Measurements of Static Heat Loss and Unloaded Q_0 on the BEPCⅡ SRF Cavities

The static heat loss and unloaded Q_0 are the most important values for both the cryogenic and the RF systems.The BEPCⅡ SRF cavity operates in a liquid helium bath contained in a vacuum insulated,liquid nitrogen cooled radiation shielded vessel.During the horizontal test at the test station,thermodynamic method is used to measure and calculate the static loss and Q_0 value of the SRF cavity.This paper has briefly introduced the method and process to measure the static loss and Q_0 value of the SRF cavity.The results under different experimental conditions are presented as important data for acceptance test of the SRF cavity.

The ECT System for RAON＇S Cavities

The ECT （eddy current test） system is in use for Nb surface quality control in many laboratories. This system can inspect Nb surthce quickly using high resolution probe. The ECT system for RAON＇s cavity was made with these features： it has a 3-axis acting probe movenlent system, it can inspect a big size of Nb sheet, which is 1,200 mm × 365 mm and it contains the analysis program that can show the result as a 2D and 3D images as well as the relative figure of the surface level. The standard sample was made with various sizes of defects using the same Nb sheet that was used to make RISP （Rare Isotope Science Project） cavity. The ECT system conditioning was carried out to optimize the ECT operation on the frequency of which the range is from 300 KHZ to 2 MHz. The result of 900 KHZ showed the strongest signal. The conditioning experiment on other parameter will be carried out in the near future.

Ultrahigh accelerating gradient and quality factor of CEPC 650 MHz superconducting radio-frequency cavity

Two 650 MHz single-cell superconducting radio-frequency(SRF)cavities used for the Circular Electron Positron Collider(CEPC)were studied to achieve a high accelerating gradient(E_(acc))and high intrinsic quality factor(Q_(0)).The 650 MHz single-cell cavities were subjected to a combination of buffered chemical polishing(BCP)and electropolishing(EP),and their E_(acc) exceeded40 MV/m.Such a high E_(acc) may result from the cold EP with more uniform removal.BCP is easy,cheap,and rough,whereas EP is complicated,expensive,and precise Therefore,the combination of BCP and EP investigated in this study is suitable for surface treatments of mass SRF cavities.Medium temperature(mid-T)furnace baking was also conducted,which demonstrated an ultrahigh Q_(0) of 8×10^(10) at 22 MV/m for both cavities,and an extremely low BCS resistance(R_(BCS))of~1.0 nΩwas achieved a2.0 K.

Electron bunch train excited higher-order modes in a superconducting RF cavity

Higher-order mode（HOM） based intra-cavity beam diagnostics has been proved effective and convenient in superconducting radio-frequency（SRF） accelerators.Our recent research shows that the beam harmonics in the bunch train excited HOM spectrum,which have much higher signal-to-noise ratio than the intrinsic HOM peaks,may also be useful for beam diagnostics.In this paper,we will present our study on bunch train excited HOMs,including a theoretical model and recent experiments carried out based on the DC-SRF photoinjector and SRF linac at Peking University.