Impurity transport in ohmically heated TFTR plasmas

The paper presents a study of impurity transport in ohmically heated TFTR plasmas by computer modelling of VUV line emissions from impurities injected using the laser blow-off technique. The results are sensitive to uncertainties in the ionization and recombination rates used in the modelling and, therefore, only a spatially averaged diffusion coefficient and parameterized convective velocity can be measured. Measurements of these transport parameters are presented for deuterium and helium discharges with I_p = 0.8–2.5 MA, n_e = (0.6-6.0) ― 1019 m–3 and Z_eff = 2-6. The diffusion coefficients are found to be in the range of 0.5-1.5 m² ƃ s^–1considerably larger than neoclassical values. Non-zero inward convective velocities are necessary to fit the data in most cases. No dependence of the diffusion coefficient on injected element, working gas species or plasma current is found, but, at a given current, the diffusion coefficient in plasmas near the density limit is smaller by approximately a factor of two than in discharges with n_e > 3 ― 10¹⁹ m^–3.