Effect of pedestal fluctuation on ELM frequency in the EAST tokamak

Abstract

The dependence of ELM frequency on heating power has been studied on the Experimental Advanced Superconducting Tokamak (EAST). It is found that the ELM frequency (f ELM ) generally increases with the power through separatrix (P sep ), indicating type-I ELM in these plasmas. However, there are two data points, named 'abnormal ELM' in the present paper, which have much lower f ELM than the 'normal ELM', while both types of ELM have similar ELM energy losses. The 'abnormal ELM' occurs at a phase with increased radiation power due to metal impurity influx events. The increased radiation power cannot explain the much lower f ELM for 'abnormal ELM', since the reduction of P sep is weaker than proportional to the observed reduction of the ELM frequency. The 'abnormal ELM' feature can be attributed to the enhanced amplitude of a coherent mode in the pedestal region. Comparing the pedestal evolutions for the two types of ELM with similar separatrix power P sep , it is actually found that the more pronounced pedestal coherent mode in the plasma with 'abnormal ELM' leads to a slower pressure pedestal recovery during the inter-ELM phase. This experimental result implies that the physical mechanism for 'abnormal ELM' is that the more pronounced pedestal fluctuation induces larger outward transport, slows down the pedestal evolution and leads to longer inter-ELM phase, i.e. lower ELM frequency.

Experimental study of ELM-induced filament structures using the VUV imaging system on EAST

“…In addition, the dependence of poloidal width on the ELM amplitude is investigated, and about 100 ELM-induced filaments are used for statistics. In the statistical process, there are different definitions of the ELM amplitude in the literature [26]. In this work, ELM amplitude is defined as the energy loss, i.e.…”

“…i The end time, ( ) t i , end at which a D decreases to ( ) exp 3 of the peak value for the i th ELM burst, is also defined. For the calculation of W ELM [26], the energy loss due to the i th ELM, ( ) W i , ELM is calculated as the difference between the maximum value and the value of W MHD at ( ) t i end in a small time interval around the i th ELM event, i.e.…”

Multi-scale interaction of pedestal instabilities in H-mode plasma on the EAST tokamak

“…show that a coherent mode (CM) [10] with frequency sweeping down appears in density and temperature (but not in magnetic) fluctuation spectra during the ELM-free phase before the first ELM. It has been shown that the CM is in the edge pedestal region and has a toroidal mode number n = 12-17 [10], with characteristics close to the resistive ballooning mode [34]. More characteristics of this mode have been presented in appendix B, including an argument of the mode rotation direction in the plasma frame.…”

A low-frequency axisymmetric oscillation in the high-confinement mode pedestal on the EAST tokamak

“…In experiment, it has been found that several turbulence modes in the ELM-free phase or inter-ELM phase are closely related to the pedestal transport [10][11][12][13][14][15][16][17][18][19][20][21][22]. In simulation, gyrokinetic codes found that several types of micro-turbulence, such as the micro-tearing mode (MTM) [23,24] and electron temperature gradient (ETG) mode [25], could be responsible for the pedestal transport.…”