Leak, O Leak, Where For Art Thou?
Allow me to explain a little bit about vacuum technology and then the extent of my frustrations will become evident. A vacuum is (with thanks to Wikipedia) ‘a volume of matter, such that its gaseous pressure is much less than standard atmospheric pressure’ and a vacuum chamber, such as the one I use, is a rigid enclosure from which air and other gases are removed by a series of vacuum pumps. The resulting low pressure, commonly referred to as a vacuum, allows researchers to conduct physical experiments or to test mechanical devices which must operate in outer space (in this case the HDLT prototype).
The vacuum chamber I use is meant to operate at a base pressure of about 1 x 10^-7 Torr. A Torr is a non-SI unit of pressure and equals about 133 Pascals (the SI unit of pressure). In Canberra, the atmospheric pressure is generally 720 Torr (it varies with your height above sea level and the weather and temperature at the time) and one standard atmosphere is approximately 760 Torr. Therefore, my vacuum chamber is meant to operate at less than a millionth of an atmosphere and hence replicates pressures comparable with those experienced by the International Space Station which orbits at between 320 and 350 km above the Earth.
So why my frustrations with my vacuum chamber? Well for most of last week, we were unable to get the pressure below 5 Torr, some 7 orders of magnitude from where it needed to be. After painstakingly removing everything (probes, windows, feedthroughs, gauges, propellant lines, etc) from the vacuum chamber we discovered that ceramic seal on the RF feedthrough that supplies power to the HDLT had cracked allowing air to leak into the chamber and counteracting the action of the vacuum pumps. Getting a replacement feedthrough from the US will take about 5 weeks. Grrr!
After removing that source of leaks, myself and the School’s vacuum technician (to whom I am extremely grateful) were able to get the pressure down to about 1 x 10^-1 Torr, still 6 orders of magnitude from where we needed to be. Still not good news. After some extensive leak testing with a cool little machine that sniffs out helium when you spray it near a weld or other potential leak location, we discovered that about 6 of the new welds on the vacuum chamber were leaking. Some were pretty bad but given that the upgrade to the vacuum chamber resulted in about 30 new welds its not surprising that a few weren’t great. All in all it took most of last week to get to this stage.
Anyway, since then the welder has repaired the offending welds and at last the base pressure in the vacuum chamber has reached the 1 x 10^-6 Torr range, which is a huge relief. Our group’s technician, Peter, has managed to repair the RF feedthrough as well (although I have ordered a replacement anyway) so I should be able to fire the HDLT up next week. Stay tuned for photos 
July 27th, 2007 at 3:34 pm
As of this afternoon the pressure in the chamber is down to 7.5 x 10^-7 Torr which is great! On Monday I plan to open the chamber and reconnect the HDLT and a few other components so I can pump the chamber down and try and get a plasma once again!