  A cryogen-free cryostat cooled using a 4 K commercial GM or pulse tube
cryocooler (PTC) displays temperature oscillations caused by the intrinsic
working principle of the regenerative cryocooler. To dampen such oscillations
usually requires either a large heat capacity or a large thermal resistance. To
understand this phenomenon better and suppress it more effectively, both the
step response characteristic and the intrinsic oscillation characteristic of
cryostat have been used to obtain the complete transfer functions of a
simulation model. The latter is used to test and optimize traditional PID
feedback control. The results showed this approach has almost no effect on the
temperature oscillation amplitude. Based on this simulation model, a novel
active method was proposed and tested numerically. Simulation results predict
the method should suppress the amplitude of the original temperature
oscillation by a factor of two.
