Increased confinement and ideal stability limits at relatively high values of the internal inductance (ℓi) 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 βN ≈ 5 at ℓi ≈ 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 βN > 5.5. Confinement is above the H-mode level with H98(y,2) ≈ 1.8. At q95 ≈ 7.5, the current is overdriven, with bootstrap current fraction fBS ≈ 0.8, noninductive current fraction fNI > 1 and negative surface voltage. For ITER (which has a single-null divertor shape), operation at ℓi ≈ 1 is a promising option with fBS ≈ 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 fNI = 0.7 and fBS = 0.4 at βN ≈ 3.5 with performance appropriate for the ITER Q=5 mission, H89βN/q952 ≈ 0.3. Modeling studies explored how increased current drive power for DIII-D could be applied to maintain a stationary, fully noninductive high ℓi discharge. Stable solutions in the double-null shape are found without the vacuum vessel wall at βN = 4, ℓi = 1.07 and fBS = 0.5, and at βN = 5 with the vacuum vessel wall.
All Science Journal Classification (ASJC) codes
- Nuclear and High Energy Physics
- Condensed Matter Physics
- internal inductance
- tokamak operation