The effect of the impurity seeding into the closed divertor on plasma detachment in the HL-2A tokamak


 Recent progress towards an increased understanding of the detached divertor physics has been made with the very closed divertor geometry in HL-2A. Non-intrinsic impurities were injected into the outer divertor chamber, and increased divertor neutral pressure and enhanced volumetric energy loss in the divertor were observed. Meanwhile the neutral pressure in the main chamber decreased slightly, and neutral compression between divertor and main chamber increased largely. This led to divertor detachment with a low level of upstream plasma line-averaged electron density (~0.5n_GW). In H-mode, a little degradation of the core confinement, characterized by decrease of plasma stored energy and pedestal pressure, and increase of edge localized mode (ELM) frequency, was observed, but the H-mode was still sustained well with H98>1. The pedestal density fluctuation was increased during detachment, implying that the enhanced pedestal transport might be responsible for the degradation. During divertor detachment phase, the impurities were well controlled in the divertor without strong radiation near X-point region, and the main plasma density did not increase but decreases slightly, this could be beneficial from the very closed divertor geometry. The experimental results suggest that, the very closed divertor geometry has the advantages of volumetric energy loss, gas pumping and impurity control in the divertor without significant effects on the plasma confinement, thus gives a wider operation window for the divertor detachment.