Progress towards sustainment of advanced tokamak modes in DIII-D

Improving confinement and β limits simultaneously in long pulse ELMy H mode discharges is investigated. The product β N H 98y serves as a useful figure of merit for performance, where β N ≡ β/(I/aB) and H 98y is the ratio of the thermal confinement time to the most recent ELMy H mode confinement scaling established by the ITER confinement database working group. In discharges with q 0 ≈ 1 (no sawteeth) and discharges with q min > 1.5 and negative central magnetic shear, β N ≈ 2.9 and H 98y ≈ 1.4 are sustained for up to 2 s. Although peaked profiles are observed, steep internal transport barriers are not present. Further increases in β N in these discharges are limited by neoclassical tearing modes (NTMs) in the positive shear region. In another recently developed regime, β N ≈ 3.8 and H 98y ≈ 2 have been sustained during large infrequent ELMs in non-sawtoothing discharges with q 0 ≈ 1. This level of performance is similar to that obtained in ELM free regimes such as VH mode. The limitation on β N and pulse length in these discharges is also the onset of NTMs.