ST40: Advancing the Physics Basis of Spherical Tokamak Reactors

The spherical tokamak ST40 [1] built and operated by Tokamak Energy Ltd, has recently demonstrated record ion temperatures above 100M K by operating at 0.5MA plasma current, 1.9 T magnetic field and 1.8MW neutral beam heating power [2]. On the way to the 100M K milestone, ST40 has also produced a wealth of data that allowed for further advances in the physics basis of spherical tokamaks. The large population of suprathermal ions from NBI proved to have an impact on both MHD and microturbulence stabilization. Gyrokinetic analysis has shown that energetic ions reduce turbulent ion transport in ST40, although remaining above neoclassical levels. Moreover, the coupling between turbulence and MHD is shown to lead to improved sawtooth control. The interplay between beam-driven Alfvénic modes of sweeping frequency (chirping modes) and H-L and L-H transitions in the global confinement of plasmas has been investigated together with the effect of beta and the ST geometry on toroidal Alfvén instabilities and ITG turbulence. Injection of Ar and Ne has led to improved confinement, confirming previous theoretical results [3]. Transport analysis has allowed the assessment of transport models including TGLF, Bohm gyro-Bohm, CDBM and CPTM. All models show predictions of the ion temperature close to the experimental values, however they fail to reproduce the scaling of confinement with toroidal magnetic field observed in STs [4]. The impact of the ST configuration on the width of the SOL is being studied with the support of the XGC1 global gyrokinetic code.