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

我们开发了一种在4.2 K下冷却功率大于0.1 W的第二级脉管制冷机（以下简称PT）。它的独特之处在于，PT和阀门电机相隔约2米，有三个延伸的柔性软管，以避免电机产生强烈的低频磁干扰。我们对PT在第二阶段的温度T下产生的磁噪声进行了表征。为了做到这一点，解决了几个关键问题：清除从逆变器到PT的电磁干扰，破坏SQUID的操作，制造高度稳定和敏感（20 nT【平方根】Hz）的小型正交磁通门磁强计（以下简称FG），在铝制杜瓦瓶的内壁上安装了一个微型FG，使其在室温下保持不振动。通过对FG的磁噪声中1.8 Hz（氦气的泵送频率）分量的测量，我们发现FG的磁噪声有两个分量：其中一个在约10 K以下变得突出，并随1/T增加，另一个比第一个小，在约10 K以上的较宽温度范围内几乎不变。前者归因于Er3Ni和HoCu2等顺磁再生材料磁化率的周期性变化，而后者归因于不锈钢构件的任何机械变化。我们进一步利用附着在第二级上的平面梯度仪（由KRISS制备的双弛豫振荡squid）进行了表征。观察到的1.8 Hz分量最大为0.5 nT，其次是第二次和第三次谐波。有趣的是，磁噪声的谐波在5阶和6阶以上逐渐消失，并在20 Hz左右（接近10阶）再次出现，这可能是由于SQUID在静态磁场中的机械振动。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