Thermal energy mitigation and toroidal peaking effects in JET disruptions

Previous investigations on JET suggest half or less of plasma stored thermal energy Wth is radiated (⁠frad,th <~0.5⁠) using either massive gas injection (MGI) or shattered pellet injection (SPI) disruption mitigation. We investigate whether the apparent incomplete frad,th is explained by radiation peaking near the injection plume. High toroidal peaking throughout the pre-thermal quench is found in argon–deuterium MGI on JET, with typically >3× higher radiation near the injector than toroidally distant. Previously unexplained toroidal bolometry measurements in neon–deuterium SPI are reproduced with similar peaking using the Emis3D radiation analysis code. These observations align with results from Alcator C-Mod and KSTAR. This peaking is not captured by previous JET studies that found poor thermal mitigation. Two sets of neon–deuterium SPI and two sets of argon–deuterium MGI are analyzed using Emis3D. In SPI, frad,th rises from no-plume estimates of 0.31 and 0.66 to lower bounds of 0.84 and 0.92, respectively, and frad,th~1 is possible. In MGI, the toroidal spread of the peaking feature is poorly constrained. frad,th up to 0.85 and 0.65 are possible using the largest possible spread, increasing from 0.42 and 0.28, although frad,th~1 does not appear to be reached. Revised mitigation estimates on JET suggest a lower melt risk to the divertor in mitigated disruptions on ITER and SPARC than previously thought. However, peaking near injectors could increase flash melting risk on nearby plasma facing components.

此前针对欧洲联合环（Joint European Torus, JET）开展的研究显示，采用大体积气体注入（massive gas injection, MGI）或碎弹丸注入（shattered pellet injection, SPI）开展破裂缓解时，等离子体储存的热能Wth中仅有半数甚至更低份额以辐射形式耗散（即辐射份额frad,th <~0.5）。本研究旨在探究表观上不完全的frad,th是否由辐射在注入羽流附近聚集所导致。针对JET上的氩-氘MGI实验，我们发现在热猝灭前的整个阶段，等离子体辐射存在显著的环向峰值特征：注入器附近的辐射强度通常较环向远端高出3倍以上。借助Emis3D辐射分析代码，我们成功复现了氖-氘SPI实验中此前无法解释的环向辐射热测量数据，且得到了相似的环向峰值分布特征。上述观测结果与阿尔卡特C-Mod（Alcator C-Mod）及韩国超导托卡马克先进研究装置（Korea Superconducting Tokamak Advanced Research, KSTAR）的已有研究结论相符。此前JET相关研究因未捕捉到该峰值特征，得出了热缓解效果不佳的结论。本研究利用Emis3D代码分析了两组氖-氘SPI实验与两组氩-氘MGI实验的数据。结果显示，在SPI场景中，若计入注入羽流的辐射贡献，frad,th将从未考虑羽流时的0.31与0.66分别提升至0.84和0.92的下限值，甚至有可能达到frad,th≈1。在MGI场景中，峰值特征的环向分布范围约束较弱；当采用最大可能的分布范围时，frad,th最高可达0.85与0.65，较未考虑羽流时的0.42和0.28有所提升，但仍未达到frad,th≈1的理想状态。基于JET实验的修正后缓解评估结果显示，相较于此前的预期，国际热核聚变实验堆（International Thermonuclear Experimental Reactor, ITER）与SPARC装置的缓解型破裂事件中，偏滤器发生熔化的风险更低。不过，注入器附近的辐射峰值可能会提升邻近面向等离子体部件发生快速熔化的风险。