Density and impurity profile behaviours in HL-2A tokamak with different gas fuelling methods

摘要

The electron density profile peaking and the impurity accumulation in the HL-2A tokamak plasma are observed when three kinds of fuelling methods are separately used at different fuelling particle locations.The density profile becomes more peaked when the line-averaged electron density approaches the Greenwald density limit nG and,consequently,impurity accumulation is often observed.A linear increase regime in the density range ne＜0.6nG and a saturation regime in ne＞0.6nG are obtained.There is no significant difference in achieved density peaking factor fne between the supersonic molecular beam injection (SMBI) and gas puffing into the plasma main chamber.However,the achieved fne is relatively low,in particular,in the case of density below 0.7nG,when the working gas is puffed into the divertor chamber.A discharge with a density as high as 1.2nG,i.e.ne=1.2nG,can be achieved by SMBI just after siliconization as a wall conditioning.The metallic impurities,such as iron and chromium,also increase remarkably when the impurity accumulation happens.The mechanism behind the density peaking and impurity accumulation is studied by investigating both the density peaking factor versus the effective collisionality and the radiation peaking versus density peaking.