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

我们研制了一种在4.2K下冷却功率大于0.1W的二级脉冲管制冷机(以下简称PT)，其独特之处在于PT和阀门电机之间的距离约为2m，并有三个加长的软管，以避免电机的强烈低频电磁干扰。我们用第二级的温度T表征了运行中的PT产生的磁噪声。为了实现这一点，已经解决了几个关键问题：从反相器到破坏SQUID操作的PT的清洁传导EMI，制造高度稳定和灵敏的(20 nT[平方根]赫兹)微型正交式磁通门磁力计(以下简称FG)，以及制造高屏蔽系数的CFRP复合圆柱形磁屏蔽以减少PT周围的静态环境磁场。在铝制杜瓦的内壁上安装了一个微型FG，这样FG就不会振动并保持在室温下。…更多Re。用FG测量了磁噪声的1.8赫兹分量(He气体的泵浦频率)，发现有两个分量：一个分量在T=10K以下变得突出，并随着1/T的增大而增大；另一个分量比第一分量小，在10K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，后者归因于不锈钢构件的任何机械变化。我们进一步使用平面梯度仪的DROS(双弛豫振荡SQIUD由KRIS制备)连接在第二级。观测到的1.8赫兹分量高达0.5nT，其次是二次和三次谐波。值得注意的是，磁噪声的谐波逐渐消失，超过5和6个数量级，并在20赫兹左右再次出现(接近10个数量级)，这可能是由于鱿鱼在静态磁场中的机械振动。较少

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