Chunhua Feng

Alternating current operation with multicycles in the CT-6B tokamak

Stable and clean AC tokamak discharges with two, four and eight cycles of alternating plasma current have been achieved in the CT-6B tokamak. Although there are differences in the plasma-wall interactions in the two halves of each AC cycle, the purity of the plasma is not significantly affected. The finite electron densities during the current reversal phases of each cycle are attributed to a non-zero particle confinement time even at zero current. The plasma parameters in the boundary region during the AC operations were measured by a triple probe system.

Edge plasma electrostatic fluctuation and anomalous transport characteristics in the Sino-united spherical tokamak

In this paper, the edge plasma parameters. including the electron temperature (T-e), electron density (n(e),), plasma potential (phi(p)), radial electric field (E-r) and the corresponding fluctuations in the Sino-united spherical tokamak (SUNIST) have been systematically measured using Langmuir probe arrays. The characteristics of the edge electrostatic fluctuations and the turbulence-induced transport are experimentally investigated. The results show that a naturally formed E-r x B-phi poloidal velocity shear layer (VSL) exists near the radial location of the limiter. Wavenumber spectrum analyses show that edge fluctuations have a radial propagation character of the drift wave turbulence, with a characteristic radial phase velocity u(phr) similar to 0.7 kms(-1) in the scrape-off-layer and u(phr) similar to 0.9-1.4kms(-1) at plasma edge, suggesting! that the. edge turbulence may originate from the core and propagate to the edge in an intermittent pattern. A radial profile of the toroidal flow velocity Mach number has been observed whose maximum is just located at the VSL position, which may imply a possible relation between the poloidal shear flow and the toroidal plasma flow in SUNIST.

Evidence for parallel flow shear instability in the edge plasma of the CT-6B tokamak

On the CT-6B tokamak, a transition from low to high plasma current discharge was obtained by feedback control of plasma current. Increased parallel flow shear was observed to be possibly correlated with enhanced fluctuation level and fluctuation-driven particle fluxes, and a more isotropic feature of edge turbulence in the perpendicular plane. A maximum of was found to be roughly located at where the poloidal velocity shear layer was located. The results suggest that instability may play an important role in edge turbulence.

The plasma current profile during current reversal in AC operation of the CT-6B tokamak

Alternating current operation with one full cycle at a plasma current level of 2.5 kA has been achieved in the CT-6B tokamak. The poloidal magnetic field in the plasma is measured with two internal magnetic probes in repeated discharges. The plasma current distribution is reconstructed with an inversion algorithm. The reversed plasma current first appears on the low field side due to decreasing poloidal beta. Two plasma current components flow in opposite directions when the net current vanishes. The existence of the magnetic surfaces and rotational transform provides particle confinement in the current reversal phase.

Electron cyclotron wave current startup for ohmic discharges on the CT-6B tokamak

The electron cyclotron wave current startup as a scheme to initiate ohmic discharges has been in, estimated with two circuits of open and shorted primary winding of transformer in the CT 6B tokamak. A diode shorted circuit is proposed for connection with ohmic discharges. Two gyrotron systems operated at different frequencies were used as the wave sources. The experimental results indicate that approximately equivalent transformer flux saving and reduction of starting voltage are achieved in the initial phase of the ohmic discharges for both circuits. An estimation of plasma resistance for the open circuit shows that no obvious anomalous resistance of the wave started plasma is observed.

Effects of Er shear on edge turbulence of the China Tokamak-6B

On the China Tokamak (CT-6B) [Nucl. Fusion 36, 1669 (1996)], application of negative limiter bias resulted in enhanced Er shear in the naturally occurring Er shear layer near the fixed limiter radius with negligible change of plasma density, electron temperature, the parallel plasma flow, and the impurity ion radiation power. In the layer, decreased turbulent fluctuation, reduced poloidal correlation and increased nonlinear coupling of the turbulence were observed to be very possibly correlated with enhanced Er shear. The results suggest that there exists interaction of Er shear with turbulence, and an Er shear-induced shift in the phase angle between density and poloidal electric field fluctuations and nonlinear three-wave coupling may play an important role in suppressing edge turbulence.

Investigation of Some MHD Events in the SUNIST Spherical Tokamak

Internal reconnection event (IRE), which is characterized by a perturbation in plasma current Ip, loop voltage Vloop, Hα radiation and magnetic perturbation dBp, was observed in the SUNIST experiment. It is found that, the fluctuation before IREs is characterized by a structure of m = 2/n = 1, then changes to m = 4/n = 1 during the IREs; and, after IREs, the mode number changes to m = 3/n = 1. An analysis in the evolution of equilibrium parameters during IREs shows that a positive spike appears in the evolution of the plasmas elongation and a negative spike in the poloidal beta of plasma. A collapse in the pressure profile, corresponding to the occurrence of IREs, is also found.

Electrode discharge assisted electron cyclotron wave current startup on the CT-6B tokamak

A novel method of electron cyclotron wave current startup has been proposed, in which a discharge between a pair of electrodes is introduced and forms a magnified toroidal plasma current in a strong toroidal field and a weak vertical field. The method has been demonstrated on the CT-6B tokamak. The experimental results are consistent with a model that is suggested.

Electron cyclotron wave current startup experiment on the CT-6B tokamak

Electron cyclotron wave current startup has been investigated in a wide range of toroidal magnetic fields on the CT-6B tokamak. Two kinds of drive mechanisms of current startup have been identified experimentally, according to the dependence of the wave started current on the vertical field. Their contributions to the current have been estimated quantitatively for different resonance scenarios. The wave started current may occur at a downshifted resonance frequency even if the resonance layer is located outside the vacuum vessel.

On the Role of Neutral Particles on Edge Turbulence in the CT-6B Tokamak

A positive limiter biasing on the CT-6B tokamak resulted in a slight increase of edge density and an obvious drop of electron temperature, which is in the temperature range where the rate coefficients for ionization processes are strongly temperature dependent. Concomitantly, edge fluctuations are enhanced, suggesting that the presence of neutral particles may play an important role in modifying edge turbulence.