Behavior of Metal Surfaces Exposed to High-Fluence Hydrogen Ion Irradiation in Vacuum Breakdown Conditions

Radio Frequency Quadrupoles (RFQs) operating in linear accelerators often face significant challenges in terms of surface degradation. Concerns are focused upon vacuum arcs or vacuum breakdowns, phenomena triggered by the exposure of the surfaces to high accelerating fields. Additionally, surface degradation in the form of blistering, caused by beam losses under hydrogen ion beam operation, exposes also a concern, not only because it can affect accelerator operation by itself, but because it has not yet been clarified if the increase of roughness due to blisters could induce an increase of breakdown rate. The work developed in this thesis, aims to address these phenomena by exploring what are the implications in terms of breakdown resistance when a material went through hydrogen ions irradiation. All the tests were sequentially done in different metals, in order to assess better alternative material candidates for future RFQ manufacturing, guaranteeing higher efficiency and durability. In this sense, seven materials were selected for the experiments: pure oxygen free copper, copper chromium zirconium, copper beryllium, stainless steel, titanium alloy, niobium and tantalum. Materials were selected based either on their higher mechanical strength compared to copper, or on their higher hydrogen diffusivity, in both cases aiming at reducing the surface blistering phenomenon. Irradiation was performed using low energy H− beam, while a DC pulsed high voltage experimental system was used to study the high-voltage conditioning process and electrical breakdown statistics. Metallurgical properties of the materials were investigated using advanced microscopic techniques to observe and characterize the different surfaces and to compare results before and after irradiation and breakdown testing. The author has identified the materials with the higher electric field handling capabilities and robustness to low-energy irradiation. Also, additional studies were specifically conducted in irradiated copper samples, where blistering growth has been identified to be dependent on crystal orientations. Furthermore, and contrarily to what it was anticipated, it has been concluded that blistering caused by irradiation have no detrimental effects in conditioning procedures. However, carbon contamination due to hydrocarbon cracking during the irradiation process, has been identified as a main concern for accelerating structures, with results showing a direct consequence on lower electric field performance with the presence of carbon. Taking in consideration the latest finding, studies involving oxygen plasma treatment, in order to remove carbon from contaminated surfaces, were conducted in CuBe2, Cu-OFE and SS316LN. Treated materials were tested with high pulsing voltages and results compared with non-irradiated electrodes and irradiated without treatment. The study has identified plasma treatment, as a successful method to remove carbon from metal surfaces, allowing irradiated materials to achieve performances comparable with the electric field strength of pristine surfaces.