  A cryogen-free cryostat cooled by a closed cycle cryocooler is compact, can
provide uninterrupted long-term operation (up to ten thousand hours) and is
suited to temperatures from 3 K to 300 K. Its intrinsic temperature
oscillation, however, limits its application in experiments requiring high
thermal stability at low temperature (below 77 K). Passive suppression methods
are effective but all suffer from drawbacks. We describe a novel, active
suppression scheme more efficient than traditional proportional-integral (PI)
control. The experimental results show that it can reduce the standard
deviation of the temperature oscillation by a further 30% compared with PI
feedback. To the best of our knowledge, this is the first time such active
suppression of temperature oscillations has been implemented with the
cryogen-free cryostat. The results also show, however, that an unwanted lower
frequency thermal noise will be generated, which appears to be the limit of the
method. Nevertheless, the approach could be used to improve the temperature
stability in all cryogen-free cryostats.
