Speaker
Guillermo Orozco
(ITED)
Description
The experimental devices ASDEX Upgrade (AUG) and Wendelstein‑7X (W‑7X) are both equipped with two neutral beam injectors each for plasma heating (up to 20 MW). Four large titanium sublimation pumps (TSPs) (4×1.5×0.2 m33) in each injector provide proper vacuum conditions (below 10-2-2 Pa) during the 10 s beam pulse with a gas feed of up to 30 Pa×m33/s. A maximum pumping speed of up to 3000 m33/s for H2 is obtained by frequent renewal of the Ti coating at the pump surfaces. This is achieved by ohmically heating 4‑m‑long hanging pairs of Ti sublimation wires above the Ti sublimation temperature (~1800 K). Each TSP contains 27 pairs of sublimators with about 9.5 kg of Ti, theoretically allowing for over 16 h of sublimation per pair, long enough to operate the injector during regular AUG experimental campaigns for as long as 20 months. However, the injectors must be opened during this time once or twice for TSP maintenance in order to shorten the residual lengthening of the sublimators which accumulates after each thermal cycle and would result in an electrical short-circuit, hampering the operation. This residual elongation is caused by material creep of the pure tantalum core of the wires. A new type of Ti sublimation wires has been tested with a TaW-2.5%w alloy core material, less prone to material creep than pure Ta, achieving over 21 h operation without wire shortening. Additionally, tests have been performed in a dedicated test facility in order to characterize the pumping performance with a controlled H2 gas feed for varying parameters of the sublimation cycle (e.g. power, duration). The results are used to simulate fully operative TSP systems for real NBI operation scenarios in AUG and W7‑X, aimed at finding an optimal sublimation strategy.
Co-author
Guillermo Orozco
(ITED, Max-Planck Institute for Plasma Physics (IPP), Garching, Germany)
