First measurements of edge impurity flow across the L-H transition in TCV

Recent upgrades of the CXRS diagnostic, together with a low power diagnostic neutral beam injector, have, for the first time, enabled accurate measurement of the changes in the impurity intrinsic flow across the transition to ELM-free H-mode on TCV. Photon statistics limited the time resolution to 12 ms, for a 30 Hz measurement frequency.
A narrow (≈5.5 mm) and deep (|u_θ| ≥ 20 km/s) poloidal rotation well develops at the last closed flux surface, together with a pedestal in the density profile < 15 mm wide. The impurity temperature profile shows after the transition a uniform increase of about 100 eV, with no evident pedestal. The radial electric field E_r, computed from the impurity kinetic profiles, revealed the development of the E_r-well characteristic of H-mode as observed in other devices, whose width agrees with previous inter-machine scaling [McDermott, Phys. Plasmas 16 (2009)]. Estimates of the main ion flow suggests that the E_r well main contributor after the transition is the main ion density gradient term.
The measured impurity flows are in reasonable agreement with neoclassical analytical [Kim, Phys. Fluids B Plasma Phys. 3, 1991] and numerical NEO [E.A. Belli and J. Candy, Plasma Phys. Control. Fusion, 50:095010, 2008] predictions.