Plasma Performance and Operational Space without ELMs in DIII-D

TL;DR

This study compares various stationary no-ELM plasma regimes in DIII-D, analyzing their operational space and performance metrics, revealing key differences in constraints, pedestal behavior, and suitability for high-performance, high-power operation.

Contribution

It provides a comprehensive comparison of no-ELM regimes in DIII-D, highlighting operational constraints and performance characteristics, especially in relation to pedestal pressure and core rotation effects.

Findings

Normalized performance favors RMP, QH, and Neg-D regimes.

Absolute performance rises linearly with plasma current and magnetic field.

QH regime achieves higher pedestal pressure than RMP, with performance linked to core rotation and carbon fraction.

Abstract

A database of DIII-D plasmas without ELMs compares the operating space and plasma performance of stationary no-ELM regimes: RMP-ELM suppression, QH-mode, I-mode, EDA H-mode, L-mode, and negative triangularity L-mode (Neg-D). Operational space is documented in terms of engineering and physics parameters, revealing divergent constraints. Some operational space discriminants (such as pedestal collisionality) are well known, while others, such as low torque & safety factor, or high power & density, are less commonly emphasized. Normalized performance (confinement quality and normalized pressure) also discriminate the no-ELM regimes and favor the regimes tolerant to power in DIII-D: RMP, QH, and Neg-D. Absolute performance (triple product $\langle p\rangle\tau$) also discriminates no-ELM regimes and is found to rise linearly with IaB and also benefits from tolerance to power. The highest normalized performance using the metric $\langle p\rangle\tau$/IaB is found in QH and RMP regimes. Though comparable QH and RMP performance is found, the pedestal pressure (p_ped) is very different. p_ped in RMP plasmas is relatively low, and the best performance is found with a high core $\langle p\rangle$ fraction alongside high core rotation, consistent with an ExB shear confinement enhancement. p_ped of QH plasmas is significantly higher than RMP, and QH performance does not correlate with core rotation. However, the highest QH p_ped are found with high carbon fraction. While normalized performance of Neg-D plasmas is comparable to QH and RMP plasmas, the absolute confinement of Neg-D is lower, owing to low elongation and low p_ped achieved thus far. Only the EDA-H, Neg-D, and L-mode scenarios have approached divertor-compatible high separatrix density conditions, with Neg-D preserving the highest performance owing to its compatibility with both high power and density.