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

The electron density profile peaking and the impurity accumulationin the HL-2A tokamak plasma are observed when three kinds offuelling methods are separately used at different fuelling particlelocations. The density profile becomes more peaked when theline-averaged electron density approaches the Greenwald density limitn_G and, consequently, impurity accumulation is oftenobserved. A linear increase regime in the density range n_e<0.6n_G and a saturation regime in n_e > 0.6n_Gare obtained. There is no significant difference in achieved densitypeaking factor f_ne between the supersonic molecular beaminjection (SMBI) and gas puffing into the plasma main chamber.However, the achieved f_ne is relatively low, in particular,in the case of density below 0.7n_G, when the workinggas is puffed into the divertor chamber. A discharge with adensity as high as 1.2n_G, i.e. n_e = 1.2n_G,can be achieved by SMBI just after siliconization as a wallconditioning. The metallic impurities, such as iron and chromium,also increase remarkably when the impurity accumulation happens. Themechanism behind the density peaking and impurity accumulation isstudied by investigating both the density peaking factor versus theeffective collisionality and the radiation peaking versus densitypeaking.