Profile evolution of edge parameters during L/H and H/L mode transitions on compass-D

Abstract

On COMPASS-D the profile evolution of electron density, electron temperature, perpendicular electric field, poloidal impurity velocity, and neutral density at the outboard mid-plane has been investigated during ohmic single null diverted (SND) discharges undergoing an L/H, H/L, and another L/H transition. This was achieved using a novel combined diagnostic consisting of thermal helium beam and Doppler spectrometry, sharing the same lines of sight. To generate the transitions first an ELM-free H-mode phase was produced by terminating the gas fuelling. Within this H-mode a stepped gas puff was applied triggering an intermediate L-mode phase followed by a second ELM-free phase. It was found that on COMPASS-D the L/H transition is not preceded by a change in velocity or electric field shear. The maximum electric field shear of ∇ ▽E _ψ ≈2×10³kV/m² develops after the L/H transition around the 95% flux surface. Although the neutral density at the separatrix is of the ordern _D ≈ 10¹⁷m⁻³ no indication of a local influence could be concluded. However, a correlation between the decay of the shear and the gas puff was observed.