Development of a measuring system for low-frequency, extremely low-level magnetic field-Introduction of high-performance magnetic shield and a adaptive vibratory noise cancelling mechanism

在本研究中，以下两个主题进行了调查发展；(1)利用磁振技术制造出人体大小、重量轻的开口圆柱形磁屏蔽；(2)自适应消除振动磁噪声的系统。我们首先使用metglass 2705M无定形带（总重量为15kg）在五壳结构中构建了半尺寸模型（直径为52cm，长度为120cm），中间间距小于1cm。横向施加交变磁场的屏蔽系数在1hz时为90900，在10hz时为21000。结果表明，利用磁振法获得的相对渗透率值较高，可以使多壳在较窄的间距下也能很好地工作。然后，我们开发了一个人体大小的四壳圆柱形屏蔽（直径=90cm，长度=250cm），其中70kg的无定形带和40kg的坡莫合金带被螺旋缠绕，形成最内层的外壳，用于防止不必要的震动场泄漏到屏蔽空间。使用SQUID磁力计测量屏蔽层中的背景磁噪声，在1hz时为54 fT/ (Hz)^<-1/2>，在10hz时为23 fT/ (Hz)^<-1/2>。残余直流磁场仅为1-2nT。以两自由度振动为前提，以线性梯度交流磁场为参参考系，研究了一种消除振动引起的非平稳磁噪声的算法。噪声抑制比为20dB。然后将该系统应用到人体大小的圆柱形磁屏蔽上。抑制比为10dB。最后，将屏蔽系统应用于心脏磁场测量，结果表明SQUID磁力计和一阶梯度仪都能成功测量人体心脏磁场。

In this study, the following two themes were investigated to develop ; (1) a human-size, light-weight, cylindrical magnetic shield with open ends by applying magnetic shaking technique, and (2) a system canceling vibratory magnetic noise adaptively.We first constructed half-sized model (diameter=52cm, length=120cm) using Metglas 2705M amorphous ribbons amounting to 15kg in a five-shell structure with narrow spacing (less than 1cm) in-between. The shielding factor for transversely applied alternating magnetic field was 90900 at 1 Hz and 21000 at 10 Hz. It shows high value for relative permeability obtained with magnetic shaking helps multi-shell to work well despite narrow spacings. We then developed a human-size, four-shell cylindrical shield (diameter=90cm, length=250cm) with the same amorphous ribbon of 70kg and Permalloy tapes of 40kg which was wound helically to form the innermost shell used to prevent unwanted leak of the shaking field into the shielded space. Background magnetic noise in the shield was measured using a SQUID magnetometer as 54 fT/ (Hz)^<-1/2> at 1 Hz and 23 fT/ (Hz)^<-1/2> at 10 Hz. The residual dc magnetic field was only 1-2nT.An algorithm to cancel nonstationary magnetic noise due to vibration was evaluated using a model system in which vibration of two order of freedom was assumed and linear gradient ac magnetic fields were used as a reference frame. Noise rejection ratio was 20dB.This system was then implemented to the human-size cylindrical magnetic shield. The rejection ratio was 10dB.Finally, the shielding system was used for a heart magnetic field measurement, showing magnetic field from a human heart was successfully measured by both a SQUID magnetometer and a first-order gradiometer.