Blower Gun pellet injection system for W7-X

Abstract

Foreseen to perform pellet investigations in the new stellarator W7-X, the former ASDEX Upgrade Blower Gun was revised and revitalized. The systems operational characteristics have been surveyed in a test bed. The gun is designed to launch cylindrical pellets with 2mm diameter and 2mm length, produced from frozen deuterium D2, hydrogen H2 or a gas mixture consisting of 50% H2 and 50% D2. Pellets are accelerated by a short pulse of pressurized helium propellant gas to velocities in the range of 100–250m/s. Delivery reliabilities at the launcher exit reach almost unity. The initial pellet mass is reduced to about 50% during the acceleration process. Pellet transfer to the plasma vessel was investigated by a first mock up guiding tube version. Transfer through this S-shaped stainless steel guiding tube (inner diameter 8mm; length 6m) containing two 1m curvature radii was investigated for all pellet types. Tests were performed applying repetition rates from 2Hz to 50Hz and propellant gas pressures ranging from 0.1 to 0.6MPa. For both H2 and D2, low overall delivery efficiencies were observed at slow repetition rates, but stable efficiencies of about 90% above 10Hz. About 10% of the mass is eroded while flying through the guiding tube. Pellets exit the guiding tube with an angular spread of less than 14°.