High sensitivity He desorption leak detection method

Abstract

A new high sensitivity leak detection method has been developed for the superconducting radio-frequency (SRF) acceleration cavities for the CEBAF linacs. The vacuum integrity of the SRF cavity pairs and the associated indium-sealed vacuum flanges is essential for the successful operation of the accelerator. These SRF cavity pairs are immersed in superfluid helium to minimize rf heat load to the cryogenic system. Conventional leak detection techniques are not appropriate to leak check these cavity pairs at superfluid helium temperature (2 K), since He will be adsorbed onto the cavity surfaces. Further, the sensitivity of the conventional leak detectors is limited to 10−11 atm cm3 s−1 at 293 K. In this article, the details of the He desorption leak detection procedure and the results with production cavity pairs are presented.

Continuous wave superconducting radio frequency electron linac for nuclear physics research

“…A new He desorption leak detection technique with a sensitivity of 10 -15 stdcm 3 -sec was routinely used to verify indium seal integrity. [49] An analysis of the production issues encountered and addressed during CEBAF construction has been provided by Mammosser. [50] These were principally associated with vacuum leaks and particulate contamination control.…”

“…Confidence in the flange surface preparation to assure leak-tight joints had to be developed. A new He desorption leak detection technique with a sensitivity of 10 -15 stdcm 3 -sec was routinely used to verify indium seal integrity [49]. An analysis of the production issues encountered and addressed during CEBAF construction has been provided by Mammosser [50].…”

Cavity Production and Testing of the First C75 Cryomodule for CEBAF

“…The field probe port is located on one of the HOM waveguides. The cavity-pair is isolated from the turbo-pump on the vertical test stand prior to cool-down, to allow for the He-desorption leak test [7]. Fluxgate magnetometers and temperature sensors are attached to the top cell of the top cavity, mid-way and at the bottom cell of the bottom cavity to verify that the residual magnetic field was less than 10 mG during cool-down.…”

A road map to extreme high vacuum

“…There is good agreement between the calculated pressure for the system using the manufacturer's specifications for the Turbo's hydrogen pumping speed and the RGA partial pressure of hydrogen compared with the extractor gauge (raw reading ~ 1.5x10 -11 Torr). The system has a wide calibration pressure range for gauges, which is 1x10 -2 -5x10 -11 Torr and was effectively used for a large number of investigations [10][11][12][13][14]. Several commercial standard gauges were incorporated in this system to facilitate this wide calibration range: including an MKS Baratron, an SRG (spinning Rotor Gauge), an Extractor and an SRS RGA.…”