Cryostat

• Schematic Overview
• Wiring
• Gas Handling

Schematic Overview

Dilution Refrigerator

Below are pictures of the dilution refrigerator built at Wesleyan University. The schematic on the right shows the major components, generally getting colder from top to bottom. The whole unit is enclosed in a vacuum space for insulation and submerged in a bath of liquid helium at 4.2 K. Moving down the chain...

The 1K pot cools to about 1.4 K by pumping away the vapor pressure in the pot. The lower pressure coexists in equilibrium with liquid at a lower temperature. Liquid helium from the bath is drawn in at a limited rate through a flow restriction replacing the evaporated liquid. The cooling power is limited by the flow restriction and pumping rate. The base temperature is limited by the pumping efficiency as the vapor pressure disappears at low temperatures.

The still works similar to the 1K pot using recycled ³He atoms for an overall lower temperature.at the limiting vapor pressure. The ³He atoms are not evaporated from liquid ³He, but from a background of liquid ⁴He. This distillation gives the stage it's name.

The mixing chamber is the lowest temperature statge. ³He atoms removed from the still are returned to the background ⁴He liquid in this chamber. Initially introduced as fairly pure ³He that floats above the ⁴He, the ³He atoms absorb heat as they dissolve back into the ⁴He. This is best thought of as the ³He atoms evaporating into the ⁴He as opposed to a vacuum. With a much lower energy difference (latent heat) the phase equilibruim occurs at a much lower temperature.

³He atoms evaporated into the ⁴He in the mixing chamber diffude back into the still for purification and ultimate return to the mixing chamber.

Features that contribute to the effectiveness of the refrigerator include...

• Efficient pre-cooling of ³He liquid returned to the mixing chamber. This is achieved by thermal exchange with the refrigeration stages and, most importantly, with the cold ³He atoms diffusing out of the mixing chamber back into the still.
• The still is actually heated to increase the circulation rate of the ³He, but not too hot to evaporate ⁴He. The distillation process is hampered by the superflow of ⁴He film up the pump tube to higher temperatures where it can evaporate. This flow should be supressed.
• All pathways for the ³He need to be carefully optimized for the best compromise between particle flow impedance (high is bad) and heat flow impedance (high is good).
• Thermal contact between stages must be minimal, and radiation shielding from the 4.2 K bath is necessary.

Wiring

Vacuum Can

 A  B  11040 1K Pot Heater
 C  D    577 1K Pot Thermometer
 E  F   1202 Film Burner
 H  J  11150 Still Heater
 K  L    642 Still Thermometer
 M  N    480 Evaporator Ring
 P  R  11222 Mixing Chamber Heater
 S  T    468 Mixing Chamber Thermometer
 U  V    236 Compensation Loop

Bath Space

 Y  Z    62 Compensation Loop
AA BB   223 Bath Thermometer 1
CC DD  8710 Bath Thermometer 2
EE FF   143 Bath Thermistor
HH JJ   370 Bath Heater
KK LL   370 Bath Heater
MM NN   370 Bath Heater

Rack Panel Connections

10-11 ---> A-B
12-13 ---> C-D
etc.
48-49 ---> MM-NN

Gas Handling

Here's picture of the top of the cryostat stand showing the major connections into the cryostat.