We were recently able to test our cryosorption pump in full, and managed to pull the vacuum system to 5· 10 -3 torr. Our test revealed several things which were previously hidden to us at higher pressures, the most obvious of which was a leak in one of our isolation valves. We attempted to clean the valve (which was horribly dirty from whatever previous function it served) but in the process we introduced a more substantial leak. We also believe we hit the ultimate vacuum we could attain without flushing the chamber with dry nitrogen before pumping, as our final vacuum was close to the partial pressure of argon in the chamber at the time of mechanical pump shutoff [UPDATE: we now no longer believe this is the case]. This was difficult to confirm, though, as we have no means to analyze the composition of the gas in the chamber under vacuum. [We now believe we were dealing with outgassing and a leak in the isolation valve (which was more severe than previously understood)]

Below is an image of the reduced vacuum system during the test.

Pump during test

In full, the test lasted for approximately seven hours and consumed approximately eight liters of liquid nitrogen. We would like to improve this efficiency by doing the following:

  • Reducing the volume of the nitrogen vessel.
  • Improving the thermal insulation of the nitrogen vessel and exposed pump head\flange.
  • Flushing the chamber with dry nitrogen before pumping (primarily to displace argon, which has a lower boiling point than nitrogen)
  • Baking out the sorption pump prior to use.

We are looking to do another cryogenic test in the near future after we have made the planned improvements to the system. Our objectives for the upcoming test are:

  • To attain our target pressure of 10 -5 torr.
  • To test our hot filament ionization gauge and to compare its nitrogen equivalent pressure reading to that of our capacitance manometer gauge in their overlapping regions of accuracy. This would give us a metric of residual gas species other than nitrogen in the chamber during pump down.
  • To identify and fix leaks that appear at lower pressures.

More photos taken during the test:

Pouring nitrogen Nitrogen pour

The pump in the nitrogen bath

in bath

Snowy pump while warming back up

Warming Pump