Cold Spray Technology Applications for SRF Cavity Thermal and Mechanical Stabilization.

The most important source of frequency shifts in SRF cavities is due to the effect of microphonics and Lorentz forces. Additional stiffening is then needed to improve the cavity’s mechanical stability and reduce the frequency shift. Gas dynamic cold spray is a coating method based on supersonic jet of powders, heavily plastically deformed during the impact. The use of Nb/Cu cold spray cladding material can provide mechanical stabilization, and thus stiffening rings may be avoided. A significant benefit of the proposed technology is also the addition of copper for thermal stability and conduction cooling, and overall cost reduction of an SRF cavity fabrication. The advantages of the cold spray technology allow it to become a novel and highly effective way of adding a fully dense copper outer layer to a niobium cavity. An additional critical feature is that the cold spray can be applied selectively only where needed, as opposed to using uniformly thicker sheet material, and could be used to join additional parts without heat. The cold-spray deposition of 99.9%-pure copper powder was performed using He and N2 propellant gases for various samples, and with technology optimizations. The thermal conductivity and residual resistivity ratio (RRR) of cold-sprayed test coupons were measured at JLAB with a standard 4-probe method while cooling the coupons down to 4.3 K from room temperature. The RRR of Cu was increased up to the range of ~50, corresponding to ~250 W/(m K) at the 4.3 K temperature. The adhesion tests demonstrated that the cold-sprayed copper layers had very good mechanical bonding to the niobium substrate in all the samples examined. In general, the Phase I results demonstrated that the developed copper on niobium cold-spray technology can be implemented for the copper-niobium SRF cavity to provide increased mechanical and thermal stability and could be used for conduction-cooled industrial SRF accelerator applications. In Phase II of the project, this new technology will be used for SRF cavity fabrication to provide mechanical and thermal stability, and for conduction-cooled industrial SRF based accelerators. The niobium sheets will be cold sprayed with copper-tungsten powder in various spraying regimes, and microstructure analysis will be carried out. Also, thermal conductivity of the resulting bimetallic materials will be measured. The results of these measurements, in their connection with the cold-spraying parameters, will allow us to fabricate a single-cell copper/niobium cavity. Nb3Sn coating and cleaning preparation of the SRF cavity for the vertical test stand (VTS) will be carried out. After the VTS testing, the single-cell cavity will be cold sprayed and tested again at both the VTS in JLAB in a liquid helium bath, and at the conduction-cooling cryostat at Euclid’s facility in Bolingbrook, IL.