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Superconducting 499.8-MHz radio frequency (rf) cavities have been proposed for the High Energy Photon Source (HEPS), a 6-GeV diffraction-limited synchrotron light source currently under construction in Beijing. Being an active third-harmonic system, two cavities shall provide 3.5-MV rf voltage and 400 kW of beam power to enable a complex rf gymnastics required by a novel injection scheme. Adopting the veteran KEKB-type 500-MHz single-cell geometry, the cavity design has been focused on optimizing its mechanical properties. Various cavity wall thickness and external stiffening mechanisms were investigated to reinforce the cavity to ensure compliance to the pressure vessel codes under different working conditions from a bare cavity to a dressed cavity in the cryomodule. Small margins on operational pressure previously reported on this type of cavity have been largely improved with slightly reduced yet acceptable frequency tunability. Extensive design optimizations have been conducted aiming for lower stress, larger elastic buckling pressure, and higher frequencies of mechanical modes to cause microphonics, while the frequency tunability, pressure sensitivity, and Lorentz force detuning were being carefully monitored. In view of the HEPS’s lifetime, cavity fatigue was examined. This constitutes a comprehensive design optimization of the 499.8-MHz single-cell superconducting cavity with improved mechanical properties. [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]

The High Energy Photon Source (HEPS) is a 6 GeV diffraction-limited synchrotron light source under construction in Beijing. It will use five 166.6 MHz superconducting radio frequency (rf) cavities to accelerate the 200 mA electron beam in the storage ring. In the current lattice design, each straight section is 6 meters long to accommodate two 166.6 MHz cavities. The cavity adopts β =1 quarter-wave geometry, and the damping of the higher order modes is realized by using an enlarged cavity beam pipe to 505 mm and a ferrite absorber installed on the beam pipe. The low rf frequency, as well as the large beam aperture, added great challenges to the design of the 166.6 MHz cavity module to fit into a strictly limited longitudinal space. In addition, heat load optimization for the transition from room temperature to the cryogenic temperature, smooth tapering from the 505 mm beam aperture to the small diameter of the gate valves for impedance control, proper collimation of the synchrotron light to prevent thermal damage of delicate components, are also demanding requirements for a compact design. Accessibilities are also preserved for tuners and vacuum components to enable online maintenance. Finally, rf field leakage in the vacuum seal groove of the cavity is carefully examined to prevent overheating on the superconducting flange, which, in the worst scenario, may cause a thermal runaway, thus limiting the achievable accelerating gradient of the cavity. This paper presents the first compact design of the 166.6 MHz srf section for HEPS. [ABSTRACT FROM AUTHOR]

In order to accommodate the luminosity improvement strategy, two superconducting 499.8 MHz radio frequency (rf) cavities have been adopted for the Upgrade project of Beijing Electron Positron Collider (BEPCII), a double-ring electron–positron collider as well as a synchrotron radiation (SR) light source. The in-house development of the 499.8 MHz superconducting cavity system was initiated at IHEP since 2008. One prototype niobium cavity with all the parts except for the flanges and the pickup port formed by spinning was fabricated and then vertical-tested successfully in July 2011. Four prototype input couplers were manufactured and high-power tested with one reaching a maximum rf power of continuous wave (cw) 420 kW. One prototype HOM damper with a satisfactory absorbing efficiency at the desired frequency band was also constructed and high power tested up to cw 4.4 kW. In the meanwhile, a frequency tuner consisting of a stepping motor and a piezoelectric oscillator was developed. Finally, the bare cavity was dressed with one input coupler, two HOM dampers and one frequency tuner, mounted in a domestic-developed cryostat and then high-power tested successfully in Oct. 2011. The achieved accelerating voltage is 2.2 MV with corresponding Q 0 measured to be 5.8 × 10 8 , beyond the designed goal. This is the first in-house developed 499.8 MHz HOM-damped superconducting cavity system in China. The design, fabrication, post processing and tests of the niobium bare cavity and its auxiliaries, as well as the horizontal high-power test of the cavity system are presented. [ABSTRACT FROM AUTHOR]