Contrasting negative triangularity discharges with H-mode scenarios in DIII-D

Negative triangularity (NT) discharges are a scenario capable of high confinement, stable ELM-free operation and have a tolerance for high beta operation (β_N ~3). A deeper investigation of the database of discharges in DIII-D reveals several differences for NT-shaped plasmas compared to positive triangularity (PT) ones. The most striking is the larger inverse scale lengths, R/LT_e and R/LT_i , for the region 0.4 < r/a < 0.8, are typically higher than PT H-modes by factor of ~2. So while NT discharges have lower pedestals, pressure and stored energy build up faster as you move into the core, resulting in the same stored energy and hence very comparable factors of H_98y2≥1 for similar discharge parameters and heating. More stored energy is in the NT confinement zone and the pressure is more peaked, p0/<p> ~2, and this contributes to neutron rates, normalized to volume, that are on par with high performance H-modes. Also, confinement can be degraded by MHD modes or fast-ion losses but these are of no more consequence than PT H-modes. Moreover, in high power NT shots, as increased heating drives β_N close to 3, even with more beams the beta typically saturates without the plasma disrupting.