Kinetic-ballooning-limited pedestals in spherical tokamak plasmas

A theoretical model is presented that for the first time matches experimental measurements of the pedestal width-height Diallo scaling in the low-aspect-ratio high-β tokamak NSTX. Combining linear gyrokinetics with self-consistent pedestal equilibrium variation, kinetic-ballooning, rather than ideal-ballooning plasma instability, is shown to limit achievable confinement in spherical tokamak pedestals. Simulations are used to find the novel Gyrokinetic Critical Pedestal constraint, which determines the steepest pressure profile a pedestal can sustain subject to gyrokinetic instability. Gyrokinetic width-height scaling expressions for NSTX pedestals with varying density and temperature profiles are obtained. These scalings for STs depart significantly from that of conventional aspect ratio tokamaks.

本研究提出了首个可匹配低纵横比高β托卡马克NSTX中台基（pedestal）宽高Diallo标度律实验测量结果的理论模型。将线性回旋动理学（linear gyrokinetics）与自洽台基平衡演化相结合后，研究证实限制球形托卡马克台基可实现约束性能的是动理学气球模（kinetic-ballooning）等离子体不稳定性，而非理想气球模不稳定性。通过数值模拟得到了全新的回旋动理学临界台基约束条件，该条件可确定台基在回旋动理学不稳定性制约下所能维持的最陡压强剖面。针对密度与温度剖面各异的NSTX台基，我们推导出了回旋动理学宽高标度律表达式。此类球形托卡马克的标度律与传统纵横比托卡马克的标度律存在显著差异。