H-mode threshold for ohmic plasmas in TCV

Under boronised wall conditions, ohmic H-mode transitions in TCV are observed in a variety of SN, DN and limited equilibria provided that in the SN configurations the ion ∇B drift be directed towards the X-point. With ohmic heating alone, since the heating power depends on the density via the loop voltage, it is difficult to determine a power threshold for the transition. A clear density threshold is, however, observed for configurations with easy access to the H-mode. This threshold can vary strongly with time and appears to be strongly influenced by wall recycling, leading to poor reproducibility in experiments designed to quantify the threshold dependence on plasma shape parameters. For a series of discharges in which recycling effects were minimised, an increase in the density threshold of 80% was observed as the upper triangularity of a SNL configuration was increased from 0.14 to 0.26. (author) 4 figs., 4 refs