Changes in local confinement after an L-H transition in DIII-D

The details of the bulk transport in DIII-D across the L-H transition have been studied with a time dependent transport analysis of a series of repeat discharges. Time dependent profiles of ne, Te, Ti, Zeff, Vϕ and Prad were obtained for these discharges. The confinement properties were observed to improve in the plasma bulk after the transition. The dominant improvement is in the electron channel. The electron diffusivity profile flattens and drops to values comparable to those of the ion diffusivity. The changes in the ion diffusivity are within the error bars, thus preventing any definite conclusions, but after the transition the data show a consistent drop in the values. The drop in the electron diffusivity does not occur abruptly at the time of the transition; rather, the improvement starts within 30 ms after the transition and the improvement continues until at least 80 ms after the transition. The nature of changes in the electron density is similar to that of changes in the diffusivities, suggesting that the density gradient length is a critical quantity in the L-H transition. Experimental data are compared with a model for the ion temperature gradient (ITG) mode. Stabilization of the ITG mode does not seem to take place in the L-H transition, but the plasma might be marginally stable against the ITG mode in both the L-mode and the H-mode