Addressing the carbon and nitrogen impurity screening issue on HL-2M snowflake minus and conventional divertor using SOLPS

This paper demonstrates the benefits of the HL-2M SF (snowflake) minus divertor on power exhaust and impurity screening compared with the LSN (lower single null) conventional divertor using SOLPS 5.0 code package. In order to estimate the effectiveness of the impurity screening capability in the HL-2M SF minus divertor, we analyze the impurity transport behavior with intrinsic carbon impurities (P-sol = 4 MW) and external injected nitrogen impurities (P-sol = 6 MW) in the LSN and SF minus divertors. The simulation results reveal that an input power of 4 MW and electron density of 2.5 x 10(19) m(-3) on the outer mid-plane can induce a nearly 80% pressure drop in the SOL (scrape-off layer) flux tube, representing the transition of the detachment regime in the SF minus divertor. For the higher power operation with Psol = 6 MW, a nitrogen gas puffing rate at 3 x 10(20) s(-1) is sufficient to exhaust excessive energy and facilitate detachment in the SF minus divertor. However, a higher gas puffing rate is necessary for the LSN divertor, indicating an improved power exhaust capability in the SF minus divertor. Furthermore, the maximum effective charge (Z(eff)) in the SF minus divertor SOL region is nearly 1.45 versus 3.5 in the LSN case with P-sol= 4 MW. The maximum Z(eff) in the SF minus divertor SOL region is below 2.4, versus 5.5 in the LSN case with the same impurity gas puffing rate when P-sol = 6 MW. Moreover, the impurity ions accumulate in the divertor region in the SF minus case, which demonstrates the effectiveness of the impurity screening capability in the HL-2M SF minus divertor. We find that the higher ionized state impurity ions contribute to the increase of Z(eff) and determine the impurity level upstream. On the one hand, the detached divertor regime has the benefit of suppressing the impurity particles ionizing to a higher ionized state. On the other hand, the ionization rate distribution and the poloidal velocity influenced by the friction and thermal forces determine the impurity transport behavior.