We present a parallel network of 16 demagnetization refrigerators mounted on a cryofree dilution refrigerator aimed to cool nanoelectronic devices to sub-millikelvin temperatures. To measure the refrigerator temperature, the thermal motion of electrons in a Ag wire—thermalized by a spot-weld to one of the Cu nuclear refrigerators—is inductively picked-up by a superconducting gradiometer and amplified by a SQUID mounted at 4 K. The noise thermometer as well as other thermometers are used to characterize the performance of the system, finding magnetic field independent heat-leaks of a few nW/mol, cold times of several days below 1 mK, and a lowest temperature of 150 μK of one of the nuclear stages in a final field of 80 mT, close to the intrinsic SQUID noise of about 100 . A simple thermal model of the system capturing the nuclear refrigerator, heat leaks, and thermal and Korringa links describes the main features very well, including rather high refrigerator efficiencies typically above 80%.
The DC-SQUID is operated in a flux-locked loop mode with XXF-1 electronics, including a second order Bessel-type low-pass filter (f3dB = 10 kHz). After a room temperature voltage preamplifier with another low-pass filter (f3dB = 1 kHz), the signal is acquired with a digital-to-analog converter.