Investigation of Lost-alpha Power due to Neoclassical Tearing Modes in the SPARC Tokamak

Using the SPIRAL Monte Carlo, full particle-orbit simulation code [Kramer PPCF 2013], we investigate the effects of neoclassical tearing modes (NTMs) on power loss in the SPARC primary reference discharge [Creely JPP 2020, Rodriguez-Fernandez JPP 2020].  We predict increased alpha-power loss in the SPARC design due to NTMs synergizing with toroidal field ripple and orbital losses. Transport through the stochastic regions associated with the 2/1 and 3/2 NTMs, located at ρ_pol≈0.86 and ρ_pol≈0.76 respectively, are expected to increase particle flux into the lossy region of the plasma (ρ_pol>0.8). Alpha particles born in the lossy region are predicted to contribute to 95% of the total lost-alpha power in the SPARC tokamak [Scott JPP 2020], so increased particle flux into this region will increase power loss, leading to lower plasma performance. We investigate this prediction using the SPIRAL code to follow ensembles of particle paths through a simulated SPARC magnetic equilibrium perturbed by NTMs of widths up to a geometric limit.