Shu-de Wan

Observation of radial propagation of electrostatic fluctuations on KT-C tokamak

The radial wavenumber spectra of electrostatic fluctuations measured by Langmuir probe arrays on the KT-5C tokamak show a quasimode-like structure which results in a net radial outward propagation of the turbulent fluctuations. The power-averaged radial wavenumber is approximately constant in the core region and increases near the limiter. The radial wavenumber spectral width stays nearly constant from core to edge and is about two times greater than the poloidal wavenumber spectral width. The measured fluctuation levels and wave action fluxes are in good agreement with model predictions (Mattor N and Diamond P H 1994 Phys. Plasmas 1 4002, Mattor N 1995 Phys. Plasmas 2 766) which suggest that drift wave propagation could be a source of edge turbulence.

Effects of electrode biasing on fluctuations and transport in the KT-5C tokamak

Electrode biasing experiments are carried out on the KT-5C tokamak to investigate the effects of the radial electric field E/sub r/ on turbulence. A modestly enhanced E/sub r/ gradient layer is formed at the plasma edge by the electrode biasing. In the E/sub r/ gradient layer, reductions in the fluctuation amplitude, radial correlation length and turbulent particle flux are observed. These results support the theoretical models on the turbulence suppression by decorrelation due to E /spl times/ B flow shear.

Modifications of plasma edge electric field and confinement properties by limiter biasing on the KT-5C tokamak

Experiments using a biased multiblock limiter in the KT-5C tokamak show that positive biasing is more effective than negative biasing in modifying the edge electric field, suppressing fluctuations and improving plasma confinement. The biasing effect varies with the limiter area, the toroidal magnetic field and the biasing voltage. By positive biasing, the edge profiles of the plasma potential, the electron temperature and the density become steeper, resulting in a reduced edge particle flux, an increased global particle confinement time and lower fluctuation levels of the edge plasma.

Observations of a quasi-coherent fluctuation mode in the KT-5C tokamak during ~90 degrees phase shift feedback

A new fluctuation phenomenon is observed through Langmuir probe measurements at the edge plasma in the KT-5C tokamak by applying a -90 degrees phase shift feedback. Using a two point correlation technique, it is found that this fluctuation mode has a longer poloidal wavelength and a definite frequency when compared with the usual edge turbulence. It is also found through bispectral analysis that this mode is a spontaneously excited quasi-coherent mode, which has almost no contribution to the cross-field particle flux

Compatible operation of the power system for steady state and pulse modes in a magnetic torus KT-5D

Compatible operation of steady state mode and pulse mode is realized in the KT-5D device. New power supplies with the operation control systems for the steady state toroidal magnetic field as well as for the vertical field are added, and the rf wave injection systems for sustaining steady state plasmas are upgraded. After the modification, the device now can work not only as a tokomak with pulsed plasma currents as it was but also as a simple magnetized torus with steady state plasma discharges. It allows more flexible and efficient experimental researches on the magnetically confined plasmas to be carried on in the same device.

Fast mega pixels video imaging of a toroidal plasma in KT5D device

A direct imaging system, viewing visible light emission from plasmas tangentially or perpendicularly, has been set up on the KT5D toroidal device to monitor the real two-dimensional profiles of purely ECR generated plasmas. This system has a typical spatial resolution of 0.2mm (1280×1024pixels) when imaging the whole cross section. Interesting features of ECR plasmas have been found. Different from what classical theories have expected, a resonance layer with two or three bright spots, rather than an even vertical band, has been observed. In addition, images also indicate an intermittent splitting and drifting character of the plasmas.

Observation of spatial intermittency in Tokamak plasma turbulence

A sharp variation at some radial positions superimposed on a slow change in the profiles of the fluctuation levels, fluctuation-driven particle and energy fluxes, which is referred as spatial intermittency, is observed in the core plasma of the 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 peaks in the profiles are located in the vicinity of low-q rational surfaces, and fluctuation spectra perpendicular to the magnetic field become more anisotropy there. The intermittency may be related to the radial variations in the nonlinear mode couplings near the low-q resonant surfaces.

Magnetically and optoelectronically isolated trigger for pulse-power applications

In this article the design of a magnetically and optically isolated trigger is discussed. Critical issues for trigger design are presented together with some experimental usages. In this trigger, an optical coupler is used to cut off the ground loop between the circuits of the preceding control system and the power supplies of the double functional device KT-5D (as a simple magnetic torus or a tokamak). A magnetic coupler is used to provide a pulse-power output for the silicon controlled rectifier. The output is a 230 mus transistor-transistor logic (TTL) with an amplitude of 3.0 V. The rising time and the trailing time are no more than 4.0 mus. The delay time between the input and the output of the trigger is 6.8+/-0.2 mus. A resistance-capacity branch is integrated into the trigger to provide an adjustable delay time of up to 72 ms. The zero quiescent dissipation character endows the trigger with a long lifetime of years dispensing with any charging or replacing batteries. It is observed that the trigger has a good stability even in a high electromagnetic circumstance (at the order of 1 T). Using it as a trigger for the silicon controlled rectifier, we realized the compatible operation of the steady state mode and the pulse mode in KT-5D.

Investigation of the Effects of the Radial Electric Field by Electrode Biasing in a Toroidal Plasma

The electrode biasing experiments were carried out on the KT‐5C tokamak to investigate the effects of the radial electric field Er on turbulence in a toroidal plasma. It is observed the radial electric field is mainly contributed by the poloidal flow both in Ohmic and in electrode biasing discharges. The changes of Er is led by the changes of poloidal flow, and it is the Reynolds stress driving the poloidal flow in Ohmic discharges in the device, but the Reynolds stress is not so important in the electrode biasing discharges. A modestly enhanced E×B shear layer is formed at the plasma edge by the electrode biasing. In the sheared E×B layer, reductions in the fluctuation amplitude, and the radial correlation length as well as the turbulent particle flux, are observed, Indicating that the turbulence suppression by de‐correlation is due to the modestly enhanced E×B shear layer.

Observation of Low Frequency Ion Mode Turbulence in Tokamak Plasma

Features of a low frequency ion mode were observed together with the electron drift wave type fluctuations by a Langmuir probe array in the bulk plasma inside the velocity shear layer in the KT-5C tokamak. The measured ion mode wavenumber, scaling of the ion mode with chord-averaged density, and the estimated ηi value suggest that the observed ion mode may be the theoretically predicted ηi mode driven by ion temperature gradient.