Abstract
166.6-MHz superconducting cavities have been chosen for the High Energy Photon Source (HEPS) as the main accelerating structures to provide 900 kW of beam power and 5.4 MV of accelerating voltage. A proof-of-principle cavity adopting the quarter-wave beta = 1 geometry was previously developed. Excellent performance was achieved in vertical tests at cryogenic temperatures. The cavity was later welded with a helium jacket, dressed with a power coupler and other ancillaries, and high-power tested in a test cryomodule. Performance degradation was observed and analyzed. Evidence from temperature sensor readout and heat loss measurement results suggested an overheating in the cavity–coupler interface region causing a “thermal runaway” and eventually quenching the cavity at its design voltage. Electromagnetic-fluid-thermal coupled simulation has thus been conducted, and the hypothesis was nicely validated. Finally, solutions were proposed including an elongated niobium extension tube at the coupler port and an optimized helium gas cooling of the power coupler’s outer conductor. These modifications have been subsequently applied on the 166.6-MHz higher-order-mode damped superconducting cavities for the HEPS. Heat loss at 4.2 K contributed by the power coupler can be largely reduced with a modest gas cooling scheme. Similar design approaches can also be applied to other non-elliptical superconducting structures with on-cavity high-power coupler mountings.
Highlights
Higher accelerating voltage and lower cryogenic losses are the main pursuit in the development of a superconducting radio frequency system for particle accelerators.1 Progress in surface treatment over the past few decades2 and the recent studies on nitrogen doping/infusion3,4 and medium-temperature baking5,6 have steadily improved the cavity performance approaching the fundamental limitations of bulk niobium (Nb)
Evidence from temperature sensor readout and heat loss measurement results suggested an overheating in the cavity–coupler interface region causing a “thermal runaway” and eventually quenching the cavity at its design voltage
Temperature readings and heat loss measurement results suggested an overheating in the cavity–coupler interface region causing a “thermal runaway” and eventually quenching the cavity
Summary
Higher accelerating voltage and lower cryogenic losses are the main pursuit in the development of a superconducting radio frequency (srf) system for particle accelerators. Progress in surface treatment over the past few decades and the recent studies on nitrogen doping/infusion and medium-temperature baking have steadily improved the cavity performance approaching the fundamental limitations of bulk niobium (Nb). Solutions are proposed consisting of an elongated Nb extension tube of the coupler port on the cavity and an optimized helium gas cooling of the coupler’s outer conductor Both the cryogenic heat loss and the interface temperature at the cavity design voltage have been largely reduced. The helium-jacket-welded cavity was subsequently processed by an established surface treatment procedure, including two times of buffered chemical polishing (BCP) to improve surface quality, heat treatment at 600 ○C to degas hydrogen, high-pressure rinsing (HPR) with ultra-pure water, and assembling with the coupler’s vacuum part in class 10 cleanroom.
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