Progress towards sustainment of advanced tokamak modes in DIII-D

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Abstract
Improving confinement and beta limits simultaneously in long-pulse ELMy HÐmode discharges is investigated. The product βNH98y serves as a useful figure-of-merit for performance, where βN ©β/(I/aB) and H98y is the ratio of the thermal confinement time relative to the most recent ELMy HÐmode confinement scaling established by the ITER confinement database working group. In discharges with qmin > 1.5 and negative central magnetic shear, βNH98y~4 is sustained for ~1Ês. Although peaked profiles are observed, strong internal transport barriers are not present. Further increases in βN in these discharges is limited by neoclassical tearing modes (NTM) in the positive shear region. In another recently developed regime, βNH98y>6 has 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 VHÐmode. The limitation on βN and pulse length in these discharges is also the onset of NTMs. 1.ÊINTRODUCTION Advanced Tokamak (AT) operating modes have been successful in improving the fusion performance of many existing tokamaks, as evidenced by the record D-D fusion reactivity achieved in DIIIÐDÊ(1), JETÊ(2), and JT-60UÊ(3). Through optimization of the plasma shape and radial profiles, AT modes lead to improved confinement and β, and higher bootstrap fraction relative to standard ELMy HÐmode. Improvements are observed in many different AT regimes such as VHÐmode, negative central shear (NCS) with an internal transport barrier (ITB), supershots, high βp, and high li. To date, however, the duration of peak performance in all of these modes is limited to a few energy confinement times (τE), generally as a consequence of evolving pressure or current profiles and eventual MHD instability. Before AT modes can be seriously considered as an operating mode for a future fusion reactor, present experiments must sustain AT performance in a controlled manner for longer pulse lengths. In this paper, we review results of recent experiments on DIIIÐD directed towards this goal The primary focus is on improving the performance and pulse length of discharges with an ELMy edge. The ELMy HÐmode is inherently steady state, with the edge pand impurity concentration regulated by the repetitive ELM events. The ELMy HÐmode regime has been studied extensively on most tokamaks and the confinement results have been compiled into a database by the ITER confinement database working group. The most recent scaling for the thermal energy confinement time is given by τκ th 98y p 0.97 1.93 0.23 19 0.41 0.08 0.2 0.67 0.63 0.0365I R (a/R) n B M P =