Plasma Performance and Operational Space without ELMs in DIII-D

Abstract

A database of DIII-D plasmas without edge-localized modes (ELMs) compares the operating space and plasma performance of stationary no-ELM regimes found in conventional tokamaks: ELM suppression with resonant magnetic perturbations (RMPs), quiescent H-mode (QH, including wide-pedestal variant), improved confinement mode (I-mode), enhanced D-alpha H-mode (EDA-H), conventional L-mode, and negative triangularity L-mode (Neg-D). Operational space is documented in terms of engineering and physics parameters, revealing divergent constraints for each regime. 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 (volume-averaged pressure p , confinement time τ , and triple product p τ ) also discriminates no-ELM regimes and is found to rise linearly with IaB (a metric for the magnetic configuration strength, the product of current I, minor radius a, and field B that has units of force), and also benefits from tolerance to power. The highest normalized performance using the metric p τ /IaB is found in QH and RMP regimes. Focusing on ITER-shaped no-ELM plasmas, Q=10 at 15 MA scaled global performance is met with two out of four considered metrics, but only thus far at high torque. 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 p 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. Considering integration with electron cyclotron heating (ECH), the operational space for RMP and QH plasmas narrow, while that for EDA-H plasmas open, and the high p ped / p regimes (EDA-H and QH) preserve the highest performance. 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. Comparison to highlighted ELMing plasmas finds a clearer pedestal correlation to plasma performance, but also reveals that the peak performance of plasmas without ELMs is significantly lower in DIII-D, owing to limits in operational space accessed so far without ELMs.