碎裂弹丸注入后托卡马克大破裂防护效率的研究

Shattered pellet injection (SPI) is the baseline concept for disruption mitigation system (DMS) in ITER and other tokamaks, the aim of which is to introduce huge amount of hydrogen isotopes or impurities uniformly into the plasma by injecting pellet fragments, thus deplete the large thermal and magnetic energy stored within the plasma homogeneously by radiative losses, so as to prevent localized energy deposition on the device (e.g. localized heat flux to plasma facing components by plasma deposition or runaway electrons beam strike, or, in a burning plasma, the fast alpha particle strike) which could cause substantial harm to the device. The efficiency of disruption mitigation is strongly influenced by the interplay between the MHD modes and injected fragment, as the ablated material from the latter cool the plasma down by dilution or radiation, thus create current perturbation and destabilize the former, while the former govern the temperature and density profile evolution which determines the ablation of the latter. The density and radiation power density asymmetry created by this interplay very much determine the outcome of the mitigation, but the understanding regarding the above process is still lacking. To address those potential problems, we will numerically explore several proposed SPI injection schemes and configurations into several different ITER and CFETR target plasmas by 3D non-linear reduced magneto-hydrodynamic (MHD) code JOREK in this research. We will look into the coupling between macroscopic MHD modes and injected fragments, and study the convective transport of injected particles by those modes. Moreover, the impact of different injection configurations and different target plasma equilibria on the density and radiation asymmetry will be investigated and compared, their further implication on the wall heat flux inhomogeneity will be analyzed. Further, a test particle module will be coupled with the MHD results to investigate the deceleration and transportation of fast particles during the mitigation. Such analysis will provide valuable insight for future SPI system in future tokamak designs.

碎裂弹丸注入是托卡马克破裂防护系统的主要手段之一。其本质是通过注入弹丸碎片快速地向等离子体内均匀地引入大量粒子，从而令其快速冷却并将原本存储于其体内的能量通过辐射均匀地耗散掉。其最终目的是通过上述方式避免能量在破裂后大量、局域地沉积在面等离子体材料上从而对装置造成损害。碎裂弹丸注入的破裂防护效率深受碎片与磁流体模式相互作用，及其导致的密度与辐射功率密度不均匀性的影响，而这方面的研究尚不深入。为更好理解注入后的物理过程，本研究将通过三维磁流体数值模拟研究磁流体模式与注入粒子的耦合及其对流输运，考察ITER及CFETR等离子体参数下各注入位形与不同靶等离子体对注入后密度不均匀性与辐射不均匀性的影响，并分析由此导致的壁表面热流不对称性。此外，本研究还将在磁流体模拟结果的基础上利用试验粒子模型考察快粒子在碎裂弹丸注入后的减速与输运行为。上述研究将为未来燃烧等离子体装置中的破裂防护策略提供参考。

散裂弹丸注入是当今与未来托卡马克中的主流破裂防护手段之一。为实现更为有效的破裂防护，我们有必要深入理解散裂弹丸注入后弹丸碎片的消融、穿深过程，宏观磁流体模式的响应及其对注入物质的输运以及上述要素对于最终破裂防护效率的影响。本项目重点开发能够准确描述多种注入位型下的碎裂弹丸在高温等离子体中消融过程的数值模块、消融释放出的杂质粒子的非电离平衡描述的数值模块，并将上述模块应用于三维约化磁流体代码JOREK中，用以考察在ITER装置的低、高约束模式下几种不同注入位型下碎裂弹丸与宏观磁流体模式之间的相互作用，探索上述磁流体模式的解稳机制及其对注入粒子的输运机制，从而探索如何实现更高的热猝灭防护效率。本项目顺利完成了上述数值模块的开发，使得对ITER及其他装置中真实的碎裂弹丸注入模拟成为可能。在弹丸碎片与磁流体模式相互作用方面，本项目的相关数值模拟表明弹丸碎片注入后导致的在有理面上的螺旋形辐射冷却与电流扰动会造成共振模式的强烈解稳，这一机制与冷却导致的环对称电流箍缩共同作用激发一系列广谱磁流体模式。我们还发现这些磁流体模式可能对注入粒子有很强的横越磁力线对流输运作用，导致良好的芯部粒子混合，从而提高破裂防护效率。这一输运作用的强弱取决于模式激发时粒子消融沉积的位置相对于模结构所在的位置。在q=1或q=2面内的消融沉积往往能导致好的芯部穿透。在辐射冷却方面，我们的非电离平衡杂质模块发现弹丸碎片周围区域刚刚消融释放的低电离态杂质对辐射功率密度贡献最为明显，而当其随时间流逝趋于电离平衡后辐射功率会趋于下降。在ITER这类大型装置中，我们发现慢速、高表面积-体积比的碎片注入模式会更快的导致电子温度的辐射崩塌，从而触发更强烈的磁流体响应与热猝灭，而反过来快速、小表面积-体积比的注入模式有利于提高碎片在触发热猝灭前的穿深。我们还发现环向多位置注入有利于缓解辐射不对称性并提高防护效率。上述成果帮助我们设计更高效的破裂防护路径。