There is a big technological gap between the SQUID sensor system and the magnetic shielding system used today. Hence, the development of a new and effective method of magnetic shielding is a key for extremely weak magnetic field measurement techniques. We have developed a human-size cylindrical magnetic shield based on a new framework: magnetic shaking enhancement using a square B-H loop material [1] and a open structure with open-end compensation [2]. The magnetic shield exploits a thin multi-shell structure which has been shown by a one-half model of the present system to be effective to obtain high shielding factor [3]. Both ends of the shield developed were left open. The sizes of the shield are: inner diameter=92 cm, outer diameter=106 cm and the length=220 cm. Weights of the magnetic material was 69 kg for Metglas 2705M amorphous ribbon and 40 kg for Permalloy tape and the total weight was about 250 kg. Fig. 1 shows a cross sectional view of the shield. Numbers in the parenthesis show numbers of layers in each shell. All coils shown are toroidal coil of 100 turns. The shield consists of four magnetic shells with narrow spacings in-between. The outer three shells are composed of helically wound Metglas 2705M amorphous ribbons and to be subjected to magnetic shaking given by a toroidal shaking coil wound on each of the shells to enhance their magnetic permeability. In the experiments, the outer two shells of the amorphous shells were subjected to the same shaking field given by a 200 Hz, 50 mA current. The third one was subjected to a shaking field given by a 536 Hz 35 mA current. The inner most shell is a passive shell made of 0.1 mm thick Permalloy tapes, which is used to attenuate unwanted leak from the shaking magnetic field. When Permalloy tapes was annealed, the tapes were wound in a coil form to have the same diameter of the outer surface of the cylindrical base material to avoid bending stress. This shell has a toroidal coil on it to demagnetize the shell while magnetic shaking for the outer three shells are kept on to reduce the earth's magnetic field. The shielding factor was measured for a transverse magnetic field of 100 mG using a large Helmholtz coil [2] under open-end compensation on and off. In this experiment, the shield was placed horizontally on the floor and perpendicularly to the earth's magnetic field direction. The results are shown in Fig. 2. The shielding factor under compensation on is reasonably high, by which one can attenuate environmental magnetic fields down to smaller than 0.1 pT/Hz~(1/2). However, a saturating nature of the comp-on curve suggests one can further improve a compensation system [3]. Leakage magnetic field from the shaking field was a problem because too high leakage field prevents flux locked loop (FLL) operation of the SQUID magnetometer. The results are shown in Fig. 3 for x (horizontal) and y (vertical) components. The level of the leakage field which was mainly from inner shaking field (536 Hz field and the second harmonics), were made small by a factor of 10 due to the Permalloy shell compared to our former results [4]. Because there is no major discontinuity in the Permalloy shell, instead many minor discontinuities spread evenly in the shell, peaking in a profile of residual dc magnetic field distribution was not found but became monatomic. We confirmed that FLL operation of a SQUID magnetometer was established in this shield. The total apparent power for magnetic shaking was only 0.51 VA.
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