Quasi-linear gyrokinetic predictions of the Coriolis momentum pinch in NSTX

This paper presents quasi-linear gyrokinetic predictions of the Coriolis momentum pinch for low aspect-ratio NSTX H-modes where previous experimental measurements were focused. Local, linear calculations predict that in the region of interest (just outside the mid-radius) of these relatively high-beta plasmas, profiles are most unstable to microtearing modes that are only effective in transporting electron energy. However, sub-dominant electromagnetic and electrostatic ballooning modes are also unstable, which are effective at transporting energy, particles and momentum. The quasi-linear prediction of transport from these weaker ballooning modes, assuming they contribute transport in addition to that from microtearing modes in a nonlinear turbulent state, leads to a very small or outward convection of momentum, inconsistent with the experimentally measured inward pinch, and opposite to predictions in conventional aspect ratio tokamaks. Additional predictions of a low beta L-mode plasma, unstable to more traditional electrostatic ion temperature gradient-trapped electron mode instability, show that the Coriolis pinch is inward but remains relatively weak and insensitive to many parameter variations. The weak or outward pinch predicted in NSTX plasmas appears to be at least partially correlated to changes in the parallel mode structure that occur at finite beta and low aspect ratio, as discussed in previous theories. The only conditions identified where a stronger inward pinch is predicted occur either in the purely electrostatic limit or if the aspect ratio is increased. As the Coriolis pinch cannot explain the measured momentum pinch, additional theoretical momentum transport mechanisms are discussed that may be potentially important.

本文报道了低环径比NSTX（National Spherical Torus Experiment，国家球形环实验装置）高约束模（H-mode）等离子体中科里奥利动量箍缩（Coriolis momentum pinch）的准线性回旋动理学（quasi-linear gyrokinetic）预测，该研究聚焦于此前已开展实验测量的相关工况。局域线性计算表明，在这类高β等离子体的感兴趣区域（中半径外侧附近），等离子体分布对仅能输运电子能量的微撕裂模（microtearing mode）最为不稳定。然而，次主导的电磁及静电气球模（ballooning mode）同样处于不稳定区间，这类模可有效输运能量、粒子与动量。假设在非线性湍流态下，除微撕裂模外，这些较弱的气球模也对输运有贡献，则由这类较弱气球模得到的准线性输运预测，会给出极弱甚至向外的动量对流，这与实验测得的向内动量箍缩不符，且与常规环径比托卡马克中的预测结果相反。针对更典型的静电离子温度梯度-俘获电子模（ion temperature gradient-trapped electron mode）不稳定性的低β L模（L-mode）等离子体的额外预测显示，科里奥利箍缩虽为向内，但强度相对较弱，且对多数参数变化不敏感。NSTX等离子体中预测到的弱箍缩或向外箍缩现象，似乎至少部分与有限β及低环径比条件下出现的平行模结构变化相关，正如此前理论研究所讨论的那样。仅在两类条件下可预测到更强的向内箍缩：一是纯静电极限（purely electrostatic limit）工况，二是环径比增大的情况。由于科里奥利箍缩无法解释实测的动量箍缩，本文还讨论了其他可能具有重要意义的理论动量输运机制。