Vertical Test of a 500-MHz SRF Cavity With a Closed-Loop Cryogenic Helium System

A vertical test for a 500-MHz superconducting radio frequency (SRF) cavity succeeded in the RF laboratory of the National Synchrotron Radiation Research Center. The test system, consisting of a vertical test cryostat, an RF power amplifier, a tunable coupler, and a low-level RF control system is integrated with the existing long-distance cryogenic transfer system and radiation shield. Relying neither on a gasbag nor a purifier, the vertical test cryostat is directly connected to the 400-W cryogenic plant that simultaneously provides liquid helium to the SRF module in the Taiwan Light Source. Applying special cleaning, drying, and purging processes, the vaporized helium gas suffers no pollution during the vertical tests. The valve on the return line for the gaseous helium regulates the helium pressure of the vertical test cryostat at an acceptable fluctuation of +/- 100 Pa. The measurement results for a KEKB-type 500-MHz SRF cavity are illustrated herein to demonstrate the integrated performance of this measurement system.