introduction to cryostat design · 2018-11-15 · introduction to cryostat design j. g. weisend ii...

Introduction to Cryostat Design

J. G. Weisend IIJuly 2014

7/7/14 Introduction to Cryostat Design J. G. Weisend II ICEC 2014 2

Course Goals

Provide an introduction to cryostat design

Examine cryostats as integrated systems rather then a group of subsystems

Provide background information and useful data

Provide pointers to resources for more detail

Show examples


Syllabus

Introduction

Cryostat Requirements

Materials Selection

Thermal Isolation and Heat Leak

Structures

Safety

Instrumentation

Seals, Feed throughs and Bayonets

Transfer Lines

Examples

References and Resources


Introduction

What is a cryostat?

A device or system for maintaining objects at cryogenic temperatures.

Cryostats whose principal function is to store cryogenic fluids are frequently called Dewars.

Named after the inventor of the vacuum flask and the first person to liquefy hydrogen


Introduction

Cryostats are one of the technical building blocks of cryogenics

There are many different types of cryostats with differing requirements

The basic principles of cryostat design remain the sameBefore we can do anything else we have to define our requirements


E158 LH2 Target Cryostat



Maximum allowable heat leak at various temperature levels

This may be driven by the number of cryostats to be built as well as by the impact of large dynamic heat loads (SCRF or target cryostats)

Alignment and vibration requirementsImpact of thermal cyclesNeed to adjust alignment when cold or under vacuum?Alignment tolerances can be quite tight (TESLA : +/‐ 0.5 mm for cavities and +/‐ 0.3 mm for SC magnets)



Number of feed throughs for power, instrumentation, cryogenic fluid flows, external manipulatorsSafety requirements (relief valves/burst discs)

Design safety in from the start. Not as an add on

Size and weight

Particularly important in space systemsInstrumentation requiredDifference between prototype and mass production



Ease of access to cryostat components

Existing code requirements (e.g. TUV or ASME)

Need, if any, for optical windows

Presence of ionizing radiation

Expected cryostat life time

Will this be a one of a kind device or something to be mass produced?



Schedule and Cost

This should be considered from the beginning

All Design is Compromise


Material Selection

Material properties change significantly with temperature. These changes must be allowed for in the design.

Many materials are unsuitable for cryogenic use.

Material selection must always be done carefully. Testing may be required.


Material Selection

Some suitable materials for cryogenic use include:

• Austenitic stainless steels e.g. 304, 304L, 316, 321• Aluminum alloys e.g. 6061, 6063, 1100• Copper e.g. OFHC, ETP and phosphorous deoxidized• Brass• Fiber reinforced plastics such as G –10 and G –11• Niobium & Titanium (frequently used in superconducting RF

systems)

• Invar (Ni /Fe alloy) useful in making washers due to its lowercoefficient of expansion

• Indium (used as an O ring material)• Kapton and Mylar (used in Multilayer Insulation and as electrical

insulation• Quartz (used in windows)


Material Selection

Unsuitable materials include:

• Martensitic stainless steels Undergoe ductile to brittletransition when cooled down.

• Cast Iron – also becomes brittle• Carbon steels – also becomes brittle. Sometimes used in

300 K vacuum vessels but care must be taken that breaks in cryogenic lines do not cause the vacuum vessels to cool down and fail.

• Rubber, Teflon and most plastics


Thermal Expansivity

Large amounts of contraction can occur when materials are cooled to cryogenic temperatures.

Points to consider:Impact on alignmentDevelopment of interferences or gaps due to dissimilar materialsIncreased strain and possible failureImpact on wiringMost contraction occurs above 77 K


Thermal Expansivity

=1/L (L/T)Results from anharmonic component in the potential of the lattice vibration


Thermal Expansivity

goes to 0 at 0 slope as T approaches 0 K

is T independent at higher temperatures

For practical work the integral thermal contraction is more useful

7/7/14Introduction to Cryostat Design

J. G. Weisend II ICEC 2014 17

Integral ThermalContraction

Material L / L ( 300 – 100 ) L / L ( 100 – 4 )

Stainless Steel 296 x 10 -5 35 x 10 –5

Copper 326 x 10 -5 44 x 10 -5

Aluminum 415 x 10 -5 47 x 10 -5

Iron 198 x 10 -5 18 x 10 -5

Invar 40 x 10 -5 -Brass 340 x 10 –5 57 x 10 -5

Epoxy/ Fiberglass 279 x 10 –5 47 x 10 -5

Titanium 134 x 10 -5 17 x 10 -5


Integral ThermalContraction

Roughly speaking

Metals – 0.5% or lessPolymers – 1.5 – 3%Some amorphous materials have 0 or even negative thermal contraction


Heat Capacity orSpecific Heat of SolidsC = dU/dT or Q/mT

In general, at cryogenic temperatures, C decreases rapidly with decreasing temperature.

This has 2 important effects:

Systems cool down faster as they get colderAt cryogenic temperatures, small heat leaks may cause large temperature rises


Specific Heat of Solids

Where is the heat stored ?

Lattice vibrationsElectrons (metals)

The explanation of the temperature dependence of the specific heat of solids was an early victory for quantum mechanics


Lattice Contribution

Dulong Petit Law

Energy stored in a 3D oscillator = 3NkT = 3RTSpecific heat = 3R = constant

Generally OK for T= 300 K or higherDoesn’t take into account quantum mechanics


Einstein & Debye Theories

Einstein explains that atoms may only vibrate at quantized amplitudes. Thus:

This results in a temperature dependent specific heatDebye theory accounts for the fact that atoms in a solid aren’t independent & only certain frequencies are possible

hnU 2/1


Debye Theory

The Debye theory gives the lattice specific heat of solids as:

As T ~ 300 K C~ 3R (Dulong Petit)At T< 10 C varies as T 3

dxe

xeTRCx

x

x

max

02

43

19


Debye Temperatures


Impact of Electronsin Metals on Specific Heat

Thermal energy is also stored in the free electrons in the metalQuantum theory shows that electrons in a metal can only have certain well defined energiesOnly a small fraction of the total electrons can be excited to higher states & participate in the specific heatIt can be shown that Ce = T


Specific Heat of Solids

The total specific heat of metals at low temperatures may be written:

C = AT3 +BT ‐ the contribution of the electrons is only important at < 4 K

Paramagnetic materials and other special materials have anomalous specific heats ‐always double check

7/7/14 Introduction to Cryostat Design J. G. Weisend II ICEC 2014

27

From Cryogenic Engineering – T. Flynn (1997)

Specific Heat of Common Metals


Thermal Conductivity

Q = ‐K(T) A(x) dt/dxK Varies significantly with temperatureTemperature dependence must be considered when calculating heat transfer rates


Thermal Conductivity of Metals

Energy is transferred both by lattice vibrations (phonons) and conduction electrons

In “reasonably pure”metals the contribution of the conduction electrons dominates

There are 2 scattering mechanisms for the conduction electrons:

Scattering off impurities (Wo = T)Scattering off phonons (Wi = T2)

The total electronic resistivity has the form :

We = T2 +T


Thermal Conductivity of Metals Due to Electrons

From Low Temperature Solid State Physics –Rosenburg

The total electronic resistivity has the form : We = T2 +T K~ 1/We


Heat Conduction by Lattice Vibrations in Metals

Another mechanism for heat transfer in metals are lattice vibrations or phonons

The main resistance to this type of heat transfer is scattering of phonons off conduction electrons

This resistance is given by W = A/T2

Phonon heat transfer in metals is generally neglected


32

From Lakeshore Cryotronics


Thermal Conductivityof Non Metals

Insulators: conduction heat transfer is completely caused by lattice vibrations (phonons)

Semiconductors: conduction heat transfer is a mixture of phonon and electronic heat transfer


Scattering Mechanisms in Phonon Heat Transfer in Crystalline MaterialsPhonon/Phonon scattering (umklapp)

Wu~ATn Exp(‐/gT)Boundary scattering

WB~1/T3 at very low temperaturesDefect scattering

WD~AT3/2

Dislocation scattering

Wdis~A/T2


Schematic Thermal Conductivity in Dielectric Crystals

From Low Temperature Solid State Physics –Rosenburg


Thermal Conductivity of Amorphous Materials

Mechanism is lattice vibrations

Thermal conductivity is quite small (lack of regular structure)

Thermal conductivity is proportional to specific heat and thus decreases with temperature


Thermal ConductivityIntegrals

The strong temperature dependence of K makes heat transfer calculations difficult

The solution is frequently to use thermal conductivity integrals

The heat conduction equation is written as:

Q = – G ( 2 – 1 )


Thermal ConductivityIntegrals

G is the geometry factor

is the thermal conductivity integral

2

1

1x

x xAdx

G

dTTKiT

i 0


Thermal ConductivityIntegralsAdvantages:

SimpleOnly end point temperatures are important. The actual temperature distribution is not. This is quite useful for heat leak calculations


From Handbook of Cryogenic

Engineering, J. Weisend II (Ed)


From Lakeshore Cryotronics


Electrical Resistivity

Ohm’s Law V=IR

R=L/A where is the electrical resistivityConduction electrons carry the current & there are 2 scattering mechanisms

Scattering of electrons off phononsScattering of electrons off impurities or defects (e.g. dislocations)


Electrical Resistivity of Metals

For T ~ phonon scattering dominates

is proportional to TFor T<< impurity scattering dominates

is constantBetween these two regions (T~ /3)

is proportional to T5 for metalsRRR = (300 K)/ (4.2K) an indication of metal purity


Electrical Resistivity of Copper From Handbook of Materials for Superconducting Machinery (1974)


Electrical Resistivity of Other Materials

Amorphous materials & semiconductors have very different resistivity characteristics than metals

The resistivity of semiconductors is very non linear & typically increaseswith decreasing T due to fewer electrons in the conduction band

Superconductivity – another course


Material Strength

Tends to increase at low temperatures (as long as there is no ductile to brittle transition)300 K values are typically used for conservative design. Remember all systems start out at 300 K & may unexpectedly return to 300 K.Always look up values or test materials of interest

7/7/14Introduction to Cryostat Design

J. G. Weisend II ICEC 2014 47

Typical Properties of 304 Stainless SteelFrom Cryogenic Materials Data Handbook (Revised)Schwartzberg et al ( 1970)

Typical Properties of 304 Stainless SteelFrom Cryogenic Materials Data Handbook (Revised)Schwartzberg et al ( 1970)

Sources of Data for the Cryogenic Properties of Material


“A Reference Guide for Cryogenic Properties of Materials”, Weisend, Flynn, Thompson; SLAC‐TN‐03‐023

Cryogenic Materials Data Handbook: Durham et al.

MetalPak: computer code produced by CryoData

http://www.htess.com/software.htm

CryoComp: computer Code produced by Eckels Engineering

http://www.eckelsengineering.com/


49

Thermal Insulation

This is key to proper cryostat design

Thou shalt keep cold things cold

The effort and cost expended on this problem are driven by cryostat requirements:

Dynamic vs. static heat loadsNumber of cryostats Operational lifetime & ability to refill cryostats (e.g. space systems)

Lowest possible static heat leak isn’t always the best answer

It is thermodynamically best to intercept heat leaks at the warmest temperature practical


Thermal Isolation

Cryostat static heat leak results from convection, radiation and conduction heat transfer

Convection heat transfer is stopped by vacuum isolation spaces

Standard Dewar design10‐6 torr or better is best for isolation vacuumVacuum pressure is frequently much better than due to cryopumping of cold surface


Superconducting RF Test Cryostat

Contains 3 isolation vacuum spaces


Radiation Heat Transfer

A major source of cryostat heat leak“Grey body” approximation

TT LHq 44

2


One way to reduce heat leak is to select low emissivity materials

Be careful of oxidation & the impact of real vs. ideal materials

(From Helium Cryogenics – S. W. Van Sciver)


Radiation Heat Transfer

Use Multilayer Insulation (MLI) or “superinsulation”inside the vacuum space to reduce heat leak

TT LHNq 44

21


Multilayer Insulation

Used in almost all cryostatsConsists of highly reflective thin sheets with poor thermal contact between sheetsDon’t pack MLI too tightly. Optimal value is ~ 20 layers / inchGreat care must be taken with seams, penetrations and ends.Problems with these can dominate the heat leak


Multilayer Insulation

Examples of Proper Seams & Penetrations in MLI Systems



“SERIES‐PRODUCED HELIUM II CRYOSTATSFOR THE LHC MAGNETS: TECHNICAL CHOICES,INDUSTRIALISATION, COSTS”A. Poncet and V. ParmaAdv. Cryo. Engr. Vol 53

“”SERIES‐PRODUCED HELIUM II CRYOSTATSFOR THE LHC MAGNETS: TECHNICAL CHOICES,

INDUSTRIALISATION, COSTS”A. Poncet and V. ParmaAdv. Cryo. Engr. Vol 53


Conduction Heat Leak

Reduce conduction heat leak by:

Using lower conductivity materials for supports e.g. G‐10Increasing length and decreasing cross sectional area of connections between room temperature & cryogenic temperaturesReducing as much as possible amount of instrumentation wiring Using heat intercepts at intermediate temperatures (less heat leak to coldest temperatures)


Recall Use of Conductivity Integrals

Q = – G ( 2 – 1 )


Structural Supports

Solution is highly dependent on cryostat requirements

Choose materials carefully

Acceptable for cryogenic temperaturesLow heat leak

Don’t over constrain supports: allow for thermal contraction

Does solution meet alignment and vibration requirements?

Must alignment be changed while cold?


Structural Support Example #1ILC Cryostat

FRP support between 300 K and Cryo tempsCavity assemblies tied to 300 mm pipe backboneAll other connections to 300 K have flex line or bellows in lineMeets alignment specs

2.2 K forward

4.5 K forward

40 K forward

2 K 2-phase

cavity

2 K return

80 K return

4.5 K return

cool down/warmup

support

coupler

300 mm


Structural Support Example #2JLab 12 GeV Upgrade Cryomodule

All components are tied to space frame which rolls into vacuum vesselConnections to 300 K done via flex lines and bellowsSimilar system planned for ESS


Structural Support Example #3Simple Top Load Cryostat

Very common for test cryostats

Everything hangs from 300 K top flange

Connections made via low conductivity piping and supports

Everything “contracts up”

Useful when precise alignment not an issue


Safety Issues

Warning: This is NOT a complete safety class

Safety considerations should be designed in from the beginning. Retrofits are time consuming & expensive


Over Pressurization Hazards

A principal hazard results from the large volume expansion that occurs when a cryogenic liquid is converted to 300 K gasAs a result large pressure rises are possibleThis could easily result in material failures, and explosive bursting


Change in volume from liquid at normal boiling point to gas at 300 K and 1 atm

Substance Vgas/Vliquid

Helium 701Parahydrogen 788

Neon 1341Nitrogen 646

Argon 779CO2 762

Oxygen 797


The solution is to always install pressure relief devices

Redundant devices should be used. Typically relief valve + burst disc

The device should be set to open at or below the MAWP of the vessel

Valves should be sized for worst case scenarios

Valves should be tested & certified before installation

Ensure that there are no trapped volumes. Remember –valves may leak or be operated incorrectly and cryogenic systems may warm up unexpectedly (vacuum spaces need relief valves as well)


Relief Devices Continued

Do not put shut off valves between system and relief valves

Ask What If ? questions

introduction to cryostat design · 2018-11-15 · introduction to cryostat design j. g. weisend ii...

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