Asymmetric halo current rotation scalings on HBT-EP and Alcator C-MOD*

Halo current (HC) rotation during disruptions can be potentially dangerous if resonant with the structures surrounding a tokamak plasma. A poloidal ExB drift-frequency-based scaling law has been proposed [1] for the rotation frequency of the asymmetric component of the HC as a function of toroidal field strength and plasma minor radius (f_rot ∝ 1/(B_T a² )). This scaling law is consistent with results reported for many tokamaks and is motivated by the faster HC rotation observed in the HBT-EP tokamak. It accurately describes the HC rotation as it evolves dynamically throughout the disruption in simple circular-limited plasmas like those on HBT-EP, and is seen to describe the average HC rotation in other plasmas like those on Alcator C-Mod. The 1/a² dependence of this scaling law, as well as a dependence on the edge safety factor (q_edge), suggest that the plasma predominantly rotates poloidally. Projections of the rotation frequency to ITER and SPARC parameters suggest the asymmetric HC rotation will be on the order of 10 Hz and 60 Hz, respectively, as opposed to the order of 2 kHz and 40 kHz observed on C-Mod and HBT-EP.

[1] A.R. Saperstein et al, Nucl. Fusion 62 (2022) 026044