MgB₂ ADR Project

ASCG is working with the Mullard Space Science Laboratory (MSSL) of University College London (UCL) and Scientific Magnetics (formerly Space Cryomagnetics) of Culham to develop, build and test a magnesium diboride (MgB₂) magnet suitable for a space-qualified Adiabatic Demagnetisation Refrigerator (ADR). The project has been awarded a three-year PPARC Industrial Programme Support Scheme (PIPSS) grant (ref. PP/C503397/1).

The project is currently in the wire development phase, led by ASCG.

What is an ADR?

ADRs are magnetic refrigerators which exploit the cooling effect of a paramagnetic material when exposed to the application and removal of a strong (several tesla) magnetic field. The application of the magnetic field produces heat in the paramagnet, which is extracted to a cold bath via a heat switch. The subsequent thermal isolation of the paramagnet and removal of the magnetic field causes it to cool to below the bath temperature.

What is the space application for ADRs?

Cryogenic detectors for wavelengths from the near infrared through to X-rays, commonly fitted to scientific spacecraft, require cooling to a temperature in the 30-100 mK range. The ADR is the system of choice to enable this for future missions, with the key reliability and lifetime advantage of being cryogen-free.

In order to use an ADR in a spacecraft and enable it to be cooled by a space cryo-cooler (rather than via an open cycle cryostat), MSSL have developed the double ADR (dADR). This cooler can achieve sub-100 mK temperatures using a magnetic field of 3 T, from a bath temperature of 4 K. This ADR technology has been designed as a self-contained package suitable for deployment in a wide range of future spacecraft, but the requirements of XEUS (a permanent space-borne X-ray observatory) have been taken as the design guidelines.

How are the coils designed?

Scientific Magnetics (formerly Space Cryomagnetics Ltd.) have collaborated in the MSSL ADR programme in the design and manufacture of the ADR magnet system. The magnet consists of two sets of coils, each producing 3 T, which are actively shielded to allow independent operation and to reduce stray field. The requirements for space-based and cryogen-free operation mean that the heat load to the cryo-cooler must be minimised. The magnet coils are therefore wound from ultra-fine conductor (0.1 mm diameter insulated) permitting an operating current of 2.3 A at full field.

The MgB₂ coils under development in this project will need to produce a comparable magnetic field (3 T) with similar heat load restrictions, and consequently the conductor will also need to be of very small diameter. The new cooling system design will permit maximum operating currents approaching 15 A at full field.

Wire development

The coil design requirements make considerable demands on the MgB₂ conductor. The provisional wire design assumes a diameter of between 0.2 and 0.3 mm, and a significant fraction of the cross-section will need to be superconducting. The preference for a react-and-wind conductor of such small dimensions requires considerable work to be done in wire reinforcement, whilst not sacrificing workability and still supplying a substantial volume of high-conductivity material to ensure stability.