Helium exchange gas based variable temperature insert for cryogen-free magnet system

A cryocooler based variable temperature inserts (VTI) has been designed and developed for measurement of physical properties at low temperature and high magnetic field. The VTI, designed using the helium exchange gas principle, needs to be integrated in the warm bore of an existing 6 T cryogen free magnet system. The lowest temperature achieved at the sample is 5 K at 34.5 kPa (∼5 psi) gaseous helium environment in the sample space. The equilibrium temperature of the sample, at the vacuum condition, is 8.7 K. The cool-down time of the sample at vacuum environment is 9 hrs whereas it takes 7 hrs in presence of helium exchange gas. The temperature of the sample was varied up to 325 K. The stability of the temperature achieved is less than 50 mK. The cooling and heating curves has been studied to estimate time required for a complete cycle of experiment. This paper will briefly present the design and performance of VTI system in temperature range of 5-325 K. (paper)