High internal inductance for steady-state operation in ITER and a reactor

Increased confinement and ideal stability limits at relatively high values of the internal inductance (l(i)) have enabled an attractive scenario for steady-state tokamak operation to be demonstrated in DIII-D. Normalized plasma pressure in the range appropriate for a reactor has been achieved in high elongation and triangularity double-null divertor discharges with beta(N) approximate to 5 at l(i) approximate to 1.3, near the ideal n = 1 kink stability limit calculated without the effect of a stabilizing vacuum vessel wall, with the ideal-wall limit still higher at beta(N) > 5.5. Confinement is above the H-mode level with H-98(y,H- 2) approximate to 1.8. At q(95) approximate to 7.5, the current is overdriven, with bootstrap current fraction f(BS) approximate to 0.8, noninductive current fraction f(NI) > 1 and negative surface voltage. For ITER (which has a single-null divertor shape), operation at l(i) approximate to 1 is a promising option with f(BS) approximate to 0.5 and the remaining current driven externally near the axis where the electron cyclotron current drive efficiency is high. This scenario has been tested in the ITER shape in DIII-D at q(95) = 4.8, so far reaching f(NI) = 0.7 and f(BS) = 0.4 at beta(N) approximate to 3.5 with performance appropriate for the ITER Q=5 mission, H-89 beta(N)/q(95)(2) approximate to 0.3. Modeling studies explored how increased current drive power for DIII-D could be applied to maintain a stationary, fully noninductive high l(i) discharge. Stable solutions in the double-null shape are found without the vacuum vessel wall at beta(N) = 4, l(i) = 1.07 and f(BS) = 0.5, and at beta(N) = 5 with the vacuum vessel wall.