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

On COMPASS-D the profile evolution of electron density, electron temperatur e, perpendicular electric field, poloidal impurity velocity, and neutral de nsity at the outboard mid-plane has been investigated during ohmic single n ull diverted (SND) discharges undergoing an L/H, H/L, and another L/H trans ition. This was achieved using a novel combined diagnostic consisting of th ermal helium beam and Doppler spectrometry, sharing the same lines of sight . To generate the transitions first an ELM-free II-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-fr ee 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 fiel d shear of delE(psi) approximate to 2 x 10(3) kV/m(2) develops after the L/ H transition around the 95% flux surface. Although the neutral density at t he separatrix is of the order n(D) approximate to 10(17) m(-3) no indicatio n of a local influence could be concluded. However, a correlation between t he decay of the shear and the gas puff was observed.