Characterization of a Low Frequency Magnetic Noise from a Two Stage Pulse Tube Cryocooler

概要

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Magnetic noise of a two stage pulse tube cryocooler(PT) has been measured by a fundamental mode orthogonal fluxgate magnetometer and by a LTS SQUID gradiometer. The magnetometer was installed in a Dewar made of aluminum at 12 cm apart from a section containing magnetic regen-erative materials of the PT. The magnetic noise shows a clear peak at 1.8 Hz which is the funda-mental frequency of the He gas pumping rate. The 1.8 Hz magnetic noise took a peak, during the cooling process, when the cold stage temperature was at (or close to) 12 K, which resembles the variation of the temperature of the second cold stage of 1.8 Hz. Hence we attributed the main source of this magnetic noise to the temperature dependency of magnetic susceptibility of magnetic regen-erative materials such as Er3Ni and HoCu2 used at the second stage. We pointed out that the su-perconducting magnetic shield by lead sheets reduced the interfering magnetic noise generated from this part. With this scheme, the magnetic noise amplitude measured with the first order gradiometer DROS, mounted in the vicinity of the magnetic regenerator, when the noise amplitude is minimum, which could be found from the fluxgate measurement results, was less than 500 pT peak to peak. Whereas without lead shielding the noise level was higher than the dynamic range of SQUID in-strumentations which is around +-10nT.