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首页> 外文期刊>Plasma physics and controlled fusion >Magnetohydrodynamic helical structures in nominally axisymmetric low-shear tokamak plasmas
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Magnetohydrodynamic helical structures in nominally axisymmetric low-shear tokamak plasmas

机译：名义轴对称低剪切托卡马克等离子体中的磁流体动力学螺旋结构

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摘要

The primary goal of hybrid scenarios in tokamaks is to enable high performance operation with large plasma currents whilst avoiding MHD instabilities. However, if a local minimum in the safety factor is allowed to approach unity, the energy required to overcome stabilizing magnetic field line bending is very small, and as a consequence, large MHD structures can be created, with typically dominant m = n = 1 helical component. If there is no exact q = 1 rational surface the essential character of these modes can be modelled assuming ideal nested magnetic flux surfaces. The methods used to characterize these structures include linear and non-linear ideal MHD stability calculations which evaluate the departure from an axisymmetric plasma state, and also equilibrium calculations using a 3D equilibrium code. While these approaches agree favourably for simulations of ITER relevant hybrid regimes in this paper, the relevance of the ideal MHD model itself is tested through empirical examination of helical states in MAST and TCV. While long lived modes in MAST do not have island structures, some of the continuous mode oscillations exhibited in high elongation experiments in TCV indicate that resistivity may play a role in further weakening the ability of the tokamak core to remain axisymmetric. The simulations and experiments consistently highlight the need to control the safety factor in hybrid scenarios planned for future fusion grade tokamaks such as ITER.
机译：托卡马克混合方案的主要目标是在大等离子体电流下实现高性能运行，同时避免MHD不稳定。但是，如果允许安全系数的局部最小值趋近于统一，则克服稳定磁场线弯曲所需的能量非常小，因此，可以创建大型MHD结构，通常占主导地位的m = n = 1螺旋成分。如果没有精确的q = 1有理面，则可以假设理想的嵌套磁通量面对这些模式的基本特征进行建模。用于表征这些结构的方法包括：线性和非线性理想MHD稳定性计算，用于评估与轴对称等离子体状态的偏离，以及使用3D平衡代码的平衡计算。尽管这些方法在本文中对ITER相关混合方案的仿真非常有利，但理想的MHD模型本身的相关性是通过对MAST和TCV中的螺旋态进行的经验检验来检验的。虽然MAST中的长寿命模式没有岛状结构，但TCV的高伸长率实验中显示的一些连续模式振荡表明，电阻率可能会进一步削弱托卡马克核保持轴对称的能力。仿真和实验一致地强调了在计划用于未来融合级托卡马克如ITER的混合场景中控制安全系数的需求。

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