Wan Shude

Modification of plasma edge fluctuations using electrostatic probes in Keda Tokamak-5C

The experiments to characterize the modification of edge fluctuations using electrostatic probes have been performed on Keda Tokamak-5C (KT-5C) [World Survey of Activities in Controlled Fusion Research, Nuclear Fusion Special Supplement (International Atomic Energy Agency, Vienna, 1991), p. 190]. The results show that the externally excited perturbation propagates in the direction of electron directional movement. The observed phenomena are mainly related to the electrons, and can be well explained by the proposed dynamic theory of the ballistic mode. The experimental correlation coefficient and the longitudinal dispersion relation are in good agreement with the proposed ballistic model.

Generation of Radial Electric Field with Electrode Biasing

Time and space resolved measurements of the radial electric field (Er) have been conducted during the electrode biasing experiments on the KT-5C tokamak. The suppression of the turbulent transport with the change of Er induced by the biased electrode is observed. It is found that the poloidal flow contributes to the main part of the Er, and the change of the poloidal flow has a lead of about 20 µs to the formation of Er. These observations suggest that a radial current, responding to an induced voltage on the electrode, drives a poloidal flow which in turn drives the radial electric field.

Steady-State Plasmas in KT5D Magnetized Torus

Steady-state plasma generated by electron cyclotron resonance (ECR) wave in the KT5D magnetized torus was studied using a fast high-resolution camera and Langmuir probes. It was found that both the discharge patterns taken by the camera and the plasma parameters measured by the probes were very sensitive to the working gas pressure and the magnetic configuration of the torus both without and with vertical fields. There existed fast vertical motion of the plasma. Tentative discussion is presented about the observed phenomena such as the bright resonance layer at a high gas pressure and the wave absorption mechanism at a low pressure. Further explanations should be found.

Dual-Electrode Biasing Experiments in KT5C Device

Based on the single biasing electrode experiments to optimize the confinement of plasma in the device of KT-5C tokamak, dual-biasing electrodes were inserted into the KT5C plasma for the first time to explore the enhancing effects of biasing and the mechanisms of the biasing. By means of applying different combinations of biasing voltages onto the dual electrodes, the changes of Er, which are the key factor for boosting up the Er×B flow shear, were observed. The time evolution showed that the inner electrode played a major role in dual-biasing, which drew larger current than the outer one. The outer electrode produced little influence. It turned out that the dual-biasing electrodes were as effective as a single one in improving the plasma confinement, for the mechanism of biasing was essentially an edge effect.

Preliminary Results of Drift Instability by Fast Optical Images in KT-5D Toroidal Plasma

Drift instability in plasma generated by electron cyclotron resonance (ECR) in KT-5D device was investigated by using a fast camera and Langmuir probes. The similarity between the distribution of light intensity from the images and the plasma pressure indicates a nearly linear relationship. The discharge images taken by the camera and the plasma parameters measured by the probes also indicate the existence of low frequency turbulent events with a time scale less than a few mini-seconds.

Effects of Dual-Electrode Biasing on E r in a Toroidal Plasma

Two externally biased electrodes were inserted into the plasma on the KT-5C tokamak to test the effects on modifying the radial electric field Er other than single biasing. Using various combinations of biasing voltage, the influences of double biasing are compared with the single biasing. It turns out that the effect of dual-biasing is also effective as a single one, but the outer electrode seems to be shielded by the inner one and show less influence. The results clearly show that the radial electric field Er changed by external biasing is intrinsically an effect localized at the edge of the plasma, which is caused by the electrode induced radial current; and dual-electrode biasing using the method similar to the single biasing seems not to be able to increase more distinctly the peaking effect on Er than the single biasing.

Excitation and Propagation of Modified Fluctuation in a Toroidal Plasma in KT-5C Device

Understanding the propagation of the turbulent perturbation in the tokamak edge plasma is an important issue to actively modify or control the turbulence, reduce the anomalous transport and improve plasma confinement. To realize active modification of the edge perturbation, a high dynamic output, broad-band, low-cost power amplifier is set up, and used to drive the active probes in the experiments on KT-5C Tokmak. By using small-size magnetic probes together with Langmiur probes, It is observed that the modified perturbation by the active probes with sufficiently driving power may spread with electrostatic mode, and electromagnetic mode as well.

Influence of the Electron Emission on the Properties of the Cathode Sheath

A simple cathode sheath model has been used to discuss the influence of the electron emission on the properties of the cathode sheath (CS). The model is applied to an argon arc plasma at atmospheric pressure with a thoriated tungsten cathode and the thermionic emission is mainly taken into account in this paper. However, the influence of the electric field intensity at the point of the cathode surface on the thermionic emission is also discussed briefly in this paper. The calculated results indicate that the electron emission has remarkably influence on the properties of the cathode sheath.

Investigation of linear ion bernstein wave in KT-5B tokamak

Linear ion Bernstein waves have been successfully excited in KT-5B tokamak by a toroidal current antenna. Ion Bernstain waves were detected by two rf Langmuir probes and identified by their exciting region and their dispersion relation. The ion temperature profile in the tokamak has been obtained from the dispersion relation of ion Bernstein wave in agreement with the measuring by an electrostatic ion probe.