Impurity transport in ohmically heated TFTR plasmas

Abstract

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 Ip = 0.8−2.5 MA, n̄e = (0.6-6.0) × 1019 m−3 and Zeff = 2-6. The diffusion coefficients are found to be in the range of 0.5-1.5 m2 · s−1, considerably 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 n̄e < 3 × 1019 m−3.

IONIZATION BALANCE FOR OPTICALLY THIN PLASMAS: RATE COEFFICIENTS FOR Cu, Zn, Ga, and Ge IONS

“…Unfortunately, detailed comparisons with these previous works are not possible. For instance, the line brightnesses of Ge ions injected by laser blowoff in the TFTR tokamak plasmas have been simulated [8] using the MIST code [9]. Details of the rates used and of the ionization balances were not published, which makes comparison impossible.…”

Nonlinear Problems of Equilibrium Charge State Transport in Hot Plasmas

“…Therefore, it is not entirely clear how m is defined. Indeed, it could be assumed that collisional and/or turbulent transport in the plasma can contribute to m along with atomic processes, e.g., as a simple sum as observed in experiments with light impurities (see, e.g., in [12,42]). for two types of equilibrium that differ in the total density profile (see Figure 7a).…”

“…In the extensive practice of modeling medium and heavy impurities and with an increase in the rates of ionization-recombination it was found that the DCM sensitivity to the empirical transport remains extremely low [7][8][9][10][11][12][13] and just unacceptable for W impurity [14,15].…”

Nonlinear Problems of Equilibrium Charge State Transport in Hot Plasmas

“…In the practice of modeling medium and heavy impurities, that is, with an increase in the rates of ionization-recombination, it was found that the DCM sensitivity to the empirical transport coefficients remains extremely low [7][8][9][10][11][12][13] and just unacceptable for W impurity [14,15].…”