Steven W. Van Sciver

Thermodynamic optimization of conduction-cooled HTS current leads

A theoretical optimization is performed for the conduction-cooling method of high Tc superconductor (HTS) current leads, which can be applied to the superconducting systems cooled directly by cryogenic refrigerators without liquid helium. The current lead is a series combination of a normal metal conductor at the warmer part and a HTS at the colder part, and is cooled by a contact with distributed or staged refrigerators instead of boil-off helium gas. An analytical method is developed to derive a mathematical expression for the required refrigerator power. By incorporating the critical characteristics of the HTS, it is demonstrated that there exist unique optimal values for the current density of HTS and the joint temperature of the two parts to minimize the total refrigerator power per unit current, for a given length of the HTS. As results of the study, the absolute minimum in the refrigerator power per unit current is presented as a thermodynamic limit and the leads cooled by a two-stage refrigerator are theoretically optimized. Some aspects in practical design are also discussed with a new and useful graphical method.

Visualization of two-fluid flows of superfluid helium-4

Cryogenic flow visualization techniques have been proved in recent years to be a very powerful experimental method to study superfluid turbulence. Micron-sized solid particles and metastable helium molecules are specifically being used to investigate in detail the dynamics of quantum flows. These studies belong to a well-established, interdisciplinary line of inquiry that focuses on the deeper understanding of turbulence, one of the open problem of modern physics, relevant to many research fields, ranging from fluid mechanics to cosmology. Progress made to date is discussed, to highlight its relevance to a wider scientific community, and future directions are outlined. The latter include, e.g., detailed studies of normal-fluid turbulence, dissipative mechanisms, and unsteady/oscillatory flows.

Cryogenic cooling system of HTS transformers by natural convection of subcooled liquid nitrogen

Abstract Heat transfer analysis on a newly proposed cryogenic cooling system is performed for HTS transformers to be operated at 63–66 K. In the proposed system, HTS pancake windings are immersed in a liquid nitrogen bath where the liquid is cooled simply by colder copper sheets vertically extended from the coldhead of a cryocooler. Liquid nitrogen in the gap between the windings and the copper sheets develops a circulating flow by buoyancy force in subcooled state. The heat transfer coefficient for natural convection is estimated from the existing engineering correlations, and then the axial temperature distributions are calculated analytically and numerically with taking into account the distributed AC loss in the windings and the thermal radiation on the walls of liquid-vessel. The calculation results show that the warm end of the HTS windings can be maintained at only 2–3 K above the freezing temperature of nitrogen, with acceptable values for the height of HTS windings and the thickness of copper sheets. It is concluded that the cooling by natural convection of subcooled liquid nitrogen can be an excellent option for compactness, efficiency, and reliability of HTS transformers.

Observed drag crisis on a sphere in flowing He I and He II

The pressure distribution on the surface of a sphere has been measured in flowing He I and He II as a function of Reynolds number. The drag coefficient was extracted by integrating the pressure distribution, using some assumptions about symmetry of the flow field. Drag coefficients are plotted against Reynolds number for both He I and He II against classical data for both smooth and nonsmooth spheres. Latest results in He II suggest that the drag crisis occurs at a Reynolds number of approximately 2×105, in fair agreement with classical data.

Optimization of operating temperature in cryocooled HTS magnets for compactness and efficiency

A new concept of thermal design to optimize the operating temperature of high temperature superconductor (HTS) magnets is presented, aiming simultaneously at small size and low energy consumption. The magnet systems considered here are refrigerated by a closed-cycle cryocooler, and liquid cryogens may or may not be used as a cooling medium. For a specific magnet application, the size of required HTS windings could be smaller at a lower temperature, by taking advantage of a greater critical current density of HTS. As the temperature decreases, however, the power input to the cryocooler increases dramatically because of the heavy cooling load and the poor refrigeration performance. Through a rigorous modeling and analysis incorporating the effect of magnet size into the load calculation, it is demonstrated that there exists an optimum for the operating temperature to minimize the power required. The optimal temperature is strongly dependent upon the magnitude of AC loss in the magnets and the assistance of heat interception.

Cryogenic systems for superconducting devices

The principles entering into the selection of cryogenic systems for superconducting magnet applications are reviewed. Types of refrigeration systems, operating temperature ranges and efficiency are all issues that must be evaluated in selecting the most appropriate cryogenic system. Specific examples of magnet systems currently under development are discussed in the context of these principles.

Natural Convection of Subcooled Liquid Nitrogen in a Vertical Cavity

An experiment to measure the natural convection of subcooled liquid nitrogen between two vertical plates has been performed. The main objective of this study is to confirm the feasibility of our recently proposed design for an HTS power transformer cooled by natural convection of subcooled liquid nitrogen. A liquid nitrogen bath is cooled to nearly the freezing temperature (63 K) at atmospheric pressure by a vertical copper heat transfer plate thermally anchored to the coldhead of a GM cryocooler. A parallel copper plate generating uniform heat flux is placed at a distance so that liquid between the two plates may develop a circulating flow by natural convection. The vertical temperature distribution on both surfaces is measured in steady state, from which the heat transfer coefficient is calculated. The experimental data are compared with the existing correlations for a rectangular cavity where each vertical surface has a uniform temperature. The discrepancy between two data sets is examined by consideri...

Sudden Vacuum Loss in Long Liquid Helium Cooled Tubes

Sudden vacuum loss in straight long tubes cooled by liquid helium is investigated. The scenario resembles an accident in a superconducting particle accelerator when the beam-cavity suddenly loses its vacuum to atmosphere. Following this accident, air will propagate down the vacuum channel and freeze on the cold walls. To warrant against catastrophes of this accident, it is vital to know the propagation speed of air in the vacuum space and the heat load on the helium bath. An experimental setup has been developed to determine these parameters. Experiments are conducted wherein a large nitrogen gas tank is rapidly vented to a high vacuum tube (~10-4 Pa) immersed in liquid helium at 4.2 K. The measurements comprise of the tube pressure to determine the propagation speed, the tank pressure to determine the gas mass flow rate into the tube, and the tube temperature to estimate the heat load. Flash solidification of the gas on the cold tube, which is apparent in the measurements, limits the propagation speed to the order of 10 m/s. Based on the mass flow measurement a heat deposition rate of 60 kW/m2 on the vacuum tube is estimated, while the heat transfer rate to the helium bath is predicted to exceed 20 kW/m2.

Chapter 5 - Visualisation of Quantum Turbulence

Abstract The application of modern flow visualisation techniques to quantum turbulence studies of helium II is reviewed. Emphasis is on those techniques that employ micron-scale particles as tracers of the flow fields. Different types of tracer particles are evaluated for this application including solid particles and particles made from solidifying hydrogen gas in liquid helium. The physics of small particles in helium II is then reviewed and discussed in the context of recent flow visualisation experiments. An interaction between suspended particles and quantised vortex lines can lead to drag force on the particles, which affects the particle velocity field in counterflow helium II. Recent experiments using the particle visualisation techniques show clear evidence of this interaction. In counterflow helium II, the particles are seen to track the normal fluid velocity field although at a reduced velocity that can be explained based on the particle-superfluid component interaction. Particle trapping by vortex lines has also been observed. On the other hand, in flowing helium II, the particles follow the total velocity field displaying a boundary layer very similar to that of classical fluids. The chapter concludes with suggestions for future experimental and theoretical investigation.

Transient heat transfer in helium II due to a sudden vacuum break

To ensure future cryogenic devices meet safety and operational specifications, significant value is gained from a developed understanding of the transient heat fluxes that result from failure of an insulating vacuum jacket around a helium II (He II)-cooled device. A novel, one-dimensional experiment is successfully performed examining the phenomena immediately following a vacuum rupture onto a cryosurface. In the experiment, a fast-opening (∼10 ms) valve isolates a rigid container of ultra high purity nitrogen (N2) gas kept at room temperature and adjustable pressure from a vertically oriented, highly evacuated (∼10−3 Pa) tube roughly 1 m in length. The bottom of the evacuated tube is sealed via a 2.54 mm thick copper disk, whose bottom surface is in intimate contact with an open column of He II (∼1.8 K). The evacuated tube, disk, and He II column share a diameter of 24 mm. Opening the valve results in a vacuum rupture. N2 gas is immediately drawn into the evacuated space and cryopumped onto the disk as a...