The push below one Kelvin
Adiabatic demagnetization refrigeration (ADR) extended the accessible temperature range for researchers. Before its practical demonstration in 1933, the lower limit was around 1 Kelvin from a pumped liquid helium bath. ADR pushed that boundary down to about 1 milli-Kelvin, enabling new experimentation. This technology remains central to modern ultra-low temperature work, including space-based observatories. For instance, a recent European Space Agency call sought an ADR system able to produce 50 mK continuously from a 2.5 K baseline, with a total mass under 5 kilograms.
Material stability and seal requirements
The paramagnetic materials used in ADR create specific demands for the surrounding hardware. Traditional water-containing salts have low stability. They must be sealed air-tight and cannot be heated. Newer materials, like KBaYb(BO3)2, are being developed for better chemical stability and cooling performance. This eliminates some old problems but places different stresses on system components. The seal for the rotating shaft introducing the magnetic field must maintain vacuum integrity while withstanding the thermal environment of the experiment, which can involve significant heat loads during the magnetization phase.
Where ferrofluid feedthroughs operate
A magnetic fluid seal provides the rotary feedthrough function in these systems. The seal is filled with magnetic fluid oriented along magnetic force lines created by the shaft, magnet, and pole pieces. It forms a sealing skin called a liquid O-ring. These seals cannot seal liquids directly. Furthermore, condensation of any liquid on the sealing section can cause failure or leakage. This makes managing the thermal gradient at the seal location a primary engineering concern, especially when the cold stage is at milli-Kelvin temperatures and the external environment is at room temperature.
Managing heat at the vacuum boundary
Water cooling integrated into a ferrofluid feedthrough addresses the heat issue directly. The cooling manages heat conducted down the rotating shaft or generated by friction at high speeds. Our seals have been used in applications where the operating atmosphere inside the vacuum device exceeds 1000°C under special specifications. While ADR systems don't reach those extremes, the principle is similar: controlling temperature at the seal preserves the magnetic fluid's properties and prevents breakdown. Effective cooling stops heat from propagating into the cold region and prevents volatile components from condensing on the seal interface.
We supply feedthroughs designed for such demanding thermal management tasks in research and industrial cooling systems.