RESUMEN
The storage ring of the high energy photon source will be driven by five 166.6 MHz ß = 1 quarter-wave superconducting cavities operating at 4 K. A higher-order-mode-damped superconducting cavity was designed with excellent rf and mechanical properties based on the successful development of the proof-of-principle cavity. The mechanical design of the dressed cavity was focused on addressing stress safety throughout the processes, tunability, frequency detuning due to pressure fluctuation, and Lorentz force, among other factors. A new liquid helium vessel was designed along with a comprehensive stiffening scheme to mitigate the surging peak stress on the cavity resulting from the significantly unequal beam pipe size. In the first batch, three cavities were manufactured, and surface preparations were carefully conducted to eliminate defects and etching traces while ensuring cleanliness. The cavity's Q0 at the design voltage of 1.5 MV reached 3.8 × 109 at 4 K, comfortably surpassing the design goal. Field emission onset was not observed during the entire test up to a peak electric field of 60 MV/m, thanks to the optimized processing procedures. Subsequently, one cavity was welded with the newly designed helium vessel and vertically tested at 2 K, achieving an rf performance comparable to the bare cavities, demonstrating the success of the jacketed cavity. This paper presents the design, fabrication, surface preparation, and cryogenic tests of the first higher-order-mode-damped 166.6 MHz ß = 1 superconducting cavity.
RESUMEN
A circular electron positron collider (CEPC) will adopt hundreds of 650-MHz superconducting cavities with high quality factor (Q) and accelerating gradient (Eacc). Two 650-MHz single-cell cavities made of fine-grain niobium were first treated via buffered chemical polishing (BCP), which was easy and convenient. However, the vertical test results could not meet the specification of the CEPC (4 × 1010 at 22 MV/m). Therefore, electro-polishing (EP) of 650-MHz single-cell cavities was conducted, which was complicated but remarkably effective. Both 650-MHz single-cell cavities achieved state-of-the-art gradients of 35 MV/m after the EP process, which is extremely high for large elliptical cavities (frequency < 1 GHz). One cavity achieved an intrinsic quality factor (Q0) of 4.5 × 1010 at 22.0 MV/m, which was higher than the CEPC spec. The other cavity obtained a lower Q0 of 3.4 × 1010 at 22.0 MV/m, which may have resulted from the cancellation of high-temperature annealing.
RESUMEN
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 ß = 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 ß = 1 superconducting cavity for HEPS. The design, fabrication, surface preparation, and cryogenic tests of the cavity are presented.