L–H transition studies on DIII-D to determine H-mode access for operational scenarios in ITER

Abstract

A comprehensive set of L–H transition experiments has been performed on DIII-D to determine the requirements for access to H-mode plasmas in ITER's first (non-nuclear) operational phase with H and He plasmas and the second (activated) operational phase with D plasmas. The H-mode power threshold, PTH, was evaluated for different operational configurations and auxiliary heating methods for the different main ion species. Helium plasmas have significantly higher PTH than deuterium plasmas at low densities for all heating schemes, but similar PTH as deuterium plasmas at high densities except for H-neutral beam injection-heated discharges, which are still higher. Changes in PTH are observed when helium concentration levels in deuterium plasmas exceed 40%. There is a strong dependence of PTH on the magnetic geometry in the vicinity of the divertor. The trend of decreasing PTH with decreasing X-point height is observed for all of the main ion species irrespective of the heating method, which appears to indicate that there is a common physics process behind this effect for all of the ion species. Helium and deuterium plasmas exhibit a significant increase in PTH for strong resonant magnetic perturbations. The application of a local magnetic ripple of 3% from test blanket module mock-up coils did not change PTH in deuterium plasmas.