Technology

A heat switch is a tunable thermal link. Heat switches are widely used in cryogenics and are a key component for every magnetic refrigerator. In a magnetic refrigerator, a heat switch is used to decouple the ADR cooling medium from its pre-cooling stage during the demagnetization cooling. The heat switch is closed to conduct the heat of magnetization, which is generated during the regeneration process, into the pre-cooling stage. There are several types of heat switches, each with different advantages and disadvantages. The most common types are mechanical switches, gas-activated heat switches and superconducting heat switches. Another distinction is between active and passive heat switches. With active heat switches, the switching process can be controlled by the operator. With passive heat switches, the switching process always happens at a certain pre-defined temperature.

With a ‘one-shot’ ADR cryostat, the cooling medium can provide cooling only for a limited amount of time, the ‘hold time’, which is determined by the cooling capacity of the medium. After the cooling medium is magnetized and the heat of magnetization has been dissipated in the pre-cooling stage, the heat switch is opened, and the cooling medium is partially demagnetized. During demagnetization of the cooling medium its temperature drops. Once the target temperature has been reached, demagnetization of the cooling medium continues at a smaller field ramp rate, which is set to compensate for thermal leaks through the support structure, thermal radiation, or customer setups. The total time available at the target temperature is the ‘hold time’. It ends once the magnetic field has been reduced to zero, i.e., when cooling medium is fully demagnetized, and the cooling process terminates. The hold time will vary as a function of temperature, so typically when the system is operated at the lowest temperature, the system may still be operated at higher temperatures by re-magnetizing the cooling medium. A major benefit of magnetic temperature control is that temperatures can be stabilized with high precision, as no valves, capillaries, or heaters are involved.

With a ‘one-shot’ ADR cryostat, the cooling medium provides cooling for a limited amount of time, known as the ‘hold time,’ which is determined by the cooling capacity of the medium. After the cooling medium is magnetized and the heat from magnetization has dissipated in the pre-cooling stage, the heat switch is opened, and the cooling medium is partially demagnetized. During demagnetization, the temperature of the cooling medium drops. Once the target temperature is reached, demagnetization continues at a slower field ramp rate, which is set to compensate for thermal leaks through the support structure, thermal radiation, or customer setups. The total time available at the target temperature is called the ‘hold time.’ It ends when the magnetic field is reduced to zero, i.e., when the cooling medium is fully demagnetized, and the cooling process terminates. The hold time will vary as a function of temperature, so typically when the system is operated at the lowest temperature, the system may still be operated at higher temperatures by re-magnetizing the cooling medium. A major benefit of magnetic temperature control is that temperatures can be stabilized with high precision, as no valves, capillaries, or heaters are involved.

A common concern regarding magnetic refrigerators is the presence of magnetic stray fields at the sample position when the cryostat is running. We pay particular attention to these magnetic fields to enable measurements of very sensitive materials and devices. In all our cryostats, we use compensated superconducting magnets to reduce the stray fields at the sample position and to minimize the magnetic interference between neighboring cooling units in multi-stage systems. In our L-Type Rapid, we maximized the distance of the ADR magnets and the sample stage to minimize the stray field. Together with our compensated magnet coils this ensures stray fields of less than 50 µT at the sample position, which is similar to the earth’s magnetic field. As in any other cryostat, mu metal shielding can be used to further reduce these small residual fields. This makes the L-Type Rapid a suitable tool to probe sensitive materials and devices, such as superconductors or quantum electronics. It is also noteworthy that the stray fields given in our specifications always refer to the maximum magnetic field in the cooling unit. In one-shot ADR cryostats, however, measurements are usually performed with the magnet operating at 0-30% of its maximum field, resulting in accordingly reduced stray fields. Similarly, in cADR systems, most of the magnets are operating at 10-30% of their maximum charge, which again leads to a reduction of the effective residual fields at the sample position.