THE EFFECT OF DIVERTOR PLATE BIASING ON IMPURITY TRANSPORT AND LEVELSIN THE CENTRAL PLASMA OF TDEV

Impurity concentration and transport are studied by interpreting line emissions in the VUV and soft X ray emission, using the MIST impurity code. Standard tokamak discharges have been used with modest divertor plate bias voltages V-b (-300 to +300 V). The intrinsic impurities stu died are oxygen and carbon. Neon, an extrinsic impurity, is introduced using hydrogen gas fuelling with a 0.3% trace of neon. Aluminium is i ntroduced by laser ablation. The radial profiles of C IV emissivity sh ow an increasing diffusivity from negative to positive biasing, while to obtain compatible changes between the VUV emissions (C TV, O VII an d Ne IX) and the soft X ray profiles, the inward convection velocity a nd the impurity concentrations must be increased as V-b increases from -300 V to +300 V. The changes in the convection predicted by six diff erent models are compared with the experimentally deduced convection. These models are based on ion mobility, ion orbit losses, anomalous vi scosity caused by turbulence, the skin size electromagnetic drift mode , neoclassical theory in the presence of turbulence and mobility relat ed to E x B turbulent diffusivity. This comparison allows the changes in the convection velocity to be related qualitatively to the ambipola r radial electric field E-amb, which may be different from the purely neoclassical value E-neo, owing to turbulence which is usually intrins ically non-ambipolar; the turbulence can also cause an anomalous visco sity, affecting the rotational velocities and hence E-amb.