High-resolution disruption halo current measurements using Langmuir probes in Alcator C-Mod

Halo currents generated during disruptions on Alcator C-Mod have been measured with Langmuir “rail” probes. These rail probes are embedded in a lower outboard divertor module in a closely-spaced vertical (poloidal) array. The dense array provides detailed resolution of the spatial dependence (~1 cm spacing) of the halo current distribution in the plasma scrape-off region with high time resolution (400 kHz digitization rate). As the plasma limits on the outboard divertor plate, the contact point is clearly discernible in the halo current data (as an inversion of current) and moves vertically down the divertor plate on many disruptions. These data are consistent with filament reconstructions of the plasma boundary, from which the edge safety factor of the disrupting plasma can be calculated. Additionally, the halo current “footprint” on the divertor plate is obtained and related to the halo flux width. The voltage driving halo current and the effective resistance of the plasma region through which the halo current flows to reach the probes are also investigated. Estimations of the sheath resistance and halo region resistivity and temperature are given. This information could prove useful for modeling halo current dynamics.在阿尔卡特C-Mod（Alcator C-Mod）托卡马克装置的等离子体破裂事件中产生的晕电流，已通过朗缪尔（Langmuir）导轨探针完成测量。此类导轨探针嵌入于装置的下外侧偏滤器模块内，采用密间距垂直（极向）阵列排布。该阵列兼具高时间分辨率（400千赫兹数字化采样率），可对等离子体刮削区内晕电流分布的空间依赖性（测点间距约1厘米）实现精细分辨。当等离子体贴靠外侧偏滤器靶板时，晕电流数据中可清晰辨识出电流接触点（表现为电流反转特征），且在多数破裂事件中，该接触点会沿偏滤器靶板垂直向下移动。此类实验数据与等离子体边界的细丝重建结果相符，借此可计算得到破裂等离子体的边缘安全因子。此外，本研究还获取了偏滤器靶板上的晕电流‘足迹’，并将其与晕流通量宽度建立关联。同时，研究团队还对驱动晕电流的电压、以及晕电流流经以抵达探针的等离子体区域的有效电阻展开了探究。文中给出了鞘层电阻、晕区电阻率与温度的估算结果。该研究成果可为晕电流动力学建模提供有价值的参考依据。