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Preparation and Construction of a Superconducting Electromagnet for Superconducting Magnetic Pump Application

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Preparation and Construction of a Superconducting Electromagnet for Superconducting Magnetic Pump Application by Anna M. Sailor A report submitted in partial fulfillment of the requirements for the degree of Bachelor of Science (Mechanical Engineering) at the University of Wisconsin-Madison December 2016 Abstract Due to the need for compact and efficient refrigeration systems in deep space detectors and telescopes, the development of a pump without moving parts is an important advancement for improving capabilities of sub-Kelvin cooling on unmanned missions. The purpose of this research is to demonstrate a proof-of-concept superconducting magnetic pump (SMP) that will replace traditional bellows-piston driven compressors. In this experiment, a low-temperature facility was designed and built to house the experimental pump. The low-temperature facility was prepared for testing and a new superconducting electromagnet was constructed and is ready for installation, magnet training, and experimental testing. A 16,000-turn, 0.8 Tesla (5 A) electromagnet was built to match its existing counterpart installed in the experiment to demonstrate bi-directional flow through a closed helium line utilizing the magnetocaloric effect and liquid helium as a working fluid. This paper outlines important aspects of the cool-down cycle experimental procedure in addition to summarizing the crucial steps for planning, preparation, and construction of the electromagnet. ii Acknowledgements I would like to extend gratitude to my family, friends, teachers, and mentors that made this research possible. Firstly, thank you to the Department of Mechanical Engineering at the University of Wisconsin-Madison that funded this project through the Faustin Prinz Research Fellowship program; to Professor Lorenz for organizing the program, and to Cathy Shults for assisting with purchasing. Thank you to the employees at the NASA Goddard Space Flight Center with whom I consulted on design and construction of the magnet, particularly Tom Hait. Thank you to Amir Jahromi who answered all my questions and offered his unlimited knowledge. Lastly, I would like to thank Professor Miller who guided me throughout this project and served as an invaluable mentor. iii Table of Contents Introduction ........................................................................................................................... 1! Background ........................................................................................................................... 2! Experimental set-up .............................................................................................................. 7! Experimental low-temperature platform ................................................................... 8! Measurements and Equipment .................................................................................. 11! Experimental procedure ........................................................................................................ 11! Evacuating the experimental loop ............................................................................. 12! Impurities in the helium line ..................................................................................... 13! Cool-down process .................................................................................................... 13! Warming process ...................................................................................................... 14! Filling cycle .............................................................................................................. 15! Experimental Results ............................................................................................................ 15! Preliminary cooling cycles ........................................................................................ 16! Temperature data ...................................................................................................... 17! Magnet construction .............................................................................................................. 17! Magnet Design .......................................................................................................... 19! Procedure .................................................................................................................. 20! Preparation ................................................................................................................ 21! Winding process ........................................................................................................ 23! Future work and conclusions ................................................................................................ 25! Bibliography ......................................................................................................................... 26! iv Introduction With the increased interest in deep space exploration comes the need for improved detectors and measurement systems. The sensitivity of instrumentation relies heavily on the ability to provide cooling capabilities at sub-kelvin temperatures. Measurement devices and detectors must operate in very low temperatures to reduce the noise introduced to the system by heat, allowing for more precise measurements. There are significant limitations to the types of refrigeration systems that can go into space and operate aboard unmanned missions. Intuitively, refrigeration systems need to have a high cooling capacity based on the heat load. In addition, refrigeration systems must be lightweight, reliable, efficient and small enough to meet the weight, volume, and fuel allowances of the mission. These limitations pose challenges to scientists and engineers due to their unique set of requirements. There are a number of different strategies for sub-Kelvin refrigeration, including dilution, superfluid pulse tube, and active magnetic regenerative refrigeration that use piston- bellows compressors to drive the working fluid in the refrigeration process [1]. Superconducting magnetic pumps (SMPs) are a suitable substitution for traditional compressors in these types of applications because of their lack of moving parts and ability to operate continuously by driving flow in both directions during one cycle. The work of the author is built upon the doctoral work conducted by A. Jahromi to demonstrate a proof-of-concept SMP. In the following sections, a brief background section is discussed, the test equipment and experimental procedure are outlined, and results of the preparation of the low-temperature facility are summarized. Following that, the process of preparation and winding of a superconducting electromagnet is explained in detail. 1 Background There are a number of important concepts that are necessary to understand to know how the experiment works and the importance of the procedural methods. These concepts are briefly explained in the section below. Helium as a Working Fluid The unique properties of 3He and 4He at sub-Kelvin temperatures provide a suitable working fluid for cryogenic applications. The more abundant isotope, 4He, is found naturally from radioactive decay of the Earth’s crust. 4He has a boiling point of 4.2 Kelvin at atmospheric pressure, and as a liquid exists in two forms [2]. At higher temperatures, liquefied helium, referred to as He I or “warmer liquid helium”, behaves as a Newtonian fluid. 4He can also exist as a superfluid at temperatures below 2.2 K, where the lambda line defines the transition from normal to superfluid on the phase diagram (see Fig.1). At temperatures below the lambda line, the liquid begins to exhibit superfluid behavior. This change is gradual and as temperature decreases, more helium transitions to He II, or “cooler liquid helium”. At 1 K, 98.7% of the mixture is He II, or superfluid [3]. Due to quantum effects that give superfluid helium strange properties, He II has no measurable resistance to flow, as observed by Kapitza [4]. The mixture below the lambda line that contains both He I and He II exhibits both viscous and inviscid properties due to the presence of both normal liquid helium and superfluid components. The thermal conductivity of superfluid 4He is orders of magnitude higher than the conductivity of copper. This property can be observed when inducing boiling in the liquid. 2 As temperature decreases across the lambda point, boiling immediately stops. That is not to say that heat no longer is escaping, but rather that heat in the liquid travels so quickly that bubbles below the surface do not have enough time to form, even though vaporization still takes place at the surface. Because the superfluid component of 4He has been shown to have no resistance to flow, it can move around the liquid at an unprecedented rate, making it a very good thermal conductor. Fig. 1. Phase diagram of Helium-4. The lambda line marks the transition from He-I to He-II fluid at temperatures below 2.2 K. [3] Superfluid 4He is also unique in that its pressure is only dependent on temperature. Due to this property, a fountain effect can be demonstrated in the following experimental setup: An inner vessel is placed inside a larger vessel, and both are filled with 4He to the 3 same halfway level. Separating the inner and outer vessel on the bottom is a porous superleak plug that only allows the superfluid component to pass through, and an open tube rising above the liquid level from the inner tube. When the inner vessel is heated, temperature and pressure
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