A quasi-continuous exhaust scenario for a fusion reactor: the renaissance of small edge localized modes

TL;DR

This paper investigates small edge localized modes (ELMs) in tokamaks, showing they can provide quasi-continuous exhaust with good confinement, and explores their stability through ideal and non-linear simulations, suggesting their suitability for future fusion reactors.

Contribution

It presents a comprehensive analysis of small ELM regimes, combining ideal ballooning stability calculations and non-linear simulations including E×B flow shear, highlighting their potential for reactor exhaust.

Findings

Small ELMs are unstable against ballooning modes near the LCFS.

Second ballooning stability correlates with enhanced confinement.

Non-linear simulations show ballooning-like fluctuations in high triangularity limit.

Abstract

Tokamak operational regimes with small edge localized modes (ELMs) could be a solution to the problem of large transient heat loads in future fusion reactors because they provide quasi-continuous exhaust while keeping a good plasma confinement. A ballooning mode mechanism near the last closed flux surface (LCFS) governed by an interplay of the pressure gradient and the magnetic shear there has been proposed for small ELMs in high density ASDEX Upgrade and TCV discharges. In this manuscript we explore different factors relevant for plasma edge stability in a wide range of edge safety factors by changing the connection length between the good and the bad curvature side. Simultaneously this influences the stabilizing effect of the local magnetic shear close to the LCFS as well as the $E \times B$ flow shear. Ideal ballooning stability calculations with the HELENA code reveal that small ELM plasmas are indeed unstable against ballooning modes very close to the LCFS but can exhibit second ballooning stability in the steep gradient region which correlates with enhanced confinement. We also present first non-linear simulations of small ELM regimes with the JOREK code including the $E \times B$ shear which indeed develop ballooning like fluctuations in the high triangularity limit. In the region where the small ELMs originate the dimensionless parameters are very similar in our investigated discharges and in a reactor, making this regime the ideal exhaust scenario for a future reactor.