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Register a new userDesign Evaluation of Serial and Parallel sub-mK Continuous Nuclear Demagnetization Refrigerators
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We present the evaluation of two different design configurations of a two-stage PrNi$_5$ continuous nuclear demagnetization refrigerator. Serial and parallel configurations of the two stages are considered, with emphasis on the attainable cooling power at sub-mK temperatures and the impact of the design choices on the operation of the refrigerator. Numerical simulations of heat transfer in the setup are used to evaluate the performance of the refrigerator as well as the technological requirements for the essential thermal links. In accord with similar findings for adiabatic demagnetization refrigerators [Shirron, emph{Cryogenics} textbf{62}, 2014], our simulations show that the performance of both configurations improves as the thermal links improve, and that the parallel configuration yields a higher cooling power than the series design for a given thermal link resistance and sample temperature.
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We have successfully developed and tested a compact shielded superconducting (SSC) magnet with a FeCoV magnetic shield. This was developed for the PrNi$_5$ based nuclear demagnetization refrigerator which can keep temperatures below 1 mK continuously (CNDR) [Toda $it{et~al}$., J. Phys.: Conf. Ser. $bf{969}$, 012093 (2018)]. The clear bore diameter, outer diameter, and total length of the SSC magnet are 22, 42 and 169 mm, respectively, and it produces the maximum field of 1.38 T at an electric current of 6 A. In order to realize both the compactness and the high shielding performance, we carefully chose material and optimized design of the magnetic shield by numerical simulations of the field distribution based on measured magnetization curves of several candidate materials with high permeability. We also measured a heat generated by sweeping the SSC magnet in vacuum to be 230 mJ per field cycle. This value agrees very well with an estimation from the measured magnetic hysteresis of the superconducting wire used to wind the magnet.
We demonstrate successful dry refrigeration of quantum fluids down to $T=0.16$,mK by using copper nuclear demagnetization stage that is pre-cooled by a pulse-tube-based dilution refrigerator. This type of refrigeration delivers a flexible and simple sub-mK solution to a variety of needs including experiments with superfluid $^3$He. Our central design principle was to eliminate relative vibrations between the high-field magnet and the nuclear refrigeration stage, which resulted in the minimum heat leak of $Q=4.4$,nW obtained in field of 35,mT. For thermometry, we employed a quartz tuning fork immersed into liquid $^3$He. We show that the fork oscillator can be considered as self-calibrating in superfluid $^3$He at the crossover point from hydrodynamic into ballistic quasiparticle regime.
The National High Magnetic Field Laboratory (NHMFL) High B/T facility at the University of Florida in Gainesville provides a unique combination of ultra-low temperatures below 1 mK and high magnetic fields up to 16 T for user experiments. To meet the growing user demand for calorimetric and thermal transport measurements, particularly on milligram-sized solid samples, we are developing scaleable thermometers based on quartz tuning fork resonators immersed in liquid $^3$He. We demonstrate successful thermometer operation at the combined extreme conditions available at our user facility, and discuss the feasibility of fast and compact thermal probes.
Optimization techniques for decreasing the time and area of adder circuits have been extensively studied for years mostly in binary logic system. In this paper, we provide the necessary equations required to design a full adder in quaternary logic system. We develop the equations for single-stage parallel adder which works as a carry look-ahead adder. We also provide the design of a logarithmic stage parallel adder which can compute the carries within log2(n) time delay for n qudits. At last, we compare the designs and finally propose a hybrid adder which combines the advantages of serial and parallel adder.
High-speed serial links implemented in SRAM-based FPGAs have been extensively used in the trigger and data acquisition systems of High Energy Physics experiments. Usually, their application has been restricted to off-detector, mostly due the sensitivity of SRAM-based FPGA to radiation faults (single event upsets). However, the device tolerance to radiation environments can be achieved by adopting dedicated mitigation techniques such as information redundancy, hardware redundancy and configuration scrubbing. In this work, we discuss the design of a bi-directional serial link running at 6.25 Gbps based on a Xilinx Kintex-7 FPGA. The link is protected against single event upsets by means of all the above-mentioned methods. A self-synchronizing scrambler is used for DC-balance and data randomization, while the subsequent Reed-Solomon encoder/decoder detects and corrects bursts of errors in the transmitted data. The error correction capability of the line code is further increased by adopting the interleaving technique. Besides, in order to completely take advantage of available bandwidth and to cope with different rates of radiation-induced faults, the link can modulate the protection level of the Reed-Solomon code. The reliability of the link is also improved by means of modular redundancy on the frame alignment block. Besides, on the same FPGA, a scrubber repairs corrupted configuration frames in real-time. We present the test results carried out using the fault injection method. We show the performance of the link in terms of mean time between failures (MTBF) and fault tolerance to upsets.
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