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

摘要

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 (0.6 × 10~(20) m~(-3) < n_e < 2.5 × 10~(20) m~(-3)), toroidal magnetic field (3.5 T < B_T < 5.4T) and plasma current (0.6 MA < I_p < 1.4 MA). In particular, data at low plasma density of n_e < 1.0 × 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_e, B_T and I_p are obtained. We found the P_(th) dependence on n_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_s, 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_e~xB_T~yI_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 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 (n_e ～ 0.6 × 10~(20) m~(-3)) 5.4 T discharge prior to L-H transition.