Cryogenic Heat Switches
Heat switches turn thermal contact on and off at low temperature, enabling multi-stage adiabatic demagnetization and controlled thermal isolation.
Why a switch for heat
Some cryogenic cycles require thermal contact to be established and then broken. Adiabatic demagnetization, for example, needs the paramagnetic pill connected to a heat sink while it is magnetized, then thermally isolated before the field is reduced. A heat switch is the component that makes and breaks this thermal link on command, without adding a permanent heat leak.
Types of heat switch
Several designs are used. A gas-gap switch introduces or removes a small amount of helium exchange gas in a gap between two surfaces; with gas present, conduction links the surfaces, and pumping the gas away isolates them. A superconducting heat switch exploits that a superconductor conducts heat poorly in its superconducting state but well in its normal state; a small magnetic field switches it between the two, turning thermal conduction on and off. Mechanical switches physically bring surfaces into and out of contact.
The superconducting switch in detail
In a metal, electrons carry most of the heat. When the metal becomes superconducting, the electrons condense into Cooper pairs that carry charge without resistance but carry very little entropy or heat, so the thermal conductivity drops sharply. Applying a field above the critical field drives the metal normal, restoring electronic heat conduction. This gives a switch with a large on-to-off conductance ratio and no moving parts, well suited to demagnetization stages.
Selection criteria
A good heat switch has a high ratio of on-state to off-state conductance, low parasitic heat leak, and fast switching. Gas-gap switches are simple and robust but limited by pumping speed. Superconducting switches are fast and clean but work only below the critical temperature of the switch metal. The choice depends on the temperature range and cycle timing of the system.
- Makes and breaks thermal contact on command
- Gas-gap switches use exchange gas in a gap
- Superconducting switches exploit low heat conduction when superconducting
- Key to multi-stage and continuous demagnetization