Measurements and analysis of magnetic noises arising from the second-stage pulse-tube refrigerator for the development of liquid helium less LTS-SQUID biomagnetic fields measurement system

我们研制了一台在4.2K下制冷功率大于0.1W的第二级脉管制冷机。其独特之处在于，PT和阀门电机之间用三根延长的柔性软管隔开约2 m，以避免电机产生强烈的低频磁干扰。我们的特点所产生的磁噪声与温度，T，第二阶段的PT在操作中。为了做到这一点，解决了几个关键问题：清除从逆变器到PT的传导EMI，该EMI破坏SQUID的操作，（20 nT[平方根]Hz）正交磁通门磁力计（以下简称FG）的微型尺寸，制作了高屏蔽系数的CFRP复合圆柱形磁屏蔽体，以降低PT周围的静态环境磁场。在铝杜瓦的内壁上安装了一个微型FG，使FG不振动，并保持在室温下。 关于我们 re.用FG测量了磁噪声的1.8Hz（He气体的抽运频率）分量，发现有两个分量：一个在T=10 K以下变得显著，并随1/T增加;另一个比第一个小，在10 K以上的很宽温度范围内几乎不变。前者归因于顺磁性再生材料（如Er_3Ni和HoCu_2）的磁化率的周期性变化，而后者归因于由不锈钢制成的构件的任何机械变化。我们进一步的特点是使用一个平面梯度DROS（双弛豫振荡SQIUD制备的KRISS）附加到第二级。观察到的1.8 Hz分量高达0.5 nT，其次是二次和三次谐波。值得注意的是，磁噪声的谐波在5阶和6阶以上逐渐消失，并在20 Hz附近（接近10阶）再次出现，这可能是由于SQUID在静磁场中的机械振动。少

We have developed a second-stage pulse-tube refrigerator (hereafter PT) with a cooling power greater than 0.1 W at 4.2 K. Its unique aspect is that the PT and a valve motor is separated by about 2 m with three extended flexible hoses to avoid strong low- frequency magnetic interferences from the motor. We have characterized magnetic noises arising from the PT in operation with the temperature, T, of the second stage. In order to perform this, several key issues have been addressed : cleaning conducted EMI from an inverter to the PT that destroys the operation of the SQUID, fabrication of highly stable and sensitive (20 nT【square root】Hz) orthogonal fluxgate magnetometers (hereafter FG) of a miniature-size, and fabrication of a CFRP composited cylindrical magnetic shield of high shielding factor to reduce static environment magnetic field around the PT. A miniature FG was installed on the inner wall of an Aluminum Dewar so that the FG should not vibrate and be kept at the room temperatu … More re. From the measurements of the 1.8 Hz (pumping frequency of the He-gas) component of the magnetic noise with FG, we found that there are two components : one becomes prominent below about T=10 K and increases with 1/T and the other is smaller than the first and almost constant over wide range of the temperature above about 10 K. The former is attributed to the periodical variation of the magnetic susceptibility of the paramagnetic regenerative materials such as Er3Ni and HoCu2, on the other hand the latter is attributed to any mechanical variation of the constructing parts made of stainless steel. We further characterized by using a planar gradiometer of DROS (double relaxation oscillation SQIUD prepared by KRISS) attached to the second-stage. The 1.8 Hz component observed was as large as 0.5 nT and followed by the second and the third harmonics. It would be interesting to note that the harmonics of the magnetic noises gradually disappear beyond the orders of 5 and 6 and appear again around 20 Hz (near 10th order) which may be due to mechanical vibration of the SQUID in the static magnetic field. Less