Advanced adiabatic demagnetization refrigerators’ temperature control system

Abstract

We will present a temperature control system for advanced adiabatic demagnetization refrigerators (AADRs). Advanced X-ray and IR detectors, which need to be cooled to temperature below 100 mK, are driving many developments in AADRs. AADRs have many advantages and are used for ground testing of detectors, as well as enabling commercial applications including semiconductor microanalysis, macromolecule spectrometry, and neutron radiation detectors. AADRs include two stages, double stage and continuous stage ADRs. These systems can use multiple paramagnetic salts within the same magnet, or multiple salt-pill/magnet stages. The present state-of-the-art has focused on component development and not integrated control systems, however, temperature control is critical for real-time analysis and maintaining the X-ray line positions. In the control system we will present, temperature and cycle control is based on feedback control of the voltage (or current ramp rate) across the magnet(s). The most complex system is a continuous ADR. This requires temperature monitoring and magnet control of four salt-pill stages and four heat switches. Additionally, the timing of the cycle is critical for continuous cooling. This control system will have four magnet power supplies, two dedicated low-temperature thermometry channels, and a four-channel mux-circuit. Control update rate will be 10 Hz. Two of the four magnet power supplies will be 5 A/5 V current sources, which can be used in series to control a single 10 A/5 V magnet. Software control, cycle timing and thermometry will be discussed.