J-TEXT Seminar No.25

时间：11月16日 8：30

地点：二楼会议室

报告人: 聂林

题目1：Experimental evaluation of Langmuir probe sheath potential coefficient

摘要1：

Systematic calibration experiment of Langmiur probe sheath potential coefficient Λ, which is a critical coefficient for estimating plasma sheath potential, has been carried out in the HL-2A & J-TEXE tokamak Flat carbon probe was used in order to obtain good I-V characteristics. It was found that in Equilibrium measurement, the estimated Λ coefficient monotonically increased from ~2.2 to ~2.9 while Langmuir probe is moved from 40 mm outside last-closed-flux-surface (LCFS) to 20 mm inside LCFS. This measured coefficient is closed to the commonly used value Λ = 2.8 for hydrogen plasmas, which is often assumed to be a constant throughout plasma. But In fluctuation measurement, which was used 30kHz swept probe, the result is different. It shows that the temperature effect can be ignored and the fluctuation of floating potential is similar with plasma potential.

题目2：Observation of nonlinear energy exchange between turbulent and zonal flows

摘要2：

The non-linear interaction between low-frequency zonal flow (LFZF), geodesic acoustic mode (GAM) and turbulence are studied at last J-TEXT experimental campaign, as well as the competition relationship of these two branches of zonal flow on turbulence energy absorption. Recent experimental results show that, there exist a strong interaction between turbulence and GAM/LFZF via non-linear triple-wave coupling. The energy transfer rates among turbulence, ZFs and GAMs were measured during Ohmic discharge, taking advantage of reciprocal Langmuir probe array. The results show that the turbulent kinetic energy is transferred from turbulence with intermediate frequency into both ZFs and GAMs, and into fluctuations with higher frequency. However, as the frequency of ZF and GAMs are quite low, as well as the energy transfer calculation needs thousands of realizations, the energy exchange between GAMs and ZFs was not clearly observed.

时间：11月16日 14：00

地点：二楼会议室

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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