Characterization of the ELM-free Negative Triangularity Edge on DIII-D

TL;DR

This study characterizes the unique edge behavior of negative triangularity plasmas in DIII-D, revealing their inherent ELM-free operation, stability limits, and potential advantages for fusion reactors due to their stable edge conditions.

Contribution

It provides the first comprehensive analysis of negative triangularity edge behavior on DIII-D, identifying critical triangularity thresholds and stability properties relevant for fusion reactor design.

Findings

Negative triangularity plasmas are inherently ELM-free at sufficient negativity.

A critical triangularity of about -0.15 marks the transition to ELM-free operation.

NT plasmas support small, steep-pressure-gradient pedestals with high core performance.

Abstract

Tokamak plasmas with strong negative triangularity (NT) shaping typically exhibit fundamentally different edge behavior than conventional L-mode or H-mode plasmas. Over the entire DIII-D database, plasmas with sufficiently negative triangularity are found to be inherently free of edge localized modes (ELMs), even at injected powers well above the predicted L-H power threshold. A critical triangularly ($\delta_\mathrm{crit}\simeq-0.15$), consistent with inherently ELM-free operation is identified, beyond which access to the second stability region for infinite-$n$ ballooning modes closes on DIII-D. It is also possible to close access to this region, and thereby prevent an H-mode transition, at weaker average triangularities ($\delta\lesssim\delta_\mathrm{crit}$) provided that at least one of the two x-points is still sufficiently negative. Enhanced low field side magnetic fluctuations during ELM-free operation are consistent with additional turbulence limiting the NT edge gradient. Despite the reduced upper limit on the pressure gradient imposed by ballooning stability, NT plasmas are able to support small pedestals and are typically characterized by an enhancement of edge pressure gradients beyond those found in traditional L-mode plasmas. Further, the pressure gradient inside of this small pedestal is unusually steep, allowing access to high core performance that is competitive with other ELM-free regimes previously achieved on DIII-D. Since ELM-free operation in NT is linked directly to the magnetic geometry, NT fusion pilot plants are predicted to maintain advantageous edge conditions even in burning plasma regimes, potentially eliminating reactor core-integration issues caused by ELMs.