Theory Weekly Highlights for March 2020

March 13, 2020

High poloidal beta (βp) DIII-D plasmas have been dynamically driven from q95 ∼ 10 to lower q95 ∼ 6 with the pre-existing internal transport barrier (ITB) and high performance persisting for five energy confinement times. Two distinct confinement states were observed: an H-mode state with a high edge pedestal, and an enhanced confinement state with a low pedestal and an ITB. Previously, the high βp scenario with an ITB and very high normalized confinement, H98,y2∼1.6, was reached only at low current with q95 ∼ 10 with βN ∼ 3 and βp ∼ 3. The higher current plasmas had βp ∼ 2, These conditions are projected to meet the ITER steady-state goal of Q= 5. The ITB is maintained at lower βp with a strong reverse shear, consistent with predictions that negative central shear can lower the βp threshold for the ITB. In resonant magnetic perturbation (RMP) experiments it was found from a scan of external amplitude that, when there are no large type-I ELMs, there is no transition to the enhanced confinement state. This is consistent with the proposed mechanism for ITB formation being a type-I ELM. Quasilinear gyro-Landau fluid predictive modeling of ITER with electromagnetic physics including the kinetic ballooning mode (KBM), suggests that only a modest negative central shear is required to achieve the ITB formation necessary for Q=5. A paper describing this, entitled “Transport at High βp and Development of Candidate Steady State Scenarios for ITER” has been published in Nuclear Fusion.

These highlights are reports of research work in progress and are accordingly subject to change or modification