The tentative schedule would have the target back in the chamber Thursday, surveyed Friday, and cooldown on Monday. These activities might all take place 1 day later but it's not likely they would slip much beyond that.
Last weekend Dave found the problem with the pump- the teflon retainer in the upper bearing had disintegrated. In the 1st figure below, you see on the left the lower (load bearing) ceramic bearing with its retainer intact. However it is dirty, there is a lot of crap that was inside the bearing which of course will eventually cause it to fail. The ceramic bearing on the right is the upper one. The teflon retainer is completely absent! Without the retainer which spaces the balls apart, the balls of the bearing can pile up on one side. This caused the pump shaft to wobble and the 2nd figure shows some slight stator scoring where the rotor scored the stator, presumably from the wobble. There are pieces of the retainer visible lying on the pump floor, and dust probably from the rest of the upper retainer.
Dave will replace all the bearings. It's not clear whether to go with completely dry, all ceramic ones, or with SS ones that have a torlon/graphite retainer and moly-disulphide (self-)lube. Dave (and I) are leaning towards the latter. In any case it's different from what was in there.
Why did the retainer fail? Not from radiation- the lower bearing was ok, and it is closer to the field. Also the pump wiring looked fine. The suspicion is that the bearing got too hot. To mitigate this Dave will run a small 1/4" copper line from the top of the pump to the suction side, to establish a small bypass flow of hydrogen in an effort to help keep the top of the pump and this bearing cold. This shorts out the pump head a little, but we think it's only going to cost a tiny fraction of our pump head so it should be ok. The rest of the pump plumbing is 3", and this little 1/4" (~1/8" id) bypass is from the top of the pump, not the outlet.
They will also do what they can to clean out the system, and will use a boroscope to look for junk that may have piled up in the J-shaped manifolds/traps. Also when we cool down again, an effort will be made to cool down thru a cold trap to try and grab any hydrocarbons that may still be in the system. Dave mentioned using an rga to look at the gas as well. It's possible the retainer failed not because it got too hot, but because we had a bad bottle of gas with crap in it. So that's why an extra effort will be made to examine the gas we have, and to use a filter on it when we condense it back in the target. With no LN2 in the hall it will be a pain to keep the trap full during cooldown, but we'll aim to do that with dewars.
The leaky 15 K feedthrough is odd- this got fixed before and tested ok. It was cold shocked and tested multiple times in September. The tgt group suspects we received a bad batch of new feedthroughs, and will probably replace this leaky one with an older feedthrough from another target with a more trustworthy batch.
The new exit window is expected to arrive in time to install. If it does, and it's closer to the 5 mil thickness we want for the nipple, and it passes muster to 100 psi in a pressure test, we will be able to get it on and use it.
We have replaced the horizontal motion limit switches. Thanks to Jiawei and Adesh for their hard work over the weekend to get these made and in place. We will try to finish adjusting them today.
Tomorrow we'll drop the cell 6-8 mm, and pre-pitch it to counteract the pitch we acquired during cooldown. The beam after-images on the exit window clearly show the beam was a little low on the nipple, as expected from our centering target raster measurements, because of the pitch. We couldn't do much about it before because we could not safely drop the target any further. That's why we'll move it tomorrow (and pre-pitch it) so we'll have less pitch cold, and more ability to deal with what's left of it. I'll post some pics of the cell and beam imprints later today.
Figure 1
Figure 2
