Confinement Scaling and Rotation Dependence in DIII-D Negative Triangularity Plasmas

While DIII-D negative triangularity (NT) plasmas have energy confinement times (τ_E) similar to positive triangularity (PT) H-mode plasmas, first-of-their-kind experiments for NT have found different scalings with normalized gyroradius (ρ_*) and collisionality (ν_*) and similar scaling with toroidal rotation. NT tokamak plasmas are a potentially transformative reactor scenario as they combine high τ_E with a stable edge (NT-edge) that inhibits H-mode, lacks dangerous edge transients, and has naturally low impurity confinement. Recent experiments in DIII-D used a modified first wall to allow operation of high-power, diverted plasmas with the largest achievable NT. A key goal of this work was determining the NT τ_E scalings as these are needed to ascertain the viability of NT reactor scenarios. Dimensionless parameter scaling experiments holding β, q, etc. constant measured ρ_* scaling that is closer to Bohm than gyro-Bohm, with the thermal transport for ions being closer to Bohm than that for electrons. The scaling with ν_* was minimal. These scaling results, though subject to some limitation due to being in a relative infancy when compared to PT scaling investigations, more closely resemble the less favorable PT L-mode scalings than PT H-mode scalings. As in many PT scenarios, τ_E decreased as injected torque went from all co-current to nearly balanced (~reactor levels). The decrease was 20-40% in q₉₅~2.8 discharges but only 10-20% in q₉₅~4.2 discharges. Even at low rotation though, excellent MHD stability was observed. Also, the favorable, low impurity confinement was maintained at multiple rotation levels. For the NT scenario, these results present challenges to its scalability while also illustrating some of its naturally attractive properties.