Low-Noise Closed-Cycle Helium Recondensing for SQUID Biomagnetic Measurement System

To reduce the running cost of a superconducting quantum interference device biomagnetic measurement system, we developed a closed-cycle helium recondensing system with a single pulse-tube cryocooler. The pulse-tube cryocooler was magnetically and mechanically less noisy and more efficient than conventional Gifford-McMahon cryocoolers. The rate of helium recondensing was 14.4 L/day with the power consumption under 8 kW. This was sufficiently larger than the evaporation rate of the cryostat of our magnetoencephalogram (MEG) system, which was similar to 8 L/day. The recondensing 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. Owing to low-loss transfer tubes for transporting the liquid helium from the cryocooler chamber to the MEG cryostat, almost all of the evaporated gaseous helium was successfully recycled. In addition, "mobile reference sensors" were implemented for monitoring the noise generated by the recondensing system. The signals from these mobile reference sensors were effectively used for noise reduction by using signal processing methods. Preliminary MEG measurements were performed. Both somatosensory evoked field and alpha waves were recorded with satisfactory signal-to-noise ratio, even during the recondensing system operation.