Cryogenics最新文献

The heat transfer performance of nitrogen PHPs with different heat transmission distances (100 mm and 500 mm) and tube configurations (single-loop and complex-loop) were experimentally investigated. Experiments were conducted in the vertical bottom heat mode with different filling ratios (15 %–70 %). The results showed that the maximum effective thermal conductivity increased proportionally with the heat transmission distance whereas the thermal resistance remained constant (0.2 K/W at a filling ratio of 31.8 %). This verified the outstanding long-distance heat transfer advantage of the nitrogen PHP. Experiments at different filling ratios showed that the maximum thermal conductivity decreased as the filling ratios increased. A filling ratio of 31.8 % was recommended. Under this operating condition, the PHP can load the maximum heat input while exhibiting relatively high effective thermal conductivity. Compared to the single-loop configuration, the complex-loop exhibited higher effective thermal conductivity, and this enhancement in thermal performance was more pronounced for the longer PHP.

Highly sub-cooled water ice, with temperatures as low as 10–20 K is not commonly utilized and, as a result, its fundamental properties remain generally unknown. Therefore, the thermal characteristics of water ice have been thoroughly reviewed in the scientific literature and compared with the performance of other solid materials, which can be potentially used as a cooling media in superconducting applications. The effectiveness of water ice as a cooling agent was demonstrated through experimental measurements of the temperature and the magnetic background field effects on the critical current of small MgB₂ solenoid immersed in water ice with temperatures ranging from 10 K to 36 K and external magnetic fields from 0 to 6 T. Increase of the solenoid’s temperature was observed when the transport current exceeded the critical threshold, which is determined by the conventional criterion of 1 µV/cm. The obtained results confirm that sub-cooled water ice is a promising, cost-effective, and safe coolant suitable for superconducting systems.

In the context of the High Luminosity upgrade of the Large Hadron Collider at CERN, a framework implementing experimental techniques and numerical analysis has been developed to systematically assess the temperature distribution in complex He II-cooled composite magnet geometries. The experiments are designed to measure the heat transfer coefficients in the magnet coil layers using coil samples in a stagnant superfluid helium bath. A numerical tool-kit has been developed to facilitate intensive parametric studies, in addition to estimation of helium content via a phenomenological model. The workflow of the tool-kit is built to handle complex geometries composed of different materials each with their temperature-dependent properties, at low computational cost. This framework has been validated with experimental data obtained from laboratory-scale experiments on impregnated coil samples, reported and discussed here. Three use cases for the developed numerical tool, with increasing levels of complexity, are presented and its results discussed.

A Stirling cooler for space use must have a 5–10 year service life. The primary limitation is the degradation of cooling performance by gas contaminants, such as H₂O, CO, CO₂, O₂, and N₂. If these gas contaminants freeze in the regenerator or the displacer’s clearance seal, they can block the helium flow for heat exchange or disturb the smooth movement of the displacer. This study verified the effect of adsorbent adapted for the 20 K-class two-stage Stirling (2ST) cooler with a correlation between the cooling performance and the residual concentration of CO₂ gas contaminant. Although the 2ST cooler was stopped by a displacer stuck during initial cooling by adding CO₂ at 4990 ppm, the cooling performance was finally recovered to a reference level below 20 K when the equipped adsorbent was activated. Gas analysis showed that the residual CO₂ concentration was successfully reduced to 426 ppm, satisfying the less than 500 ppm requirement.

Hysteresis losses in the heat transfer between compressing or expanding gas and the adjacent wall is said to play an important role in Stirling machines, where it increases the amount of required p-V work. Previous studies have linked hysteresis loss with the pressure phase shift. In the context of this research, the effect of the pressure phase shift on the net p-V work in a single space is examined.

A Sage model of a single space piston-cylinder device is used to investigate the underlying mechanisms of the pressure phase shift. The Sage model is validated using an experimental piston seal rig. In addition, the time dependence of heat transfer is discussed along with how it affects the pressure phase shift, using an iterative model. The Schmidt equations were manipulated to determine the phase shift between pressure and volumetric oscillation in an ideal Stirling refrigerator.

The results of this investigation are surprising. It was found that even in the case of an idealized Stirling refrigerator, the phase shift between pressure and volume is non-zero in order to produce a refrigeration effect.

Epoxy resins are widely used in cryogenic applications due to their distinguished mechanical and electrical insulation properties. To avoid generating enormous amounts of waste that affect the environment and cause economic losses, covalent adaptable networks can be introduced into the epoxy structure to make the thermosetting network reversible. In this study, ester bond and hexahydro-s-triazine structure were adopted in epoxy structures. Two new degradable epoxy resins (HPBE and HPHE) with variances in the content of benzene rings were synthesized which were confirmed by FTIR and NMR. After curing, the thermal, mechanical, thermal conducting properties of the resulting materials were examined. The products exhibited tensile strength and flexural strength all above 120 MPa at 77 K, with glass transition temperature of 89.1 °C and 86.9 °C, respectively. Meanwhile, HPBE and HPHE demonstrated favorable thermal stability, with thermal conductivity comparable to that of the commonly used epoxy resin DGEBA. These resins could be degraded under mild conditions and demonstrated high hydrolysis efficiency (approximately 90 %). This investigation presented a viable method for the application of degradable thermosetting materials in cryogenic environments and the possibility for repairing of high-value equipment.¹

Dilution refrigerator provides continuous ultralow temperature environments as low as 10 mK. It has been widely used in a variety of important applications such as quantum computations. The silver powder heat exchanger in a dilution refrigerator plays a crucial role in realizing ultralow temperatures on the order of 10 mK by precooling the circulation of helium mixtures. To study the silver powder heat exchanger quantitatively, we have proposed an efficient numerical solution method for its thermodynamic model. This method utilizes constraint conditions cleverly to convert the initial value problem of differential equations into a boundary value problem, allowing us to solve the control equations using the existing ODE function quickly. Furthermore, we demonstrate the application of this method in the evaluation and design of silver powder heat exchanger. The research results of this paper have certain significance for the development of dilution refrigerator.

The SMC (Short Model Coil) R&D program was started at CERN around 2007 to develop the Nb₃Sn technology. The small magnet structure allowed relatively cheap and fast testing of various superconducting coils. One of the key questions to be answered, was related to the relation between the pre-stress and the magnet’s performance. To measure this dozens of strain gauges were installed on the coils, the axial tie-rods and the external shell. The experimental results of the strain measurements during all stages of the load: room temperature (RT) pre-stress, cool-down, powering, warm-up were analyzed in an extensive report [56]. A repeatable pattern of a decreasing strain after the warm-up, compared to the value before the cool-down, was observed on the external cylinder for all the tested coils. Values from 2 % to 50 % were reported.

In this work a viscoelastic model was used to explain this effect. The Nb₃Sn coil was treated as a composite material with decreasing stiffness due to mechanical damage. The Generalized Maxwell Solid model (Prony series model) was employed, including one spring and one damper, leading to a relatively simple model characterized by only two parameters. The two constants of the viscoelastic model were found: 1st – the relative relaxation moduli $\alpha$ based on a calibration curve derived from the experimental results of the SMC program and the 2nd one – relaxation time $\tau$ – based on minimizing the computational cost, by finding the asymptotic solution in one integration step. The model showed the capability of explaining the strain drop (loss of pre-stress) of more than 80 %. In addition to the viscoelastic effects, the role of friction coefficient was studied revealing the possibility of explaining up to 14 % of the strain drop. Yet, to fit with the experimentally measures strains on the SMC cylinder, especially during the RT pre-load, the most-probable value of the friction coefficient should be $\mu <0.4$. The strong impact of the stiffness of the G-10/G-11 laminate used to spread the load on the coil was found, indicating the need of knowing the elastic properties of this material very precisely. In addition, the experimentally measured strain values showed strong asymmetric, both in plane and along the magnet’s axis, revealing the potential sensitivity to the geometric imperfections and the need for 360° magnet models.

Residual-resistance-ratio (RRR) of Cu stabilizer in REBCO coated conductor is an important design parameter for REBCO magnets. Cu stabilizer with high RRR is especially beneficial for quench protections of REBCO magnets. In this work, we study RRR of electroplated Cu stabilizer in commercial REBCO tapes. We present RRR of over 180 samples measured for the quality assurance programs of REBCO magnet projects at the National High Magnetic Field Laboratory, USA (NHMFL). To investigate the factors that influence RRR, several samples were analyzed extensively by using scanning electron microscopy, secondary ion mass spectroscopy, and inductively coupled plasma mass spectroscopy. We found that RRR is strongly correlated with the grain size of Cu, which suggests that resistivity at low temperatures is dominated by grain boundary resistivity. In addition, low RRR corresponds to high concentration of chlorine impurity. This is explained by that higher chlorine impurity hindered the grain growth in the self-annealing process at room temperature which resulted in smaller grain size and low RRR. Annealing at 300C significantly enlarged the grain size and enhanced RRR. Due to the concern of critical current degradation, however, annealing is not recommended as a practical method to improve RRR of Cu in REBCO tapes.

High Temperature Superconductors (HTS) are very promising materials for possible application in future fusion magnets, with significant R&D progress on HTS conductors in recent years. However, since geometric and thermo-physical characteristics of HTS and LTS conductors differ significantly, some doubts have arisen if the approaches successfully used in numerical simulations of the thermal–hydraulic behavior of LTS conductors would be sufficient also for HTS, particularly in cases when fast transient processes (such as e.g. quench) are considered. In order to provide data for better understanding of the quench phenomenon in HTS conductors as well as for testing different numerical approaches and proper tuning of the numerical codes, a dedicated experimental campaign (Quench Experiment) was carried out at the SULTAN test facility within the international collaboration between the EUROfusion consortium and China. Our present study is a part of the work on analysis and interpretation of the data collected during this experiment. Simulations of the chosen experimental run were performed using two THEA models with different levels of complexity. Uncertain model parameters (thermal resistances and copper RRR) were explored across a wide range. Our goal was to identify the possibly simple model that accurately reproduces the experimental results.