Wang Yumin

Density Profile and Fluctuation Measurements by Microwave Reflectometry on EAST

A microwave reflectometry system operating in the V-band frequency with extraordinary mode polarization has been developed on the EAST tokamak. The reflectometry system, using a voltage-controlled oscillator (VCO) source driven by an arbitrary waveform generator with high temporal resolution, can operate for the density profile measurement. The result of the bench test shows that the output frequency of the VCO has a linear dependence on time. The dispersion of reflectometry system is determined and reported in this paper. The evolution of a pedestal density profile during the L-H transition is observed by the reflectometry in H-mode discharges on EAST tokamak. A frequency synthesizer is used to replace the VCO as microwave source for density fluctuation measurements. The level of density fluctuation in the pedestal shows an abrupt decrease when the plasma enters into H-mode. A coherent mode with a frequency of about 100 kHz is observed and the mode frequency decreases gradually as the pedestal evolves.

Present Status of the Electron Cyclotron Emission Measurements on HT-7 and EAST

Present status of the diagnostics for electron cyclotron emission measurements on both HT-7 and EAST is reported. A 16-channel heterodyne radiometer system and a 20-channel grating polychromator, have been installed on HT-7 and EAST. A 32-channel heterodyne radiometer system, with a wider frequency coverage and better spatial resolution, is being built. In order to provide a reliable electron temperature profile, an in-situ absolute calibration system is currently being built. With the ECE measurements, study on the electron heat transport and the anomalous Doppler resonance is conducted.

Observation of Pedestal Plasma Turbulence on EAST Tokamak

Pedestal plasma turbulence was experimentally studied by microwave re∞ectometry on EAST tokamak. The characteristics of edge pedestal turbulence during dithering L-H tran- sition, ELM-free H-mode phase and inter-ELM phase have recently been studied on EAST. An edge spatial structure of density ∞uctuation and its dithering temporal evolution is observed for the flrst time on the EAST tokamak during the L-H transition phase. A coherent mode usually appears during the ELM-free phase prior to the flrst ELM on EAST tokamak. The mode fre- quency gradually decreases as the pedestal evolves. Analysis shows that the coherent mode is in the pedestal region inside the separatrix. In plasma with type-III ELMs, a precursor mode before ELM is usually observed. The frequency of the precursor was initially about 150 kHz and gradu- ally decreased till the next ELM. The mode amplitude increases or shows saturation before ELM. In the plasma with compound ELMs composed of high and low frequency ELMs, the precursor was also observed before the high frequency ELM while the harmonic oscillations with frequencies of 20 kHz, 40 kHz and 60 kHz appear before the low frequency ELM.

Measurement of Magnetic Island Width by Multi-Channel ECE Radiometer on HT-7 Tokamak ⁄

A 16-channel electron cyclotron emission (ECE) radiometer has been employed to observe the (m;n) = (2;1) magnetic island structure on HT-7 tokamak, where m and n represent the poloidal and toroidal mode number respectively. The results indicate that the island width is about 7 cm when the magnetic island is saturated during the m=n = 2=1 mode. The location of resonance surface can be determined by plotting the contour of ECE relative ∞uctuation. This method could be applied to the HT-7 and EAST campaigns in the future for the research of neoclassical tearing modes (NTMs).

Preliminary Study of the Magnetic Perturbation Effects on the Edge Density Profiles and Fluctuations Using Reflectometers on EAST

The resonant magnetic perturbation (RMP) coils have been successfully designed and installed on the Experimental Advanced Superconducting Tokamak (EAST). Using the reflectometer systems, the density profile and the density fluctuations during magnetic perturbations (MPs) phase have been investigated. During the experiments, two different cases are studied separately: steady MPs and rotating MPs. In both cases, a strongly density pump-out has been observed. In the steady MPs cases, an enhancement of the low frequency (<60 kHz) density fluctuations in H-mode phase has been observed. The plasma density boundary out-shifts ~ 5% caused by the MPs. The pedestal density gradient is reduced by 50%, while the radial location nearly stays unchanged. In the rotating MPs, the line-averaged density, the Dα emission at the divertor region and the spectrum of the density fluctuations are modulated. The results suggest that the low frequency (<60 kHz) density fluctuations may contribute to the strong density pump-out.

Experimental Determination of the ICRF Power Depositing on the Electrons in HT-7

This paper reports for the first time an experimental study on the power deposition profile of the ion cyclotron range frequencies (ICRF) power depositing on electrons in HT-7. The fast Fourier transform (FFT) analysis and the break in slope (BIS) method are utilized to obtain the information of the power deposition. The results indicate that the electrons were heated directly, and the electrons absorbed around 20% of the input power of the discharge of interest.