Observation of Pedestal Plasma Turbulence on EAST Tokamak

Pedestal plasma turbulence was experimentally studied by microwave reflectometry on EAST tokamak. The characteristics of edge pedestal turbulence during dithering L-H transition, ELM-free H-mode phase and inter-ELM phase have recently been studied on EAST. An edge spatial structure of density fluctuation and its dithering temporal evolution is observed for the first time on the EAST tokamak during the L-H transition phase. A coherent mode usually appears during the ELM-free phase prior to the first ELM on EAST tokamak. The mode frequency 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 gradually 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.     

Ideal MHD Stability Prediction and Required Power for EAST Advanced Scenario

The Experimental Advanced Superconducting Tokamak(EAST) is the first fully superconducting tokamak with a D-shaped cross-sectional plasma presently in operation.The ideal magnetohydrodynamic(MHD) stability and required power for the EAST advanced tokamak(AT) scenario with negative central shear and double transport barrier(DTB) are investigated.With the equilibrium code TOQ and stability code GATO,the ideal MHD stability is analyzed.It is shown that a moderate ratio of edge transport barriers’(ETB) height to internal transport barriers’(ITBs) height is beneficial to ideal MHD stability.The normalized beta βN limit is about 2.20(without wall) and 3.70(with ideal wall).With the scaling law of energy confinement time,the required heating power for EAST AT scenario is calculated.The total heating power Pt increases as the toroidal magnetic field BT or the normalized beta βN is increased.     

Comparison of methods for turbulence Doppler frequency shift calculation in Doppler reflectometer

The Doppler reflectometer(DR), a powerful diagnostic for the plasma perpendicular velocity(u⊥) and turbulence measurement, has been widely used in various fusion devices. Many efforts have been put into extracting the Doppler shift from the DR signal. There are several commonly used methods for Doppler shift extraction, such as the phase derivative, the center of gravity, and symmetric fitting(SFIT). However, the strong zero-order reflection component around 0 k Hz may interfere with the calcula...     

Experimental study of double tearing mode on EAST tokamak

The double tearing mode(DTM)in a high βN(βN>1.5)discharge with internal transport barrier on EAST was investigated.A 15 kHz tearing mode(TM)(m≥3,n=3)appears at outer q=2 surface at first,which is stable in the highβN phase.Then a 2 kHz TM(m=2,n=1)occurs at inner q=2 surface.Soon after,high βN collapsed with the crash of ELM,and DTM formed during the collapse of βN.The positions of the two islands of the DTM are consistent with the q=2 surface.The temperature fluctuations are the strongest between the two magnetic islands.A statistical analysis of high βN discharges operating with the reversed magnetic shear configuration in the EAST 2015-2018 campaign revealed the existence of the DTM in many discharges.During the DTM phase,all βN are reduced by 10%-30% within 0.1 s.However,there are two different βN behaviors thereafter-with and without βN recovery.Studying the physical mechanism of βN recovery during the DTM phase will benefit steady-state operation with reversed shear configurations in the future.     

Observation of Electron Energy Pinch in HT-7 ICRF Heated Plasmas

Inward energy transport （pinch phenomenon） in the electron channel is observed in HT-7 plasmas using off-axis ion cyclotron resonance frequency （ICRF） heating. Experimental results and power balance transport analysis by TRANSP code are presented in this article. With the aids of GLF23 and Chang-Hinton transport models, which predict energy diffusivity in experimental conditions, the estimated electron pinch velocity is obtained by experimental data and is found reasonably comparable to the results in the previous study, such as Song on Tore Supra. Density scanning shows that the energy convective velocity in the electron channel has a close relation to density scale length~ which is qualitatively in agreement with Wang＇s theoretical prediction. The parametric dependence of electron energy convective velocity on plasma current is still ambiguous and is worthy of future research on EAST.     

Sawtooth Control Experiments on HT-7 and EAST Tokamak

Sawtooth control experiments were performed on HT-7 with a limiter configuration and on the Experimental Advanced Superconducting Tokamak (EAST) with a double null con-figuration. The sawtooth period can be modified by lower hybrid wave (LHW) and ion cyclotron resonance frequency (ICRF). Different sawtooth behavior was observed with the same plasma density, LHW power but different plasma currents on HT-7. There was a dwell time between LHW added and the sawtooth stabilization on HT-7 while the sawtooth stabilization immediately happened when LHW power was injected on EAST. The possible mechanism of the sawtooth control is discussed in this paper. All the experimental results have proved that power deposition of the radio frequency (RF) should be the key factor for sawtooth control.