT-6B Tokamak is estimated by the history method for oxygen impurity lines. The time variations of line emissions of OII-OV1 in VUV region are measured by one-meter grazing incidence vacuum spectrometer and the electron density is measured by using FIR HCN laser interferometer. The switch-on time of each line emission is used for estimating electron temperature. The influences of oxygen impurity influx rate and confinement time on intensity and switch-on time of line emissions are surveyed numerically. The estimated electron temperatures arc around 3.6, 5.6, 7.3, 10.8 and 21 eV, respectively at 0.36, 0.56, 0.66, 0.90 and 1.4 ms after the start of discharge, which indicate the plasma temperature in central region.

A currentless plasma has been obtained in a tokamak device using a powerful microwave at electron cyclotron frequency generated by gyrotron systems. Its electron density is up to 8×1012cm-3; the electron temperature is up to 50 eV; the particle confinement time is about. 0.5ms; the heating efficiency is above 25%. The effect of the plasma on the toroidal breakdown and the heating mechanism have been studied.