An upper pressure limit for low-Z benign termination of runaway electron beams in TCV

TL;DR

This paper models the particle balance during the benign termination of runaway electrons in TCV, revealing how runaway electron density influences free electron levels and heat flux spread, but the cause of the neutral pressure limit remains unclear.

Contribution

The study introduces a model linking runaway electron density and free electron levels to plasma behavior during benign termination in tokamaks, based on experimental data from TCV.

Findings

Runaway electron density significantly impacts free electron levels.

Higher neutral pressures increase target particles for ionization.

The cause of the neutral pressure limit is still unknown.

Abstract

We present a model for the particle balance in the post-disruption runaway electron plateau phase of a tokamak discharge. The model is constructed with the help of, and applied to, experimental data from TCV discharges investigating the so-called ``low-Z benign termination'' runaway electron mitigation scheme. In the benign termination scheme, the free electron density is first reduced in order for a subsequently induced MHD instability to grow rapidly and spread the runaway electrons widely across the wall. We show that the observed non-monotonic dependence of the free electron density with the measured neutral pressure is due to plasma re-ionization induced by runaway electron impact ionization. At higher neutral pressures, more target particles are present in the plasma for runaway electrons to collide with and ionize. Parameter scans are conducted to clarify the role of the runaway electron density and energy on the free electron density, and it is found that only the runaway electron density has a noticeable impact. While the free electron density is shown to be related to the spread of heat fluxes at termination, the exact cause for the upper neutral pressure limit remains undetermined and an object for further study.