Prevention of disruption driven runaway electrons with a passive non-axisymmetric coil in the SPARC tokamak*

A significant challenge to the development of the tokamak as a fusion energy source is the unwanted formation of relativistic electron beams during a plasma disruption. These runaway electrons (RE) are driven by an inductive electric field during the current quench (CQ), and high current machines capable of confining burning plasmas may be particularly susceptible. The SPARC tokamak is designed as a compact (R₀ = 1.85 m and a = 0.57 m), high-field (B₀ = 12.2 T) tokamak capable of reaching Q > 2 in D-T fueled H-mode plasmas. With a flattop current of I_p = 8.7 MA, and a minimum expected disruption duration of τ_CQ > 3.2 ms, a loop voltage of ~5 kV may lead to RE beam currents of up to 4.3 MA if not mitigated. This work examines a non-axisymmetric runaway electron mitigation coil (REMC) that is passively driven by the disruption CQ and excites MHD modes which produce stochastic fields and thus scatter energetic electrons. A 3D finite element code (COMSOL) determines the current induced in the REMC from the plasma current decay (up to 590 kA) and resulting external magnetic field perturbations. The NIMROD 3D MHD code then models the excitation of plasma MHD in the presence of this perturbation. Orbit-following code ASCOT5 computes advection and diffusion coefficients of the radial transport of REs in a stochastic field. Finally, the 1D radial transport solver in the DREAM framework evolves the electric field and the RE generation throughout the disruption. Whereas candidate REMC toroidal symmetries of n = 2 and 3 show little to no mitigation of RE formation, the n = 1 REMC excites a rich spectrum of saturated plasma modes early (0.7 ms) in the CQ leading to a near fully stochastic magnetic field and complete prevention of RE beam formation. Design of the REMC is proceeding, including structural support against electromagnetic loads, and options for a disabling switch.