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.

本研究借助SPIRAL蒙特卡洛（SPIRAL Monte Carlo）全粒子轨道模拟代码[Kramer，PPCF 2013]，针对SPARC基准参考放电[Creely，JPP 2020；Rodriguez-Fernandez，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α粒子功率损失的影响。基于接近热猝灭（thermal quench）的H模（H-mode）等离子体参数，选取具备夸张模宽的TF波纹、m/n=2/1与3/2型NTMs模型扰动，加载至模拟的SPARC磁平衡位形（magnetic equilibrium）中，并对其中的示踪粒子（marker particles）开展轨道追踪。3/2型与2/1型NTMs分别位于ρpol~0.76与ρpol~0.86处，其位置恰好可强化α粒子向ρpol~0.8以外的等离子体外部损失区域的输运，而该区域内生成了超过95%的损失α粒子[Scott，JPP 2020]。研究结果表明，总α粒子功率损失率从最低的1.73%小幅提升至最高的2.34%；同时受NTMs与TF波纹的影响，α粒子表面功率密度会在低场侧（lowfield side）中平面附近的第一壁（first-wall）上形成局部热斑。本研究为典型平顶稳态（flattop operation）运行工况下，环向对称第一壁的α粒子表面功率密度设定了保守上限，并为SPARC三维限幅器（limiter）表面的设计需考量NTMs的影响提供了理论依据。