Progress towards sustainment of advanced tokamak modes in DIII-D

Abstract

Improving confinement and β limits simultaneously in long pulse ELMy H mode discharges is investigated. The product βN H98y serves as a useful figure of merit for performance, where βN ≡ β/(I/aB) and H98y 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 q0 ≈ 1 (no sawteeth) and discharges with qmin > 1.5 and negative central magnetic shear, βN ≈ 2.9 and H98y ≈ 1.4 are sustained for up to 2 s. Although peaked profiles are observed, steep internal transport barriers are not present. Further increases in βN in these discharges are limited by neoclassical tearing modes (NTMs) in the positive shear region. In another recently developed regime, βN ≈ 3.8 and H98y ≈ 2 have been sustained during large infrequent ELMs in non-sawtoothing discharges with q0 ≈ 1. This level of performance is similar to that obtained in ELM free regimes such as VH mode. The limitation on βN and pulse length in these discharges is also the onset of NTMs.