Achieving Steady-State Conditions in the High-Beta Hybrid Scenarios in DIII-D

The natural attributes of the hybrid scenario, especially the anomalously broad current profile, with $q_{min} \agt 1$, allows steady-state conditions with zero surface loop voltage to be achieved at 1 MA plasma current in DIII-D. Using efficient central current drive, the surface loop voltage is driven down to zero for $>1\tau_R$, with $\sim$ 50\% bootstrap current fraction when $\beta_P$ is increased above 1.9. Interestingly, good alignment between the current drive and plasma current profiles is not necessary as the hybrid regime self-organizes the current density profile. Steady-state hybrid plasmas can achieve $\beta_N=3.6$ for the full duration of the NB pulse ($>1\tau_R$) without exciting the m/n=2/1 tearing mode, corresponding to $\beta_T$ up to 3.4\%. The thermal energy confinement time is excellent, with confinement factors up to $H_{98y2}=1.6$ even during strong EC heating. A 0-D physics model demonstrates that attractive scenarios with $Q_{fus}$=3.5-3.8 exist for steady-state operation in ITER and FNSF.