Pulse Tube Cryocoolers vs. Gifford-McMahon Cryocoolers
Three main differences exist between Pulse Tube (PT) cryocoolers and Gifford-McMahon (GM) cryocoolers.
In general, GM cryocoolers are less expensive than PT cryocoolers of similar cooling power.
Both GM and PT cryocoolers are mechanical refrigerators that do have some level of vibration. Two
locations exist on the cold head where vibration is important: The room temperature mounting flange and
the 2nd stage (see photo).
The performance of PT cryocoolers is orientation-dependent. PT cryocoolers only function properly when
they are operated in a purely vertical orientation with the 2nd stage pointing down. GM cryocoolers will lose
some cooling power when the cold head is not operated vertically, but the base temperature will not be
affected. GM cryocoolers can operate in any orientation.
0.2 W @ 4.2K GM cold head
0.5 W @ 4.2K Pulse Tube cold head
The room temperature mounting flange provides the interface for a vacuum shroud or vacuum chamber. Vibrations at the room temperature mounting flange will be transmitted to the vacuum shroud and anything else the shroud is physically contacting.
This is important if the cryocooler is rigidly mounted on other instruments that are sensitive to vibrations. An example would be a cryocooler installed in the evacuated sample compartment of an FTIR spectrometer. Any vibrations from this room temperature mounting flange would be transmitted through the vacuum shroud to the spectrometer. These vibrations may cause damage or misalignment to the delicate optics inside the spectrometer.
When a cryocooler is mounted on a delicate instrument such as the FTIR spectrometer, a PT system would be a better choice than a GM system because the vibration level at the room temperature mounting flange of a PT system is significantly lower than in a GM system.
The sample being cooled is usually mounted onto the 2nd stage. Any vibrations from the 2nd stage will be transmitted directly to the sample.
GM cold heads have vibrations at the 2nd stage on the order of 20 microns along the axis of the cold head. GM cold heads have vibrations on the order of ~5 microns perpendicular to the axis of the cold head.
Pulse tube cold heads have vibrations on the order of 4-7 microns along the axis of the cold head and ~2 microns perpendicular to the axis of the cold head.
Unlike GM cold heads, PT cold heads do not have any internal moving parts to add to vibration levels of the cold head. However, expansion of the compressed Helium gas inside each stage of the cold head does contribute to the vibration level at the 2nd stage. This is true for PT and GM cold heads.
The valve motor on GM cold heads is always bolted to this room temperature mounting flange. As the motor turns, the valve is opened and closed. This same motor also drives the pistons (displacers) inside the 1st and 2nd stage cylinders.
The vibrations from this valve motor will contribute to the overall level of vibrations of the cold head. The movement of the displacers may also contribute to the vibrations at this flange on a GM cold head.
The valve motor on some PT cold heads does have moving parts to open and close the rotary valve, although a “remote valve option” is available for the SHI RP-062B and all Cryomech PT systems allowing the valve motor to be mounted externally from the cold head, reducing vibrations from the motor.
A PT system would be a better choice if:
a) Vibrations at the sample must be less than 20 microns.
b) Vibrations from the cold head need to be isolated from other equipment that is interfaced with the cryostat.
A GM system may be a better choice if:
a) Vibrations on the order of 20 microns at the sample are acceptable.
b) The cryostat will not be rigidly mounted to any equipment that is sensitive to mechanical vibrations.
c) A base temperature of 4K or below is necessary but a PT system is beyond the budget limitations.
d) The cold head will not be oriented vertically with the 2nd stage pointing down.
If vibrations at the sample must be less than 5 microns, Janis has a special design for a vibration isolated exchange gas cooled system. This system includes a 4K, 7K, or 10K GM cold head with a special exchange gas vibration isolation system that reduces the vibration at the sample mount to less than 50nm. The cryocooler does not make physical contact with the sample area so vibrations from the cold head are essentially isolated from the sample. Because there is no physical contact between the cold head and the sample mount in this system, there is no incentive to use a more expensive PT cryocooler when a GM cryocooler will offer the same vibrational performance at a lower price.