Shape Memory Alloy connectors for Ultra High Vacuum applications: a breakthrough for accelerator technologies

Beam-pipe coupling in particle accelerators is nowadays provided by metallic flanges that are tightly connected by several screws or heavy collars. Their installation and dismounting in radioactive areas contribute to the radiation doses received by the technical personnel. Owing to the increased proton-beam intensity and luminosity of the future High-Luminosity LHC (HL-LHC), radioactivity in some specific zones will be significantly higher than in the present LHC; the presence of the technical staff in these areas will be strictly controlled and minimized. Remote interventions are being considered, too. Shape Memory Alloys (SMAs) offer a unique possibility to generate tight connections and fast clamping/unclamping by remotely changing the temperature of the junction unit. In fact, SMAs exhibit unique strain and stress recovery capabilities which are related to reversible phase transition mechanisms, induced thermally or mechanically. In this PhD work, a novel Ultra-High Vacuum (UHV) coupling system based on SMAs for applications in particle accelerators was proposed. In particular, ring-shaped beam-pipe couplers were analyzed. Such SMA-based connectors can provide significant benefits in terms of space occupancy, bi-material joining, and above all, possible remote thermal activation. Specifically, bolt-free SMA couplers can be used for beam-pipe joining without the necessity of a connection flange and are smaller and lighter than the traditional coupling systems used at CERN (standard and quick-connect Conflat® flanges). The coupling performance of the proposed connectors was investigated by finite element (FE) simulations and experimental measurements. In particular, the tightening properties of the SMA-based devices, in terms of contact pressure and clamping/unclamping mechanisms, were studied for different values of the initial clearance between SMA sleeve and vacuum pipe by Strain Gauge (SG) and Digital Image Correlation (DIC) tests. Moreover, a design method aimed at optimizing the ring/pipe contact interface was proposed; it involves the FE results and a vacuum sealing model. The results revealed that the contact pressure is not significantly affected by the assembly clearance due to the plateau in the stress-strain response of the material. In addition, the thermal dismounting (cooling stage) and the subsequent re-clamping (heating stage) were always obtained by exploiting the two-way shape memory recovery capabilities of the alloys. The leak rate measurements, carried out to assess the sealing performance of the couplings, showed that the constraints for UHV applications are easily satisfied (leak rate < 10-10 mbar l s-1) even after several thermal activation cycles and long-term room temperature aging. Furthermore, few selected prototype SMA-based UHV chambers have been exposed to a high-energy mixed particle field (up to ~140 kGy of absorbed dose). Preliminary results, in terms of post-irradiation measurements after cool down (radioactive decay), have revealed that leak tightness and thermal dismounting are unaffected by irradiation. Thanks to these features, possible applications within the CERN accelerator complex were considered.