EDA H-mode in ASDEX Upgrade: scans of heating power, fueling, and plasma current

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Abstract
Electron cyclotron resonance heating (ECRH) staircase discharges in strongly shaped plasmas were performed at the full-tungsten ASDEX Upgrade tokamak to investigate the enhanced D α (EDA) H-mode, a high-confinement regime without edge localized modes (ELMs) that exhibits numerous desirable qualities for future reactors. Heating power, fueling, and plasma current scans reveal rich dynamics as the plasma traverses different confinement regimes. The L-H transition typically occurs with a brief I-phase, sometimes followed by a short nonstationary ELM-free H-mode, before the quasi-coherent mode (QCM) sets in, marking the start of the EDA H-mode. After the pedestal fully develops, the plasma remains stationary until the heating power is raised above a certain threshold, causing ELMs. A novel criterion based on the normality of the divertor shunt current distribution is introduced to identify phases with ELMs, showing general applicability under a wide range of discharges and conditions. The no-ELM power boundary is found to increase with fueling, and too little deuterium gas puff results in a pathological nonstationary ELM-free H-mode without the QCM. Empirical scalings are derived for core, pedestal, and global parameters in EDA H-mode. These show, for example, that pedestal electron pressure increases sublinearly with power and almost quadratically with current. Line-averaged density is approximately proportional to plasma current but very weakly affected by power and fueling, whereas energy confinement time decreases sublinearly with power and increases supralinearly with current. The EDA H-mode achieves several reactor-relevant dimensionless parameters, most notably high Greenwald fraction and confinement enhancement factor over the entire heating power range. This dataset constitutes a versatile resource to plan EDA experiments in present and upcoming devices, also serving as a testbed for validating physics-based theories and models of the regime. Overall, the EDA H-mode remains promising and could become an important no-ELM scenario in future reactors such as SPARC and the full-tungsten ITER.

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