Modeling Expansion of the DIII-D Steady-State Operating Regimes

Equilibrium and transport-code-based modeling are being used to evaluate the benefits for DIII-D fully noninductive operation (f_NI=1) at high β_N and either high q_min or high ℓ_i of increasing the triangularity (δ), the plasma density, and/or the toroidal field strength. The δβ_N increase (to above 6) of the ideal n=1 stability limit enabled by a δ increase from 0.65 to 0.85 would ensure the required stability margin at f_NI=1. However, increasing δ at fixed q₉₅ results in lower bootstrap current fraction (f_BS) as the total bootstrap current increases more slowly than I_p. Core-edge coupling physics at the ITER pedestal pressure would be possible at B_T>3 T and n_e>0.85f_G. Achieving the ITER collision rate, though, requires the high B_T but lower n_e. Although the n_e gradient should most efficiently generate bootstrap current, at fixed β_N simply increasing n_e does not increase f_BS because the collision rate increases. f_NI=1 operation at high ℓ_i is examined in detail using transport-code-based modeling, initially by validating the TGLF transport model against experimental discharges.