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Large-scale electric aircraft is a disruptive technology to address the environmental impact of air travel. Fault current limitation is crucial to realise the safety and reliability of the electric aircraft, in particular for large-scale electric aircraft using DC distribution network. This paper investigates the behavior of the resistive SFCL under different cryogenic cooling systems including liquid nitrogen and helium gas circulation cooling systems for the electric aircraft DC distribution network. Electromagnetic and thermoelectric models were used to predict the characteristics of resistive SFCL. The simulation results demonstrate that it is promising to design resistive SFCL using liquid nitrogen cooling as well as helium gas circulation cooling systems, which offer flexibility for SFCL cryogenic cooling system for electric aircraft applications.

Current quantum systems based on spin qubits are controlled by classical electronics located outside the cryostat at room temperature. This approach creates a major wiring bottleneck, which is one of the main roadblocks toward truly scalable quantum computers. Thus, we propose a scalable memristor-based programmable DC source that could be used to perform biasing of quantum dots inside of the cryostat (i.e. in-situ). This novel cryogenic approach would enable to control the applied voltage on the electrostatic gates by programming the resistance of the memristors, thus storing in the latter the appropriate conditions to form the quantum dots. In this study, we first demonstrate multilevel resistance programming of a TiO2-based memristors at 4.2 K, an essential feature to achieve voltage tunability of the memristor-based DC source. We then report hardwarebased simulations of the electrical performance of the proposed DC source. A cryogenic TiO2-based memristor model fitted on our experimental data at 4.2 K was used to show a 1 V voltage range and 100 uV in-situ memristor-based DC source. Finally, we simulate the biasing of double quantum dots enabling sub-2 minutes in-situ charge stability diagrams. This demonstration is a first step towards more advanced cryogenic applications for resistive memories such as cryogenic control electronics for quantum computers.

International audience; For the HL-LHC project, a 90 mm NbTi cos(2θ) double aperture quadrupole magnet with an operating gradient of 120 T/m at 1.9 K has been designed as an option to replace the 70 mm aperture LHC quadrupole MQY. CEA in collaboration with CERN designed and manufactured a single aperture short model magnet with a magnetic length of 1.215 m at 1.9 K called MQYYM. The MQYYM cold test occurred at CEA at 4.2 K in a vertical cryogenic station. During the power test, the operating gradient at 1.9 K has been reached after two training quenches. All along the test, magnetic and mechanical measurements were done using respectively a rotating probe and strain gauges. This paper describes the performance of the MQYYM at 4.2 K and gives an analysis of the data acquired during the test, including training behavior, quench detection, protection and field quality measurements.

We are developing a new instrument, the RF Josephson arbitrary waveform synthesizer (RF-JAWS), for communications metrology and quantum information applications. An important aspect of the RF-JAWS design is the accurate and traceable characterization of its superconducting devices. In this article, we present a procedure for characterizing microwave superconducting devices in a cryogenic RF probe station via a vector network analyzer (VNA) calibrated with a custom cryogenic calibration kit colocated with the superconducting device under test (DUT) in the cryogenic environment. By de-embedding lossy and dispersive RF interconnects linking the superconducting DUT to the measurement apparatus at room temperature, we characterize the DUT exactly at the cryogenic on-wafer reference plane. More importantly, we operate our VNA with an external modulated source and our procedure features metrology-grade multiline thru-reflect-line calibration and absolute power and phase corrections, as opposed to the more common relative scattering-parameter correction. In addition, we apply an X -parameter model to account for impedance mismatch in cryogenic Josephson microwave sources. Our techniques are also suitable for cryogenic characterization of microwave superconducting devices for solid-state quantum computers and could help to optimize the quantum-classical interfaces in these systems.

© 2022 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. One of the most enticing candidates for next-generation computing systems is the memristor. Memristor-based novel architectures have demonstrated considerable promise in replacing or augmenting traditional computing platforms based on the Von Neumann architecture, which faces many issues in the big-data era, as well as in newly developed neuromorphic tasks. Although the current classical computing architecture is unlikely to be abandoned in the foreseeable future, the growing trend of neuromorphic, quantum, and bio-inspired computing schemes calls for more specialized beyond Von Neumann platforms. Memristors showcase multiple advantages in terms of small area footprint, energy efficiency, high endurance, bio-compatibility, and their inherent synaptic and neuromorphic behavior. The topic of this work is to present the memristive devices that meet the requirements for the implementation of the novel beyond Von Neumann applications and examine their switching mechanism and material selection, as well as to conduct a performance comparison between the fabricated devices paving the way for future computing applications.

Superfluid helium has been used for cooling superconducting magnets in nuclear fusion and high energy physics. This paper presents a simplified final stage process of 1.8 K superfluid helium cryogenic system and its key component sub-atmospheric heat exchanger. Influence of variable thermal parameters of sub-atmospheric heat exchanger on the system performance is studied. The application of sub-atmospheric heat exchanger can increase refrigerating capacity by 61.34% under given conditions. The helically finned-tube heat exchanger (HFHE) is selected, and three-dimensional steady-state numerical simulations of flow and heat transfer performance have been carried out. Nu and f of tube side are 4.5-6 times higher and 95-97% lower than those of shell side, respectively. It is found that HFHE with low fin height and HFHE with filler (nylon rope) have better comprehensive heat transfer and flow performance, and PEC can reach 1.65 and 1.41 respectively under given conditions.

At scaled nodes, Back End of Line (BEOL) interconnect resistance increases super-linearly and creates performance and energy bottlenecks. While it is known that metal resistance deceases with temperature, understanding and quantifying the behavior of interconnects from a foundry process design kit (PDK) at low temperature is critical for designing circuits and systems operating at cryogenic temperatures including Quantum and Superconducting Computers and other High-Performance Systems. In this letter we use parametric path based ring oscillators along with reference ring oscillators and Metal-on-Metal Capacitors for 22nm Fully Depleted Silicon on Insulator to measure RC delays of BEOL elements from room temperature to 4.2K. The extracted resistance of the elements show reduction of up to 30% for M1/M2, 33% for M3-M6 and 19% for Via1. With simple assumptions for interconnect structure, we perform least-squares fit of Fuchs-Sondheimer-Mayadas-Shatzkes (FS-MS) models with line-Edge roughness incorporated in the model using the Namba Model. The empirically fitted parameters track well with literature. The fitted model is within 3.7% error of all experimental results.

In order to fully understand switching transients of SiC power MOSFET, an easy-to-implement transient model is built to evaluate the switching characteristics regarding to the influences of parasitic elements and circuit parameters. Under two testing conditions of room temperature and cryogenic temperature, it is shown that all the simulations generally fit the measurements well with different operating currents and voltages. For the switching energy loss during the turn-on and turn-off transients, further analysis shows that the relative error is less than 20% between the experiments and simulations. By considering the zero-loss superconductor and its equipped cryogenic environment, it can be well expected to explore a new concept of superconducting power electronics involved with superconducting and semiconducting devices in one system.

In superconducting system, current lead connecting cryogenic device and normal temperature copper cable is the main leakage heat source of entire system. Its performance directly affects cooling cost of cryogenic system. Multi-current leads have advantages of eliminating Joule heat and reducing heat conduction leakage. As a commonly superconducting material, MgB2(magnesium diboride) has broad application prospects. In this paper, the current lead of 200 A MgB2 cable and normal temperature copper cable are designed and calculated. The heat transfer power of superconducting section and the conventional current lead section are 17.90 W and 18 W, respectively. The numerical simulation method is used to verify and analyze calculation results. The temperature at interface between normal temperature copper wire and current lead is lower than 70 K.

Memory devices for low-temperature electronics have great application potential in emerging fields like quantum computing, deep-space exploration etc. However, the practical non-volatile memory device for cryogenic use still remains as a challenge today. We propose a delicately designed resistive memory device with bi-layer structure. The resistive switching layer is covered by a thermal enhancement capping layer which confines the Joule heat for better switching performance and also serves as an oxygen vacancy reservoir. This structure gives the device the multilevel resistive switching capability within a large temperature range from 8 K to 300 K. The endurance more than $10^{{6}}$ cycles at 8 K is demonstrated. Combined with kinetic Monte-Carlo simulation, the role of the thermal enhancement capping layer is analyzed.

We present compact models that capture published cryogenic temperature effects on silicon carrier mobility and velocity saturation, as well as fully depleted silicon on insulator (FDSOI) and fin field effect transistor (FinFET) devices characteristics within the industry-standard Berkeley short-channel IGFET model (BSIM) framework for cryogenic IC applications such as quantum computing. For the core model charge density/surface potential calculation, we introduce an effective temperature formulation to capture the effects of the band tail states. We also present a compact model that corrects the low-temperature threshold voltage for the band-tail states, Fermi–Dirac statistics, and interface traps. New temperature-dependent mobility and velocity saturation models are accurate down to cryogenic temperature. In addition, we propose that experimentally observed ${I}_{D}$ dependence of subthreshold swing (SS) at cryogenic temperatures is a consequence of the expectedly higher rate of Coulomb scattering of free carriers.

The construction of magnets with ever higher magnetic fields requires ever stronger and tougher reinforcement materials. Nitronic stainless steels are sometimes used for this purpose. Avoiding the sensitization temperature of these steels during fabrication is of critical importance because excessive exposure to this temperature may make them sensitive to corrosion. Even though these steels have exceptionally high mechanical strength and satisfactory toughness for use at room temperatures, low cryogenic temperatures may reduce their toughness to the degree that they are rendered unsatisfactory as reinforcement materials. This low toughness at cryogenic temperature in Nitronic steels often stems from the microstructure established during fabrication, even when the heat treatment profile follows the required protocol. Even materials that have not previously been exposed to sensitization temperatures during fabrication may later be exposed during the construction of magnets. Consequently, quality control of reinforcement materials must include both toughness tests and microstructure analyses. This paper reports recent studies of the effect of component volume and microstructure on cryogenic temperature toughness in Nitronic-type stainless steels. By comparing four different shells, we established that the presence of grain boundary particles caused lower impact fracture energy but had no significant impact on mechanical strength.

As a high-efficiency cooling technology for high-temperature superconducting coils, we have begun research and development to examine the feasibility of a cooling assist technology that maintains a cryogenic state by combining the magnetic force generated by the superconducting coil with magnetic refrigeration technology. Magnetic refrigeration requires the magneto caloric material to change the magnetic field. In many cases, the magnetic field change is obtained by moving the magnetic source, but moving the superconducting coil is not a good idea. Although it is possible to generate a change in the magnetic field by turning on and off the power supply of the high-temperature superconducting coil, it is unlikely to be established as a system that assists cooling of the superconducting coil because the coil generates heat due to AC loss. Therefore, it was considered that the magnetic field change can be obtained if the magnetic force generated from the superconducting coil can be controlled by the magnetic shield. As a verification of the principle, it was clarified experimentally that a magnetic field change can be obtained by repeatedly inserting and removing the magnetic shield into and from the gap between the magnetic field generation source and the magneto caloric material, and the temperature change can be extracted by the magneto caloric effect. In the experiment, the temperature change obtained when a magnetic shield was inserted into the air gap was measured by a simple test device using an iron-based magneto caloric material having high magneto caloric effect performance at room temperature and a permanent magnet. The principle verification confirmation test was performed using several types of magnetic shielding materials such as Permalloy bulks and the electrical steel sheets. In addition, numerical analysis is performed on the magnetic shielding effect, and the shielding effect is improved by increasing the thickness of the shielding material and the possibility of using high temperature superconductors as magnetic shielding materials were also analyzed. In this study, we report the possibility of cooling the high-temperature superconducting coil with high efficiency by combining the magnetic field created by the superconducting coil and magnetic refrigeration.

We describe our initial approach and progress towards a reliable and useful technique to connect multiple flexible superconducting cables using a cable-to-cable connector. The test structure comprises multiple components, including flexible thin-film superconducting cables, a bridging connector, a silicon well chip, and a polyimide membranes with Cu pillars. The thin-film superconducting cables used in this work contain 12 parallel Nb lines. A bridging connector which was used to connect two superconducting cables had a stripline structure with 12 parallel superconducting Nb transmission lines with 300 $\mu$ m pitch among them. A silicon well chip and a polyimide membrane with Cu pillars have been used in the assembly for high-precision micro-mechanical alignment and to apply uniform pressure on the contact points of cables and the bridging connector. DC resistances of signal traces of connected flexible cables were measured at temperature ranging from 293 K to 4.2 K in an evacuated cryogenic environment. We present the details of the design, fabrication, assembly, and test results, which show that this is a promising cable-to-cable connection approach.

BabyIAXO, a 20 m-long telescope aiming at the search for solar axions, is planned since late 2017 as a smaller scale but fully functional demonstrator of the International Axion Observatory (IAXO). It essentially comprises a superconducting magnet, optics and X-ray detectors, installed on a Sun-tracking drive system providing rotation and tilting of the detector. The preliminary design of the BabyIAXO magnet has been developed with time and the most recent modifications are reported. The magnet concept is based on using Al-stabilized NbTi conductor in a common-coil winding layout, fully resembling the baseline IAXO design and providing two free bores of 700 mm diameter and 10 m length with a magnetic field of about 2 T. The key challenges for construction and operation of the conduction cooled 40 MJ BabyIAXO magnet are addressed, such as procurement of the conductor, quench protection of the magnet components and magnetic shielding of vacuum and cryogenic equipment. Following the lab-scale experiments carried out to verify the magnet concept, their relevance and main results are further discussed before taking up the construction of the full project anticipated in 2020.

China Southern Power Grid Corporation started a project to develop a 10-Mvar high-temperature superconducting (HTS) dynamic synchronous condenser (DSC). As the first stage of the project, a 300-kvar HTS DSC prototype was developed to verify some key technologies, such as HTS rotor magnets, helium gas circulating cooling and rotating signal acquisition. In this prototype, conduction-cooled REBCO coils were installed in the rotor and air-core copper windings were used in the stator. Cold helium gas at around 20 K was used as the coolant circulating between the rotor and the external cryogenic unit through a cryogenic rotary coupling. The REBCO coils were conduction cooled by distributed helium-gas tubes embedded in the cover plates of the coils. The rotor magnet was successfully charged up to 200 A under both static and rotating conditions. For the requirement of an HTS DSC, rapid excitation tests from 170 A to 200 A were carried out at both 500 rpm and 1500 rpm.

This paper presents a 1-2 K cryogenic system for cooling the superconducting nanowire single-photon detector (SNSPD). The system is based on the Stirling-type pulse tube cryocooler (SPTC) and the Joule-Thompson (JT) cryocooler technologies and thus named as the hybrid cryogenic system. It eliminates any moving component at the cold end which endows it with evident advantages over the Gifford-McMahon (GM) cryocooler in terms of low vibration, low electromagnetic interference (EMI) and long life. It can operate at 1-2 K and has an expected mean-time-to-failure of 10 years. The overall weight is below 30 kg, which makes it an attractive cryocooler candidate for the space applications. Furthermore, the operating temperature can be adjusted conveniently for the SNSPD other than being at a fixed temperature as the superfluid helium does. The design approaches and system integration are described in detail, and the performance characteristics presented and discussed. The cooling system has an experimental cooling temperature of 1.52 K. It is also expected to reach 1 K and below provided that the further performance improvement is conducted.

By rapid development of high temperature superconductors (HTS), the HTS magnet has become an enabling technology for compact, high magnetic field tokamaks requiring larger current densities and higher magnetic fields where low temperature superconductors (LTS) have limitations. Thus, a preliminary conceptual design study on HTS magnet for toroidal field (TF) coil, a major component of compact and high field tokamak, is presented to investigate the possibility of the use of an HTS. General features relevant to a large-scale superconducting magnet are discussed such as electromagnetics, mechanical stress, protection, and cryogenics. Two types of HTS tapes are used to design TF coils for a tokamak that targets 9.2 T at its major radius 3.3 m: 1) Design A , using an HTS tape used in previous high field HTS magnets; and 2) Design B , using the same HTS tape with twice the performance. The results convey the possibility of HTS TF coil with currently available technology, but with the expense of a large amount of HTS tapes inferring that a reduction in the HTS tape cost is required. Improvement on the HTS TF coil design is needed using the unique characteristics of HTS. Hence, some of the challenges for HTS fusion magnet are addressed including performance degradation of conductors, quench detection and protection method, and joint for demountable TF coil.

The recent trends in High Temperature Superconductor (HTS) development and HTS based applications have led to the exploration of various methods for achieving cryogenic temperature suitable for the superconducting cables and magnet characterization. An existing test facility at ITER-India has been augmented to deliver 80 K helium for testing, particularly for thermo-hydraulic characterization of superconducting cables. Such characterization is very important for the prediction of performance during operation to ensure adequate mass flow rate through the cable for heat load removal and to maintain the cable stability. The facility is capable of supporting the test on a superconducting short sample at 80 K with a mass flow rate up to 35 g/s and pressure up to 7 bar(a) using helium as a fluid. The functional acceptance of the developed test facility has been performed with the first experiment conducted on a short coiled shaped mock sample cable-in-conduit conductor (CICC). The longitudinal friction factor obtained from the measurement has been compared with the existing correlations for Reynolds number ranging between 50-400. The wound CICC short sample has been analyzed using Computational Fluid Dynamics (CFD) approach, considering the bundled region as a porous media to cover a wide range of various thermo-hydraulic and geometrical parameters involved in the test sample. A comparison of the experimental and the CFD simulation results is presented.

As air traffic has been increasing in recent years, the environmental impact of aviation is more obvious, forcing governments to impose stringent regulations on emissions. In order to meet these regulations, and reduce the carbon footprint, research has been directed towards the all-electric and hybrid aircraft, where the use of cryogenic HTS machines and cables has been proposed to reduce the overall size of the aircraft. With the cryogenic system already in place, research has been exploring the use of power electronics at lower temperatures in order to obtain systems with higher power densities and lower losses. In this paper several power semiconductor devices are tested at room and cryogenic temperature in order to evaluate their performance at lower temperature. One of the tested devices, a 'CoolMOS' superjunction MOSFET, is used in a single voltage source phase-leg which is experimented with at room and cryogenic temperature to evaluate its efficiency as a primary indication of its usefulness in the All-Electric Aircraft.

We present a design overview of the MIT 1.3-GHz LTS/HTS NMR magnet (1.3G) with a newly designed 835-MHz REBCO insert (H835) as a replacement for the 800-MHz REBCO insert (H800) that was damaged when it quenched during operation in 2018. The new H835 is designed to contribute 19.6 T in a background field of 10.93 T by an LTS NMR magnet that normally rated at 11.74 T (500 MHz): combined, 1.3G generates a total field of 30.53 T corresponding to a proton resonance frequency of 1.3 GHz. H835 is designed to operate stably while meeting 1.3G design constraints. We have also designed H835 to protect it from permanent damage in an improbable event like a quench. Key design features are: 1) a single-coil formation, composed of 38 stacked metal-co-wound no-insulation and 2 stacked no-insulation double-pancake coils, all with mechanically improved cross-over sections; 2) enhanced thermal stability; and 3) reduced current margin with a detect-and-heat method. This paper includes: 1) electromagnetic and mechanical design of H835; 2) cryogenics overview; 3) quench protection strategy; and 3) discussion on the next steps to successfully complete 1.3G.

Glass Fiber Reinforced Plastic (GFRP) is usually employed as the insulator material of superconducting cable terminal, owing to its excellent mechanical properties and insulation strength at cryogenic temperature. However, in the presence of a void defect, both the insulation life span and mechanical strength of GFRP decrease significantly at superconducting temperature conditions. In this paper, efforts have been made to investigate the degradation process of GFRP at cryogenic temperature and give insight into its degradation mechanism. By comparing PD activities at 278.7 K, 223 K and 153 K in initial phase of aging, it is found that PDs are inclined to be inhibited at lower temperature. Experiments indicate that the deposited charge accumulation and mechanical properties degradation, instead of chemical aging and PD energy, play a crucial part in dielectric degradation. At cryogenic temperatures, the local heat accumulation brought by PDs increases the material internal mechanical stress, and the accumulation of deposited charges causes instantaneous energy release. On the basis of the decrease of the mechanical properties of the material, the crack expands rapidly, which leads to the dielectric failure of the material. In conclusion, the degradation of GFRP at cryogenic temperature is the result of the combined effects of deposited charge accumulation and mechanical properties decline.

The superconducting nanowire single photon detectors (SNSPD) are now widely used in space quantum communication, satellite laser ranging, and quantum key distribution, etc. However, a very low temperature around 2 K is required for its operation and performance improvement. We are developing a 1.6 K hybrid cryocooler to meet this demand, which is composed of a three-stage Stirling-type pulse tube cryocooler (SPTC) and a Joule-Thomson cryocooler (JTC). The hybrid cryocooler has no moving component at the cold end and is driven by the long life linear compressor, thus realizing high reliability, low vibration and long operation life time. The cryogenic system, including the coupling structure with the SNSPD, is descripted in detail. The thermodynamic enthalpy analysis of the last-stage of the hybrid cryocooler is carried out, by which the upstream pressure and precooling temperature are optimized to meet the required base temperature and cooling capacity with less input power. The model is described and the experiments coupling with a SNSPD simulated load are carried out. The results show that the hybrid cryocooler can achieve 16.3 mW at 1.6 K, 20 mW at 1.8 K and 25 mW at 2.1 K, respectively, and thus suggests a promising space cryocooler for cooling the SNSPDs.

A Chinese group is collaborating with CERN in producing the HL-LHC MCBRD magnets (D2 corrector). Before delivery to CERN, the MCBRD magnets should be trained and cold tested at 4.5 K to check that the magnets are up to standard. To meet the requirements of cold testing, the IMP team made an upgrade plan for the existing cryogenic test facility and designed 4 m long anti-cryostats for the rotating coil. To control the temperature difference and avoid nitrogen contamination during the pre-cooling, the IMP team developed an indirect cooling method which can pre-cool the magnet with LN2 by radiation and convection through gaseous helium. With this method, the LN2 does not touch the magnet and even not need a circulation of the cold helium gas. The first Chinese produced magnet which is called MCBRDP2 has been tested on this stand. The details of the cryogenic test facility, the indirect cooling method, and their performances during MCBRDP2 cold testing are discussed in this paper.

This article presents the design and evaluation of the first cryogenic switched capacitor array (SCA)-based waveform digitizer (CryoSCA) for HPGe detectors for low-background physics experiments. The chip has integrated 16 channels and was fabricated using a 0.18- $\mu \text{m}$ CMOS process with modified BSIM3v3 model parameters at 77 K. Each channel employs two 32-cell sample blocks working in the ping–pong mode, a 256-cell storage array, 32 parallel Wilkinson-type ADCs with 12-bits dynamic range, and registers. The chip was fully evaluated and showed promising performance at 300 K and 77 K. The measured power consumption of each channel increased from 3.3 mW at 300 K to 3.6 mW at 77 K. The averaging integral nonlinearity (INL) over 1–V dynamic range was measured to be 0.3% and 0.2% at 300 K and 77 K, respectively. The static noise decreased from 0.8 mV at 300 K to 0.6 mV at 77 K. The leakage current decreased from 18.2 fA at 300 K to 2.2 fA at 77 K.

Operating circuits under cryogenic conditions is effective for a large spectrum of applications. However, the refrigeration requirement for the cooling of cryogenic systems introduces serious issues in terms of power dissipation. Gain-cell embedded dynamic random access memory (GC-eDRAM) is a low-area, logic-compatible embedded memory alternative to static random access memory (SRAM), which has the potential to provide ultralow-power operation under cryogenic conditions due to the lower leakages at these temperatures. In this article, we present the first comparative design exploration of GC-eDRAM under cryogenic conditions performed with transistor models characterized based on actual silicon measurements under temperatures as low as 77 K. Our study shows that the two-transistor (2T)-based GC-eDRAM configurations turn out to be the best solutions for very low-temperature operation. In particular, the 2T mixed GC-eDRAM configurations allow read sensing margin improvements (up to 99%) within the 2T-based configurations while at the same time excel in terms of data retention time (+44%) and power consumption (−27%) when compared to more complex GC-eDRAM topologies. Moreover, even better improvements in terms of area (−73%), leakage power (−97%), retention power (−76%), and energy (−66%) are observed when compared to conventional 6T-SRAM.

There are axial and radial electromagnetic forces between the stator and rotor of axial flux disk motor besides the useful torque. The electromagnetic forces act on the rotor and are crucial to the stable operation in magnetic levitation applications driven by disk motor. This article concerns the behaviors of axial and radial electromagnetic forces of disk motor using for cryogenic submersible pump. It is measured experimentally that the electromagnetic properties of motor materials, including the electrical conductor and magnetic conductor materials, in room temperature (RT) and liquid nitrogen (LN2) environment. Then, the 3D finite element model (FEM) of the three-phase asynchronous disk motor in RT environment is built which is also validated by the RT experimental tests. Finally, the behaviors of axial and radial electromagnetic forces in LN2 environment are investigated using the modified FEM taking account of the effect of cryogenic environment on the motor material. It is concluded that the axial electromagnetic force decreases monotonously with the increasing air-gap thickness or the decreasing rotating speed. When the rotor exhibits radial eccentricity, there is a restoring radial electromagnetic force, trying to pull the rotor back to the central position. The radial electromagnetic force increases with the increasing rotational speed and radial eccentric displacement.

In this report, a pulsed electro-acoustic (PEA) device is modified to test the charge profile in epoxy resin at 77 K (-196°C) and successfully obtains the charge signal. A piezoelectric material suitable for cryogenic application is chosen for the measurement. In the experiment, the polarization and depolarization tests are operated with different voltage supply procedures. After initial wave recovery, the waveforms are discernable enough to identify the charge information. It is found that the charges are much less active at the low temperature with both slow dissipating rate and low injection quantity, comparing to the case at 293 K.

The avalanche ruggedness of power devices becomes a crucial issue to ensure the safe operation of the power conversion systems, particularly under the extreme temperature conditions. In this article, the avalanche capability of SiC planar/trench MOSFETs is systematically evaluated and analyzed over the temperature range of 90 to 340 K. Importantly, the essential mechanisms and temperature dependence of avalanche failure under cryogenic conditions are further explored by combining many analysis methods such as TCAD simulations, the unclamped inductive switching characterizations, and the transient junction temperature prediction. The highest avalanche energy density of 171.24 mJ/mm2 at 90K indicates the great application potential of SiC planner mosfet in cryogenic electronics. Moreover, the safe avalanche operation boundary (AOB) model is established over the cryogenic temperature range. The relevant analysis method and AOB model can be used to accurately evaluate and quantitatively predict the avalanche capability of SiC planar/trench mosfet s for the cryogenic converter design.

This article provides for the first time a very extensive experimental characterization coupled with a fully analytical modeling in order to investigate, in a systematic and comprehensive way, the sensing performance of a two-port surface acoustic wave (SAW) resonator from room down to cryogenic temperatures. The motivation behind this work is twofold: to quantitatively assess the temperature sensitivity of the SAW technology for cryogenic applications and to gain a better understanding of the underlying physics in terms of the equivalent-circuit elements. Although the measurement-based analysis is developed by focusing on a SAW from Murata as a case study, the developed investigation methodology is independent of the considered technology and extensible to other SAW types. A cryogenic system based on a closed-loop helium refrigerator is used to cool the tested SAW from 300 K down to 20 K with a step of 10 K. At each studied temperature, the scattering parameters are measured using a vector network analyzer over a narrow frequency band around the nominal resonant frequency of 423.2 MHz, spanning from 420 MHz to 425 MHz with a small step of 3.125 kHz. The measured scattering parameters are then transformed into the admittance ones, as they are more useful for sensing performance assessment and for equivalent-circuit model extraction. The extracted model is successfully validated through the achieved good agreement between measurements and simulations of the temperature- and frequency-dependent behavior of the studied resonator.

Cryogenic computing can achieve high performance and power efficiency by dramatically reducing the device’s leakage power and wire resistance at low temperatures. Recent advances in cryogenic computing focus on developing cryogenic-optimal cache and memory devices to overcome memory capacity, latency, and power walls. However, little research has been conducted to develop a cryogenic-optimal core architecture even with its high potentials in performance, power, and area efficiency. In this article, we first develop CryoCore-Model, a cryogenic processor modeling framework that can accurately estimate the maximum clock frequency of processor models running at 77 K. Next, driven by the modeling tool, we design CryoCore, a 77 K-optimal core microarchitecture to maximize the core’s performance and area efficiency while minimizing the cooling cost. The proposed cryogenic processor architecture, in this article, achieves the large performance improvement and power reduction and, thus, contributes to the future of high-performance and power-efficient computer systems.

Dynamic random-access memory (DRAM) peripheral nMOSFETs with various channel length and channel doping concentration are analyzed at liquid nitrogen temperature (LNT). Important device parameters in circuit designs such as subthreshold mismatch, subthreshold swing (SS), transconductance, and drain-induced barrier lowering (DIBL) are analyzed at temperatures ranging from room temperature to LNT. In devices with relatively low channel doping, exacerbation of threshold mismatch at cryogenic temperatures is alleviated. Devices with lower doping exhibit better SS and transconductance characteristics. DIBL improved in short-channel devices at cryogenic temperatures, and the degradation of the DIBL with low doping is relatively small.

Current mirrors (CMs) are essential building blocks for biasing electronic circuits. The present work demonstrates that errors in mirroring ratios increase at cryogenic temperatures, for all types of SiGe HBT and CMOS CMs. The sources of errors are attributed to the mismatch in operating conditions and a combination of process variations and layout effects. In particular, the effects of layout on device current are found to increase significantly at low temperatures, which can potentially contribute more to the CM error than process variations. Among all investigated CM configurations, the simple CM based on large geometry SiGe HBTs is the most versatile option with small errors, a large operational range, and a simple layout.

We show that fast technical fluctuations of microwave signals can be strongly suppressed by cryogenic resonators. The experiments were carried out with sapphire resonators cooled to approximately 6 K at frequencies around 11 GHz. Each sapphire crystal was shaped like a spindle with the rotational and crystal axes aligned within a degree. Noise suppression factors in excess of 50 dB were measured at 10-kHz offset from the carrier. This was achieved by rotating the sapphire spindle and moving the coupling probes relative to its surface. Microwave signals with reduced fast technical fluctuations allow the high-precision tests of fundamental physics, as well as the development of better radars.

This investigation examines the physical mechanisms that can increase the variability of both p-n junctions and silicon–germanium heterojunction bipolar transistors (SiGe HBTs) at cryogenic temperatures. The important operative mechanisms responsible for device parameter variability include bandgap narrowing due to heavy doping, mechanical stress, and the Ge profile. The impact of direct tunneling on cryogenic parameter variability in SiGe HBTs is also examined. Measurement results are compared with TCAD simulations to provide additional insights, and possible mitigation methods are discussed.

Temperature monitoring is an essential task in cryogenic engineering to ensure the safety of equipment, where highly sensitive and reliable sensors are required to provide real-time and accurate information about temperature. Titanium (Ti)–copper (Cu)-coated fiber Bragg grating (FBG) temperature sensors with a gradient of material properties to minimize thermal stresses are hence fabricated by a combination of Ti/Cu magnetron sputtering and Cu electroplating. The thermal characteristics and thermal fatigue behavior of the sensors are investigated. The sensors exhibit significantly higher sensitivity than that of the bare FBGs at cryogenic temperatures down to 79 K, with good repeatability and stability. Their response to the temperature changes can be precisely described by a quadratic-polynomial equation. No obvious variations in the zero-point, the reflection spectrum, and the temperature sensitivity of the sensors exposed to thermal fatigue from 77 K to room temperature are observed before fatigue failure which could be attributed to the formation and growth of subcritical cracks within the fiber rather than that of fatigue cracks within the metallic coatings, as a result of the cyclic thermal stresses induced by the large differences in coefficient of thermal expansion between the silica optical fiber and the metallic coating materials. The results demonstrate that the multilayer metal-coated FBG sensors provide great potential for temperature measurements or temperature compensation in cryogenic engineering by proper selection of metallic coatings with a gradient of material properties.

High-temperature superconducting (HTS) power equipment and tests at present require operating temperatures higher than the liquid nitrogen (LN2). LN2 is an important cryogenic refrigerant and liquid insulation material, which is widely used in HTS equipment. Although pressurized LN2 can achieve temperatures higher than 77 K, it requires the use of sealed pressure vessels. Thus, a liquid cryogen that can offer temperatures higher than LN2 at atmospheric pressure, excellent electrical insulation performance, and high thermal conductivity has to be explored. Tetrafluoromethane (CF4) is another cryogenic refrigerant and electrical insulation material. The mixture of LN2 and liquid CF4 has excellent thermal properties. When mixed in appropriate proportions, it has a melting point of 50 K and a boiling point of 145 K. The LN2/CF4 mixture is used in the insulating layer of the superconducting energy pipeline to provide a low-temperature environment (85–100 K) for the HTS dc cable. However, the insulation characteristics of the LN2/CF4 mixture at cryogenic temperatures have not been studied, quantitatively. In this article, a test platform is developed to measure the breakdown voltage of the liquid mixture. The dc breakdown voltages of LN2/CF4 liquid mixture under varying mixing proportions are obtained at cryogenic temperature. The insulation failure probability is estimated using a two-parameter Weibull statistical method. The results reveal that the breakdown strength of the LN2/CF4 mixture is lower than that of pure LN2. As the proportion of CF4 increases, the breakdown field strength of liquid mixture first decreases and then increases. When the mole fraction of CF4 is 60%, the average breakdown strength decreases to the lowest value (14.07 kV/mm), about half of that of pure LN2 (28.26 kV/mm). The results of this article can provide a reference for the applications of LN2/CF4 liquid mixture in HTS power equipment and a scheme to expand the working temperature range of superconducting applications.

This article presents the cryogenically cooled application for wide bandgap (WBG) semiconductor devices. Characteristics of silicon carbide (SiC) and gallium nitride (GaN) at cryogenic temperatures are illustrated. SiC MOSFETs exhibit increased on-state resistance and slower switching speed at cryogenic temperatures. However, cryogenic cooling provides low ambient temperature environment and thus enables the SiC converter to operate at lower junction temperature to achieve higher efficiency compared to room temperature cooling. A cryogenically cooled MW-level SiC inverter prototype is developed and demonstrated the feasibility of operating high-power SiC converter with cryogenic cooling. GaN HEMTs exhibit more than five times on-state resistance reduction and faster switching speed at cryogenic temperatures which makes GaN HEMTs an excellent candidate for cryogenic power electronics applications. The significantly reduced on-state resistance of GaN devices provides the possibility to operate them at a current level much higher than rated current at cryogenic temperatures. A GaN double pulse test (DPT) circuit is constructed and demonstrated that GaN HEMTs can operate at nearly four times of rated current at cryogenic temperatures. Challenges of utilizing WBG device with cryogenic cooling are discussed and summarized.

This paper reports a pioneering demonstration platform of a cryogenic propulsion unit at liquid nitrogen temperature. A high temperature superconducting (HTS) machine is connected with a cryogenic power rectifier in a generator mode to prove the feasibility of a cryogenic propulsion unit for electric aircraft propulsion applications. Machine operation was carried out with a special focus on the total heat dissipation inside the HTS AC windings. Different types of 2G HTS tapes were tested to provided data for stator coils’ design. The transient operation was carried out to represent a short-circuit failure in one of the power electronic devices. The test shows AC loss performance of the HTS windings using calorimetric method during the short circuit event, indicating the importance of developing protection schemes for the cryogenic propulsion units to prevent damage to the HTS components. The paper provides initial insights into the interaction between superconducting machines and cryogenic power electronics within a cryogenic propulsion unit. It is an important first step to understand and further develop cryogenic propulsion technology for future electric aircraft.

As Moore’s Law nears its end, we are searching for alternative technologies and architectures to further increase performance. Cryogenic computing has gained considerable attention in the last couple years, due to the highly ideal performance of CMOS circuits at very low temperatures. While cryogenic operation can provide impressive performance benefits, it introduces a new trade-off space which must be examined. Additionally, it does not eliminate current bottlenecks for performance, such as the memory wall. Processing-in-Memory architectures present an interesting opportunity. They are suitable to operate at cryogenic temperatures, enable lower cooling costs via extreme energy efficiency, and enable compute and memory capabilities in this regime with relatively minor adjustments to the architecture.

This letter demonstrates the first CMOS logic compatible cryogenic memory solution operating from 4 to 300 K designed in the 28-nm high-K metal gate (HKMG) CMOS. With the growing applications of cryogenic systems from quantum computing to space electronics, there is a need for memory capable of reliable functionality. While the prevailing low-temperature memories suffer from temperature scalability and integrability, the proposed test chip of 1-kb 2T hybrid gain cell-based embedded DRAM macro overcomes these issues while providing $10^{6} \times $ better retention time, 1.3-GHz peak frequency at 4 K, sub nW/kb array refresh power, and $1.7\times $ energy efficiency at 4 K compared to 300 K. This is due to the near absence of leakage, improved ON current, and subthreshold slope which leads to enhanced performance of critical path circuits at lower temperatures.

Cryogenically cooled low-noise amplifiers (LNAs) have had a profound impact on experimental science. For instance, these amplifiers allow us to communicate with distant spacecraft, probe the history and composition of the universe through radio astronomy, study basic phenomena through low-temperature physics research, and read out the state of quantum systems as required for quantum computing. These devices-which can achieve noise performance within an order of magnitude of the fundamental limits imposed by quantum mechanics-find use from low frequencies through several hundreds of gigahertz. Without cryogenic LNAs, whole branches of experimental science simply could not exist.

Cryogenic dielectrics are a crucial component for applied superconducting systems such as high-temperature superconductor (HTS) cables. Here two types of polymer nanocomposites were investigated as dielectrics for cryogenic applications. Both polyimide (PI) and polyamide (PA) nanocomposites showed exceptional performance as dielectrics, with PI as the stronger material. Significant dielectric strength improvement was observed for samples tested in the cryogenic environment when compared to those tested at the room temperature. The PI exhibited a high dielectric strength of 347 ± 67 kV/mm at 92 K, while its nanocomposites were in the range of 261-280 kV/mm. The performance change of these dielectrics was influenced by a number of factors including the density of free charge carriers, localized heat generation and material degradation, thermal contraction of polymers, and polymer-nanoparticle interfacial changes at cryogenic temperatures. The findings from this research can help advance the understanding of breakdown failures in cryogenic dielectrics.

International audience; This work presents the performance and low-frequency noise (LFN) of 22-nm fully-depleted silicon-on-insulator (FDSOI) CMOS technology. The experimental measurements and the analysis are performed as a function of temperature for the first time, focusing on cryogenic applications, down to 4.2 K. The back bias impact on device performance is evaluated. The results reveal that the threshold voltage tuning is found to be temperature independent, allowing extra drain current improvement. This is particularly interesting for short channel devices, whose drain current gain with temperature lowering is expected to be smaller in comparison with long channel MOSFETs. LFN is characterized by means of time-domain current sampling measurements. Moderate and strong inversion regimes are investigated. The carrier number with correlated mobility fluctuations model can well describe the 1/f noise behavior down to 4.2 K. The physical origin behind the drain current noise-to-signal power augmentation with temperature lowering could be mainly attributed to the normalized transconductance improvement.

This article investigates the potential for high power density, high-efficiency power conversion using GaN-based flying capacitor multilevel power converters at low temperature. Specific focus is given to the application of hybrid aircraft powered by liquefied natural gas. Building off of prior characterization of GaN switches over temperature, this article outlines the impact of near-cryogenic operation on the losses of a complete 1-kW inverter. Specific focus has been given to the characterization of the losses in ceramic capacitors and inductors at near-cryogenic operation. A 1-kW GaN-based three-level power converter was designed and successfully tested from room temperature down to $-$ 140 $^{\circ }$ C. Along with the first demonstration of a flying capacitor multilevel converter and associated components at such low temperature, a breakdown of losses over temperature is included to facilitate a better understanding of the opportunities and challenges introduced by this article.

In this article, the temperature behavior of a silicon ring resonator is experimentally investigated for cryogenics. The ring resonator shows a linear sensitivity of 63.9 pm/K in the temperature range from 180 K to 300 K. And the experimental results agree well with the simulated data. The response time of the sensor is investigated by thermal analysis and a fast response of 325 ms is experimentally achieved. In addition, the developed sensor shows high performance in repeatability and stability. Based on our work, for the first time, silicon based ring resonators are applied to cryogenic temperature sensing with the prominent advantages of compactness, chip-scale integration and complementary metal oxide semiconductor (CMOS) compatibility. This work proves that silicon ring resonators are a potential platform for cryogenic monitoring.

This article presents the development of a cryogenic bandgap reference (BGR) circuit in a standard 0.18- $\mu \text{m}$ CMOS process for high-purity germanium (HPGe) detectors for low-background physics experiments. It provides a temperature-independent reference for mixed-signal circuits such as a ramp generator. A reliable cryogenic model is essential for the circuit design. The parameters of metal–oxide–semiconductor field-effect transistors (MOSFETs) and bipolar junction transistors (BJTs) were characterized and extracted at cryogenic temperatures from dc measurements on various geometries. MOSFETs had a general increase in threshold voltage and carrier mobility, a decrease in subthreshold slope at 77 K, with respect to 300 K. A new set of compact model parameters for MOSFETs and BJTs under cryogenic temperatures have been extracted. The relative errors between simulation and measurement were below 10% in almost all the bias regions for various device dimensions at 77 K. Two different BGR circuits were designed with the modified compact model parameters and evaluated from 77 K to 300 K. Temperature coefficient of less than 0.2 mV/K has been achieved over the whole range from 300 K to 77 K.

According to quantum transport simulations, source-to-drain tunneling (SDT) has been recognized as the main cause leading to subthreshold swing ( SS ) saturation and degradation of short-channel MOSFETs at cryogenic temperatures. Generally, at a given low temperature, the steeper constant SS of thermionic currents may be overwhelmed by less-steep SDT currents at lower gate bias, degrading the average SS . Our simulations show that the SS of SDT currents is insensitive to temperatures for a MOSFET with a given channel length, accounting for SS saturation as lowering temperature. This work reveals the key points differentiating the possible reasons (interface traps, band tail and SDT) of SS saturation at cryogenic temperatures. We also study the impacts of carrier effective masses, gate-SD underlapping and a tunneling barrier at the source junction on SS saturation. SDT may pose a potential challenge and limit for scaling cryogenic CMOS of a quantum processor.