Favourable scenarios established by SMBI for the realization of the ELMy H-mode at HL-2A

The ELMy H-mode plasmas realized with the supersonic molecular beam injection (SMBI) are studied in relation to the energy confinement and the heating power for the L-H transition (P_L-H) in the HL-2A tokamak. A database is assembled for this study based on the ELMy H-mode discharges during the experimental campaigns in the period 2009-2013 at the HL-2A tokamak. The statistical results show that the SMBI is favourable for reaching the H-mode by reducing the heating power at the L-H transition and for the H-mode performance by improving the energy confinement compared with the ordinary gas puffing (GP). The reduction of P_L-H is about 20% when the density is low, and the energy confinement enhancement factor of H_H98y2=τ_E/τ_th,98y2 ≈ 1.5 is achieved with the SMBI. Note that in the database the density dependence of P_L-H is non-monotonic with the n_e,min ≈ 3×10¹⁹ m^-3 at which the P_L-H is minimum. Most of P_L-H data are on the low density branch where the P_L-H increases with the decrease in density. The minimum of the P_L-H in HL-2A is comparable to the ITPA multi-machine threshold power scaling P_{thr_scal08}. The physics behind the reduction of the P_L-H with the SMBI is also investigated in relation to the change of the density gradient at the plasma edge, the gas fuelling efficiency, and the recycling.