Measurements of Helium transport in JET

A perturbative measurement of helium confinement in deuterium plasmas was made using neutral helium beams as a central source. Evolution of the helium ion density profile was measured using charge exchange spectroscopy. It was found that the influence of delayed photon emission from hydrogen-like helium, the plume effect, must be included in the deduction of helium densities. Central fuelling into L-mode discharges was found to produce peaked helium ion density profiles. This peaking disappeared within 50 ms of termination of central fuelling. Simulation using 1 1/2-D impurity transport code reproduces the central peaking during fuelling with a core diffusion coefficient of 0.3±0.1 m2/s. On the other hand, the rapid loss of peaking can only be simulated with a diffusion coefficient four to five times greater. This discrepancy is not understood. Iron ablation into the same discharge was used to compare low- and medium-Z impurity transport. Again using the transport code, the core diffusion coefficient is found to be 0.125 M2/s. In H-mode experiments, it has not been possible to produce peaked helium density profiles. As in the L-mode discharges, this suggests rapid loss of helium from the plasma core. (author). 10 refs, 2 figs