Siliconization for wall conditioning and its effect on plasma performance in HL-2A tokamak

Wall conditioning with siliconization has been performed in the HL-2A tokamak to improve the plasma performance. It was based on the plasma chemical vapor deposition by using a four-anode glow discharge cleaning system and gas mixture of 90% He + 10% SiH4. It was found that the main components of the residual gas were CO, H2 and H2O. The amount of H2O was reduced from 49.5% to 24.5%. After siliconization, the impurity fluxes released from the first wall were reduced, especially the oxygen level (till to 10%) and high z impurities like Cu were decreased significantly. The total radiated power measured by the bolometer was decreased from 70% to 35%. A high density exceeding the Greenwald limit and energy confinement improvement have been achieved. The plasma operational region of HL-2A has been effectively broadened.