Threshold power for the transition into H-mode for H, D, and He plasmas in TCV

The threshold power for the transition into H-mode with hydrogen (H), deuterium (D), and helium (He) as majority ion species has been evaluated from a series of dedicated experiments on the tokamak TCV. Identical plasma configurations with a single-null X-point and favorable direction of the ion ∇B drift have been chosen. The input power was varied via the plasma current and L–H transitions were obtained with Ohmic heating alone. Under these conditions and for electron densities in the range of 6–7 · 1019 m−3 the threshold power compared to D increased by 1.75 for H and 1.45 for He, respectively. For D and He, the measured power levels are in good agreement with the predictions of the commonly used scaling law. In the case of H, transitions into H-mode were observed already at power levels of about 80% of the expected threshold power.Our results have also been analyzed on the basis of a physics-based scaling, which includes more parameters and applies to all ion species. Using the case of D as reference, we find that the increase in threshold power for He follows the predictions. For H there is a noticeable disagreement which may partly be explained by uncertainties in the relevant plasma parameters. The new scaling implies a strong dependence on the values of the electron temperature at the separatrix. For the present study, only data up to a normalized radius of 0.95 were available. More precise measurements of the edge temperature profiles may help to resolve the issue.