Characterization and performance improvement of the large titanium sublimation pumps in the AUG and W7‐X neutral beam injectors

The experimental devices ASDEX Upgrade (AUG) and Wendelstein 7‐X (W7‐X) are equipped with two neutral beam injectors each for plasma heating with up to 20 MW. Four large titanium sublimation pumps (TSPs) (4 × 1.5 × 0.2 m3) in each injector provide proper vacuum conditions (below 10−2 Pa) during the 10 s beam pulse with a gas feed of up to 30 Pa × m3/s. A maximum pumping speed of up to 3000 m3/s for H2 is obtained by frequent renewal of the Ti coating at the pump surfaces. Ti is sublimed from 4 m long ohmically heated hanging pairs of Ti sublimation wires. So far, all pairs of wires were operated with constant current during 120 s, despite of the large spread of the resistance of the wires. Tests have been performed in a dedicated test facility in order to characterize the pumping performance for H2 with different parameters for sublimation power and time. The data gathered can be used to simulate fully operative TSP systems for real NBI operation scenarios in AUG and W7‐X, so as to find an optimal sublimation strategy for the TSPs.