IMPURITY EDGE TRANSPORT IN THE VICINITY OF THE BERYLLIUM BELT LIMITERS IN JET

The radial distributions of the low charge states of Be near the JET beryllium belt limiters have been measured experimentally. When interpreted with an impurity production/transport code, these measurements elucidate the plasma behaviour near the limiter, and the source and sink action of the limiter on the impurity ions. We have considered here 3 MA ohmically heated, limiter discharges with n(e)BAR = 1-4 X 10(19) m-3. A model which satisfactorily reproduces the measured profiles required the following assumptions: (a) an n(e)(r) profile just inside the LCFS (0 to approximately 10 cm radially inwards) which extrapolated smoothly from the SOL (probe) measurements; (b) a T(e)(r) outer edge profile which extrapolated smoothly from the core (ECE) measurements; (c) a cross-field impurity diffusion coefficient D(perpendicular-to almost-equal-to 0.5 m2/s in the outer edge; (d) no strong inward or outward pinch action in the outer edge; (e) impurity production by physical sputtering, rather than by evaporation; (f) sputtering due to parallel plasma flow onto the limiters; no significant direct perpendicular flow onto the limiters, as has been observed for larger limiters.