Thursday, September 27, 2007
PPMS cryopump regeneration
We have been having a lot of trouble with the PPMS cryopump. The manual claims we should only need to regenerate the pump once every 500 uses, but we are needing to regenerate every ~30 uses. Ernest at QD says that, every time we regenerate in dewar, we should run the regeneration command 5 or 6 times, and every time we regenerate out of dewar, we should run it 4 times, then do an in dewar regneration after finishing the out of dewar regeneration. I've modified the regeneration sequences accordingly, and we should all remember to do an in dewar regeneration after finishing an out of dewar regeneration.
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In our PPMS, CryoRegs are much more often necessary than every 30th time. In fact, it seems to me that with age (the system is 10 years old now) the getter material decays, or is clogged somehow; and we had lately Trouble with the regeneration heater itself (burnt through) - which makes me guess that perhaps the heater did not work properly anymore since a while.
However, I recommend to check the following next time when you have de-installed the cryopump (for an out-of-dewar regeneration): The heater current is fed into the steel tube through a wire feedthrough, which is sealed by a rubber ring. You can easily disconnect the wires at the outside and the inside of the feedthrough and then you can take out the feedthrough.
Also, I recommend to replace, clean and grease the rubber ring. Also, when you take off the steel tube (Held by 4 screws, which are easily removable) you will see that there is a rather thin rubber ring which seals it against the atmosphere. I recommend to replace this rubber ring too, and clean and grease the whole affair.
We had the case for some time that the vacuum WITHOUT activated cryopump was quite good, but the pump itself didn't work well. Here it can be a slipped flapper valve motor or slipped end Switch, but in our case it was such that the screws which hold the flapper valve plate against ist axis were untight and the flapper valve did not close completely - it always sucked a little, but did not suck well anymore when needed. Well, that sucked! Simply tightening these two Allen-key screws helped. At this occasion, I also replaced, cleaned and greased the rubber ring in the aluminium plate of the flapper valve, and its seat.
Also check if the overpressure valve at the other end of the Cryopump Setup is tight. This is another rubber ring, which may decay with time. I did not have the proper size here in new rubber, and therefore simply smeared grease into the overpressure valve (pull it open a bit with a soft tool for this, I used a toothpick).
Good luck!
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