Experimental study of the role of the edge radial electric field on the access to H-mode at ASDEX Upgrade

The underlying mechanism for the transition from L- to H-mode (LH transition) has been of interest for experimental and theoretical studies since the discovery of the H-mode. The gradients of the radial electric field, E_r , at the plasma edge are often considered to be responsible for the edge transport barrier and the access to H-mode.

In this study the edge parameters of L-mode plasmas which have significantly different LH power thresholds were compared. Plasmas with favorable and unfavorable drift configuration, deuterium versus hydrogen, and plasmas with the outer divertor leg on the horizontal target versus on the vertical target were examined. Special focus was put on the measurement of the outer E_r gradient, which connects the confined plasma and the scrape-off layer (SOL). For this a new diagnostics, based on the active spectroscopy of HeII, was used.

It is found that prior to the LH transition the outer E_r gradient normally shows little variation and only steepens once the LH transition has occurred. The evolution of the edge E_r profile follows the main ion pressure gradient. In L-mode also the toroidal plasma rotation has a non-negligible impact on the E_r profile. In particular, the intrinsic rotation increases with increasing edge collisionality, while E_r in the confined plasma decreases. The E_r profile in the SOL changes concomitantly, which indicates a strong interaction between SOL flows and the edge toroidal rotation. The direction of the intrinsic edge rotation follows the Pfirsch-Schlüter flows on the high field side of the SOL. A comparison with neoclassical theory suggests that the main ion poloidal rotation is on the neoclassical level, although in detail deviations are observed. In particular, the impact of rotation cannot explain the observed shallower E_r well in unfavorable compared to favorable drift configuration. E_r measurements in hydrogen show no dominant role of the inner or the outer E_r gradient in the LH transition.