The physics of ELM-free regimes in EUROfusion tokamaks

TL;DR

This paper investigates ELM-free operational regimes in EUROfusion tokamaks, focusing on negative triangularity and quasi-continuous exhaust, highlighting their potential for stable, long-term tokamak operation.

Contribution

It advances understanding of ELM-free regimes, especially NT and QCE, and develops access models based on ideal-MHD for these regimes across multiple tokamaks.

Findings

QCE regime achieves pedestal top values comparable to ELMing H-mode.

Minimum separatrix density correlates with experimental measurements.

QCE shows potential as an operational scenario for ITER and future reactors.

Abstract

The development of operational scenarios without large Type-I ELMs is of utmost importance for the stable operation and longevity of future tokamaks. The EUROfusion tokamak exploitation program has therefore made the understanding of ELM-free regimes a major topic of exploration across all its contributing devices (ASDEX Upgrade, JET, MAST-Upgrade, TCV, and WEST). An integrated program to investigate a range of Type-I ELM-free regimes has been developed covering the enhanced D-alpha (EDA), magnetic perturbations (MP), negative triangularity (NT), quasi-continuous exhaust (QCE), quiescent H-mode (QH), the baseline small ELMs (SE), I-mode, and X-point radiator (XPR) regimes. This contribution focuses on the development and understanding of the NT and QCE regimes on ASDEX Upgrade, JET, and TCV. The importance of transport via ballooning modes in both regimes is highlighted, as well as the progress in developing access models based on ideal-MHD. In the case of the QCE, this can also be expressed as a minimum separatrix density, which corresponds well to experimentally measured separatrix densities. Particular focus is paid to the performance of the QCE in terms of the achieved pedestal top values, which, when appropriately normalised, do not differ significantly from ELMy H-mode plasmas. This, combined with the predicted minimum separatrix density for the 15 MA ITER baseline plasma, highlight the relevance of the QCE as a potential operational scenario for both ITER and future reactors.