Effects of neoclassical tearing modes and toroidal field ripple on lost alpha power in the SPARC tokamak

Abstract Using the SPIRAL Monte Carlo, full particle-orbit simulation code [Kramer PPCF 2013], we investigate the effects of neoclassical tearing modes (NTMs) and toroidal field (TF) ripple on alpha power losses in the SPARC primary reference discharge [Creely JPP 2020, Rodriguez-Fernandez JPP 2020]. Model perturbations for TF ripple and the m/n = 2/1 and 3/2 NTMs with widths selected based on an H-mode thermal quench upper-bound are added to a simulated SPARC magnetic equilibrium through which marker particles are tracked. The 3/2 and 2/1 NTMs are located at ρpol ∽ 0.76 and ρpol ∽ 0.86 respectively, well positioned to increase alpha particle transport into and within an outer lossy region of the plasma beyond ρpol ∽ 0.8 where over 95% of lost alpha particles are born[Scott JPP 2020]. Total alpha power losses are shown to increase modestly from 1.73% lost at a minimum to 2.34% lost at a maximum, and alpha particle surface power densities form localized hotspots on the first-wall near the low-field side midplane due to NTMs and TF ripple. We establish an upper-bound for first-wall alpha surface power densities on a toroidally symmetric wall simulating a locked 2/1 mode scenario and motivate the consideration of NTMs in the design of three dimensional limiter surfaces for SPARC.