Preparation and Construction of a Superconducting 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- 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, training, and experimental testing. A 16,000-turn, 0.8 (5 A) electromagnet was built to match its existing counterpart installed in the experiment to demonstrate bi-directional flow through a closed line utilizing the magnetocaloric effect and 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.

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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.

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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!

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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 . 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.

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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 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 ”, 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 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 . This property can be observed when inducing boiling in the liquid.

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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 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

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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 increase due to the higher concentration of normal helium liquid that is a function of both temperature and volume, causing the liquid level to rise. To reestablish a balance the superfluid component from the surrounding vessel enters through the superleak on the bottom while fluid leaves from the top of the inner vessel through the small tube, giving the fountain effect its name. The unique properties of liquid helium at sub-Kelvin temperatures indicate that it is advantageous to use as a working fluid, as its behavior can be taken advantage of in cryogenic applications.

Magnetocaloric Effect

The magnetocaloric effect is a phenomenon that is utilized in this cryogenic application. In short, it is the adiabatic heating or cooling of a paramagnetic material in the presence of a . A paramagnetic material without the presence of a magnetic field has randomly arranged poles, and the introduction of a magnetic field aligns the magnetic ions. The entropy of a paramagnetic material can be thought of as a combination of the entropy due to the magnetization and demagnetization of ions, and the entropy due to changes in the temperature of the material. The components of entropy are important to define when looking at the magnetization and demagnetization of a paramagnetic material from an isothermal and adiabatic standpoint demonstrated in Fig. 2. In an isothermal process when a paramagnetic material is subject to a magnetic field, the temperature between State 1

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(before) and State 2 (after the magnetic field is produced) is constant. In this case, the thermal entropy remains constant, but the entropy due to the magnetic ion ordering decreases as the poles align and become more ordered, meaning the total entropy of the system decreases, implying that heat was transferred out of the system to maintain constant temperature. In an adiabatic process, the total entropy of the system is conserved from State 1 to State 2. Due to the ordering of magnetic poles, the magnetic component of the system entropy decreases. To maintain constant entropy through the process, this means that the thermal entropy component of the system must increase from State 1 to State 2. An increase

Fig. 2. Adiabatic heating and cooling of a paramagnetic material depends on the presence of a magnetic field. [5] in the thermal entropy indicates that the temperature will also increase from State 1 to State

2. Thus, the temperature of a paramagnetic material can be controlled in an adiabatic process by the presence of a variable magnetic field.

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Superconducting Magnetic Pump

The SMP that was built by A. Jahromi takes advantage of the superfluid properties of sub-Kelvin helium fluid and the magnetocaloric effect to induce the flow of liquid helium in a closed system. A schematic of the closed-loop helium line in which the experiment is conducted is shown in Fig 3. Two are situated around a closed line of helium liquid. Each electromagnet consists of an aluminum mandrel with superconducting -

Titanium windings. Within the hollow middle of each mandrel is an open container with porous paramagnetic material, Gadolinium Gallium Garnet (GGG), that allows helium liquid to pass through. The are connected in series and are separated by a Vycor

“superleak”, a material that only allows superfluid to pass through. The two magnets are part of a closed helium line.

Fig. 3. Simple schematic of the superconducting magnetic pump. The two mandrels are wound with . The hollow center is filled with a canister of crushed paramagnetic material, and separated by a Vycor superleak.

As current is sent through one of the magnets, the paramagnetic material heats up due to the magnetocaloric effect. As the magnetic entropy decreases, the thermal entropy of the material increases, resulting in an increase in temperature. Because the liquid helium inside the closed loop is in good thermal contact with the paramagnetic material, it experiences the same temperature change as the crushed GGG. This process is nearly reversible, because the

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magnetic field can be reversed and the magnetic entropy will increase, resulting in a decrease in thermal entropy and a subsequent decrease in the temperature of the paramagnetic material. During both the magnetization and demagnetization periods, the helium fluid present in the salt beds will experience the same temperature change as the paramagnetic material. During operation, when the system is running at temperatures below 2.2 K, this will result in a change in the fraction of normal fluid and superfluid present in the closed loop. As the helium fluid heats up slightly, the fraction of superfluid helium in the system will decrease. The higher concentration of normal helium fluid will increase the pressure within the system. As noted previously, one way that He II (superfluid) differs from He I (normal fluid) is that its pressure is only a function of temperature, while normal helium will see an increase in pressure as the temperature increases and the volume changes. When one electromagnet is magnetized at a time, there is an instantaneous pressure imbalance across the Vycor superleak separating the two magnets. To reestablish equilibrium in the closed loop, superfluid helium passes through the Vycor superleak and normal fluid flows around the closed loop in the same direction. In this way, the current sent through the electromagnets induces the direction of flow by controlling the temperature of the paramagnetic material.

Flow can be induced in either direction by magnetizing either magnet to establish a gradient over which fluid will flow to reestablish steady state operation.

Experimental set-up

The low-temperature facility was built in-house to run this experiment. Amir Jahromi, a researcher at the University of Wisconsin-Madison modeled, designed, and built the system

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over the course of his doctoral career. The system remains intact and the experiment continues to be run after his departure from the university.

Experimental low-temperature platform

The following section describes the equipment and test rig that was used to run and collect data from the SMP experiment. A general layout of the system is provided in Fig. 5.

A commercial two-stage cryocooler is employed to initially cool the system through an open helium loop that feeds a small 1 K pot refrigerator. This provides the environment necessary for the closed helium loop through which the experiment is run. The two fluid circuits are independent of one another, but the commercial cryocooler must reach its maximum cooling rate before the experiment can be conducted.

The cryocooler used in this experiment is a Cryomech pulse tube model 410. Two heat exchangers at the face of each Dewar within the system reach temperatures of 40 K and

4 K, respectively. The designer chose to use a pulse tube cryocooler rather than a traditional cryogenic refrigerator because of the favorably low level of vibrations caused by the cryocooler. It is important to subject the test rig to the least amount of vibrations and electrical noise as possible to reach sub-Kelvin temperatures.

The setup consists of three stages of nested Dewar tanks that house the closed helium pump line and the 1 K pot. The open helium line runs through the top of the Dewar cooling the system at 40 K and 4 K heat exchange platforms. A Pfeiffer TMH 071 turbo vacuum pump continually pulls a vacuum on the outer-most Dewar, and the system is held under vacuum for the duration of the experiment. An Alcatel 2021-D dual stage rotary vane

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vacuum pump purges the exhaust line of the open helium line exiting the 1 K pot. A Joule-

Thompson restrictor valve throttles helium into the 1 K pot, allowing it to reach the desired temperature within the pot.

The closed helium line contains the SMP experiment within the 1 K pot environment.

A separate tank of helium is used to provide the working fluid to the experimental line

(closed loop). A long drive shaft is installed at the head of the outermost Dewar to control the valve leading to the closed line. When the c commercial cryocooler has reached the desired temperature, and the J-T valve throttles the temperature of helium in the 1 K pot to about

1.4 K at the third heat exchanger in the system, helium can be sent through the secondary line and the closed loop will be filled. A dual stage b a vacuum pump and pressure relief valve are

Fig. 4. The inside of the low-temperature facility installed in the closed line to evacuate the line containing the experiment. (a) - drive shaft controlling the closed loop. (b) - 1 K pot. (c) – of impurities and relieve pressure during filling electromagnets - one behind the other. in case of an emergency. The two superconducting magnets that are employed in this experiment are thermally fixed to the 4 K heat exchanger.

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] 5 SchematicSMPofthe experiment [ . 5 Fig.

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Measurements and Equipment

Multiple pieces of equipment are used for measurement and data collection in the experimental setup. Instruments to measure temperature, pressure, voltage, current, capacitance, and flow rates, as well as equipment to supply current to the superconducting magnets are employed and are described in the following section.

Two flow meters are installed at the inlet of the open helium line and monitor the flow of helium. They help determine if there are any obstructions in the line due to impurities at low temperatures. Temperature sensors are installed at each heat exchanger platform, at the 1 K pot, and at both superconducting magnets. Pressure sensors monitor the vacuum in the Dewar. An analog capacitance bridge measures the pressure drop across the Venturi flow meter installed in the closed helium loop.

In addition to the devices installed to measure various points in the system, a Gorman

Star Winder was used to make the superconducting magnets that are thermally anchored at the second heat exchanger plate, which the closed helium line runs through. For a more detailed description of the equipment used and the methods of data collection, see Jahromi’s doctoral thesis [5].

Experimental procedure

In addition to modeling, designing, and constructing the low-temperature facility,

Jahromi designed the experimental procedure to run this experiment. This section is dedicated to elaborating on points in the procedure that need to be better explained, or

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clarifications that make running the SMP experiment more clear. These clarifications were determined after the author ran the experiment using Jahromi’s published procedure.

Evacuating the experimental loop

Before cooling cycles can be run in the experiment, the closed loop experimental helium line must be completely evacuated. One of the reasons for this is the additional load that the cryocooler would have to cool, in addition to concerns about contaminants in the line. If you are not sure whether the line is evacuated, a number of filling and purging cycles should be run to ensure that all helium is removed from the line. To do this, alternately fill the helium line and pull a vacuum on the closed loop by opening BV1 and BV2 (See Fig. 5).

During the filling stage, pressure in the helium tank (helium tank B) is set to 10 psi. When transitioning from the filling stage to the purging stage, it is important to first CLOSE BV1, then OPEN BV2. Care should be taken to open BV2 slowly so that no oil from the vacuum pump enters the intake line. When transitioning to the filling stage, the user should first

CLOSE BV2 to the vacuum pump, then OPEN BV1 to the helium tank. This will ensure that the vacuum pump will not pump out helium from the pressurized helium tank. When the helium line is sufficiently evacuated, the following procedure should be followed to close the helium line. First CLOSE NP1, the valve controlling the closed loop using the handle at the top of the Dewar, then CLOSE BV5, and BV2. Then OPEN BV1, so that the line is pressurized up to BV5 and evacuated from BV5 into the closed helium line. When closing the experimental loop, the user should use the utmost care to operate the handle of the drive shaft. The handle is constructed such that a heat strap anchors the handle. Closing the handle

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should take about 3 half-turns to completely open and close. DO NOT twist the handle beyond a comfortable torque. Twisting the handle too much can break the handle and render the valve useless.

Impurities in the helium line

One complication that can occur in the experiment is contamination in the open helium line. If care is not taken to close all of the ball valves connecting the helium tank to the line running through the Dewar, air will enter the system when the helium tank is changed. This can cause damage to the system and require a significant amount of time to cycle through cooling and warming stages to completely purge the line of contaminants.

Contamination of the helium line poses a major problem to the J-T restrictor valve at the opening of the 1 K pot. If particles such as water are in the system, they can damage the small restrictor valve when they cool expand in the state. Foreign particles limit the lowest temperature that the 1 K pot can reach. When the helium line is contaminated, it is necessary to run a number of cooling and warming cycles to dislodge foreign particles. It will be evident when the line is completely purged, because the desired temperature and flow rates will be observed.

Cool-down process

The process to cool the Dewar facility requires about a week to transition from room temperature to sub-Kelvin temperature. After transitioning from to liquid at about 4 K, the cooling process speeds up due to the improved thermal capabilities of liquid helium. An

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additional phase change at about 2.2 K begins the transition from normal to superfluid. For cooling at high temperatures, the helium tank supplying the open line can be set at 60 psi.

When 2 K is reached in the 1 K pot, the pressure should be set to 30 psi. At high temperatures, the higher pressure is more desirable, while below 2 K, when transition to superfluid begins, the high pressure at the J-T valve can overwhelm the line and cause the system to heat up.

Warming process

Warming the cryocooler is a relatively simple process. Once the commercial cryocooler is turned off, the system will naturally reach room temperature after a long period of time. The user should make sure to monitor the pressure at the helium tank until the minimum temperature is at 4 K, above the transition to a liquid state. If the system warms up too quickly below 4 K, it can be subject to huge pressures that will damage the system.

Above the liquid transition, the pressure in the helium tank can be set and left alone.

When the system is sufficiently warm, the operator has the option to add gas to the outer Dewar to speed up the cooling process. For instance, when the minimum temperature in the system reaches above 70 K, the user can carefully add about a liter of pure nitrogen gas to the outer Dewar. To accomplish this and speed up the warming process, first close GV1, the valve pulling a vacuum on the outermost Dewar. Then, carefully add the volume of nitrogen gas, no more than a room temperature liter’s worth in by way of GV3, taking care to only allow nitrogen gas to enter. This step is not necessary, but will help warm the system faster.

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Filling cycle

The final step for preparing the low-temperature facility to run the SMP experiment

(once the 1 K pot has reached the desired temperature) is to fill the experimental helium line.

This step is difficult because the low-temperature facility must cool the helium entering the experimental line while maintaining low temperature within the whole system. If too much helium is allowed to enter at once, it will overwhelm the system. If the system warms up above 4 K, the transition to gas, it can have very detrimental effects on the experiment. To successfully fill the experimental helium line without overwhelming the system, the following procedure must be followed. First, NP1 is opened, and BV5 is opened and closed to send “pulses” of helium into the line. BV5 only needs to be opened a crack to allow helium to flow inside. It is imperative to observe the temperature and flow rate as you open the valve. The helium feed rate in tank 2 should stay at about 1 – 2 psi. The flow rate of helium in the open line will decrease, but should return to the same value. At this point you can continue to crack BV5 open. The operator should never let the flow rate of helium drop below 100 mL/min. After the SMP experiment is run, the user must evacuate the experimental helium line before warming the entire system.

Experimental Results

The author was successfully able to purge the open helium line of contaminants and fill the experimental helium line in preparation for running the SMP experiment. The following section details the results of the preliminary experiment that prepared the low temperature facility for future SMP experimental analysis.

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Preliminary cooling cycles

Because of impurities in the helium line, it was necessary to perform a number of cooling cycles to purge the open helium line of impurities. It was evident that this would be required because the expected flow rate and temperature values were not achieved after the first cool down. When the author ran the first cool-down cycle after re-connecting the helium tank, the lowest temperature that could be reached at the 1 K pot was 3.4 K at Helium flow rate of 170 mL/min. The expected temperature and flow rate at ideal conditions is 1.43 K at

800 mL/min. It took seven cool-down cycles, each taking about a week and a half, to completely purge the helium line of all contaminants and reach the desired temperature and flow rates in the system. Table 1 shows the minimum temperature and maximum flow rates that were achieved through the seven cool-down cycles that it took to reach ideal working conditions.

Table 1. The minimum temperature and maximum flow rate achieved by the seven cool-down cycles to reach optimal working conditions demonstrated that there were impurities in the line initially.

Cool down He flow rate Temp cycle (mL/min) (K) 1 170 3.4 2 560 3.0 3 449 2.57 4 511 2.34 5 781 1.43 6 797 1.43 7 797 1.43

The significant number of cool-down cycles was required to dislodge particles that were stuck in the helium line. The process of cooling to sub-Kelvin temperatures and warming back to room temperature dislodged particles, as can be seen from Table 1. As the cool-down cycles progressed, the flow rate of helium increased, and the minimum 16

temperature achieved at the 1 K pot dropped. This indicated that cycling the system did, in fact, purge the unwanted particles from the open helium line.

Temperature data

The rate of cooling throughout the system varies at each heat sink. Fig. 6 shows a general pattern observed when cooling the system. It is shown that the temperature at each magnet lags that of the rest of the system. This is because the salt pills containing paramagnetic GGG have a very high heat capacity, so they change very slowly when subjected to changes in the environment.

Fig. 6. Temperature time trace for a cool-down cycle from room temperature

The 40 K and 4 K heat exchanger plates also respond relatively quickly to the cryocooler, while the 1 K pot lags behind.

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Fig. 7. The lowest temperature reached at the 1 K pot is 1.43 K. The two magnets lag in reaching the temperature because of the high heat capacity of GGG.

Shown in Fig. 7 is a closer look at the temperature trace of the magnet canisters at sub-Kelvin temperatures. While the 1 K pot reaches a constant minimum temperature due to the J-T valve at the inlet, the canisters filled with paramagnetic material take longer to reach steady-state temperature because of the high heat capacity of paramagnetic GGG.

Magnet construction

After the low-temperature facility was restored to its former working condition, it was necessary to address the construction of a new superconducting magnet; of the two that were installed in the low-temperature facility, one had broken. Construction of the new magnet would be the most time-consuming task of the project yet. Not only does it require precise measurements and a confident hand, but also any small misstep in the procedure can ruin the magnet and render it useless, destroying weeks or months of preparation. It is unclear how

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the original magnet broke, as there are many potential modes of failure. Some will be discussed in the following section.

Magnet Design

A comprehensive design study of the magnet was conducted by A. Jahromi during the preliminary design and construction of the low temperature facility, and can be referenced in his doctoral thesis [5]. Analyses of note include consideration of eddy currents in the canister body that are not included in this work.

The magnets built for this experiment are only required to produce a magnetic field of

0.8 Tesla to demonstrate proof-of-concept. Each electromagnet consists of a mandrel with superconducting Niobium-Titanium wire wrapped around connecting to leads. The NbTi wire that is wrapped around the mandrel has an insulated outer diameter of 102 microns.

Within each copper sheath are 54 individual NbTi filaments that measure 7 microns in diameter. A diode pocket is located on one flange that is 25 mm in length that houses the diodes that protect the magnet from unwanted quenching. Leads coming out of the diode pocket are made of thicker NbTi wire and protected by heat shrink tubing.

To ensure that the new magnet would meet the design requirements of the last magnet, it was necessary to size it appropriately based on the working magnet in the low- temperature facility. A simple application of Ampere’s Law along the magnet will confirm that the general design will supply the required magnetic field. This calculation is necessary to match the magnetic field of the new magnet to the functional one already installed. The dimensions of the new magnet and the functional installed magnet are outlined in Table 2.

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It’s important to note that the dimensions and the numbers of turns of each electromagnet is different, but that the magnetic field produced is roughly the same according to Ampere’s

Law. This calculation determines the smallest magnet that needs to be made to successfully demonstrate the experiment.

Table 2. The parameters of the previously installed and working magnet were used to design the new magnet

Working New magnet magnet Turns 8,500 16,000 Length (cm) 8 cm 11.9 cm Radius (mm) 28.5 mm 25 mm Current (A) 5 A 5 A Magnetic field (T) 0.7 T 0.8 T

It was important to design the new magnet with the parameters of the working magnet in mind to build the minimum viable device that will successfully demonstrate the concept.

Procedure

The procedure for constructing the electromagnet is as follows. First, a mandrel was machined out of 6061 aluminum bar stock. The surface and flanges of the mandrel were lined with electrically insulating tape, and a thin layer of epoxy was applied to the joining edge.

The NbTi wire was stripped at one end and temporarily secured to the outside of the flange.

Over the course of roughly five hours, the magnet was continually wound while potting with layers of degassed epoxy. After a setting period, the wires were secured to two diodes and soldered together. The diodes were set in the diode pocket and all wires buried in epoxy.

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Preparation

There was a significant amount of time that was spent preparing for the construction

of the magnet. It was crucial that any work done to the magnet was extremely precise and

practiced, as to not ruin any past work. Because of this, most of the time spent working on

the magnet was either practicing or preparing for future work. The mandrel used in this

electromagnet application was machined out of 6061 aluminum bar stock. The winding

length measures 119.9 mm with an outer diameter of 50.3 mm. Flanges on either side of the

mandrel are used to mount the electromagnet to the workbench and to the experiment, with a

machined “diode pocket” on one flange that houses the diode circuitry.

The body of the mandrel was taped with insulating Kapton tape to ensure that no

shorting occurred in the magnet, and that there was consistent thermal communication

throughout the whole electromagnet. This process was

extremely time consuming and tedious. It was absolutely

necessary to ensure that there were no air bubbles or air

pockets under the tape or in any other place within the

magnet after production. The Kapton tape must to be

placed as precisely as possible, with as few overlaps as

possible to maintain constant thermal communication

between the magnet and the mandrel. In lieu of traditional

Fig. 8. A thin layer of epoxy (not pictured) was needed for the gaps in the roll tape, the author used Kapton sheets that are typically Kapton tape at the flange corner. used in 3D printing applications to cut custom pieces with

as few overlappings as possible. One additional design consideration to take into account is

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the orientation of the mandrel during the winding process. It was important to overlap the

Kapton tape on the flanges in such a way that the wire that traverses to the edge of the mandrel will not strip the tape. As shown in Fig. 8, a small gap between the taped faces remains in the edge joining the body of the mandrel and the flange. A small amount of degassed epoxy was placed in this gap to ensure that no shorting would occur between the wire and the mandrel.

The process for degassing epoxy was especially critical during the construction of the magnet. A Pyrex vacuum chamber was used to remove from the epoxy mixture. The vacuum pump and vacuum chamber were separated by a copper tube. This was submersed in a bath to ensure that no epoxy entered into the pump and clogged the machine. The degassing process should take no more than 10 minutes to complete when using small batches of less than 25 g total of epoxy; after the initial phase of violent bubbling, the mixture is ready. It is generally good practice to mix small batches of epoxy frequently every 45 minutes or so when potting the magnet, and to solicit batch-mixing help from a professor or student during the winding process.

It is also worth noting that one of the potential failure modes of the last magnet was the use of a different type of epoxy to fill the diode pocket. The differences in thermal expansion rates of the two types of epoxies could have added too much strain to the wire and caused it to break. To eliminate this error, there was only one type of epoxy that was used during the construction of the new magnet.

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Winding process

A CNC Gorman Star Winder machine

was used to wind the magnet. This machine

allows the user to program the position to

begin winding at, the pitch corresponding to

wire diameter, total width that the wire

Fig. 9. A CNC Gorman Star Winder was used to traverses, the number of turns requires, slow wind the electromagnet

and fast rotational speeds, and the number of slow turns at the beginning and end of the

process. When programming the winding machine, it was necessary to adjust the parameters

with the correct wire diameter and corresponding wind length to correct for slight errors in

the machine. After winding a number of magnets with copper wire, two practice passes were

completed with NbTi wire to check that

parameters were correctly entered into the

machine, and that the wire traversed smoothly

across the length of the magnet. One aspect of the

winding process that makes it so difficult is that

once winding has begun, the process cannot stop.

The magnet must be continually wound and

brushed with epoxy until it is finished so that the

epoxy hardens in a consistent manner around the

mandrel. This magnet was designed to have Fig. 10. Once the winding process began, the mandrel was continually potted with epoxy. An 16,000 turns, which operating at a winding speed acid brush was used for application.

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of 60 rpm, took about 5 hours to wind. A relatively low winding speed of 60 rpm was chosen to ensure that no problems arose and the wire would smoothly layer on top of itself. After the winding process is complete, the epoxy needs at least 7 days to completely cure. The mandrel

spun for an additional 2 days at 30 rpm after

winding was complete to ensure that the epoxy

hardened consistently along the magnet.

It is extremely important to place the wire

leads coming off the windings in an advantageous

place while the magnet is curing. Any epoxy that the

leads are touching while it is wet will anchor them

to whatever surface they are touching and

potentially ruin the magnet. The wires should be

temporarily secured to the outside of the flange well Fig. 11. The finished magnet has 16,000 turns and produces a magnetic field of 0.8 away from any epoxy during the curing process. Tesla with 5 A. Having long free lengths of NbTi wire is also useful when stripping the wire leads to connect to the back-to-back diodes. After the magnet is completely cured and the wire leads are stripped, they can be soldered to either end of the twisted diodes. Utmost care should be taken during the soldering process, the final step before the magnet is complete. The stripped NbTi wire is difficult to solder so it should be wrapped around the diode leads to improve electrical communication. After the diode circuitry is complete, the diode should be fixed in the diode pocket with degassed epoxy, and any free length of wire should be buried. As a final step, heat shrink tubes should be added to

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protect the leads coming out of the diode pocket. The final magnet is shown in Fig. 11, and a close-up of the diode pocket is shown in Fig. 12.

Future work and conclusions

This experiment is far from completed.

Now that the final magnet has been made, there is an extensive process to train the magnet and install it into the low-temperature facility. First, Fig. 12. Detail of the finished diode pocket. All the magnet should be installed into the low- free wires are buried in epoxy and the leads are protected by heat shrink tubing. temperature facility. A number of cool-down cycles should be completed to begin training the magnet to operate at sub-Kelvin temperatures. There is a degree of wire shifting that may occur during this process. After the magnet has been trained, the magnetic field that the magnet can product should be measured.

Only after this extensive preparation process is complete will the experiment be conducted to demonstrate bi-directional flow.

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Bibliography

[1] A. E. Jahromi and F. K. Miller. “Modeling, development, and experimental validation

of a Joule-Thomson superfluid refrigerator using a pulse tube cryocooler”.

Cryogenics, vol. 61, pp. 15-24, 2014.

[2] W. E. Keller. “Helium-3 and Helium-4”. Science, vol. 166, pp. 1501, Dec. 1969.

[3] S. W. Van Sciver. Helium . Springer Science & Business Media, 2012.

[4] P. Kapitza. “Viscosity of liquid Helium below the I-point”. Nature, 1938.

[5] A. E. Jahromi. “Development of a proof of concept low temperature superfluid

magnetic pump with applications.” Ph.D. thesis, University of Wisconsin-Madison,

2015.

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