Low Noise Closed-Cycle Helium Re-Condensing for SQUID Biomagnetic Measurement System

To reduce the running cost of a superconducting quantum interference device biomagnetic measurement system, a closed-cycle helium re-condensing system using a single pulse-tube cryocooler was developed. The pulse-tube cryocooler was magnetically less noisy and more efficient than conventional Gifford-McMahon cryocoolers. The rate of re-condensing helium was 14.4 L/day with a power consumption of less than 8 kW. This was sufficiently larger than the evaporation rate of the cryostat of our magnetoencephalogram (MEG) system, which was around 8 L/day. The re-condensing system was positioned beside a magnetically shielded room (MSR) and exchanged gaseous and liquid helium directly with the cryostat of the MEG system in this MSR. Because of the low-loss transfer tubes used to transport the liquid helium from the cryocooler chamber to the MEG cryostat, almost 100% of the evaporated gaseous helium was successfully recycled. Additionally, "mobile reference sensors" were implemented to monitor the noise from the re-condensing system. The signals from these mobile reference sensors were effectively used for noise reduction by signal processing. Preliminary MEG measurements were performed. Alpha waves were recorded with a sufficient signal-to-noise ratio even while the re-condensing system was operating.