Compact, low-cost higher order mode absorbers formed by cold spray of metal matrix composites

A charged particle beam passing through an accelerating cavity excites a spectrum of resonant, higher order modes (HOM) which can negatively disrupt the beam. Suppression of HOMs represents a significant challenge for current and future accelerators, particularly high peak and/or average current machines such as Free Electron Lasers (FEL) and Energy Recovery Linacs (ERL), which have kW- levels of estimated HOM power per cavity. While current HOM RF absorber materials have satisfied the requirements for cryogenic and room temperature operation; robust, low-cost manufacturing technologies which can generate large area RF absorbers are needed for future high current accelerators. This proposal outlines a method to fabricate novel Al/SiC metal matrix composites for use as an RF absorber using cold spray technology. This process can be used to directly bond full density absorbers onto the inside of large, water-cooled beam line absorbers. The low temperatures associated with cold spray can result in the formation of nanostructured, non-equilibrium phases with enhanced microwave absorbing properties. We performed coupon level spray trials using 2 separate cold spray vendors, then metallographically examined the coatings. We down selected the best vendor and cold sprayed the inside of a brazed, 80 mm diameter copper vacuum assembly, followed by inspection and post-spray machining. The particulate, RF and vacuum characteristics of the assembly were measured. While we observed ceramics particles in the coating, initial 3GHz pillbox results show a metal-like coating behavior. With this proof of principle demonstration, we plan to coat the inside diameter of a large (~12”) beam line absorber (BLA), such as those required for the electron storage ring (eSR) of the Electron-Ion Collider (EIC). In Phase II, RadiaBeam will fabricate an EIC-sized BLA copper/stainless steel housing, performing the required dimensional and vacuum checks. In year I, we will perform coupon-level cold spray development to maximize the RF absorption of our coating. In year 2, we will cold spray the inside diameter of the EIC BLA assembly, then test the RF, particulate and vacuum properties of this full-scale device. This work is to be performed with support from Jefferson National Lab, who will provide guidance on the design, fabrication, RF measurement and data analysis. Cold spray HOM absorbers could reduce the capital and operational costs associated with superconducting RF accelerators via low-costs RF absorbers and optimized cryogenic thermal management, respectively. This novel, low-temperature powder consolidation process enables the fabrication of non-equilibrium materials which have received significant interests for corrosion and wear applications. With additional development, these composite materials can be similarly engineered to have specific RF properties. If successful, this process could also be applied to high power microwave systems and radar-absorbing aerostructure.