Liquid helium cryostat for SQUID-based MRI receivers

We describe a liquid helium cryostat, developed to cool SQUID-based receivers in low field MRI systems. The cryostat has a 4 L liquid helium capacity, a hold time of over 3 days and accommodates 10 cm diameter receiver coils. New vacuum insulation methods reduce the noise level by at least an order of magnitude compared to existing commercial designs. The minimum detectable field at 425 kHz, with a 5 cm diameter circular coil, was estimated to be 0.018 fT/Hz^1/2 from Q-factor measurements and 0.035 fT/Hz^1/2 by direct measurement with a SQUID amplifier. Further measurements indicated that most of this field noise probably originates with dielectric losses in the cryostat’s fibreglass shells.     

A Numerical Treatment of the rf SQUID: I. General Properties and Noise Energy

We investigate the characteristics and noise performance of rf Superconducting Quantum Interference Devices (SQUIDs) by solving the corresponding Langevin equations numerically and optimizing the model parameters with respect to noise energy. After introducing the basic concepts of the numerical simulations, we give a detailed discussion of the performance of the SQUID as a function of all relevant parameters. The best performance is obtained in the crossover region between the dispersive and dissipative regimes, characterized by an inductance parameter β′ _L ≡2π LI ₀/Φ ₀≈1; L is the loop inductance, I ₀ the critical current of the Josephson junction, and Φ ₀ the flux quantum. In this regime, the lowest (intrinsic) values of noise energy are a factor of about 2 above previous estimates based on analytical approaches. However, several other analytical predictions, such as the inverse proportionality of the noise energy on the tank circuit quality factor and the square of the coupling coefficient between the tank circuit and the SQUID loop, could not be well reproduced. The optimized intrinsic noise energy of the rf SQUID is superior to that of the dc SQUID at all temperatures. Although for technologically achievable parameters this advantage shrinks, particularly at low thermal fluctuation levels, we give examples for realistic parameters that lead to a noise energy comparable to that of the dc SQUID even in this regime.      

High Dynamic Range SQUID Readout for Frequency-Domain Multiplexers

A 16-SQUID array has been designed and fabricated, which shows 0.12 μΦ ₀ Hz^−1/2 flux noise at 4.2 K. The readout amplifier based on a cryogenic silicon-germanium bipolar transistor employs short-delay negative flux feedback and reaches 7 MHz bandwith for a 1 Φ _0p-p signal. The −1 dB compression is reached approximately at 4.2 Φ _0p-p amplitude when the signal frequency is 1 MHz. In the feedback mode the flux noise is anomalously increased to 0.35 μΦ ₀ Hz^−1/2.      

Performance of Stirling-type non-magnetic and non-metallic co-axial pulse tube cryocoolers for high-Tc SQUIDs operation

A set of Stirling-type non-magnetic and non-metallic co-axial pulse tube cryocoolers, intended to achieve portable cryogen-free systems with very low interference for high-T_c SQUIDs operation, have been designed and tested in TIPC/CAS. The key feature is that all cooler components in the vicinity of SQUIDs pick-up loops are made of non-magnetic and non-metallic materials, in order to eliminate complicated interference and realize direct couple with SQUIDs. The cooling options, cooler interference and corresponding solutions are reviewed briefly, and then we focus our attention on the cryogenic design and selection of the materials. Over 30 cooler samples have been fabricated and tested systematically. A typical cooling power of over 100 mW at 80 K with 70 W input electrical power has been achieved. Detailed cooling performance and elementary interference characteristics of the coolers are also analyzed and evaluated.     

用于冷却HTc SQUIDs的脉冲管制冷机系统的设计分析

设计出一套以冷却HTc SQUIDs(高温超导量子干涉仪)为目的的基于Stirling型同轴脉冲管制冷机的低噪声低温系统并进行了系统化的试验。为消除制冷机产生的复杂干扰并实现与器件的直接耦合,所有处于SQUIDs拾波线圈附近的制冷机部件都由无磁、非金属和电绝缘的材料制作。详细描述了制冷机部件材料的选择以及制冷系统的设计与优化,并对系统的综合性能进行了分析与评价。初步试验显示系统的制冷性能和干扰特性满足冷却HTc SQUIDs的基本要求。     

A Numerical Treatment of the rf SQUID: II. Noise Temperature

We investigate rf SQUIDs (Superconducting QUantum Interference Devices), coupled to a resonant input circuit, a readout tank circuit and a preamplifier, by numerically solving the corresponding Langevin equations. The quantity of interest is the noise temperature T _N . We use an analytical expression T _N0,opt, which is already optimized for the parameters of the input circuit, and vary the model parameters of the remaining circuit to minimize T _N0,opt. We also compare T _N0,opt to numerical simulations of the full circuit and find good agreement. The best device performance is obtained when β′ _L ≡2π LI ₀/Φ ₀ is in the range 0.5–0.9; L is the SQUID inductance, I ₀ the junction critical current and Φ ₀ the flux quantum. For a tuned input circuit we find an optimal noise temperature T _N0,opt≈3Tf/f _c , where T, f and f _c denote temperature, signal frequency and junction characteristic frequency, respectively. This value is close to the optimal noise temperatures obtained by approximate analytical theories carried out previously in the limit β′ _L ≲1. We study the dependence of T _N0,opt on various model parameters away from their optimum values, and often find much lower values of T _N0,opt than predicted by the analytical theory. We finally discuss implications for devices that can be implemented experimentally.      

无磁非金属微型同轴脉冲管制冷机的设计分析

分析了脉冲管制冷机系统中存在的主要干扰源 ,并给出了其相应的解决方法 ,在此基础上设计并制作了一台无磁非金属微型同轴脉冲管制冷机。常规脉冲管制冷机中由磁性或金属材料所产生的电磁干扰和感应涡流 ,是将微型脉冲管制冷机应用到高灵敏度高温超导量子干涉仪冷却上的最大障碍。作者采用一系列无磁非金属电绝缘的材料完全替代脉冲管中的磁性或金属材料 ,以实现脉冲管制冷机的低电磁干扰化、甚至无电磁干扰化 ,并最终实现利用脉冲管制冷机 ,有效冷却包括高温超导量子干涉仪在内的对电磁干扰要求极严格的高温超导器件     

An efficient helium circulation system with small GM cryocoolers

We developed a helium circulating system that re-liquefies all the evaporating helium gas and consumes far less power than conventional systems, because warm helium gas at about 40 K collected high above the surface of the liquid helium in the Dewar is used to keep the Dewar cold, and because cold helium gas just above the liquid helium surface is collected and re-liquefied while still cold. A special transfer tube with multi-concentric pipes was developed to make the system operate efficiently. The system can produce up to 35.5 l/D of liquid helium from the evaporated helium using two 1.5 W@4.2 K GM cryocoolers.     

Tunable Resonators for Quantum Circuits

We have designed, fabricated and measured high-Q λ/2 coplanar waveguide microwave resonators whose resonance frequency is made tunable with magnetic field by inserting a DC-SQUID array (including 1 or 7 SQUIDs) inside. Their tunability range is 30% of the zero field frequency. Their quality factor reaches up to 3×10⁴. We present a model based on thermal fluctuations that accounts for the dependence of the quality factor with magnetic field.      

Rapid flaw depth estimation from SQUID-based eddy current nondestructive evaluation

A scheme to rapidly estimate the depth of an unknown flaw from finite element two dimensional calculations using sheet inducer geometry is described. The linear relationship between the phase of the defect magnetic field (difference between the magnetic field arising from eddy currents in flawed and unflawed regions of a plate) and the depth of a flaw is utilized along with two experimental SQUID-based eddy current nondestructive evaluation line scans—one with a flaw at a known depth for reference and the other with the unknown flaw. The results are in good agreement with experimental data obtained with a double-D coil.