Rotation of non-axisymmetric halo current in disrupting plasmas

Large rotating halo current (HC) generated during tokamak disruptions may damage next step fusion devices, like ITER and SPARC, if the non-axisymmetric HC rotation resonates and amplifies mechanical stresses on structures surrounding the plasma. An empirical estimate for the expected HC rotation, based on data from C-Mod, NSTX, ASDEX Upgrade, DIII-D, and JET, projected the average HC rotation in ITER will be above 20 Hz [1]. However, the scaling parameters used to define this empirical scaling were insensitive to variations of the toroidal magnetic field. In this presentation, we present a new drift-frequency-based scaling law 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)) [2]. This scaling law is motivated by the faster HC rotation observed in the HBT-EP tokamak, with B_t = 0.35 T, while also being consistent with observations from other tokamaks used previously. The new scaling indicates non-axisymmetric HC will rotate more slowly in ITER (10 Hz) and will rotate near 60 Hz in SPARC, similar to that previously estimated [4]. The new scaling is also important in light of models for the rotation rate [3], because it implies the mechanism for HC rotation must be ExB flow. Even within a single HBT-EP discharge, as the minor radius decreased, the HC rotation increases approximately as 1/a² demonstrating that the rotation associated with the open field lines of the HC must be primarily poloidal but slower than the speed previously estimated [1].

[1] C.E. Myers et al 2018 Nucl. Fusion 58 016050.

[2] A.R. Saperstein et al 2022 Nucl. Fusion 62 026044.

[3] A.H. Boozer 2015 Phys. Plasmas 22, 102511.

[4] R. Sweeney et al 2020 J. Plasma Phys. 86, 865860507.