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. In Phase I, we will concentrate on thermal and stress analysis of the SRF cavities coated by copper, and on technology development, working with cold spray industry. Cold spray sputtering with copper on niobium coupons will be developed; bonding strength measurements, tensile and thermal shock tests will be carried out. Analysis of the obtained test results and optimization will allow to fabricate and carry out high-power tests of the new SRF cavities with the copper cladding in Phase II of the project. The cold spray technology can provide the possibility of adding external cladding to SRF cavities for stiffness or thermal stability without compromising the superconducting properties of the inner layer. It could also be applied selectively to engineer stiffness locally, for example to reduce Lorentz force detuning. In addition, it could be applied to provide a first layer of copper on Nb for conduction cooling, or to join pre-formed external parts to cavities without the use of heat. The proposed type would be of direct benefit to the Jefferson National lab JLEIC facility, Brookhaven National Lab eRHIC, and the Fermilab PIP-II project.