I-mode pedestal relaxation events in the Alcator C-Mod and ASDEX Upgrade tokamaks

TL;DR

This study compares pedestal relaxation events in I-mode plasmas across Alcator C-Mod and ASDEX Upgrade tokamaks, revealing their conditions, energy impacts, and relation to plasma parameters, with implications for future fusion devices.

Contribution

It provides a comparative analysis of PREs in two tokamaks, linking their occurrence and energy loss to plasma parameters and testing a model for energy fluence predictions.

Findings

PREs occur mainly near the H-mode transition.

Energy loss increases as pedestal collisionality decreases.

The model bounds PRE energy fluence, aiding future device projections.

Abstract

In some conditions, I-mode plasmas can feature pedestal relaxation events (PREs) that transiently enhance the energy reaching the divertor target plates. To shed light into their appearance, characteristics and energy reaching the divertor targets, a comparative study between two tokamaks $-$ Alcator C-Mod and ASDEX Upgrade $-$ is carried out. It is found that PREs appear only in a subset of I-mode discharges, mainly when the plasma is close to the H-mode transition. Also, the nature of the triggering instability is discussed by comparing measurements close to the separatrix in both devices. The PRE relative energy loss from the confined region increases with decreasing pedestal top collisionality $\nu_{\mathrm{ped}}^*$. In addition, the relative electron temperature drop at the pedestal top, which is related to the conductive energy loss, rises with decreasing $\nu_{\mathrm{ped}}^*$. Finally, the peak parallel energy fluence due to the PRE measured on the divertor in both devices is compared to the model introduced in [1] for type-I ELMs. The model is shown to provide an upper boundary for PRE energy fluence data, while a lower boundary is found by dividing the model by three. These two boundaries are used to make projections to future devices such as DEMO and ARC.