On the minimum transport required to passively suppress runaway electrons in SPARC disruptions

In [V.A. Izzo et al 2022 Nucl. Fusion 62 096029], state-of-the-art modeling of thermal and current quench (CQ) MHD coupled with a self-consistent evolution of runaway electron (RE) generation and transport showed that a non-axisymmetric (n = 1) in-vessel coil could passively prevent RE beam formation during disruptions in SPARC, a compact high-field tokamak projected to achieve a fusion gain Q > 2 in DT plasmas. However, such suppression requires nite transport of REs within magnetic islands and re-healed flux surfaces; conservatively assuming zero transport in these regions leads to an upper bound of RE current ~1 MA compared to ~8.7 MA of pre-disruption plasma current. Further investigation finds that core-localized electrons, within r/a < 0.3 and with kinetic energies ~0.2-15 MeV, contribute most to the RE plateau formation. Yet only a relatively small amount of transport, i.e. a diffusion coefficient ~18 m^2/s, is needed in the core to fully mitigate these REs. Properly accounting for (i) the CQ electric field's effect on RE transport in islands and (ii) the contribution of significant RE currents to disruption MHD may help achieve this.

在[V.A. Izzo等人2022年发表于《核聚变（Nucl. Fusion）》62卷096029号的研究中]，针对热猝灭与电流猝灭（current quench, CQ）磁流体动力学（magnetohydrodynamics, MHD）的前沿建模，结合逃逸电子（runaway electron, RE）产生与输运的自洽演化，研究表明：在SPARC——一款预计可在氘氚（DT）等离子体中实现聚变增益Q>2的紧凑型高场托克马克——的等离子体破裂过程中，非轴对称（n=1）堆内线圈可被动阻止逃逸电子束的形成。然而，此类抑制效果需要磁岛与重愈合磁通面内的逃逸电子发生有限输运；若保守假设上述区域内无输运，则逃逸电子电流的上限约为1 MA，而破裂前等离子体电流约为8.7 MA。进一步研究发现，位于r/a<0.3区域、动能约为0.2~15 MeV的芯部局域电子，对逃逸电子平台的形成贡献最大。但仅需在芯部施加相对少量的输运（即扩散系数约为18 m²/s），即可完全缓解此类逃逸电子。若能合理考虑（i）电流猝灭电场对磁岛内逃逸电子输运的影响，以及（ii）显著逃逸电子电流对破裂磁流体动力学的贡献，或可实现这一目标。