H-mode access during plasma current ramp-up in TCV

A recent TCV experiment has investigated the dependence of the L H transition threshold power on the plasma current ramp-rate and the X-point height above the divertor target, which both have previously been seen to affect the transition behaviour. Systematic scans in ohmically heated plasmas do not show any dependence on the plasma current ramp-up rate. In contrast, the threshold power is found to increase by a factor of two while the X-point is moved from about 10 cm up to 35 cm above the vessel floor. However, further increase, up to 60 cm, does not lead to any further increase of the required power. The Fundamenski et al model is tested against the measurements. Estimates of the Wagner number (Wa) at L H transitions are generally close to unity, in accordance with the model. In contrast, estimates of Wa before the L H transition, i.e. in L-mode, do not show the expected evolution towards unity.