Effect of detachment on Magnum-PSI ELM-like pulses: I. Direct observations and qualitative results

TL;DR

This study investigates how increasing neutral pressure in a linear plasma device affects ELM-like pulses, showing that higher pressure leads to more energy dissipation in the volume and can prevent plasma-target interaction.

Contribution

It provides direct observations of ELM-like pulse behavior during detachment in a linear plasma device, highlighting the stages of energy dissipation and the impact of neutral pressure.

Findings

Higher neutral pressure increases energy removal from ELM-like pulses.

At sufficient pressure, ELM-like pulses do not affect the target, dissipating energy in the volume.

The dissipation process occurs in three observable stages.

Abstract

Conditions similar to those at the end of the divertor leg in a tokamak were replicated in the linear plasma machine Magnum-PSI. The neutral pressure in the target chamber is then increased to cause the target to transition from an attached to a detached state. Superimposed to this steady state regime, ELM-like pulses are reproduced, resulting in a sudden increase in plasma temperature and density, such that the heat flux increases transiently by half an order of magnitude. Visible light emission, target thermography, and Thomson scattering are used to demonstrate that the higher the neutral pressure the more energy is removed from the ELM-like pulse in the volume. If the neutral pressure is sufficiently high, the ELM-like pulse can be prevented from affecting the target and the plasma energy is fully dissipated in the volume instead (ID 4 in Table 1). The visible light images allow the division of the ELM-plasma interaction process of ELM energy dissipation into 3 "stages" ranging from no dissipation to full dissipation (the target plasma is detached). In the second publication related to this study, spectroscopic data is analysed with a Bayesian approach, to acquire insights into the significance of molecular processes in dissipating the plasma energy and particles.