Chapter 142 - Design and manufacture of a low-current ADR magnet for a space application

Abstract

Adiabatic demagnetization refrigerator (ADR) systems offer a convenient method for achieving temperatures below 50 mK. The magnet system described in this chapter is a key component of the ADR engineering and qualification model for the proposed European Space Agency's (ESA) XEUS mission, in which the ADR will be used to cool x-ray detectors. A low-operating current ADR magnet has been constructed as part of the development of the XEUS x ray observatory satellite. The components of the magnet system are designed and developed with emphasis on the satellite requirements, and on constraints on the overall length and the interactions between the two ADR stages. The leads—which supply current to the magnet coils—operate in vacuum, and therefore have to be thermally anchored to the 4.5 K stage of the cryocooler. However, the cooling power available from the cryocooler is very limited, so it is vital to minimize the heat load from the current leads. For this reason, the magnet has been designed to operate at very low current, using very small NbTi superconducting wire, running at 60% of its short sample performance. For quench protection, the solenoids and coils are subdivided into sections with resistor and diode protection circuits. The magnet system consists of two major solenoid coils and eight secondary coils. All are cooled by conduction through heat shunts to the 4.5 K stage cryocooler: there are no liquid cryogens. Thermal modelling and eddy current analysis have been used to determine the optimum geometry for the heat shunts.