Optimization of Large Titanium Sublimation Pumps for the Neutral Beam Injection System on AUG and W7-X

The neutral beam injection (NBI) system for the magnetic confinement experiments ASDEX Upgrade (AUG) and Wendelstein 7-X (W7-X) require a vacuum system with a high pumping speed of up to 3000 m
³
/s for H
₂
and a large capacity with a gas feed of up to 30 Pa m
³
/s and 10 s. It must ensure a maximum vacuum level of 10
^-2
Pa in any section of the neutral beam line in order to avoid reionization losses. The large titanium sublimation pumps (TSPs) of the NBI system at AUG have been proved suitable for operation within the permanent stray magnetic field of W7-X (up to 30 mT) if they are operated with 200-500-Hz sine ac instead of dc for the ohmic heating of the 4-m long hanging pairs of titanium sublimators. During each heating cycle, the pair of sublimators experiences a reversible thermal expansion of around 50 mm and, in addition, the tantalum core of the sublimators is subject to material creep, leading to residual lengthening. When this lengthening reaches 70 mm after 80-100 thermal cycles (3.5-4.5 h operation time), the wires contact the bottom of the NBI vessel causing a short circuit. In order to extend the availability and operation time of the TSP, a modification of the sublimator with a tantalum-tungsten core with 2.5% W is tested, for which less creep is expected. The first results confirm a reduction in the creep rate by a factor of ~20. This allows sublimation of the whole Ti inventory without intermediate maintenance of the TSP. Furthermore, possible short-circuit mitigation strategies are discussed.