Xuanzong Yang

Quasi-linear transport inferred from density fluctuation spectra

Expressions for the quasi-linear electrostatic electron particle and heat fluxes are derived in the wave frequency and electron collisionality regimes appropriate to the interior of the TEXT tokamak. These are expressed in terms of moments of the density fluctuation spectrum accounting for poloidal plasma rotation. Minimal assumptions are made as to the origin of the turbulence. The fluxes are then evaluated for an appropriate discharge condition on TEXT, using the density fluctuation frequency spectrum and equilibrium plasma potential measured by a heavy ion beam probe (HIBP). These are compared with the experimental fluxes for two measurements of the normalized rms poloidal wave number k-theta,rms-rho-s. For k-theta,rms-rho-s almost-equal-to 0.1, which is an upper bound of the HIBP two-point phase shift measurements, the electrostatic fluxes are negligible and the rms frequency of the fluctuations is much larger than the Doppler shifted electron diamagnetic drift frequency. However, for k-theta,rms-rho-s almost-equal-to 0.3, as supported by far-infrared (FIR) scattering and as expected for drift waves, the electrostatic fluxes are sufficient to account for the observed fluxes in the region of low collisionality. In addition, the rms frequency (consistent with FIR scattering) agrees with the Doppler shifted electron diamagnetic drift frequency. However, the computed fluxes fall increasingly short of the observed fluxes with increasing radius.

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.

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.

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.

Electrostatic fluctuations at the edge of the CT-6B tokamak

Langmuir probes and a Mach probe were used for measuring edge fluctuations and parallel plasma flow, respectively, on the CT-6B tokamak. A maximum radial gradient in parallel flow was observed to be located roughly at the maximum Er shear layer (or, namely, the poloidal velocity shear layer). Er shear is found to be very possibly consistent with a regulating effect on edge turbulence features. Non-linear analysis indicates that non-linear phase coupling in turbulence may be influenced by Er shear, or that there exist coherent structures induced by strong Er shear, and the phase coupling processes show an intermittent character. The results provide some new observations for a better understanding of the basic mechanisms of edge turbulence.

PLASMA CURRENT PROFILE DURING CURRENT REVERSAL IN A TOKAMAK

Alternating current operation with one full cycle and a current level of 2.5 kA have 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 current distribution is reconstructed with an inversion algorithm. The inversed current first appears on the weak field side. The existence of magnetic surfaces and rotational transform provide particle confinement in the current reversal phase.

Plasma current modulation system and AC operation in the CT-6B tokamak

To perform alternating current (AC) operations in the CT-6B tokamak, a newly developed plasma current modulation system (PCMS) has been designed for the ohmic heating circuit. The modulation system is reliable, simple in technique, easy to operate, and has four operation modes to modulate CT-6B plasma current. Reproducible multicycle AC operation at a current of /spl sim/5 kA and a fast current ramped down and then up with a linear rate of 6 kA/ms AC discharge with trapezoidal waveform have been achieved using PCMS. Feedback control of the plasma current plateau, plasma current weak modulation, and the plasma transition from a low to high current for the studies of the anomalous transport and the plasma edge turbulence have also been demonstrated.