When a mixture of 3He and 4He is cooled below 870 mK, it separates into two phases. The lighter 'concentrated phase' is rich in 3He and the heavier 'dilute phase' is rich in 4He. The concentration of 3He in each phase depends upon the temperature. Since the enthalpy of the 3He in the two phases is different, it is possible to obtain cooling by evaporating the 3He from the concentrated phase into the dilute phase.
These concentrated and dilute phases separate and a phase boundary established in the mixing chamber, where the cooling process takes place.
To establish continuous cooling one must promote the flow of 3He across the phase boundary in a continuous process. This is achieved by raising the temperature of the dilute phase to ~700 mK outside of the mixing chamber in the still. The vapour pressure of 3He at this temperature is two orders of magnitude higher than that of 4He allowing 3He to be preferentially pumped using external room temperature mechanical pumps or charcoal sorption pumps. This exhausted 3He can be returned to the system, condensed on the 1K pot, pre-cooled at the still and then further cooled through heat exchange with the exiting stream using a continuous heat exchanger ~150 mK and a series of silver sinter step heat exchangers from 100 mK to 20 mK, before being reintroduced to the mixing chamber to continue the process.
To protect the cooling platform from heating the dilution unit and 1K pot are housed in a vacuum with a radiation shield from either the still or 100 mK cold plate that surrounds the heat exchangers and sample space below the mixing chamber.
With careful design temperatures below 5 mK are achievable with a dilution refrigerator.
Access to the sample :
Gaining access to the sample is made by removing the insert from the cryostat, warming it to room temperature and removing the inner vacuum chamber (IVC) tail to gain access to the mixing chamber base. Samples are typically mounted on the mixing chamber or on a sample holder fixed to the mixing chamber. To shorten the turnaround time a sliding seal may be fitted to allow rapid pre-cooling of refrigerator insert and minimising the loss of 4He.