Temperature Control and Noise Reduction in our Compact ADR System for TES Microcalorimeter Operation

Abstract

We have been developing a compact adiabatic demagnetization refrigerator, keeping ground application and future missions in mind. A salt pill fabricated in-house, a superconducting magnet with a passive magnetic shield around it, and a mechanical heat switch are mounted in a dedicated helium cryostat. The detector stage temperature is regulated by PID control of the magnet current, with a $\mathrm{d}I/\mathrm{d}t$ term added to compensate the temperature rise due to parasitic heat. The temperature fluctuation of the detector stage is 1–2 $\upmu$Krms, and the hold time was extended by about 15 % thanks to the $\mathrm{d}I/\mathrm{d}t$ term. Bundle shields of the harnesses between the cryostat and the analog electronics boxes were connected to the chassis at both ends, and the analog electronics boxes were grounded to the cryostat through the bundle shields. This reduced the readout noise to 16 pA/$\sqrt{\mathrm{Hz}}$ in the 10–60 kHz range. Using this system, an energy resolution of 3.8 $\pm$ 0.2 eV (FWHM) was achieved at 5.9 keV.