Scaling of H-mode threshold power and L-H edge conditions with favorable 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 < BT < 5.4?T) and plasma current (0.6?MA < Ip < 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 (Pth) and local plasma edge conditions with , BT and Ip are obtained. We found the Pth dependence on is nonlinear and exhibits a 'U-shape', which is not sensitive to Ip variation, and affected by BT mainly at low density. The characterized L?H local edge conditions include the amplitude and gradient scale length of Te, ne profiles. In all, the scaling of L?H Pth 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 Te at ? = 0.95, and Ln, LT 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 ? = 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 Te pedestal emerging in a low density 5.4?T discharge prior to L?H transition.