A Noise Thermometer for Practical Thermometry at Low Temperatures

The application of a magnetic-field-fluctuation thermometer (MFFT) is described for practical thermometry in the low-temperature range. The MFFT inductively measures the magnetic noise generated by Johnson noise currents in a metallic temperature sensor. The temperature of the sensor is deduced from its thermal magnetic noise spectrum by applying the Nyquist theorem, making the thermometer in principle linear over a wide range of temperatures. In this setup, a niobium-based dc SQUID gradiometer detects the magnetic field fluctuations. The gradiometer design optimizes the inductive coupling to the metallic temperature sensor, yet equally ensures sufficient insensitivity to external magnetic interference. In order to obtain a highly sensitive and fast thermometer, the SQUID chip is placed directly onto the surface of the temperature sensor. The compact setup of the gradiometer/temperature sensor unit ensures good conditions for thermal equilibration of the sensor with the temperature to be measured, a factor that becomes increasingly important in the temperature range below 1 K. The first direct comparison measurements of the MFFT with a high-accuracy realization of the Provisional Low Temperature Scale of 2000 (PLTS-2000) are presented. Special emphasis is given to the investigation of the linearity, speed, and accuracy of the MFFT.     

基于小波理论的漏磁检测的噪声消除

利用噪声信号和测试信号对各个尺度上的波谱的不同表现，应用小波理论对漏磁法检测海底管道的腐蚀和缺损情况中的信号噪声进行消除，根据相应的实验得出了一些海底管道缺陷检测的具体特征。     

Magnetic Impurities in Glass and Silver Powder at Milli- and Microkelvin Temperatures

In search of an ideal paramagnet for thermometry at very low temperatures, we have studied the magnetic behaviour of impurity moments (localized Fe³⁺, impurity concentration x = (180±10) ppm) dissolved in a structural borosilicate glass, in the temperature range 0.07 T 300 mK by means of dynamic ac susceptibility, and at 1.6T 300 K by static dc magnetization. In order to improve the thermal coupling of the insulating glass at the lowest temperatures, it was pulverized and mixed with silver powder of submicron grain size; the composite was subsequently compacted to a cylindrical sample by applying a pressure of a few kbar. This contact method which is applicable to other materials with bad thermal conductivity as well improved the accessible minimum temperature for the glass down to 0.1 mK. At low temperatures, we observe a Curie Weiss law for the dynamic susceptibility of the magnetic Fe³⁺ impurities in the glass down to 0.6 mK, a broad maximum of at T 0.38 mK and a decrease towards even lower temperatures. Compared to the frequently used, highly diluted metallic spin glasses, the magnetic behaviour of the glass makes inductance thermometry applicable in a much larger temperature range. In addition, we have investigated the low temperature magnetic properties of a sample of compacted silver powder. The small amount of paramagnetic impurities in the Ag particles (x = (4 ± 1) ppm) exhibits a low temperature susceptibility which can be described by the Kondo effect with an unexpectedly small Kondo temperature of T _K 1 mK.     

Short Communication: Improvements to INRIM Johnson Noise Thermometer

This paper presents a progress report for the Johnson noise thermometry experiment which is under development at the Istituto Nazionale di Ricerca Metrologica. In order to aim at an uncertainty level better than 10⁻⁵ and to reduce the measurement time, a new setup has been developed. In particular, several modifications have been applied to the experiment described by Callegaro et al. (Metrologia 46: 409, 2009) to improve the traceability to voltage, resistance and frequency standards, and new amplifiers have been designed in order to expand the working frequency range up to 20 kHz, with a sensing resistor of 1 kΩ, while maintaining amplifier induced systematic errors to an acceptable level.     

噪声温度计用放大器设计与性能评估

低噪声、低失真、高共模抑制比的放大器对于噪声法测量热力学温度至关重要。设计了由前置放大、仪表放大和缓冲放大3级组成的噪声温度计用高性能放大器。其中,采用具有超低噪声的结型场效应对管IF9030作为前置差分放大级的输入;研制了自动化测量对管转移特性的装置,用于在大批量对管中挑选匹配度更优的器件;采用互相关技术压低电路中不相关的噪声,从而确保能够准确测量到被放大器背景噪声淹没的非线性失真信号;重点评估了放大器在1MHz带宽内的共模抑制比和放大器受双频激励时的非线性失真。测量结果表明,放大器的背景噪声为1.2nV/Hz1/2;100kHz带宽内的共模抑制比优于90dB,100kHz至1MHz带宽内的共模抑制比会随着频率的增加而降低,但不低于70dB;放大器受频率为400kHz、401kHz,幅度有效值为8μV的双频信号激励时,产生的二阶交调失真信号比双频信号低79dB,二阶谐波失真信号比双频信号低85dB。     

Dipole Modeling of Magnetic Flux Leakage

In this paper, we present an analytical model to represent the 3-D magnetic flux leakage (MFL) field due to the occurrence of a surface-breaking defect in a ferromagnetic specimen. This situation is frequently encountered in the nondestructive evaluation (NDE) of energy pipelines using the MFL technique. The model is derived from first principles, and utilizes the concept of dipolar magnetic charge induction to yield the 3-D MFL field in terms of surface integrals. The magnetic flux density in the specimen is assumed to be in the saturation region, and the permeability is assumed to be locally constant in the vicinity of the defect. The model uses just two geometric parameters and is capable of reproducing results that have been obtained experimentally in the literature. 3-D MFL field simulations obtained from the model facilitate a better understanding of the effect of a surface-breaking defect on the magnetic field in its vicinity. Furthermore, we simulate and analyze the 3-D MFL field in the 3-D space around the defect. This analysis yields numerous properties regarding the spatial characteristics of the three orthogonal components of the MFL field of the defect.      

A Model for Noise Power Nonlinearities in Perpendicular Magnetic Recording

Noise power versus linear density is modeled focusing solely on the magnetization fluctuations. The model examines the effect of mid- to high-density trunctation of the recorded magnetization transitions. At these densities, the medium becomes increasingly erased and the low density linear noise slope can vanish or even change sign. The slope of the total noise power versus density depends on a tradeoff of decreasing transition noise and increasing (AC) erasure noise. At high densities, the noise becomes dominated by the large medium erasure noise. The role of head-medium parameters is illustrated.      

A Space Mapping Methodology for Defect Characterization From Magnetic Flux Leakage Measurements

We present an inversion methodology for defect characterization using the data from magnetic flux leakage (MFL) measurements. We use a single tangential component of the leakage field as the MFL response. The inversion procedure employs the space mapping methodology. Space mapping is an efficient technique that shifts the optimization burden from a computationally expensive accurate (fine) model to a less accurate (coarse) but fast model. Here the fine model is a finite-element method (FEM) simulation, while the coarse model is based on analytical formulas. We achieve good estimation of the defect parameters using just a few FEM simulations, which leads to substantial savings in computational cost as compared to other optimization approaches. We examine the efficiency of the proposed inversion technique in estimating the shape parameters of rectangular and cylindrical defects in steel pipes. Our results show good agreement between the actual and estimated defect parameters.      

钢管漏磁在线检测技术的研究

介绍了钢管漏磁检测的基本原理，对钢管在线漏磁检测系统进行总体设计。分析了漏磁场理论模型和讨论了影响缺陷漏磁信号的一些因素及补偿方法，针对该系统的特点设计了高速数据采集板，借助Windows系统平台，在所研制硬件的基础上，采用多线编程和虚拟设备驱动技术编制了数据采集、分析、状态显示、实时控制等面向对象、多功能模块化的软件，详细叙述了漏磁信号数据分析的方法和过程。这种系统具有检测速度快、数据吞吐量大、效率高、钢管缺陷分辨率高等特点。     

A Precision Silicon Transistor Thermometer

This article describes a mass-producible electronic thermometer employing an inexpensive transistor as a temperature sensor. The instrument features ±0.1°C accuracy from -50 to + 125°C; ±0.02°C stability throughout a 1000-day 125°C temperature cycle test; and probes that are freely interchangeable with no calibration by user. Probes need be factory-calibrated at only one temperature, and are based on a novel low-thermal-mass hermetic transistor package. Also described are the theoretical analysis and experiments carried during development. It is shown that sensor transistor Vbe will vary almost linearly with temperature if collector current is an appropriate quadratic function of absolute temperature. Effects of various current functions on Vbe linearity are theoretically analyzed and experimental results given for comparison. The technique used to find the optimum current function is explained, and the circuit which generates the function is described. It is shown that using a more expensive function generator and a 3-point sensor calibration will yield ±0.01°C accuracy. Also discussed is the degree to which different commercially available transistors conform to the theoretical predictions. Criteria are given for selecting an appropriate transistor type.     

一种便携式多通道精密测温仪

介绍了利用数字式温度传感器 DS1 6 2 4构成的多路温度测量仪器。它具有线路简单 ,测量精度高的优点 (在 - 5 5～ 1 2 5°C内 ,精度优于 0 .0 5°C) ;带有计算机接口 ,能够实现长时间连续测量。     

Residual Magnetic Flux Leakage: A Possible Tool for Studying Pipeline Defects

Simulated defects of different shapes and sizes were created in a section of API X70 steel line pipe and were investigated using a residual magnetic flux leakage (MFL) technique. The MFL patterns reflected the actual shape and size of the defects, although there was a slight shift in their position. The defect features were apparent even at high stresses of 220 MPa when the samples were magnetized at those particular stresses. However, unlike the active flux technique, the residual MFL needs a sensitive flux detector to detect the comparatively weaker flux signals.     

场量测量在变交漏磁检测技术中的应用

介绍了以AD637为基础建立的交变漏磁场测量调理电路,完成了基于A3515霍尔元件的漏磁场场量测量传感器的设计,阐述了交变漏磁场测量的工作原理和测试方法,通过对钢管矩形裂纹的测试结果表明,该设计实现了测试的目的,在对缺陷信号进行分析和处理的基础上,提出了基于"缺陷环"的识别方法.     

Magnetic Flux Penetration and Motion in Antiferromagnetic Superconductors

The paper reviews a concept of induced spin-flop domain inside vortices in an antiferromagnetic superconductor. Such phenomenon may occur when an external magnetic field is strong enough to flip over magnetic moments in the core of the vortex from their ground state configuration. The formation of the domain structure inside vortices modifies the surface energy barrier of the superconductor. During this process the entrance of the flux is stopped and a newly created state exhibits perfect shielding. Such behavior should be visible as a plateau on the dependence of flux density as a function of the external magnetic field. The end of the plateau determines the critical field, which has been called the second critical field for flux penetration. Moreover, it is predicted and described how this phenomenon modifies flux creep in layered superconductors. The various scenarios of changing the creep regime from thermal to quantum and vice versa at constant temperature are discussed.     

Elastic Coupling Through Flux Lines and Magnetic Interaction

We show that elastic coupling through flux lines gives an interaction energy between two superconductors or magnetic pieces, which is inversely proportional to the distance between the two bodies. First, we present some details on the penetration depth of the magnetic field in layered superconductors and, especially, on the vortex lines.     

A Kinetic Inductance Ammeter with Coplanar Waveguide Input Structure for Magnetic Flux Focusing

We propose a multiplexible kinetic inductance ammeter, which uses a high-quality-factor, superconducting, lumped-element, kinetic inductance resonator as a current sensor, a short, superconducting coplanar waveguide (CPW) for current input, and a CPW transmission line for the sensor readout. The resonator consists of an interdigitated capacitor and a superconducting loop that inductively couples to the input CPW. Current running through the central line of the input CPW generates magnetic fields which are focused into the gaps of the input CPW. These magnetic fields can be measured collectively as the magnetic flux through the superconducting loop. The kinetic inductance of the superconducting loop depends on the screening current for the magnetic flux, so the input current is converted to a change in the frequency of the resonator. We analyze the response and noise of a kinetic inductance ammeter with a high-resistivity NbN loop.     

Nuclear demagnetization cryostat at University of Florida Microkelvin laboratory

Two nuclear-demagnetization cryostats, each containing 173 mol of copper coolant in a 8 T magnetic field and based on an Oxford Instruments model 1000 dilution refrigerator, have been under development at the University of Florida Microkelvin Laboratory for studies of nuclear magnetic ordering in metals and in solid³He, properties of superfluid³He at T0, and mesoscopic systems, and for development of primary thermometers based on population differences in nuclear quadrupole systems. The performance of the first cryostat is analyzed with emphasis on improvements to be made.     

Measurement of Microkelvin Temperature Differences in a Critical-Point Thermostat

The density of a pure fluid near its critical point is extremely sensitive to temperature gradients. In the absence of gravity, this effect limits the fluid's homogeneity. For example, at 0.6 mK above the critical temperature, the microgravity experiment Critical Viscosity of Xenon (CVX) can allow temperature differences no larger than 0.2 K, corresponding to a gradient of 10^–5 K·m^–1. The CVX thermostat, which consists of a thick-walled copper cell contained within three concentric aluminum shells, was designed to achieve such a small temperature gradient. However, asymmetries not included in the thermostat's model could degrade the thermostat's performance. Therefore we measured the temperature gradient directly with a miniature commercial thermoelectric cooler consisting of 66 semiconductor thermocouples. We checked the results with a half-bridge consisting of two matched thermistors. The measurement was made along a thin-walled stainless-steel cell whose conductance was much lower than that of the copper cell, thus amplifying the temperature differences by a factor of 60. When the thermostat was controlled at a constant temperature, the steel cell's static temperature difference was 5±1 K. (The value inferred for the copper cell is 0.08 K.) Ramping the thermostat's temperature at a rate of 1 × 10^–5 K·s^–1 increased the temperature difference to 0.36 mK. These results demonstrate the feasibility of achieving extremely low temperature gradients.     

带阻塞喷射的大流量排气管降噪处理

针对高温高压燃烧实验间中的排气噪声,在前期降噪尝试和频谱测量确定了噪声源基础上,设计了由多孔共振腔和多孔排气管组合的消声器,消除由临界孔板产生的刺耳的高频阻塞噪声,降低噪声总声压级。经实验测定,噪声的折合声压级降低了21 dB,取得明显降噪效果。不同消声器的组合为实验室的排气降噪提供了参考。