Effects of Neoclassical Tearing Modes and Toroidal Field Ripple on Lost Alpha Power 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) and toroidal field (TF) ripple on alpha power losses during steady-state operation of 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 exaggerated widths selected based on an H-mode plasma approaching thermal quench 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 lowfield side midplane due to NTMs and TF ripple. We establish a conservative upper limit for first-wall alpha surface power densities on a toroidally symmetric wall for typical, flattop operation and motivate the consideration of NTMs in the design of three dimensional limiter surfaces for SPARC.

本研究采用SPIRAL蒙特卡洛（SPIRAL Monte Carlo）全粒子轨道模拟程序[Kramer等，《等离子体物理与受控核聚变》(PPCF) 2013]，针对SPARC基准参考放电[Creely等，《等离子体物理学报》(JPP) 2020；Rodriguez-Fernandez等，《等离子体物理学报》(JPP) 2020]的稳态运行工况，探究新经典撕裂模（neoclassical tearing modes，NTMs）与环向场（toroidal field，TF）波纹对阿尔法粒子功率损失的影响。基于接近热猝灭（thermal quench）的高约束模（H-mode）等离子体条件，选取放大后的模宽，为环向场波纹以及m/n=2/1、3/2型新经典撕裂模构建模型扰动，并将其叠加至模拟的SPARC磁场平衡位形中，随后对该位形内的标记粒子进行轨迹追踪。其中3/2型与2/1型新经典撕裂模分别位于极向磁面坐标ρpol≈0.76与ρpol≈0.86处，其位置恰好可增强阿尔法粒子向ρpol≈0.8以外的等离子体外层损耗区的输运，而该区域内诞生了超过95%的损耗阿尔法粒子[Scott等，《等离子体物理学报》(JPP) 2020]。研究结果表明，阿尔法粒子总功率损失率从最低的1.73%小幅升高至最高的2.34%；受新经典撕裂模与环向场波纹的影响，阿尔法粒子表面功率密度在第一壁的低场侧中平面附近形成局域化热点。本研究为典型平顶运行工况下的环向对称壁第一壁阿尔法粒子表面功率密度设定了保守上限，并提出在SPARC的三维限制器表面设计中应考虑新经典撕裂模的影响。