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 del B drift in the favourable direction for H-mode access. The study is performed over a broad range of plasma density (0.6 x 10(20) m(-3) < <(n)over bar>(e) < 2.5 x 10(20) m(-3)), toroidal magnetic field (3.5 T < B-T < 5.4 T) and plasma current (0.6MA < I-p < 1.4MA). In particular, data at low plasma density of <(n)over bar>(e) < 1.0 x 10(20) m(-3), 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 <(n)over bar>(e), B-T and I-p are obtained. We found the P-th dependence on (n) over bar (e) 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 (n) over bar (x)(e) B-T(y) I-p(z). 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 psi = 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 psi = 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 ((n) over bar (e) similar to 0.6 x 10(20) m(-3)) 5.4 T discharge prior to L-H transition.