MEASUREMENTS OF IMPURITY TRANSPORT IN JET

Impurity transport has been studied in JET using the laser blow-off technique. Results are presented for Ohmic, L-mode and H-mode plasmas. In alt cases impurity transport was found to be small inside r/a approximately 0.3 with values for D approximately 0.03-0.3 m2 s-1 close to neoclassical predictions. Outside r/a approximately 0.4, the transport was faster than neoclassical with values for D approximately 0.3-0.6, 3-5 and 0.8-1.2 m2 s-1 in H-mode. L-mode and Ohmic plasmas, respectively. These values apply only between sawtooth crashes, during the sawtooth phase itself (approximately 100-mu-s) the transport is greatly perturbed over the central region allowing the impurities to quickly leave or enter this region. In an Ohmic plasma the impurity confinement time tau(imp) was between 250 and 350 ms. This value was reduced to approximately 150-200 ms in the L-mode plasma due to an increase of D in the anomalous transport region. During H-mode plasmas, on the other hand, very long impurity confinement times, of the order of several seconds, were observed. This was explained by a sharp increase of the ratio V/D in a thin region near the plasma edge and a general reduction of D in the outer-half of the plasma radius.