In this paper, we review the thermal plasma confinement and transport properties observed and predicted in low aspect ratio tokamaks, or spherical tokamaks (STs), which can depart significantly from those observed at higher aspect ratio. In particular, thermal energy confinement scalings show a strong, near linear dependence of energy confinement time on toroidal magnetic field, while the dependence on plasma current is more modest, the opposite of what is seen at higher aspect ratio. STs have revealed a very strong improvement in normalized confinement with decreasing collisionality, much stronger than at higher aspect ratio, which bodes well for an ST-based fusion pilot plant should this trend continue at an even lower collisionality than has already been accessed. These differences arise because of fundamental differences in transport in STs due to the more extreme toroidicity (i.e. reduced region of bad curvature), and to the relatively larger shearing rates, both of which can suppress electrostatic drift wave instabilities at both ion and electron gyroradius scales. In addition, electromagnetic effects are much stronger in STs because they operate at high βT. Gyrokinetic (GK) studies, coupled with low- and high-k turbulence measurements, have shed light on the underlying physics controlling transport. At lower βT, both ion- and electron-scale electrostatic drift turbulence may be responsible for transport. At higher βT, microtearing, kinetic ballooning, and hybrid trapped electron/kinetic ballooning modes increasingly play a role, and they have a much stronger impact in the core of ST plasmas than at higher aspect ratio. Flow shear affects the balance between ion- and electron-scale modes. Non-linear GK simulations find regimes where the electron heat flux decreases with decreasing collisionality, consistent with the experimental global normalized confinement scaling. The ST is unique in that the relatively low toroidal magnetic field allows for localized measurements of electron-scale turbulence, and this coupled with turbulence measurements at ion-scales has facilitated detailed comparisons with GK simulations. These data have provided compelling evidence for the presence of ion temperature gradient and electron temperature gradient turbulence in some plasmas, and direct experimental support for the impact of experimental actuators like rotation shear, density gradient and magnetic shear on turbulence and transport.
本论文回顾了低环径比托卡马克（即球形托卡马克，ST）中热等离子体约束特性及输运现象的观测与预测结果，这些特性可能显著偏离高环径比装置中的行为。具体而言，热能量约束定标显示，能量约束时间对环向磁场的依赖接近线性且较强，而对等离子体电流的依赖则相对较弱，这与高环径比装置中的趋势相反。球形托卡马克在碰撞率降低时表现出归一化约束性能的显著提升，其改善幅度远大于高环径比装置；若在更低碰撞率下仍保持该趋势，则对基于ST的聚变 pilot 装置十分有利。这些差异源于ST中极端环径比（即 bad curvature 区域减小）导致的输运机制根本不同，以及相对更大的剪切率，二者均可抑制离子和电子回旋尺度上的静电漂移波不稳定性。此外，由于ST运行于高β_T（环向β），电磁效应显著增强。回旋动理学（GK）模拟结合低k和高k湍流测量，揭示了底层输运物理。在较低β_T下，离子和电子尺度的静电漂移湍流可能主导输运；而在较高β_T下，微撕裂模、动理学气球模以及混合捕获电子/动理学气球模的作用逐渐增强，且这些模在ST芯部的影响远大于高环径比装置。流动剪切调节离子与电子尺度模之间的平衡。非线性GK模拟发现，随着碰撞率降低，电子热通量减小，这与实验观测到的全局归一化约束定标一致。ST的独特之处在于其相对较低的环向磁场使得对电子尺度湍流的局域测量成为可能，结合离子尺度湍流测量，为与GK模拟的详细对比提供了条件。这些数据强有力地证明了在某些等离子体中存在离子温度梯度（ITG）和电子温度梯度（ETG）湍流，并直接支持了旋转剪切、密度梯度和磁剪切等实验控制手段对湍流及输运的调控作用。