Blob properties in L- and H-mode from gas-puff imaging in ASDEX upgrade

Abstract

Blob properties are studied in the scrape-off layer of the tokamak ASDEX Upgrade with a fast camera. The gas-puff imaging technique is used to investigate the detection rate as well as the blob size and velocity scaling. The experiments were performed in L- and H-mode phases of the same discharges to study the change in blob properties after the L-H transition. In both regimes the detection rate is of the order of a few thousand blobs per second, which is compatible with the picture of blob generation by edge micro instabilities. The blob size increases in H-mode, while the radial velocity decreases slightly. The changes are, however, not indicating a drastic change in the blob dynamics in both phases. The experimentally found blob properties were compared to predictions from a novel blob model including effects due to a finite ion temperature, which should be more appropriate for the conditions in the SOL of fusion plasmas.