Preliminary results of the optimization of the large titanium sublimation pumps for the W7-X neutral beam injection system

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 high pumping speed of up to 3000 m3/s for H2 and large capacity with a gas feed of up to 30 Pa×m3/s and 10 s. It must ensure a vacuum level of a maximum of 10-2 Pa in any section of the neutral beam line in order to avoid re-ionization losses. The large Titanium sublimation pumps (TSP) of the NBI System at AUG have been proven 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 ohmical heating of the 4 m long hanging pairs of Titanium sublimators [1]. During each heating cycle the pair of sublimators experiences a reversible thermal expansion of around 50 mm and additionally 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 being tested, for which less creep is expected. The first results confirm a reduction of the initial creep rate by a factor of ~ 10. Furthermore, possible short-circuit mitigation strategies are discussed.