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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. [ABSTRACT FROM AUTHOR]

Superconducting β = 1 quarter-wave geometry of 166.6 MHz has been proposed as main accelerating cavities for the high-energy photon source (HEPS) storage ring. HEPS is a 6-GeV kilometer-scale synchrotron light source promising ultralow emittance currently being constructed in Beijing. A proof-of-principle cavity has been previously in-house developed and vertical-tested achieving excellent cryogenic performances. A helium jacket was subsequently designed adopting a simple structure and an integrated magnetic field shielding. The mechanical properties of the dressed cavity were largely improved focusing on pressure sensitivity, stress, and Lorentz force detuning, while frequency tunability and microphonics were carefully monitored. Elastic buckling and fatigue were also examined. The helium jacket was later fabricated and welded directly with the niobium cavity. Mounted with an input power coupler and tuners, the dressed cavity was finally assembled in a modified cryomodule and tested at both 4.2 and 2.0 K. The favored low-pressure sensitivity and small Lorentz force detuning were confirmed. Microphonics spectrum was scanned and measured, presenting good consistencies with simulation predictions. Finally, the cavity was successfully tuned to the operating frequency of 166.6 MHz at cold and its frequency could be regulated within a ±3-Hz range. This article covers the mechanical aspects in the development and horizontal cryogenic tests of the first dressed 166.6-MHz superconducting cavity. [ABSTRACT FROM AUTHOR]

Superconducting 166.6 MHz β = 1 cavities of quarter-wave geometry have been chosen for a high energy photon source, a 6 GeV diffraction-limited synchrotron light source currently under construction in Beijing. Five cavities will provide the required 5.4 MV radio frequency (rf) voltage and 900 kW beam power. Each cavity will be equipped with one fundamental power coupler (FPC), delivering a minimum rf power of 180 kW to the beam. A 50 Ω coaxial structure with one planar warm window was employed. Its location was carefully selected to avoid electron bombardments on the ceramic window due to potential cavity field emission while preserving the required strong coupling. Focusing on optimized heat loads, a compact geometry was pursued to allow assembly with the cavity in a class 10 clean room, thus minimizing contamination. Two prototype FPCs have been fabricated and examined with high power on room-temperature test stands. The couplers were tested up to 50 kW continuous wave (cw) limited by the available solid-state amplifier. The rf conditioning was conducted initially in travelling-wave mode and later in a standing-wave setup with the variable phase of the reverse wave. All rf surfaces were thus exposed to a high field equivalent to 200 kW cw traveling wave. Being the most critical component, the window-inner-conductor assembly was conditioned up to 150 kW at 650 MHz on a hybrid test stand. Multipacting barriers were encountered as predicted and can be processed by rf conditioning. The design, fabrication, and high-power tests of the first 166.6 MHz FPCs are presented. [ABSTRACT FROM AUTHOR]

A low-frequency superconducting cavity is needed in main accelerators for storage ring light sources with ultralow emittance. A compact 166.6 MHz superconducting proof-of-principle cavity was designed adopting a quarter-wave β = 1 geometry for a High Energy Photon Source (HEPS). It is a 6 GeV diffraction-limited synchrotron light source currently being developed at the Institute of High Energy Physics. The cavity is exceedingly compact in size yet possessing a low resonant frequency. The nearest higher order mode is largely separated from the fundamental, making the cavity an attractive geometry for effective damping of these modes in high current accelerators such as HEPS. The achieved accelerating voltage of 3.0 MV is well beyond the designed 1.5 MV and required 1.2 MV for HEPS operation. High surface electromagnetic fields were reached with excellent rf and mechanical performances, and multipacting barriers were easily processed. This constitutes the first demonstration of a compact low-frequency β = 1 superconducting cavity for HEPS. The design, fabrication, surface preparation, and cryogenic tests of the cavity are presented. [ABSTRACT FROM AUTHOR]