The effect of divertor plate biasing on impurity transport and levels in 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 Vb (-300 to +300 V). The intrinsic impurities studied are oxygen and carbon. Neon, an extrinsic impurity, is introduced using hydrogen gas fuelling with a 0.3% trace of neon. Aluminium is introduced by laser ablation. The radial profiles of C IV emissivity show an increasing diffusivity from negative to positive biasing, while to obtain compatible changes between the VUV emissions (C IV, O VII and Ne IX) and the soft X-ray profiles, the inward convection velocity and the impurity concentrations must be increased as Vb increases from -300 V to +300 V. The changes in the convection predicted by six different models are compared with the experimentally deduced convection. These models are based on ion mobility, ion orbit losses, anomalous viscosity caused by turbulence, the skin size electromagnetic drift mode, neoclassical theory in the presence of turbulence and mobility related to E*B turbulent diffusivity. This comparison allows the changes in the convection velocity to be related qualitatively to the ambipolar radial electric field Eamb, which may be different from the purely neoclassical value Eneo, owing to turbulence which is usually intrinsically non-ambipolar; the turbulence can also cause an anomalous viscosity, affecting the rotational velocities and hence Eamb