Summary of MHD instabilities and internal transport barriers for the condition of $q_{min} \simeq$ 2 in the EAST tokamak
Summary of MHD instabilities and internal transport barriers for the condition of $q_{min} \simeq$ 2 in the EAST tokamak
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DOI：
10.7498/aps.72.20230721
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发表时间：
2023
影响因子：
1
通讯作者：
Xu Ming;Xu Li-Qing;Zhao Hai-Lin;Li Ying-Ying-Li-Ying-Ying-2224215740;Zhong Guo-Qiang;Hao Bao-Long;Ma Rui-Rui-Ma-Rui-Rui-2222642933;Chen Wei
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文献类型：
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作者：
Xu Ming;Xu Li-Qing;Zhao Hai-Lin;Li Ying-Ying-Li-Ying-Ying-2224215740;Zhong Guo-Qiang;Hao Bao-Long;Ma Rui-Rui-Ma-Rui-Rui-2222642933;Chen Wei
内部输运势垒(ITB)结构的建立和维持是磁聚变等离子体的重要保障。简要总结了建立和维持Q_(Min)为2的ITBS的相关物理过程：离轴锯齿(OAS)和双撕裂模不稳定性、快离子诱导的Alfvén本征模、热压梯度诱导的低频模(LFMS)不稳定性等.首先，“离轴锯齿”(OAS)的爆发是评价Q_(Min)倒转的重要判据.详细地给出了OAS的激发条件、分类和前驱模的结构.并且崩塌事件由$m$/$n$=2/1双撕裂模式(DTM)的磁重连触发。其次，在OAS振荡过程中，快离子容易激发β诱导的Alfvén本征模(BAE)和反向剪切Alfvén本征模(RSAE)。两种Alfvén波的环向模数为1$\leq n\leq$5，位于1.98$\leq R\leq$2.07 m处，归一化小半径为0.2$\leq\rho$0.45。研究了qmin-simeq2条件下的激发条件，并考虑了热压梯度、快离子分布函数和环状流或流剪切三个不同的物理量。第三，振荡过程中的压力梯度会激发低频模式的不稳定性。采用广义鱼骨式色散关系(GFLDR)求解LFMS的基本特征：1 LFMS尺度频率与离子抗磁频率的关系；2 LFMS具有Alfvén极化方向；3 LFMS为反应型动力学气球模式。LFMS的激发不依赖于快离子，快离子发生在较高的压强梯度区：α-(1+-tau)(1+-eta_i)，$-tau=T_e/T_i，$-eta_i=L_(N_I)/L_(T_I)。最终实现了对OAS的抑制和ITBS的建立。对于EAST中Q_(Min)\simeq$2的情况，包含三个重要的过程：1与垂直注入(NBI1R)相比，切向注入(NBI1L)更容易抑制OAS：2在OAS振荡过程中可以抑制微不稳定，反向剪切的$Q$-剖面更有利于ITBS结构的建立；3ITBS的建立伴随着Alfvén波不稳定性的激发(较大环向模数：1$\leq\leq$5)，ITBS的持续伴随着热离子温度梯度诱导的不稳定性(中位数：5$\leq n\leq$10)。因此，了解OAS的建立和抑制、Alfvén波不稳定性的激发和快离子的重分布以及与之相关的热压梯度不稳定性，对于建立ITBS具有重要意义。
Establishment and sustainment of the structure of internal transport barriers (ITBs) is an important guarantee for the magnetic fusion plasmas. The related physics process for the establishing and sustaining of ITBs with $q_{min} \simeq$ 2 is simply summarized:the "off-axis sawteeth" (OAS) and double tearing modes instability, fast ions induced Alfvén eigenmodes, thermal pressure gradient induced low-frequency modes (LFMs) instability, etc. Firstly, the burst of "off-axis sawteeth" (OAS) is an important criterion for the evaluating of reversed $q$-profile with $q_{min} \simeq$ 2. The excitation conditions, classification and the structure of precursor mode of OAS are given in detailed, and the collapse event is triggered by the magnetic reconnection of $m$/$n$=2/1 double tearing modes (DTM). Secondly, the beta-induced Alfvén eigenmodes (BAEs) and reversed shear Alfvén eigenmodes (RSAEs) are easily excited by the fast ions during the oscillation of OAS. The toroidal mode numbers of the two kinds of Alfvén waves are 1 $\leq n \leq$ 5, which are located at 1.98 $\leq R \leq$ 2.07 m with normalized minor radius 0.2 $\leq \rho \leq$ 0.45. The excitation conditions are investigated for the condition of $q_{min} \simeq$ 2:and three different physical variables of thermal pressure gradient, fast ions distribution function, and the toroidal flow or flow shear are considered. Thirdly, the low-frequency modes (LFMs) instabilities are excited by the pressure gradient during the oscillation of OAS. The general fishbone like dispersion relationship (GFLDR) is adopted for solving the basic features of LFMs:① the frequency of LFMs scale with ions diamagnetic frequency; ② the LFMs has the Alfvén polarization direction; ③ the LFMs are reactive-type kinetic ballooning modes. The excitation of LFMs is not relied on the fast ions, which is taken place at higher pressure gradient regime $\alpha \propto (1 + \tau) (1 + \eta_i)$, $\tau=T_e/T_i$, $\eta_i=L_{n_i}/L_{T_i}$. In the end, the suppression of OAS and establishment of ITBs are achieved. Three important processes are contained for the condition of $q_{min} \simeq$ 2 in EAST:① the tangential injection (NBI1L) of NBI is more easier for the suppression of OAS in comparison with the perpendicular injection (NBI1R); ② the micro-instability can be suppressed during the oscillation of OAS, and the reversed shear $q$-profile is more favorable in the establishment of the structure of ITBs; ③ the establishment of ITBs is accompanied by the excitation of Alfvén waves instabilities (bigger toroidal mode number:1 $\leq n \leq$ 5), the sustainment of ITBs is accompanied by the thermal ions temperature gradient induced instability (median size:5 $\leq n \leq$ 10). Therefore, understanding the establishment and suppression of OAS, the excitation of Alfvén wave instability and the redistributed fast ions, the related instability of thermal pressure gradient, which are important for the establishment of ITBs.