Processing and testing equipment for Coaxial Test Resonators

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Studies to optimize and understand superconducting radio-frequency (SRF) performance have been primarily focused on elliptical 1.3GHz cavities for electron acceleration. A new wave of hadron SRF linear accelerators are being developed that will operate with continuous wave (cw) rf power. The cavities for hadron accelerators utilize the transverse electric and magnetic (TEM) accelerating mode and must accelerate a wide range of particle velocities from 5% to ~60% the speed of light and with rf frequencies ranging from 100MHz to 700MHz typically.  The PI on this proposal has used a previous Discovery grant and RTI funds to support students and equipment to install an rf induction oven, to source two coaxial cavities for rf cryogenic testing [5] at multiple frequencies and to explore the performance of Nb and new materials with muSR and beta-NMR [1,2,3,4,6]. All the equipment is geared to support student based research programs and leverages TRIUMF SRF and material science infrastructure. The funds requested here will allow us to augment the coaxial resonator program by expanding our processing and testing equipment. Present TEM mode bulk Niobium cavities almost exclusively use Buffered Chemical Polish (BCP) as a step to remove ~150microns of damaged layer after fabrication or to reset the rf surface of the cavity between processing steps. The elliptical cavity community primarily uses Electropolish (EP) surface removal  since comparative studies show that the high field performance is improved over the BCP technique. Also, certain new heat treatments like N2 doping [8] applied to improve the performance of elliptical cavities need a post-treatment EP to optimize the treatment.  Due to the complex geometry of TEM mode cavities, EP has been done in only a few Labs [7] and no studies have been done to compare the performance of EP vs BCP on the same TEM cavity. Further, no TEM mode cavity has been treated with N2 doping. We propose to purchase equipment through this RTI to enable EP on the two coaxial cavities [15] to allow us to, for the first time, compare EP vs BCP on the same TEM cavity. Further we plan to use the EP process to allow N2 doping of the TEM mode cavities also for the first time. To augment the study we propose to purchase equipment and parts for a TEM mode temperature mapping (T-map) system to enable us to discern between localized or global losses in the cavities. Such information is crucial to better define rf surface resistance vs rf frequency. We also request funds to procure parts to add accurate low field magnetic probes to allow also H-mapping of the cavity during cooldown. The magnetic probes in combination with T-map will give us detailed information on geometry dependent flux trapping. The RTI funds requested in this proposal will provide a unique set of tools to enable student led discovery science and provide breakthroughs in the rapidly evolving SRF technology sector.