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

The largest barrier to achieving >100 MV/m gradients in RF linacs is RF breakdown effects, believed to be caused by motion of defects associated with the pulsed heating and high fields. Data suggests that the use of high strength, high conductivity materials can improve linacs performance. Capturing these gains requires new design and manufacturing strategies which retain the hardened RF surface while maintaining the alignment and vacuum properties associated with high temperature brazing. We aim to improve the performance of splitcell structures through a combination of laser peening LP, copper alloying and electron beam welding EBW. LP introduces residual compressive stress into the surface and is a proven method to improve the fatigue properties of highstress, missioncritical aerospace structures. Alloying is required to increase softening temperature of copper, but careful thermomechanical engineering is needed to achieve high conductivity. Finally, low temperature joining by EBW is required to retain engineered material properties. Combined, these technologies offer a new paradigm for scalable, highgradient design and manufacturing. We demonstrated that LP could be used to engineer the hardness and electrical conductivity of pure and alloyed copper. We peened 8 unique materials, including a custom run of a high conductivity/high strength alloy. We also peened a 3D RF surface, demonstrating technological feasibility. We performed chemical etching and EBW studies for most alloys and found that LP was compatible with current practices. Overall, LP increased hardness of all copper with negligible to minor loss in conductivity. After 400°C heat treatment, we found that these hardness/conductivity properties changed with the magnitude and direction dependent on the alloy chemistry. We will continue development of our combined LP + alloying efforts, working with our copper foundry vendor and laser peening subcontractor. We will perform outgassing measurements, optimize etching procedures, fabricate a split cell structure with elastically averaged registration features and EBW a vacuum hermetic RF device. Testing will be performed with TE013 mushroom cavity experiments at SLAC in Year 1. In Year 2, we will perform 3cell high power RF testing of a completed laser peened EBW RF structure. The processes developed in this project will pioneer a new strategy to linacs manufacturing, with applications in both scientific and industrial accelerators. This will increase our market share of scientific and industrial accelerators. In addition, high strength/high conductivity laser peened copper also has potential applications in power transmission, electric vehicles and reusable rocket propulsion systems.