Laser shock peening of electron beam welded of elastically averaged, split-cell accelerator fabricated from hard Cu alloy

Holistic manufacturing methods are required to meet the demands of new, high gradient accelerators. Currently, the largest barrier to reliably achieving >100MV/m is the breakdown rate (BDR), which is believed to be caused by movements of crystal defects associated with the pulsed surface heating and large electric fields. Recent data suggests that room temperature, high gradient operation benefits from materials and processes which restrict defect movement through a combination of hardening mechanisms (cold work, solution, etc). Furthermore, fabrication must incorporate design features and processes which retain the hardened RF surface, while also maintaining the alignment features and vacuum properties required from modern accelerator structures. We propose to machine and Electron Beam Weld (EBW) a split-cell accelerating structure from commercially available, hardened copper alloy with a yield stress of >300MPa and a conductivity of >95% IACS. To guarantee our material properties on the critical structure features (i.e. irises), we propose to laser peen the near-net shape structure prior to final machining. Laser peening induces deep (>mm) residual compressive stresses and is currently used to enhance the fatigue properties of structurally critical aerospace components such as turbine blades and landing gear. Furthermore, laser peening is a clean process which can selectively harden surfaces with grazing angle as low as 20o and can be delivered via mulit-axis robot articulation, giving a wide range of processable geometries. In Phase I, we will laser-peen a series of coupons from various copper alloys (Cu-Ag, Cu-Cr, Cu-Zr) to determine the surface hardening characteristics. The coupons will be vacuum annealed to determine the surface hardness after accepted accelerator bake-out processes. From these results, we will downselect the optimum alloy and peening parameters. One-half of split-cell X-band, 3-cell resonant test will then be machined, laser peened, etched and mock electron beam welded at RadiaBeam. The structure surface will be inspected using SEM and Vickers microhardness to ensure no welding-induced softening at the RF surface. In Phase II, we will fabricate a complete, 26-cell accelerator based on the CLIC-G-OPEN design using the downselected, laser-peened Cu alloy and hot test the structure at SLAC. The high energy community places a strong emphasis pushing the limits of high gradient accelerators, as advanced manufacturing technologies can minimize the length and cost of future facilities. Similar demands required in industry for compact accelerators used for nuclear material interrogation, compact FELs and medical accelerators. The experimentally developed, material-focused results derived from this project will support current high gradient research and assist in reducing the costs of future research and industrial accelerator systems.