A cryopump is a capture pump and requires regeneration when capacity is reached. Indications of the pump having reached capacity include a loss of pumping speed and a warming of the 22 kelvin cryopanel.

Regeneration Procedure

1. Isolate pump from vacuum chamber - usually a gate valve is used. Ensure that valve control is in protected mode if there is an interlock system in use.
2. Turn off the cold head power. If a portable compressor is used, this should be turned off as well. All of the cold heads currently in use are made by APD. If an APD brand compressor is not being used, then the cold head motor-power is obtained from a small, motor-power box made in-house. This is also the case for the new target room cold heads that are running off of the large rotary screw compressors used for the He liquifier. Power for the gamma-cave cold head is integrated into the vacuum instrumentation system.
3. Obtain a source of dry nitrogen. For example, at the low energy end of the tandem, there is a 1/4" polyethylene tube with a rubber stopper available. Place the stopper in the end of the roughing port for the pump and open pump to the nitrogen to purge the pump. Pumps which do not have their own roughing ports, such as the gamma-cave pump, must be vented and pumped through the chamber. The pumps have a check valve that will open to release pressure. It is a good idea to verify that this valve opens within a few minutes. Once the valve opens, place something soft and clean, such as an o-ring or the wooden shaft of a q-tip in the valve to keep it open so that the nitrogen can flow through the pump. A penny works in a pinch.
4. If a heat tape is installed around the pump's vacuum jacket, plug the tape in. A heat tape is not necessary, but will accelerate the process.
5. Allow the pump's internal cryopanel to reach ambient temperature. This should take at least 2 hours. If time is not critical, allow 4 to 6 hours for warm-up.
6. After warm-up is complete, stop the nitrogen purge, and pump out the cryopump using an oil free roughing system. I prefer the small rotary pump with the liquid nitrogen trap. Try to avoid using a sorption pump, as they have difficulty pumping the same gasses that the cryopump has trouble with. Pumping should continue until 1) the cryopump vacuum chamber is less than 50 mTorr and 2) a rate of rise test of the cryopump vacuum chamber indicates that dP/dt < 15 mTorr/min. A rate of rise test is conducted by valving off the pumping station so that the pump/chamber is no longer being pumped and monitoring the rate at which the pressure in the pump/chamber rises.
7. With the roughing port and gate valve both shut, restart the cryopump (cold head and compressor if used). The first indication of a successful regeneration will be a reduction in pressure in the cryopump. Allow the cold head to operate until the temperature gauge indicates something cold. No really, the temperature gauges used are H2, vapor-pressure thermometry systems and rely on the liquification of the H2 to reduce pressure in the system. Thus almost any drop in temperature is due to some liquification of the H2, and thus the cryopanel is probably cold enough for our use. It is a good idea to mark the "typical" temperature after getting a newly rebuilt pump installed on your system. Then, after each regeneration, simply compare the new temperature to the mark. If, after a regeneration, the temperature gauge does not return to the mark exactly, that doesn't mean it's necessarily bad - it may be starting to degrade a bit. Wait a few days and see if it drops the last little bit. By far, the best indication of a functional pump is the effect it has on your chamber pressure. If you get good vacuum, then complainest thou not.
8. Open the gate valve. Observe the vacuum on the nearest penning gauge. Verify that the vacuum in the chamber improves, or at a minimum, does not worsen.

Return to Vacuum Resources Page
Last updated: 22 Sep 2000
Send any comments to: pbarber@mailer.fsu.edu