Recent high-q_min scenario development efforts on the DIII-D tokamak

Steady-state fusion pilot plant concepts are often designed around scenarios with elevated safety factor profiles. Exclusion of low order rational surfaces is thought to prevent dangerous 2/1 tearing modes and disallows sawtooth crashes. High-q_min plasmas in DIII-D can achieve safety factor values set forth in studies like CAT-DEMO¹ (q_min > 2, q₉₅ ~ 5–6.5), but have yet to reach requisite β_N (~3.5–4.5). The stage 1 “Shape and Volume Rise” (SVR) divertor, which increases elongation, triangularity, and volume, was predicted to improve high-q_min plasma performance via increased confinement quality, β_N limits, and pedestal pressure. However, experiments have found stronger shaping alone is insufficient to raise pedestal pressure, a key part of the predicted improvements. Two additional gyrotrons were installed concurrently with the SVR such that ~3.5 MW of electron cyclotron heating and current drive (ECH&CD) power was available. In the discharge evolution, shortly after ECCD injection, unintended internal transport barriers (ITBs) formed that were not observed in reference plasmas. These ITBs were all followed by tearing modes that locked, some of which cause full disruption, others of which unlock and minimally reduce confinement. Plasmas of the latter variety achieved, even surpassing in some cases, the flattop β_N targets. Qualities of high-q_min ITBs, MHD stability, and potential methods to create stable ITBs without NTMs are explored.

[1] R.J. Buttery et al. NF 61 046028 (2021)