The effect of plasma beta on error-field penetration in radio-frequency wave heated plasmas in EAST
Cheng Ye, Youwen Sun, Huihui Wang, Yueqiang Liu, Tonghui Shi, Qing Zang, Tian-Qi Jia, Qun Ma, S. Gu, Nan Chu, Kaiyang He, M. Jia, Xue-Min Wu, Pengcheng Xie, Hui Sheng, Hua Yang, Lian-Sheng Huang, Biao Shen, Miaohui Li, Jinping Qian
Abstract
Abstract The plasma-beta effect on the n = 1 resonant magnetic perturbation (RMP) field penetration in purely radio-frequency (RF) wave heated discharges has been investigated in EAST. The experimental results show that the dependence of the threshold RMP coil current for field penetration, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>RMP,th</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:math> , on the total absorbed power P tot scales as approximately <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>I</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>RMP,th</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>∝</mml:mo> <mml:msubsup> <mml:mi>P</mml:mi> <mml:mrow> <mml:mrow> <mml:mtext>tot</mml:mtext> </mml:mrow> </mml:mrow> <mml:mrow> <mml:mn>0.30</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , indicating that the error-field tolerance is improved with increasing RF power. This is benefited by the increased electron perpendicular flow dominated by a counter-current electron diamagnetic flow with increasing RF power. However, theoretical scaling in cylindrical geometry overestimates the power index. Assuming an additional term <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mi>β</mml:mi> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> <mml:mrow> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:msub> <mml:mi>β</mml:mi> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:msubsup> </mml:math> for the normalized beta in the scaling, it is shown that the fitted <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>α</mml:mi> <mml:mrow> <mml:msub> <mml:mi>β</mml:mi> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:math> from the experimental observation is around −1, indicating a degradation effect of plasma beta. To clarify the underlying physics of the plasma-beta effect that was not included in the theoretical scaling in cylindrical geometry, the MARS-Q code with full toroidal geometry is employed for simulation of nonlinear field penetration (Liu et al 2013 Phys. Plasmas 20 042503). The MARS-Q simulation results reproduce the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>β</mml:mi> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> </mml:msub> </mml:math> dependence well, and hence the P tot scaling of the threshold current in experimental observations. The main reason for this is that the net total torque, which is mainly contributed by the neoclassical toroidal viscosity (NTV), increases with increasing plasma <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>β</mml:mi> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> </mml:msub> </mml:math> . The results demonstrate that the nonlinear toroidal coupling effect via NTV torque plays an important role in determining field penetration, even in cases with relatively low <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>β</mml:mi> <mml:mrow> <mml:mtext>N</mml:mtext> </mml:mrow> </mml:msub> <mml:mo>∈</mml:mo> <mml:mo stretchy="false">[</mml:mo> <mml:mn>0.3</mml:mn> <mml:mo>,</mml:mo> <mml:mn>0.6</mml:mn> <mml:mo stretchy="false">]</mml:mo> </mml:math> , which is far less than the no-wall beta limit.