Cryogenically, we look ok. Loop temps and pressures look pretty normal. We have 13-14K, and ~24 psia in loops 2 & 3. The pressure is a little low but it has been lower, it is determined by a coarse regulator on the house helium supply and dows change by a few psia. The return temp is fine (~17K). So it smells a little like a helium leak.
Unfortunately the archiver seems to have bellied up last evening and so we were blind during this event. I can't look at the histories of any target pressures to correlate them with anything. We restarted the archiver this morning once we realized it was dead, but this does not help us diagnose the problem.
Working on the theory it might be a leak in one of the cells, we tried closing the fill valves one at a time on loop2 and 3. Over a period of a couple minutes no change in pressure was observed. So if it is a leak it's a small one. We were hoping that if we could isolate the leak to one loop, then that loop could be valved out of the system and we could carry on with the cooling power of a single loop. That would get us back up this afternoon. If neither loop is leaking then the problem is more likely a hardened O-ring on the scattering chamber. That would be a rather major disaster since it would likely be some days before things would cool down enough up there to change it out. It's a major job that requires hours and hours of proximity work right next to the hottest areas up there. The O-rings do turn hard as a rock when exposed to enough radiation, and we have certainly been providing enough of that! I think the O-rings were last changed during the August down, to mitigate this problem to the extent possible. We knew this might happen anyway, perhaps it finally has.
Called Mike Seely. He says that the leak is probably pretty small if it is a leak in the cells. Otherwise the SC vacuum would be even worse than it is. So he is coming in to completely pump out the loops one at a time to see if that will get us our vacuum back. If it does then we can get back up as described above. Otherwise we have to let things cool down and look for a problem in the O-rings.
A copy of this log entry has been emailed to: saw@jlab.org