A self-contained, fully portable, Josephson junction voltage reference standard system has been designed, developed and tested. The system relies on an active, closed-cycle refrigerator (CCR) cryocooler system and completely eliminates the reliance on liquid helium for cooling the Josephson Junction Array (JJA) chip to the required superconducting temperature of approximately to 4.2 K. The CCR based system has performance capabilities comparable to the liquid helium Dewar-based system and is packaged to operate as a portable system in environments like the US Army's calibration vans or calibration labs that do not have access to liquid helium.
The use of a CCR based cryocooling system brings forth many challenges not found in the classic Dewar-based system. This work identifies the principal challenges for achieving an operating system, and provides unique solutions to overcoming two areas of significant concern, thermal management and electromagnetic noise.
Dewar based systems provide three-dimensional convective cooling. While very effective in cooling, they are inappropriate for portable labs and are subject to evaporation. The challenge for an active system is in providing adequate thermal management to ensure sufficient cooling despite having only one-dimensional conductive cooling. An extensive study was conducted into various methodologies for mounting the chip in the new system and ensuring that superconducting temperatures were obtained. The surface roughness of a conventional machined surface is in the range of several micrometers. The random peaks and valleys of the surface offer insufficient contact area between the cryocooler cold-head and the Josephson junction array chip, resulting in higher than superconducting temperatures at the JJA chip surface. Several approaches researched to increase the thermal contact conductance included thermal grease, adhesives and other high conductivity interstitial materials. The solution provided in this research is an elegantly simple technique, which eliminated the introduction of viscous materials or adhesives, thereby improving the maintainability of the chip. An innovative chip-mount was designed and machined using a state-of-the-art diamond turning technique to achieve a surface roughness of lower than 5 nm and completely eliminate the use of any foreign material. The diamond turned surface attained an operational temperature of 4.2 ± 0.2 K indicating a 30% improvement in the ability to cool the JJA chip.
The second area of interest is understanding the presence of magnetic fields and electromagnetic noise in the vicinity of the JJA chip and eliminating or greatly reducing them. High permeability MuMetal® magnetic shields were designed and installed to reduce the presence of magnetic field by up to 90%, and the nature of magnetic field noises were experimentally quantified. Performance deterioration due to the presence of electromagnetic noise induced by the cryocooler, motor and pump was expected, but for the first time detailed experiments were conducted to measure the magnetic fields in the system, understand their effects, and systematically eliminate or reduce them.
The final system was assembled and the performance was verified using standard Josephson voltage standard (JVS) system practices. A comparison of different operational parameters for the CCR based system was done with the laboratory based liquid helium cooling system, and the results were found to be comparable. The values for critical current and the step amplitude for the JJA chip (# 2629B11) were reported by the chip manufacturer to be 110 &mgr;A and 29 &mgr;A, respectively, tested with a liquid helium system; the same values were measured to be 112 &mgr;A and 27 &mgr;A, respectively, while operating in the UNC-Charlotte JVS system. The measured values were within the experimental repeatability of 5% and the nature of characteristic I-V curves and voltage steps were similar to the measurements made in the liquid helium system. These comparisons demonstrated the operational capabilities of the UNC-Charlotte CCR based Josephson voltage standard system.
|Advisor:||Cuttino, James F.|
|Commitee:||Farahi, Faramarz, Sharer, Deborah L., Smith, Stuart T.|
|School:||The University of North Carolina at Charlotte|
|Department:||Mechanical Engineering (PhD)|
|School Location:||United States -- North Carolina|
|Source:||DAI-B 74/06(E), Dissertation Abstracts International|
|Subjects:||Electrical engineering, Mechanical engineering|
|Keywords:||Cryocooler, Josephson junction, Noise suppression, Standard, Voltage|
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