Helium Technology and Low Temperature Section

Major activities

1. Dilution Refrigerator

Dilution refrigerator is a device that can achieve and maintain continuous refrigeration in the milli-Kelvin temperature range with substantial cooling power. It takes advantage of the unique low-temperature behaviour of the 3He-4He mixtures below about 0.87K producing phase separation between two isotopes. A lighter and rich in3He called concentrate solution floating on top of the 4He rich phase called dilute solution respectively. In order to maintain the finite solubility of 3He in superfluid 4He, the lighter 3He atoms cross the phase boundary over to dilute solution below results in cooling.Recently, we have indigenously designed, developed and tested a dilution refrigerator for producing milli-Kelvin temperature. The base temperature obtained was below 50mK in the mixing chamber.

Initially, the project was aimed at developing technologies relevant to dilution refrigerator so each component were designed, fabricated and tested in the laboratory. In this context, we have developed a computer programme SIDFO for initial design and optimization of dilution refrigerator as it greatly reduces the risk of serious design flaws. The infrastructure specific to very low temperature work has been developed for the purpose. Efforts to achieve still lower temperature would be on since low temperatures allow study of quantum phenomena such as superfluid phase transition of 3He, quantum hall effect,semiconductor-based nano-materials, low temperature detectors superconducting tunnel junctions, low temperature nuclear orientation and NMR experiments and so on.

2. Development of Cryogen-free cryostat for Recondensation of helium vapour

A small scale 4He liquefier has been designed and constructed that is solely based on the cooling of a two-stage 4K pulse tube cryo-cooler. In a further expansion a cryogen-free super-fluid cryostat with closed loop helium circulation system is under development. This cryostat is uniquely qualified to provide the cooling requirement along with an option for magnetic field for materials research, neutron scattering, NMR studies etc. for many branches of physics. Unusual phases of matter such as superconductivity at low temperatures and much subtle behaviour of materials that are obscured by thermal motion at relatively higher temperature can be studied in great detail low temperature regime (~ 1.8 K). The closed cycle super-fluid cryostat will be extremely useful to the experimental nuclear physics community. Some of the very central research areas involve MRI, NMR and SQUIDs. Its usefulness arises from the fact that it operates continuously, it can provide a substantial cooling power along with a magnetic field at temperatures from around 1.8 K and it can run uninterrupted for as long as several months. Most existing superconducting systems employ a liquid helium refilling system that is costly, troublesome and must be maintained by a skilled technician. To replace the standard refill system with helium re-condenser presents a challenge.