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 β_N ≈ 5 at ℓ_i ≈ 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 β_N > 5.5. Confinement is above the H-mode level with H_98(y,2) ≈ 1.8. At q₉₅ ≈ 7.5, the current is overdriven, with bootstrap current fraction f_BS ≈ 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 ≈ 1 is a promising option with f_BS ≈ 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 q₉₅ = 4.8, so far reaching f_NI = 0.7 and f_BS = 0.4 at β_N ≈ 3.5 with performance appropriate for the ITER Q=5 mission, H₈₉β_N/q₉₅² ≈ 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 β_N = 4, ℓ_i = 1.07 and f_BS = 0.5, and at β_N = 5 with the vacuum vessel wall.