Design and Construction of a Helium 4 Dipper

Kirsten Blagg

Physics Department Colorado School of Mines, Golden CO

(Dated: December 11, 2017)

2

CONTENTS

Introduction 3

Material Properties 4

Specific Heat 4

Thermal Conductivity 7

Thermal Expansion 10

Magnetic Susceptibility 11

Material Properties for Common Cryogenic Materials 12

Design 16

Top Electronics Box 16

Main Body 21

Sample Stage 23

Wiring 28

Conclusion 29

Appendix A: Design Drawings 30

Appendix B: Purchase List 38

Acknowledgments 40

References 40

3

INTRODUCTION

Low temperature physics offers insight into a variety of interesting phenomena in materials

research, particle physics, solid state physics, and much more. There are multiple methods

to reach very low temperatures. Commercially available helium 4 cryostats can be used

to reach approximately 1 Kelvin and helium 3 - helium 4 dilution fridges can reach the

milikelvin range. These commercially available options (Oxford, Blue Forge) are simple

and effective; however, they tend to be extremely expensive and difficult to modify[1][2].

Additionally, larger dilution cryostats can take six hours or more to cycle a sample from room

temperature down then back to room temperature again. Many interesting applications

mentioned above can be accessed at the temperature of helium evaporation, 4.2K. A simple

cryostat, commonly called a dipper, can be inserted directly into a standard helium 4 dewar

allowing experiments to be run at 4.2 Kelvin. In addition to acting as a stand alone cryostat,

a helium 4 dipper can preform preliminary tests quickly and effectively compared with colder

dilution fridges. Standard helium dippers place the sample directly in liquid helium and use

evaporation to directly cool the sample. While, this offers a simple design, it does not allow

for temperature variation or for thermoelectric measurements. To this end, the sample must

be placed in vacuum. While this complicates the dipper design, it offers more flexibility and

experimental possibilities compared with a typical helium dipper.

The primary concerns in the design and use of a helium 4 dipper are cooling a sample

down to low temperature and conserving expensive helium 4. In general, the sample needs to

reach a stable thermal equilibrium with the surrounding liquid helium, measurements need

to be taken on the cold sample, and heat sources need to be minimized. For any cryostat,

the main sources of heat are: the conduction of heat along walls, wires, and tubes, thermal

radiation from room temperature to cold components, conduction by gas particles, thermal

acoustic oscillations, Joule heating, and additional electric and mechanical vibrations. Each

heat source must be offset by the cooling power of the cryoliquid. While heat transfer cannot

be completely avoided, careful attention to materials and construction is critical to ensuring

efficient use of helium.

The following paper describes basic design schematics, design challenges, material con-

siderations, and guidelines important in the design process of any fridge. It also includes a

detailed step by step guide to the construction of a vacuum 4.2K dipper.

4

MATERIAL PROPERTIES

As with any design and assembly, particular attention needs to be paid the properties

of the material used. In the construction and use of a helium fridge the most important of

these properties are specific heat, thermal conductivity, and thermal expansion. Magnetic

susceptibility also becomes important for experiments involving the production of magnetic

fields and magnetic measurements. The workability, cost, and strength of the materials also

impacts design choices. The cryogenic application of the dipper causes further challenges in

material selection because materials must operate over a wide temperature range. There-

fore, it is critical to consider the temperature dependence of material properties. In the

following sections the theoretical basis and and specific material considerations of specific

heat, thermal conductivity, thermal expansion, and magnetic susceptibility are addressed

for cryogenic applications.

Specific Heat

The specific heat, the amount of heat required to increase the temperature of the material,

is primarily controlled by the excitation of electrons and phonon vibrations. The specific

heat determines how much energy it takes for the lower part of the dipper to cool from

room temperature to 4.2K. This translates to the amount of helium boiled off each time the

dipper is inserted into the dewar.

In insulating materials lattice vibrations called phonons cause excitations. At high tem-

peratures phonons can be modeled as independent oscillators with three degrees of free-

dom. Electrons which remain bound to the atoms have negligible excitations compared

with phonons at high temperatures. These degrees of freedom (phonon modes) freeze out at

low temperatures below the Debye temperature, θD = hωD

kBwhere h is Plank’s Constant, ωD

is the Debye frequency, and kB is Boltzmann’s Constant. While these three phonon modes

freeze out at low temperatures, the electronic excitations do not. Thus, at very low temper-

atures, far below the Debye temperature, electronic contribution to specific heat becomes

important even for insulators. The specific heat of insulators at low temperatures has a

cubic dependence on temperature (C = βT 3). Thus, the specific heat is generally very low

at low temperatures.

5

In metals, the specific heat is determined by the excitation of phonons and electrons

which are free to conduct through the material. At low temperatures the electron energy

states remain close to the Fermi energy and only electrons near the Fermi energy conduct

heat. The specific heat is given by the cubic dependence of the phonon vibrations summed

with a linear temperature dependence of the electron excitations, C = βT 3 + γT where β

and γ are both constants. At very low temperatures phonon modes freeze out, just as in

the case of insulators, and electron excitations dominate (C ≈ γT )[3]. Figure 1 compares

the temperature dependence of metals and insulators.

FIG. 1. Comparison of the heat capacity of silicon and copper as a function of temperature cubed.

Both materials approach the law of Dulong and Petit at high temperature. At a middle temperature

range both materials follow the Debye model and match a cubic temperature dependence. At low

temperatures the metal departs from the Debye model due to the contribution of electron specific

heat. [4]

At low temperatures, some metals become superconductors and the electrical resistivity

of the material drops to zero. The specific heat of phonons is not changed by the transition

into the superconducting state. However, the electron excitations now have an additional

degree of freedom corresponding to the possibility of forming a Cooper Pair and entering the

superconducting state. This new degree of freedom increases the specific heat at the critical

temperature of the material. Below the critical temperature the temperature relation of the

electron specific heat is given by an exponential decay (Figure 2). Due to the zero resistivity

of superconductors, they are often use in wiring, magnets, and devices in fridges.

6

FIG. 2. The heat capacity of a superconducting metal versus temperature. Above the critical

temperature, the heat capacity is dominated by a linear relation to temperature. Once the critical

temperature is reached, there is a increase in the heat capacity and the heat capacity follows an

exponential relation to temperature[5].

FIG. 3. The specific heat of common cryogenic materials is plotted with respect to temperature.

Metals are plotted with solid lines, metallic alloys with dot-dashed lines, thermal insulators with

dotted lines, and gases with dashed lines [6].

7

Thermal Conductivity

Thermal conductivity (κ) is defined as the heat flow (.q) per unit area under a temperature

gradient (5T ),.q = −κ5T . The thermal conductivity of the material is critical cooling the

sample and conserving Helium. Materials with a high thermal conductivity are required to

connect the sample to the surrounding liquid helium. Materials with low thermal conduc-

tivity are required to reduce heat flow from room temperature down to the liquid helium to

prevent wasteful boil off.

Heat is carried through the material by the conduction of electrons and phonons. This mo-

tion is defined by the limiting of conduction through scattering processes: phonon-phonon,

phonon-defect, electron-impurity, electron-electron, and electron-magnetic impurity. Con-

ductivity is thus strongly dependent on the material and temperature.

Electron scattering is negligible in insulators because of the low electronic conductivity.

Thus, thermal conductivity is defined by phonon-phonon or phonon-defect scattering. At

very low temperatures, far below the Debye temperature, the low number of thermally ex-

cited phonons decreases phonon-phonon scattering and the majority of the scattering takes

place off defects, dislocations, and grain boundaries. In metals, electronic conductivity

dominates; though electron and phonon conductivity is comparable in disordered alloys.

At high temperatures thermally excited phonons limit the conduction of electrons through

phonon-electron scattering. At low temperatures electrons primarily scatter off defects and

impurities since there are very few phonons. Figure 4 shows how the temperature depen-

dence of the thermal conductivities changes as scattering processes become more or less

dominant. Since the conduction of both electrons and phonons at low temperatures is lim-

ited by defects and impurities the thermal conductivity varies from sample to sample and

between manufacturing processes. Therefore, the purity, tempering, fabrication, and history

of a material is important. A comprehensive review of thermal conductivities for elements

has been outlined in literature[7], but variations between samples still occur (Figure 5).

8

FIG. 4. A schematic outline of the dominant scattering processes in insulators and metals versus

temperature. While the temperature and exact characteristic of these shifts in scattering are

material dependent, the schematic reveals the importance of temperature on scattering processes

and on thermal conductivity. The graph shows the thermal conductivity of AlN with respect to

temperature. The trendline changes as the different scattering processes turn off as the temperature

decreases. [8]

9

FIG. 5. The thermal conductivity versus temperature of common cryogenic materials for T > 2K.

We note that there is additional dependence on purity and defects [6].

10

Thermal Expansion

Materials at high temperatures expand and at low temperature contract. This expan-

sion and contraction caused by the aharmonic potential the atom experiences due to the

electrostatic forces of its neighbors (Figure 6). At low temperatures the atoms remain in

the parabolic harmonic region since the amplitude of the atomic vibrations is small. How-

ever, as temperature increases, the thermal vibrations of the atoms grow and experience

an asymmetric potential. This causes the atoms to spread out as they spend increasingly

more time at longer distances leading to the expansion of the lattice spacing. The thermal

expansion coefficient (α) depends on the material and is given by the ratio of the change in

length (l) by the change in temperature (T ) divided by the length at a constant pressure

(P ), α = 1l( ∂l∂T

)P . This characterizes a material’s expansion/contraction under temperature

change.

FIG. 6. A typical atomic potential as a function of distance between the atoms or ions in a lattice

is plotted above. The blue dot represents the average location of an atom at a cold temperature

and the red atom represents the average location of an atom at hot temperatures. The expansion

is seen in the difference of the two averages [3].

When designing a helium dipper, the dipper manufacturing occurs at room temperature,

but the dipper must be heated and cooled for each experiment repeatedly expanding and

11

FIG. 7. The thermal expansion versus temperature of common materials used in cryogenic

manufacturing[6].

contracting the materials. The thermal contraction of common materials is given in Figure

7. For connections between the same materials all components will expand and contract

equally, so thermal expansion of materials is relatively unimportant. In connections between

different materials, the different rates of expansion and contraction can cause connections

to leak or break due to strain at the joints. In general, the material with the largest thermal

expansion coefficient should be on the outside and the material with the smallest coefficient

towards the center. It is important to remember that seals and contacts made at room

temperature will not necessarily hold at low temperatures. In order to ensure contact a low

expansion material such as molybdenum or tungsten can be used as a washer. In this design,

we have minimized connections between dissimilar materials to avoid leaks and additional

stress.

Magnetic Susceptibility

The magnetic susceptibility of a material (χ) defines the magnetization of that material

in response to an external magnetic field. The magnetic susceptibility is a dimensionless

constant given by χ = M/H, where M is the magnetization of the material and H is the

magnetic field strength. Often cryogenic experiments require the use of strong magnetic

12

fields, upwards one Tesla. Additionally, some cryostats employ the use of SQUIDs (Super-

conducting Quantum Interference Devices) for very sensitive magnetic field measurements.

If the materials used, particularly the material surrounding the sample or wiring have high

susceptibilities, they can produce a magnetic field influencing the sample and skewing any

magnetic measurements preformed. Again temperature considerations are important; mate-

rials which are considered non-magnetic at room temperature can have appreciable magnetic

susceptibilities at low temperatures. In addition, for sensitive experiments magnetic impu-

rities in materials can cause interference and noise. If the cryostat will be used to preform

experiments in a magnetic field the magnetic properties, including magnetic defects, of con-

struction materials must be carefully analyzed.

Material Properties for Common Cryogenic Materials

The properties of common manufacturing and cryogenic materials are given in Table 1.

While exotic materials have better properties with respect to cryostat design, the machining

ease, availability, and cost of common materials makes them the leading choice. These more

ideal materials, such as Invar in the case of thermal expansion and Suprasil in the case of

magnetic susceptibility can be used in small applications such as washers, connections, or

stages. However, readily available and easily machinable materials such as stainless steel

and copper are used for the bulk of this design.

TABLE I. The specific heat, thermal expansion and thermal conductivity of common cryogenic

materials. Magnetic susceptibility is not listed due to its extreme variability based on particular

alloy and purity.

Material Specific Heat Thermal Expansion Thermal Conductivity

(J/gK) (δL/L) (300-4K) (W/m)

Copper 7x10−5 370x10−5 200-1000(RRR 20-100)

Aluminum 3x10−4 462x10−5 6

Stainless Steel 0.4-0.5 331x10−5 0.4

Brass 2x10−4 397x10−5 7

13

The materials required for a cryostat fall broadly into two categories: materials which

conduct heat and materials which minimize heat conduction. All materials used were also

chosen due to low magnetic susceptibilities, low specific heat, and reasonably small thermal

expansion coefficients.

Stainless steel provides a durable, machinable, affordable material with a low thermal

conductivity even at low temperatures. However, different alloys, purities, and manufac-

turing can greatly affect the properties of the material. Most 300 series stainless steels are

adequate for cryogenic applications; in particular, 304 and 316 stainless are classified are

cryogenic stainless steels. While these stainless steels are idea, most important is that the

stainless steel chosen is an austenitic stainless steel. When austenitic steels cool, the iron

remains in the form of austenite (face center cubic structure) allowing the material to retain

its strength. All other stainless steels exhibit an impact ductile / brittle transition. Addi-

tionally, non-austenitic steels, due to the crystal phase of the iron, exhibit a high magnetic

susceptibility at cryogenic temperatures making them poor material choices.

304L stainless steal was chosen for the main body of this cryostat design. 304 stainless

steel is part of the 300 series of steels composed of austenic chromium-nickel alloys. 304

is the most common grade with 18% chromium and 8% nickel. As seen in table II 304

stainless steels have a low thermal conductivity over a wide temperature range compared

with other alloys. The low carbon content of L stainless steels (0.03% carbon as opposed to

0.05% carbon) allows for easy welding. Additionally, many cryogenic and vacuum parts such

as flanges are made of 304L stainless steel. Thus, by using 304L stainless steel for dipper

construction, commercially available parts can easily be welded to the dipper.

While stainless steel offers a good material for preventing thermal conduction, copper’s

high thermal conductivity and availability make it an ideal material for providing thermal

contact between the liquid helium and the sample. However, the high cost and difficult

machinability of copper requires a design which minimized copper use. Similar to stain-

less steel, copper’s many different alloys, purities, and fabrication processes can have very

different properties particularly at low temperatures (Table III/Figure 8). OFHC (Oxygen

free high conductivity copper) is widely used in cryogenic applications because of its high

conductivity at low temperatures. C1100 offers an comparable thermal conductivity and

can be used in place of OFHC. For this design, since a small amount of copper was required,

scrap copper was used due to cost and availability over OFHC and C1100 copper.

14

TABLE II. The thermal conductivities of stainless steel alloys at room temperature and at 4.2K

Stainless Steel Alloy Thermal Conductivity (W/m ∗K)

Room Temperature 4K

304 0.3 14.5

316 0.3 14.5

16 Cr 20 Ni 0.4 13.6

15 Cr 26 Ni 1.0 11.2

Armco Iron 13 76

TABLE III. The thermal conductivity, magnetic susceptibility, and strength of different copper

alloys

UNS Number Material Thermal Conductivity (4K) Magnetic Susceptibility (4K)

(W/mK)

C10100, C10200 Oxygen Free, Annealed 183 −2.98x10−6

C11000 Electroytic Tough Pitch, Annealed 325 2.53x10−5

C17000-C17300 Beryllium Copper, Annealed 20 - 0.02 1.82x10−3

C22000 Commercial Bronze Annealed 6.1 7.63x10−6

C36000 Free Cutting Brass 2 −1.41x10−2

Copper and 304L stainless steel made up the majority of the materials used in this design.

Other materials used for wiring, connections, and electronics will be covered in the following

sections.

15

FIG. 8. The temperature dependence of the thermal conductivity of different copper alloys.

16

DESIGN

The helium 4 dipper is a multi-part assembly consisting of a top box at room temperature

for electronics, a long tube through which electronic wiring is run to reach the sample, and a

bottom sample container and stage. Figure 9 shows a diagram and pictures of the completed

helium dipper. The following subsections describe the design of each part as well as an

explanation of the intent and restrictions which influenced the design choice.

FIG. 9. Schematic and picture of the helium 4 dipper outlined in this manual.

Top Electronics Box

In order to run experiments at low temperatures a variety of electronics need to be con-

nected to the sample at 4.2K. This involves wiring (covered in a following section) and

electronic hook ups. A large, 8.7in x 5.7in x 2.2in, aluminum enclosure box (Mouser Elec-

tronics) houses the connections and wiring. Since changing wiring and connections after

construction of the fridge can be difficult, it is important to have a large number of versa-

17

tile connections such that the same wiring can be used in a variety of experiments. There

are a variety of hook up options available through distributors such as Mouser electronics,

Newark, and Digikey. These hook ups can be tailored to any specific project. Our enclosure

box houses 24 coaxial cable feedthroughs purchased from Newark electronics (Figure 10).

Coaxial hook ups allow for a diverse set of connections, a large bandwidth, and a variety

of input/output adapters. In order to prevent electrostatic build up on the enclosure, it

is important to have coaxial feedthroughs with floating insulation rather than connections

without a grounding loop. Each coaxial feedthrough is attached to a switch (Newark) which

connects the central pin to either a wire going down to the sample stage (on) or to a ground-

ing pin (off). An electronic schematic is shown in Figure 11. This allows each hook up to

be turned on only when in use ensuring unused hook ups and wires do not cause additional

noise and thermal conduction to the sample. Each coaxial grounding pin is connected to

a single coaxial connection which can be connected to an external grounding bar (Figure

12). It is important when wiring to avoid grounding loops which can short the circuit, cause

inaccurate measurements, and prevent current.

The enclosure box is connected to the vacuum can tube with a 25KF four way cross

and hermetically sealed wire feedthroughs. Originally, a high vacuum epoxy (Torr Seal)

was used to create a vacuum seal between the top enclosure box and the tube of the main

body. However, this proved to only hold a vacuum to 10−3Torr and thus was replaced

with hermetically sealed feedthroughs. Since a single feedthrough with both dc and coax

lines is expensive, the wiring to the top box was broken into two feedthroughs, one 19 pin

circular connector and a BNC connector feedthrough (Figure 13). The larger top enclosure

is connected via clamps to the 19 pin hermetically sealed flange. The BNC connector leads

4 connections to one side of the four way cross. This is connected to 4 BNC hookup via a

small breakout box. In order to connect the 25 KF cross to the 40 KF hermetically sealed

DC connection and the 50 KF BNC connector an adapter was used. The remaining side

of the cross is used to pump down the vacuum with a standard roughing pump and turbo

pump.

18

FIG. 10. Schematics and pictures of the enclosure box with 24 coax connections and 24 switches

is shown above. One additional coax connector is added on the side to connect the ground to an

external grounding bar.

19

FIG. 11. A wiring schematic of the coax connections and an image of the wiring in the box. Each

coax plug is connected to a switch which toggles between a wire going to the sample stage and a

grounding bar.

FIG. 12. The enclosure box with all coax connections wired to switches connecting the plug to

either ground or the sample. Wiring choices are described in a following section. A strip of copper

was used as a grounding bar to ease soldering.

20

FIG. 13. Assembly of the hermetically sealed connection between the main body of dipper and the

top electronics box. This allows for a vacuum sealed lower sample stage and main body. The top

electronic box connects via wiring to the 19 pin connector, while the main body is connected on

the opposite side.

21

Main Body

Connecting the room temperature enclosure to the sample at 4.2K is a vacuum can

made of a 50 inch long 304L stainless steel tube with a one inch outer diameter and a wall

thickness of 0.060 inch (McMaster Carr). The tube must fit in the opening of the helium

dewar and allow the sample to reach the bottom of the dewar. The length and diameters

choosen for this design was optimized for insertion into a Cryofab CMSH 100 LHe dewar

(Matheson Trigas). Since the tube goes from room temperature to liquid helium at 4.2K,

heat flow along the can walls must be minimized. Since significant thermal conductivity

requires more cooling power from the helium, the vacuum tube is designed to minimize heat

conduction. The 304L Stainless steel used has a low thermal conductivity at both high and

low temperatures. Additionally, a thin tube further minimizes the conduction by decreasing

the area in which heat can flow.

In order to control temperature and run thermoelectric measurements, the tube is pumped

down to a few militorr with a roughing pump. The vacuum has the additional benefit of

minimizing the conduction of heat through gas particles. The pump is attached to the

stainless steal tube with a 25KF flange vacuum hose adapter (Kurt Lesker) to the four way

cross at the top of the can. While there are other insulation options available such as MLI

sheets, these are difficult to effectively install in the narrow vacuum can.

Finally, in order to prevent thermal radiation from room temperature, radiation shields

made of high conductivity copper are equally spread throughout the tube. The shields,

copper baffles, consist of small thin disks which just fit in the inner diameter of the stainless

steal tube, seen in Figure 13. To hold the baffles in place the copper disks are brazed to

a thin (0.060 inch) stainless steel tube using Stay-Silv 45. Each disk has two large holes

through which wires can be threaded. To increase radiation shielding the copper is buffed

until shinny. The rod of baffles is threaded with wire and slid into the long main body

tube (Figure 14). These copper baffles act as a radiation shield ensuring that the 4.2K part

of the fridge is not in direct radiation contact with the room temperature portion. Since

thermal radiation goes as the temperature difference to the fourth, even small decreases

in the temperature difference can drastically decrease parasitic heating. By minimizing

conduction and radiation heat sources can be reduced and helium usage optimized.

A 50KF flange with a one inch vacuum tube adapter (Kurt Lesker) was used to connect

22

the vacuum tube to the top of the dewar. While the top of the dewar is fitted with a triclover

flange, a 50KF offers a reasonably good seal and more fitting options (Figure 15). However,

an adapter can be custom made with a triclover flange offering a better fit. The flange was

attached along the main body tube between the top KF flange and the bottom sample stage

such that it is free to slid along the length and can be tighten with an o-ring seal. This

allows the top of the dewar to be sealed when the dipper is in the dewar minimizing leakage.

FIG. 14. Above are pictures and schematics of the copper baffles used as radiation shields.

FIG. 15. 50KF flange with vacuum tube adapter used to seal the dipper to a liquid helium dewar.

23

Sample Stage

The sample stage should be in good thermal contact with the surrounding liquid helium

and easily accessible for sample placement and wiring. In order to exchange samples a

vacuum grade demountable seal must connect the sample can to the main body. There are

many options available for demountable cryogenic seals: Conflat flanges, Indium corner and

face joints, Helicoflex gaskets, Kapton and Mylar gaskets, and conical seals. Flange seals

and gaskets are simple and effective, but too large to fit in the mouth of the helium dewar.

Indium seals are often used in cryogenic applications, but these seals need to be replaced

frequently, are expensive, and are environmentally toxic. This design uses a simple conical

seal consisting of a short 304L stainless steel cone with a 5 to 15 degree incline and holes

that allow wiring to reach the sample stage and a 304L stainless steel can which connects to

the can with vacuum grease (Kurt Lesker) (Figure 16 and Figure 17). The cone is welded to

the bottom of the long stainless steel tube with a vacuum tight weld. The cone and can seal

when the dipper pumped down to vacuum using the pressure difference to hold a vacuum

tight seal.

The sample stage is made of a copper rod and a copper paddle (Figure 18). The stage

is connected to the vacuum can by treading the copper rod and allowing the sample stage

to screw into the cone. This allows sample paddles of different configurations to be inter-

changed. The similar thermal contraction of stainless steel and copper ensures that the

sample paddle remains connected even at low temperature. Connecting the stage to the

upper part of the dipper simplifies wiring and minimizes the potential for breakage.

A sample chip holder was designed in order to connect the sample to the paddle and to

the electronics. While, there are sample chips available for purchase (Montana Instruments)

these are often expensive and cannot be customized. To fabricate the chip holder EAGLE

(a electronic circuit board design program) was used to fabricate a PCB board to which

a microD connector could be soldered, a sample could be placed, and connections made

between the sample and small connector pads (Figure). The PCB is attached to the paddle

using 0-80 screws. To create thermal contact between the sample chip and the sample paddle

the center of the PCB board was removed so that the sample was placed directly in contact

with the copper paddle.

Finally, the sample stage must be in thermal contact with the liquid Helium outside the

24

bottom stainless steel can. The stainless steel cone and can have a low thermal conductivity

and thus do not provide good thermal contact. Furthermore, the vacuum surrounding the

sample stage prevents conduction through gas particles. To ensure that the sample stage

and thus the sample reaches 4.2K a copper wire is threaded from outside through the bottom

of the vacuum can then wrapped around the rod of the sample stage. The hole is then sealed

by brazing. This design allows the sample to be in contact with the surrounding helium and

in vacuum.

Multiple designs for the bottom can and sample stage were considered. Attaching the

sample stage to the bottom can with the use of a copper cold finger allowed for good thermal

contact with the liquid helium at the bottom of the dewar, but the wiring and the sample

would be separated until sealed with the vacuum making electrical connections to the sample

difficult. Attempting to have an upper stage connecting to a lower cold finger would require

extremely precise machining and careful calculation of thermal expansion. Copper is too

malleable and difficult to machine to be used to construct a thermally conductive cone and

can. The final design provides a simple way to connect the sample stage to the liquid helium

and remained anchored to the upper can and electronics.

25

FIG. 16. The figure shows the cone portion of the conical seal. The cone has a threaded hole to

hold the sample stage and four holes to thread the wiring.

26

FIG. 17. The figure shows the can portion of the conical seal consisting of a stainless steal can

with a bevel at the top matching the angle of the cone seal.

27

FIG. 18. The figure shows the schematic and pictures of the copper sample stage.

28

Wiring

To conduct experiments wiring must run from the room temperature enclosure to the

sample at 4.2K. The material, diameter, and number of wires should be chosen to reduce

the heat conduction along the wires. A material with a large thermal resistivity but a small

electrical resistivity at a temperature range from 300K to 4.2K is ideal. Typical copper

wiring, while having a low electrical resistivity, has a high thermal conductivity so is a poor

option. Thin Manganin and Phosphor Bronze wires have low electrical resistivity and high

thermal conductivity (Table IV).

TABLE IV. The table shows the thermal conductivity and electrical resistivity of common wire

material and 300K and 4K.

Material AWG Thermal Conductivity (W/m ∗K) Resistivity (Ω/m)

300K 4K 300K 4K

Copper (C110) 30 400 300 0.32 0.003

Copper (C110) 34 400 300 0.81 0.0076

Phosphor Bronze 32 48 1.6 4 3.3

Phosphor Bronze 36 48 1.6 10 8.6

Manganin 30 22 0.5 9.7 8.6

Manganin 36 22 6 39 35

Quad-twist 36 Phosphor Bronze wire was purchased from Lakeshore electronics and sol-

dered to 14 of the coax connections. 25 C Coaxial Cable from Lakeshore electronics were

soldered to 4 coax hookups, Thermal couple wire for Lakeshore to two, and plain copper

wire to 4 for a magnet and heater. Due to the thermal contraction of the wires at low tem-

peratures, all wiring needs to have extra length, slack, and large loops to prevent the wires

for breaking while cooling. The wiring should be long to the radiation shield and coiled up

at low temperature region to take up less space. Additionally, using twisted pairs or wire

loops can simplify wiring and reduce electronic noise. To further reduce thermal conduction,

all wires should be heat sunk by wrapping them around copper shielding and the sample

stage (Table V). To ensure accurate measurements wires should be heat sunk adjacent to

29

any sensors.

TABLE V. The table show recommended heat sinking for common cryogenic wiring.

Heat sinking length (mm)

Material 0.21mm2 0.032mm2 0.031mm2 0.005mm2

(24 AWG) (32 AWG) (36 AWG) (40 AWG)

Copper 688 233 138 80

Phosphor Bronze 38 13 7 4

Maganin 20 7 4 2

Thermal conduction through the electrons traveling down the wires is unavoidable. How-

ever, allowing each connection to be turned off reduces the heat conduction through electrons

and minimizes another source of heat.

CONCLUSION

As an alternative to larger commercially available cryostats, we have designed an effective

helium 4 dipper which can cool samples to 4.2K by inserting the cryostat directly into the

dipper. In contrast to many dipper designs this dipper keeps the sample in vacuum allowing

the sample to be heated and thermoelectric measurements to be made. The design concerns

described in this paper can be applied to any cryostat design and modified for a variety of

experiments.

30

APPENDIX A: DESIGN DRAWINGS

Appendix A provides the Solidworks Drawings for each of the machined components of

the helium 4 dipper. All units are given in inches.

31

50.

00 7

0.0

7.0

0 4.7

6 1.

25

1.

00

Cop

per B

affe

ls

Con

e/C

onic

al

Seal

Can

Sam

ple

Stag

e

Top

Elec

troni

cs

Box

4 W

ay 2

5 KF

Fl

ange

Dip

per B

ody

AA

BB

22

11

Heliu

m 4

Dip

per

DO

NO

T SC

ALE

DRA

WIN

G

Cry

oSH

EET 1

OF

1

Kirst

en B

lagg

UNLE

SS O

THER

WIS

E SP

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IED

:

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LE: 1

:12

WEI

GHT

:

REV

DW

G.

NO

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ASIZE

TITLE

:

NA

ME

DRA

WN

FIN

ISH

MA

TERI

AL

304L

Sta

inle

ss S

teel

, Alu

min

um, C

oope

r

INTE

RPRE

T G

EOM

ETRI

CTO

LERA

NC

ING

PER

:

DIM

ENSI

ON

S A

RE IN

INC

HES

32

8.5

0

5.7

0

.5

5 .2

7

1.1

0

1.1

0

.60

5.7

0

2.2

0

1.

01

Epox

ed

to 2

5 KF

fla

nge

Coa

x ca

ble

hook

upSwitc

h

Gro

und

ing

coax

hoo

kup

AA

BB

22

11 Top

Elec

troni

cs

Box

DO

NO

T SC

ALE

DRA

WIN

G

Brea

kout

Box SH

EET 1

OF

1

Kirst

en B

lagg

UNLE

SS O

THER

WIS

E SP

ECIF

IED

:

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LE: 1

:4W

EIG

HT:

REV

DW

G.

NO

.

ASIZE

TITLE

:

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ME

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WN

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ISH

MA

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AL

Alu

min

um

INTE

RPRE

T G

EOM

ETRI

CTO

LERA

NC

ING

PER

:

DIM

ENSI

ON

S A

RE IN

INC

HES

33

.1

5

.80

.0

6

.50

Braz

ed u

sing

Stay

-Silv

45

and

Sta

y-sil

v W

hite

Bra

zing

Flux

to 0

.06

dia

met

er

stai

nles

s ste

el tu

be

Hole

s to

allo

w

wiri

ng to

reac

h th

e bo

ttom

sa

mpl

e

.05

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0

.0

6

AA

BB

22

11 Cop

per B

affe

l/ Ra

diat

ion

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ld

DO

NO

T SC

ALE

DRA

WIN

G

CuB

affle

SHEE

T 1 O

F 1

Kirst

en B

lagg

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LE: 2

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per

INTE

RPRE

T G

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ETRI

CTO

LERA

NC

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:

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ON

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RE IN

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34

.1

5

.2

5

.8

615

1.

00

1.

11

.30

1.

00

.8

615

.75

.875

.50

.8

750

.7

5

Wel

ded

to lo

ng

stai

nles

s ste

el tu

be

bod

y

Thre

aded

hol

e fo

r cop

per

sam

ple

stag

e

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s for

w

iring

to

feed

thro

ugh

8 d

egre

e an

gle

man

ufat

ured

w

ith th

e us

e on

a

sin b

ar a

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forc

e ga

uge

Inne

r and

out

er

dia

met

er

mat

ch th

e st

ainl

ess s

teel

tu

be b

ody

for

ease

of

wel

din

g

AA

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22

11 Stai

nles

s Ste

el

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e

DO

NO

T SC

ALE

DRA

WIN

G

SSC

one SH

EET 1

OF

1

Kirst

en B

lagg

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SS O

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:

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LE: 1

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TITLE

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AL

304L

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ss S

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INTE

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T G

EOM

ETRI

CTO

LERA

NC

ING

PER

:

DIM

ENSI

ON

S A

RE IN

INC

HES

35

1.

25

1.

13

1.

06

Wel

ded

cap

to

the

botto

m

of th

e tu

be

1.

06

1.

25

1.

13

.70 4.0

0

.75

.70

1.

25

1.

13

Tube

Cap

Ass

embl

y

AA

BB

22

11 Stai

nles

s Ste

el

Can

DO

NO

T SC

ALE

DRA

WIN

G

SSC

anSH

EET 1

OF

1

Kirst

en B

lagg

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THER

WIS

E SP

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LE: 1

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ASIZE

TITLE

:

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ME

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WN

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ISH

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AL

304L

Sta

inle

ss S

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INTE

RPRE

T G

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ETRI

CTO

LERA

NC

ING

PER

:

DIM

ENSI

ON

S A

RE IN

INC

HES

36

1.

00

.2

5

3.0

0

.10

Top

tread

ed to

be

scre

wed

into

stai

nles

s st

eel c

one

Plac

men

t of

sam

ple/

sam

ple

chip

car

rier c

an b

e on

top

or o

n bo

ttom

AA

BB

22

11 Sam

ple

Stag

e

DO

NO

T SC

ALE

DRA

WIN

G

CuS

tage

SHEE

T 1 O

F 1

Kirst

en B

lagg

UNLE

SS O

THER

WIS

E SP

ECIF

IED

:

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LE: 1

:1W

EIG

HT:

REV

DW

G.

NO

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ASIZE

TITLE

:

NA

ME

DRA

WN

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ISH

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AL

Cop

per

INTE

RPRE

T G

EOM

ETRI

CTO

LERA

NC

ING

PER

:

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ENSI

ON

S A

RE IN

INC

HES

(1) K

F-25

4-w

ay c

ross

(2) 1

9-pi

n he

rmet

ical

ly

seal

ed K

F-25

circ

ular

con

nect

or (4

) Ele

ctro

nics

Box

(5

) KF-

25 h

inge

cla

mp

(6) K

F-40

hin

ge c

lam

p (7

) K

F-50

hin

ge c

lam

p (8

) KF-

40 to

KF-

25 c

onic

al

redu

cer

(9) K

F-50

to K

F-25

con

ical

redu

cer (

10)

KF-

50 c

ap (1

1) K

F-25

cap

Wire

Sea

l

15

11

5

5

9

7

10

82

6

4

15

11

5

5

9

7

10

82

6

4

1

5

115

5

9

7

108

2

6

4

1

5

115

5

9

7

108

2

6

4

15

11

5

5

9

7

10

82

6

4

15

11

5

5

9

7

10

82

6

4

15

11

5

5

9

7

10

82

6

4

15

11

5

5

9

7

10

82

6

4

37

.062

.920

.440

.394

.046

1.130

1.500

.118 .394

TRUE R.050

.070

.150

.920

.240

.440

.051

A A

B B

2

2

1

1

WEIGHT:

SW988C

PROPRIETARY AND CONFIDENTIALTHE INFORMATION CONTAINED IN THIS DRAWING IS THE SOLE PROPERTY OF <COMPANY NAME >. ANY REPRODUCTION IN PART OR AS A WHOLE WITHOUT THE WRITTEN PERMISSION OF <COMPANY NAME> IS PROHIBITED.

COMMENTS:

SHEET 1 OF 1

Q.A.

MFG APPR.

ENG APPR.

CHECKED

DRAWN

DATENAMEDIMENSIONS ARE IN INCHESTOLERANCES:FRACTIONALANGULAR: MACH BEND TWO PLACE DECIMAL THREE PLACE DECIMAL

NEXT ASSY USED ON

APPLICATION DO NOT SCALE DRAWING

FINISH

MATERIAL

REV.

ADWG. NO.SIZE

SCALE:2:1

1.00

.25

3.00

.10

Top treaded to be screwed into stainless steel cone

Placment of sample/sample chip carrier can be on top or on bottom

A A

B B

2

2

1

1

Sample Stage

DO NOT SCALE DRAWING

CuStageSHEET 1 OF 1

Kirsten Blagg

UNLESS OTHERWISE SPECIFIED:

SCALE: 1:1 WEIGHT:

REVDWG. NO.

ASIZE

TITLE:

NAME

DRAWN

FINISH

MATERIALCopper

INTERPRET GEOMETRICTOLERANCING PER:

DIMENSIONS ARE IN INCHES

PCB board designed in

EAGLE

Cut out for sample

placement

38

APPENDIX B: PURCHASE LIST

Below lists the parts and materials purchased for the manufacturing of the helium 4

dipper described in this manual. Most parts can be purchased from various distributors.

Extra raw material was purchased in order the ease machining and allow for practice parts

and welding.

39

TABLE VI.

Part Quantity Supplier Cost

Stainless Steel Tube, 60” length, 1” OD, 0.88” ID 1 Kurt Lesker 124.20

Triclover Flange, 3” 1 Brewer’s Hardware/Amazon 36.25

Stainless Steel Cylinder, 1-3/8” Dia, 0.5’ Length 1 McMaster Carr 21.13

Stainless Steel Tube, 1-1/4” OD, 10.084 ID, 0.5’ length 1 McMaster Carr 19.34

Copper Cylinder, 1” Dia, 1’ Length 1 Scrap Material NA

Thin Stainless Steel Tube, 60” length, 0.06” OD 1 Kurt Lesker 68.00

Enclosure, 8.7” x 5.7” x 2.2” 1 Mouser Electronics 74.84

Four Way Cross, 25 KF Flange 1 Ebay 50.00

25 KF Flange 2 Kurt Lesker 20.52

BNC Hookups 30 Newark 73.50

Switches 30 Newark 69.30

Thermocouple Wire 3 m Lakeshore Electronics 81.00

Coax Cable 25 ft Lakeshore Electronics 173.00

Phosphor Bronze Wire 50 ft Lakeshore Electronics 300.00

Vacuum Grease 1 Amazon 6.55

Torr Seal 1 Kurt Lesker 63.75

Brazing Supplies 1 Amazon 29.16

PCB 1 Ebay 2.99

62GB-12E14-19SN Circular Connector 1 Newark 33.22

Flat Ribbon Cable Black 20 Conductors 1 Digikey 37.78

Reducer conical KF-25 to KF-50 1 Ideal Vacuum 48.00

Hinge Clamp KF-50 1 Ideal Vacuum 10.95

Centering Ring KF-50 1 Ideal Vacuum 10.85

KF-25 Cap 1 McMaster Carr 10.53

KF-25 Ring 4 McMaster Carr 33.60

KF-25 Hinge Clamp 1 McMaster Carr 33.75

40

Thank you to Mike Manz and Randy Bachman in the Colorado School of Mines Physics

Machine Shop for their invaluable expertise on machining and welding all of the components

of this assembly.

Thank you to Jonathan Watson and Devon Gonzales in the Colorado School of Mines

Brazing and Welding Lab for help with the copper to stainless steel brazing.

[1] E. T. Swartz, Review of Scientific Instruments 57, 2848 (1986).

[2] A. M. Putnam, D. A. Geller, and V. Alexis, Physica B: Condensed Matter 194-96, 57 (1994).

[3] F. Pobell, Matter and Methods at Low Temperatures, 3rd ed. (Springer, 2007).

[4] J. W. Rohlf, Modern Physics from a to Z0 (Wiley, http://hyperphysics.phy-

astr.gsu.edu/hbase/thermo/heatrf.html, 1994) an optional note.

[5] “The great soviet encylopedia,” (The name of the publisher) 3rd ed.

[6] P. Duthil, arXiv preprint arXiv:1501.07100 (2015).

[7] C. Y. Ho, R. W. Powell, and P. E. Liley, Journal of Physical and Chemical Reference Data 1,

279 (1972).

[8] G. A. Slack, R. A. Tanzilli, R. Pohl, and J. Vandersande, Journal of Physics and Chemistry

of Solids 48, 641 (1987).