Runaway electron deconfinement in SPARC and DIII-D by a passive 3D coil

The operation of a 3D coil (passively driven by the current quench loop voltage) for the deconfinement of runaway electrons is modeled for disruption scenarios in the SPARC and DIII-D tokamaks. Nonlinear MHD modeling is carried out with the NIMROD code including time-dependent magnetic field boundary conditions to simulate the effect of the coil. Further modeling in some cases uses the ASCOT5 code to calculate advection and diffusion coefficients for runaway electrons based on the NIMROD-calculated fields, and the DREAM code to compute the runaway evolution in the presence of these transport coefficients. Compared with similar modeling in Tinguely, et al [2021 Nucl. Fusion 61 124003], considerably more conservative assumptions are made with the ASCOT5 results, zeroing low levels of transport, particularly in regions in which closed flux surfaces have reformed. Of three coil geometries considered in SPARC, only the n = 1 coil is found to have sufficient resonant components to suppress the runaway current growth. Without the new conservative transport assumptions, full suppression of the RE current is maintained when the TQ MHD is included in the simulation or when the RE current is limited to 250kA, but when transport in closed ux regions is fully suppressed, these scenarios allow RE beams on the order of 1-2MA to appear. Additional modeling is performed to consider the effects of the close ideal wall. In DIII-D, the current quench is modeled for both limited and diverted equilibrium shapes. In the limited shape, the onset of stochasticity is found to be insensitive to the coil current amplitude and governed largely by the evolution of the safety-factor prole. In both devices, prediction of the q-prole evolution is seen to be critical to predicting the later time effects of the coil.

本研究针对SPARC与DIII-D托卡马克装置的破裂场景，对用于逃逸电子（Runaway Electrons）去约束的三维线圈（由电流猝灭环电压被动驱动）的运行特性开展建模。研究采用NIMROD程序开展含随时间变化磁场边界条件的非线性磁流体动力学（Magnetohydrodynamics，MHD）建模，以模拟该三维线圈的作用效果。部分场景下的进一步建模采用ASCOT5程序，基于NIMROD程序计算得到的磁场，求解逃逸电子的对流与扩散系数；同时借助DREAM程序，结合上述输运系数计算逃逸电子的演化过程。相较于Tinguely等人[2021，《核聚变》(Nucl. Fusion) 61, 124003]中的同类建模工作，本研究基于ASCOT5结果采用了更为保守的假设：将低水平输运过程置零，尤其是在闭合磁面重新形成的区域。在SPARC装置考虑的三种线圈构型中，仅n=1模线圈具备足够的共振分量，可抑制逃逸电流的增长。若不采用新的保守输运假设，当模拟中纳入电流猝灭（Current Quench，TQ）磁流体动力学过程，或将逃逸电流限制在250千安以内时，仍可实现逃逸电流的完全抑制；但当闭合磁通区域的输运过程被完全抑制时，此类场景下将出现量级为1~2兆安的逃逸电子束。本研究还开展了额外建模，以探究闭合理想壁的影响。在DIII-D装置中，本研究针对限制器位形与偏滤器位形两种平衡构型开展了电流猝灭建模。在限制器位形下，磁场随机化的起始时刻对线圈电流幅值不敏感，主要由安全因子分布的演化过程决定。对于两台装置而言，准确预测安全因子分布的演化过程，是精准预测线圈后续作用效果的关键。