Scenario Development and MHD Stability of Negative Triangularity Plasmas in DIII-D*

Novel negative triangularity (NT) L-mode scenarios have been developed to study stability and performance as functions of q₉₅, at q₉₅=3 and 4. Furthermore, novel access to a state with very small or no sawteeth and no fishbones was developed in NT L-mode, similar to hybrid scenarios in positive triangularity (PT). This state was previously observed transiently during high performance NT shots (H₉₈=1.15). Cases at both q₉₅s described here encounter m/n=2/1 tearing modes (TMs) at ~90% normalized pressure (b_N) of previously predicted ideal wall limits ~3.1. Establishing a stationary hybrid-similar scenario at q₉₅=4 to study stability required attempting early 3/2 TM onset, observed late in flattop at first. Experiments described here achieve good performance in NT L mode on flattops of up to 5 current relaxation times, τ_R≈0.6 s. This duration is sufficient to study MHD stability and scenario performance in stationary conditions, limited only by hardware availability at 7-10MW NBI, 2.2MW ECH. Confinement at both q₉₅ values reaches standard H-mode levels (H₉₈~0.95) without n=2 tearing modes. At q₉₅=4, n=2 TMs saturate at low amplitude and decrease confinement by ~13%, consistent with PT plasmas at comparable q₉₅. At q₉₅=3, n=2 TMs grow to large amplitude, causing b_N collapses. This behavior is comparable to the impact of n=2 TMs in the q₉₅=4 PT hybrid vs the q₉₅=3 ITER baseline scenario. Projections based on 0.5D rho* scaling shows both cases perform well with high-field reactor parameters akin to the ARC design (B_t=9 T, I_P=7.5-10 MA), yielding 400MW of fusion power, with fusion gain Q~10, b_N~1.5, and low input power P_aux~40 MW. These results mark significant progress toward development of NT fusion reactor scenarios and identify avenues for improvement. The latter include larger, optimized plasma shapes to improve performance, better n=1 error field correction, increased density to reduce fast ion losses, and tradeoffs between TMs and sawtoothing regimes.