J-TEXT Seminar No.27

时间：12月12日 9：00

地点：二楼会议室

报告人: 陈伟（核工业西南物理研究院副研究员）

题目：Kinetic Electromagnetic Instabilities in a plasma with weak magnetic shear

摘要：

Kinetic Alfvén-ballooning instabilities are very common in magnetized plasmas both in space and laboratory. In present-day fusion and future burning plasmas, they are easily excited by fast particles and/or pressure gradients. They can not only cause the loss and redistribution of fast particles but also affect plasma confinement and transport. The physics associated with them is an intriguing but complex area of research. In this talk, we will report two experimental results in HL-2A Ohmic and NBI plasmas with weak magnetic shears（s～0 and low pressure gradients（α<0 .3）. Firstly, low-frequency oscillations with f=15-40 kHz and n=3-6 were detected by many kind diagnostics in high-density Ohmic regimes. Secondly, the low-n AITG instabilities with f_BAETAE and n=2-8 are found to be unstable in the NBI plasmas with weak shears and low pressure gradients. The measured results are also consistent with the GFLDR-E, and the modes are more unstable |s| is smaller in low pressure gradient regions. These modes have possibly opposite effects on the ITB formation. The trapped electrons potentially enhance these modes unstable. It is worth emphasizing that the study of AITG/KBM with or without fast particles should be paid more attention because they link to the ITB and H-mode pedestal physics for weak magnetic shears.

时间：11月16日 14：00

地点：313楼会议室

报告人1: 钟武律

题目：Experimental observation of turbulence transition and a critical gradient threshold for trapped electron mode in tokamak plasmas

摘要：

In HL-2A and J-TEXT ohmic confinement regimes, an electrostatic turbulence with quasi-coherent characteristics in spectra of density fluctuations was observed by multi-channel microwave reflectometers. These quasi-coherent modes were detectable in a large plasma region (r/a ~0.3-0.8). Their frequencies were in the range of 30–140 kHz. They propagated in the electron diamagnetic direction. In the plasmas with the quasi-coherent modes, trapped electron mode (TEM) was predicted to be unstable by gyrokinetic simulations. The combined experimental results demonstrated that the mode is a quasi-coherent TEM. It was shown that the TEM is survived in the linear ohmic confinement regime of plasmas. The quasi-coherent TEM was replaced by broad-band fluctuations when the plasma transits from linear to saturated ohmic confinement regime. The observation was strongly related to the turbulence transition from TEM to ion temperature gradient mode. A critical gradient threshold for TEM excitation in electron temperature gradient was found. The study directly revealed the effect of TEM on particle transport.

报告人2: 赵开君

题目：Co-current toroidal rotation-driven and turbulent stresses with resonant magnetic perturbations in the edge plasmas of the J-TEXT tokamak

摘要：

The acceleration of the co-current toroidal rotations around resonant surfaces by resonant magnetic perturbations (RMPs) through turbulence is presented. With RMPs, the flat electron temperature T_e profile due to magnetic islands appears around resonant surfaces. When the resonant surface is closer to the last closed flux surface, the flat T_e profile vanishes with RMPs. In both cases, the toroidal rotations significantly increase in the direction of the plasma current around resonant surfaces. The characteristics of turbulence are significantly affected by RMPs around resonant surfaces. Turbulence intensity profile changes and the poloidal wave vector k_θ increases with RMPs. The power fraction of the turbulence components in the ion diamagnetic drift direction increases with RMPs. The measurements of turbulent Reynolds stresses are consistent with that the toroidal flows can be driven by turbulence. The estimations of the energy transfer between turbulence and toroidal flows suggest that turbulence energy transfers into toroidal flows. The result has the implication of the intrinsic rotation being driven by RMPs via turbulence.

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