I-mode pedestal relaxation events at ASDEX Upgrade

TL;DR

This paper investigates I-mode pedestal relaxation events at ASDEX Upgrade, revealing their characteristics, stability conditions, and similarities to type-I ELMs, with implications for future reactor designs like ARC.

Contribution

It provides the first detailed analysis of I-mode pedestal relaxation events, including stability, transport properties, and divertor energy deposition, with projections for reactor applications.

Findings

Events are associated with edge profile relaxations at the pedestal top.

Transport during events resembles type-I ELMs in timescale and filament behavior.

More energy is deposited on the outer divertor target during these events.

Abstract

The I-mode confinement regime can feature small edge temperature drops that can lead to an increase in the energy deposited onto the divertor targets. In this work, we show that these events are associated with a relaxation of both electron temperature and density edge profiles, with the largest drop found at the pedestal top position. Stability analysis of edge profiles reveals that the operational points are far from the ideal peeling-ballooning boundary. Also, we show that these events appear close to the H-mode transition in the typical I-mode operational space in ASDEX Upgrade, and that no further enhancement of energy confinement is found when they occur. Moreover, scrape-off layer transport during these events is found to be very similar to type-I ELMs, with regard to timescales ($\approx$ 800 $\mu$s), filament propagation, toroidally asymmetric energy effluxes at the midplane and asymmetry between inner and outer divertor deposited energy. In particular, the latter reveals that more energy reaches the outer divertor target. Lastly, first measurements of the divertor peak energy fluence are reported, and projections to ARC - a reactor designed to operate in I-mode - are drawn.