Density Limits as Disruption Forecasters for Spherical Tokamaks

Fusion power output from spherical tokamaks would benefit from increased confined plasma density, but there exists a limit on the density before confinement is lost and the plasma current is disrupted. This density limit has long been characterized by a simple, global Greenwald limit proportional to the plasma current and inversely proportional to the cross sectional area of the plasma. It is shown that in the database of discharges from the NSTX and MAST spherical tokamaks, the likelihood of disruption does increase above the Greenwald limit, and especially in the plasma current rampdown phase. The physics of the density limit has been recently theoretically explored through local criteria. Several of these are tested using the disruption event characterization and forecasting (DECAFTM) code for their potential effectiveness as disruption warning signals. For a limited set of NSTX discharges, a local island power balance criteria was found to be less reliable, presently, than the Greenwald limit. An empirical critical edge line density and a boundary turbulent transport limit were both tested for MAST-U, which has an electron density profile measurement with high spatial resolution in the outer part of the plasma. Both were found to have similar dependencies on key plasma parameters. In a limited set of MAST-U discharges that appear to disrupt due to rising density at values under the Greenwald limit, crossing of the boundary turbulent transport limit occurred close to the time of disruption. Finally, these limits were evaluated for their potential use in real-time, and it was found that with the necessary real-time inputs and with refinement through further testing, these limits could be implemented in a real-time disruption forecasting system.

球形托卡马克（spherical tokamaks）的聚变功率输出可通过提升约束等离子体密度得到优化，但在约束丧失与等离子体电流破裂前，密度存在上限。长期以来，该密度极限以简单的全局格林沃尔德极限（Greenwald limit）为表征，其与等离子体电流成正比，与等离子体横截面积成反比。研究表明，在NSTX与MAST球形托卡马克的放电数据库中，破裂发生概率确实在超过格林沃尔德极限后升高，尤其在等离子体电流下降阶段。近期，已有学者通过局部准则对密度极限的物理机制开展理论探索。本研究借助破裂事件表征与预测（DECAFTM）代码，对多款此类准则作为破裂预警信号的潜在有效性进行了测试。针对有限组NSTX放电实验，当前结果显示局部岛功率平衡准则的可靠性不及格林沃尔德极限。针对MAST-U——其在等离子体外部区域具备高空间分辨率的电子密度剖面测量能力——我们分别测试了经验临界边缘线密度与边界湍流输运极限，二者均表现出对关键等离子体参数的相似依赖关系。在有限组因密度上升且该密度低于格林沃尔德极限而发生破裂的MAST-U放电实验中，边界湍流输运极限的跨越时刻与破裂时刻极为接近。最后，我们评估了上述极限用于实时预警的潜力，结果显示，在配备必要实时输入数据并经进一步测试优化后，这些极限可应用于实时破裂预测系统。