Precision Electrochemical Fabrication of Corrugated Waveguides

Abstract

Advancements in high energy physics require continuous innovations in hardware to support the generation, amplification, transmission, modulation, and detection of radio frequency (RF) electromagnetic waves. One particular hardware component, waveguides, have garnered increasing interest due to their integral function in the transmission, amplification, and/or manipulation of electromagnetic waves. Waveguides are typically high-purity hollow metal structures that require precision fabrication. Waveguides operating in higher than conventional frequency ranges, e.g., 30 to 300 GHz, are of particular interest to the high energy physics community given the scaling of gradient and shunt impedance with frequency. These higher frequencies necessitate waveguides exhibiting features, such as corrugations, with significantly smaller dimensions. However, traditional manufacturing approaches are inadequate, in terms of manufacturing precision and cost, to meet these requirements. Thus, to satisfy the purity and dimensional tolerance requirements of waveguides operating at high frequencies, novel fabrication techniques are required.Faraday Technology, in consultation with Argonne Wakefield Accelerator (AWA) group, addressed this need by developing an economic fabrication process and apparatus for electroforming high-purity cylindrical copper waveguides with internal corrugations. Aluminum alloy mandrels with corresponding corrugations served as the substrate for copper electroforming. The electroforming approach was achieved within a custom electrolyte solution that was formulated by eliminating or minimizing conventional chemical additives – which is typically required to achieve uniform, conformal, and/or void-free deposits with direct current (DC) electroforming strategies. The custom, low-additive electrolyte was employed to mitigate any possibility of impurity (i.e. additive) inclusion within the copper electroform and thus, ensure high-purity copper waveguides. The low-additive electrolyte consisted of a high-purity copper sulfate solution consisting of 95–100 g/L CuSO4•5H2O, 210–215 g/L H2SO4, 60–70 ppm Cl− and 350 ppm polyethylene glycol. To compensate for the absence of traditional additives, pulse-modulated waveforms were employed during copper electrodeposition to selectively control ionic transport as well as the subsequent deposit morphology and thus facilitate complete copper filling of the corrugation valleys, as seen in the accompanying figure. Preliminary scale-up efforts from ~2-inch to ~6-inch copper electroformed waveguides were demonstrated and confirm the versatility of the pulse-modulated electroforming strategy. Upon mandrel removal, a cold test of the ~2-inch copper waveguide using a vector network analyzer (VNA) was conducted. The results of the S-parameter measurements and the bead-pull test indicate reasonable agreement with the design by CST simulation, which validates the novel pulse-modulated electroforming approach.Acknowledgements: The U.S. Department of Energy (DOE) provided financial support for this work under the contract award number: DE-SC0020782. Figure 1