Performance of a helium circulation system for a MEG

We report the performance of a helium circulation system (HCS) for a magnetoencephalography (MEG) that re-liquefies all the evaporating helium gas using two 1.5 W GM cryocoolers operating at 4.2 K. The MEG with the HCS was used to measure human brain responses for over one and a half years without any noise problems. The noise level is below 10 fT/Hz1/2 for 2-40 Hz, below 30 fT/Hz1/2 at 1 Hz, and 200 fT/Hz1/2 at 50 Hz, which is the power supply frequency. As the amount of liquid helium used decreases less than one percent, the maintenance cost of the MEG becomes less than one-tenth of the previous cost.