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圆形截面等离子体是最基本的托卡马克等离子体形态,是磁约束聚变实验研究的基础位形.本文基于HL-2A装置限制器位形放电实验,研究了托卡马克圆形截面等离子磁流体动力学（MHD）平衡和MHD不稳定性.研究表明,当q0=0.95时,m/n=1/1内扭曲模总是不稳定的.轴安全因子q0和边缘安全因子qa的组合决定了等离子体的平衡位形,也影响着平衡的MHD稳定性,但其不稳定性增长率与比压(β)的大小相关联.在qa>2和q0稍大于1的条件下,可以容易地实现内扭曲模和表面扭曲模的稳定.但当q0超过1较多时,等离子体又变得不稳定, 且等离子体(扭曲)不稳定性强度随q0的继续增高而增强.随着极向比压(βp)的增加,MHD不稳定性会增强,MHD平衡位形横向拉长, Shafranov位移增加,这反过来又有抑制不稳定性的作用.计算发现, HL-2A圆形截面等离子体的运行比压极限约为βNc≅2.0. 较高的q0不利于MHD稳定性,引起比压极限降低.当q0=1.3时,我们得到最大βN约为1.8.最后,基于现有的圆形横截面等离子体,讨论了影响运行β的一些关键因素以及可期实现的高比压和理想比压极限之间的关系问题.
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关键词:
- 圆截面等离子体 /
- 磁流体(MHD)平衡 /
- MHD稳定性 /
- 扭曲模 /
- 比压极限
Circular cross-section plasma is the most basic form of tokamak plasma and the fundamental configuration for magnetic confinement fusion experiments. This article is based on the HL-2A limiter discharge experiments to study the magnetohydrodynamic (MHD) equilibrium and MHD instability of tokamak plasmas in circular cross-section. The results showed that when q0=0.95, the internal kink mode of m/n=1/1 is always unstable. The increase in plasma β (=the ratio of thermal pressure to magnetic pressure) can lead to the appearance of external kink modes. The combination of axial safety factor q0 and edge safety factor qa determines the equilibrium configuration of the plasma and also affects the MHD stability of the equilibrium, but its growth rate is also related to the size of β. Under the condition of qa>2 and q0 slightly greater than 1, it is easy to achieve the stabilization of internal and surface kink modes. However the plasma becomes unstable again and the instability intensity increases as q0 continues to increase when q0 exceeds 1. As the poloidal specific pressure (βp) increases, the MHD instability develops, the equilibrium configuration of MHD elongates laterally, and the Shafranov displacement increases, which in turn has the effect for suppressing instability. Calculations have shown that the maximum β value imposed by the ideal MHD mode in a plasma with free boundary in tokamak experiments is proportional to the normalized current IN (=Ip(MA)/a (m)B0(T)), and the maximum specific pressure is calibrated as β(max)~2.01IN. The operational β limit of HL-2A circular cross-section plasma is approximately βNc≈2.0. A too high q0 is not conducive to MHD stability and leads to a decrease in the β limit. When q0=1.3, we obtain a maximum β_N of approximately 1.8. Finally, based on the existing circular cross-section plasma, some key factors affecting the operational β and the relationship between achievable high β and the calculated ideal β limits were discussed.-
Keywords:
- circular cross-section plasma /
- magnetohydrodynamic (MHD) equilibrium /
- MHD instability /
- kink mode /
- β limit
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