One Friday afternoon (troubles always come up at the last minute on Friday
right before you are ready to go home) I was wrapping up some stuff in my
office getting ready for a long vacation, one of my technicians knocked my
door with much stronger force than usual. When I opened the door I saw a
panic face in front of me. He told me that the magnet we were pumping was
puffing out He fume. We found that the pump was dead because the power cord
was burned out (we used a diffusion pump). The vacuum meter indicated a
reading only in 10^-3 region. I immediately shut the magnet volve, replaced
the power cord and started the pump again while watching the big flume of
He shooting out of the safety vent on the bridge. Fortunately, we just
filled the Dewar in the morning, and I figured the magnet would not be
quenched for another hour or so. Also fortunately we discovered the
problem quickly enough that the oil was not sucked into the the vacuum line.
After 15 minutes of repumping, I opened the magnet volve again and noticed
that the He fume was gradually diminishing. What a relief! About two hours
later, no more jume came out of the vent. We left the pump running over the
weekend and everything seemed to be under control.
Funny thing is after this episode, the vacuum has never degraded after every
He fill. It is almost six months since then and we've never needed to
pump that magnet again (knock on wood). Can anyone explain it?