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

TL;DR

This paper presents high-resolution measurements of halo currents during plasma disruptions in Alcator C-Mod using embedded Langmuir probes, revealing detailed spatial and temporal current distributions and their relation to plasma boundary reconstructions.

Contribution

It introduces a dense array of Langmuir probes for detailed, high-speed measurements of halo currents, providing new insights into their spatial structure and plasma resistivity during disruptions.

Findings

Detailed spatial resolution of halo current distribution (~1 cm)

Correlation of halo current movement with plasma boundary reconstructions

Estimates of sheath resistance and plasma resistivity during disruptions

Abstract

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.