Scaling of H-mode threshold power and L–H edge conditions with favourable ion grad-B drift in Alcator C-Mod tokamak

This paper presents the results from a systematic study of low-to-high confinement transition (L–H transition) in Alcator C-Mod lower single null plasmas, with ion ∇ B drift in the favourable direction for H-mode access. The study is performed over a broad range of plasma density , toroidal magnetic field (3.5 T < B T < 5.4 T) and plasma current (0.6 MA < I p < 1.4 MA). In particular, data at low plasma density of , which were not included or carefully examined by earlier C-Mod studies, are highlighted in our analysis. With the large set of data, the scaling of H-mode threshold power ( P th ) and local plasma edge conditions with , B T and I p are obtained. We found the P th dependence on is nonlinear and exhibits a ‘U-shape’, which is not sensitive to I p variation, and affected by B T mainly at low density. The characterized L–H local edge conditions include the amplitude and gradient scale length of T e , n e profiles. In all, the scaling of L–H P th and edge conditions are complex, neither of which can be simply represented by a relation of the type . Local edge conditions are also evaluated in low-power L-mode and compared with the L–H companion. Noticeable differences between L-mode and L–H are mainly observed in T e at ψ = 0.95, and L n , L T near separatrix, while other parameters are found to be not significantly changed. Comparison with resistive-ballooning mode theory yields reasonable good agreement, such that the experimental data at ψ = 0.95 fit into the corresponding L-mode and H-mode domains, as well as L–H boundary predicted by theory. Finally, we see an edge T e pedestal emerging in a low density 5.4 T discharge prior to L–H transition.