Progress towards sustainment of advanced tokamak modes in DIII-D

摘要

Improving confinement and beta limits simultaneously in long-pulse ELMy H- mode discharges is investigated. The product (beta)(sub N)H(sub 98y) serves as a useful figure-of-merit for performance, where (beta)(sub N) (triple-bond) (beta)/(I/aB) and H(sub 98y) is the ratio of the thermal confinement time relative to the most recent ELMy H-mode confinement scaling established by the ITER confinement database working group. In discharges with q(sub 0) (approximately) 1 (no sawteeth) and discharges with q(sub min) > 1.5 and negative central magnetic shear, (beta)(sub N) (approximately) 2.9 and H(sub 98y) (approximately) 1.4 are sustained for up to 2 s. Although peaked profiles are observed, steep internal transport barriers are not present. Further increases in (beta)(sub N) in these discharges is limited by neoclassical tearing modes (NTM) in the positive shear region. In another recently developed regime, (beta)(sub N) (approximately) 3.8 and H(sub 98y) (approximately) 1.8 has been sustained during large infrequent ELMs in non-sawtoothing discharges with 1(sub 0) (approximately) 1. This level of performance is similar to that obtained in ELM-free regimes such as VH-mode. The limitation on (beta)(sub N) and pulse length in these discharges is also the onset of NTMs.