Scenario development for high β_p low torque plasma with q_min above 2 and large-radius internal transport barrier in DIII-D

A recent experiment on DIII-D, which was conducted by the joint research team from DIII-D and EAST, has extended the previous high ${\beta }_{\text{p}}$, high q _min regime, which has been tested in the 2013 DIII-D/EAST joint experiment, to inductive operation at higher plasma current ($I_{\text{p}}=0.8$ MA) and significantly higher normalized fusion performance ($G=H_{89}{\beta }_{\text{N}}/q_{95}^2=0.16$). The experiment aims at exploring high performance scenario with $q_{\text{min}}>2$ and reduced torque for long pulse operation, which can be potentially extrapolated to EAST. The effort was largely motivated by the interest in developing a feasible scenario for long-pulse high performance operation with low torque on EAST. Very high confinement, H ₈₉  =  3.5 or $H_{98,\text{y}2}=2.1$ with ${\beta }_{\text{N}}\sim 3.0$, has been achieved transiently in this experiment together with $q_{\text{min}}>2$ and reduced NBI torque ($3\sim 5$ N m). The excellent confinement is associated with the spontaneous formation of an internal transport barrier (ITB) in plasmas with $I_{\text{p}}=0.8$ MA at large minor radius (normalized $\rho \sim 0.7$) in all channels (n _e, T _e, T _i, $V_{\varphi }$, especially strong in the T _e channel). Fluctuation measurements show a significant reduction in the fluctuation levels, including AE modes and broadband turbulence, at the location where an ITB forms. Linear gyrokinetic simulations also support the decrease of the growth rate of the most unstable mode during strong ITB formation. The simulation implies that strong suppression of turbulence and a positive feedback loop may be active in this process and is responsible for the spontaneous formation of large-radius ITB. In an unstable ITB phase, an ELM crash is observed to have a positive effect on transient formation of large-radius ITB. The formation of this kind of ITB is found to have a shielding (protecting) effect on the core plasma while isolating the perturbation due to ELM crash. (paper)