Introduction to Cryostat Design
J. G. Weisend II July 2014 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
Introduction to Cryostat Design 7/7/14 2 J. G. Weisend II ICEC 2014 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 to Cryostat Design 7/7/14 3 J. G. Weisend II ICEC 2014 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 to Cryostat Design 7/7/14 4 J. G. Weisend II ICEC 2014 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 same Before we can do anything else we have to define our requirements
Introduction to Cryostat Design 7/7/14 5 J. G. Weisend II ICEC 2014 E158 LH2 Target Cryostat
Introduction to Cryostat Design 7/7/14 6 J. G. Weisend II ICEC 2014 Cryostat Requirements
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 requirements Impact of thermal cycles Need 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)
Introduction to Cryostat Design 7/7/14 7 J. G. Weisend II ICEC 2014 Cryostat Requirements
Number of feed throughs for power, instrumentation, cryogenic fluid flows, external manipulators Safety requirements (relief valves/burst discs) Design safety in from the start. Not as an add on
Size and weight Particularly important in space systems Instrumentation required Difference between prototype and mass production
Introduction to Cryostat Design J. G. 7/7/14 8 Weisend II ICEC 2014 Cryostat Requirements
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?
Introduction to Cryostat Design J. G. 7/7/14 9 Weisend II ICEC 2014 Cryostat Requirements
Schedule and Cost This should be considered from the beginning
All Design is Compromise
Introduction to Cryostat Design J. G. 7/7/14 10 Weisend II ICEC 2014 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.
Introduction to Cryostat Design 7/7/14 11 J. G. Weisend II ICEC 2014 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 lower coefficient of expansion • Indium (used as an O ring material) • Kapton and Mylar (used in Multilayer Insulation and as electrical insulation • Quartz (used in windows) Introduction to Cryostat Design 7/7/14 12 J. G. Weisend II ICEC 2014 Material Selection
Unsuitable materials include: • Martensitic stainless steels Undergoe ductile to brittle transition 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
Introduction to Cryostat Design 7/7/14 13 J. G. Weisend II ICEC 2014 Thermal Expansivity
Large amounts of contraction can occur when materials are cooled to cryogenic temperatures. Points to consider: Impact on alignment Development of interferences or gaps due to dissimilar materials Increased strain and possible failure Impact on wiring Most contraction occurs above 77 K
Introduction to Cryostat Design 7/7/14 14 J. G. Weisend II ICEC 2014 Thermal Expansivity
=1/L (L/T) Results from anharmonic component in the potential of the lattice vibration
Introduction to Cryostat Design 7/7/14 15 J. G. Weisend II ICEC 2014 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
Introduction to Cryostat Design J. G. 7/7/14 16 Weisend II ICEC 2014 Integral Thermal Contraction
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
Introduction to Cryostat Design 7/7/14 17 J. G. Weisend II ICEC 2014 Integral Thermal Contraction
Roughly speaking Metals –0.5% or less Polymers –1.5 –3% Some amorphous materials have 0 or even negative thermal contraction
Introduction to Cryostat Design 7/7/14 18 J. G. Weisend II ICEC 2014 Heat Capacity or Specific Heat of Solids
C = 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 colder At cryogenic temperatures, small heat leaks may cause large temperature rises
Introduction to Cryostat Design J. G. 7/7/14 19 Weisend II ICEC 2014 Specific Heat of Solids
Where is the heat stored ? Lattice vibrations Electrons (metals) The explanation of the temperature dependence of the specific heat of solids was an early victory for quantum mechanics
Introduction to Cryostat Design J. G. 7/7/14 20 Weisend II ICEC 2014 Lattice Contribution
Dulong Petit Law Energy stored in a 3D oscillator = 3NkT = 3RT Specific heat = 3R = constant Generally OK for T= 300 K or higher Doesn’t take into account quantum mechanics
Introduction to Cryostat Design 7/7/14 21 J. G. Weisend II ICEC 2014 Einstein & Debye Theories
Einstein explains that atoms may only vibrate at quantized amplitudes. Thus: U n 1/ 2h
This results in a temperature dependent specific heat Debye theory accounts for the fact that atoms in a solid aren’t independent & only certain frequencies are possible
Introduction to Cryostat Design J. G. 7/7/14 22 Weisend II ICEC 2014 Debye Theory
The Debye theory gives the lattice specific heat of solids as: 3 x T max ex x4 C 9R dx x 2 0 e 1
As T ~ 300 K C~ 3R (Dulong Petit) At T< 10 C varies as T 3
Introduction to Cryostat Design 7/7/14 23 J. G. Weisend II ICEC 2014 Debye Temperatures
Introduction to Cryostat Design 7/7/14 24 J. G. Weisend II ICEC 2014 Impact of Electrons in Metals on Specific Heat
Thermal energy is also stored in the free electrons in the metal Quantum theory shows that electrons in a metal can only have certain well defined energies Only a small fraction of the total electrons can be excited to higher states & participate in the specific heat
It can be shown that Ce = T
Introduction to Cryostat Design 7/7/14 25 J. G. Weisend II ICEC 2014 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
Introduction to Cryostat Design 7/7/14 26 J. G. Weisend II ICEC 2014 Specific Heat of Common Metals
From Cryogenic Engineering – T. Flynn (1997)
7/7/14 Introduction to Cryostat Design 27 J. G. Weisend II ICEC 2014 Thermal Conductivity
Q = ‐K(T) A(x) dt/dx K Varies significantly with temperature Temperature dependence must be considered when calculating heat transfer rates
Introduction to Cryostat Design 7/7/14 28 J. G. Weisend II ICEC 2014 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) 2 Scattering off phonons (Wi = T ) The total electronic resistivity has the form : 2 We = T + T
Introduction to Cryostat Design J. G. 7/7/14 29 Weisend II ICEC 2014 Thermal Conductivity of Metals Due to Electrons
From Low Temperature Solid State Physics –Rosenburg
2 The total electronic resistivity has the form : We = T + T K~ 1/We
Introduction to Cryostat Design J. G. 7/7/14 30 Weisend II ICEC 2014 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
Introduction to Cryostat Design 7/7/14 31 J. G. Weisend II ICEC 2014 From Lakeshore Cryotronics
Introduction to Cryostat Design 7/7/14 32 J. G. Weisend II ICEC 2014 Thermal Conductivity of 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
Introduction to Cryostat Design J. G. 7/7/14 33 Weisend II ICEC 2014 Scattering Mechanisms in Phonon Heat Transfer in Crystalline Materials
Phonon/Phonon scattering (umklapp)
n Wu~AT Exp(‐/gT) Boundary scattering
3 WB~1/T at very low temperatures Defect scattering
3/2 WD~AT Dislocation scattering
2 Wdis~A/T
Introduction to Cryostat Design 7/7/14 34 J. G. Weisend II ICEC 2014 Schematic Thermal Conductivity in Dielectric Crystals
From Low Temperature Solid State Physics –Rosenburg Introduction to Cryostat Design J. G. 7/7/14 35 Weisend II ICEC 2014 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
Introduction to Cryostat Design J. G. 7/7/14 36 Weisend II ICEC 2014 Thermal Conductivity Integrals
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 )
Introduction to Cryostat Design J. G. 7/7/14 37 Weisend II ICEC 2014 Thermal Conductivity Integrals G is the geometry factor 1 G x 2 dx A x x1
is the thermal conductivity integral
T i KT dT i 0
Introduction to Cryostat Design 7/7/14 38 J. G. Weisend II ICEC 2014 Thermal Conductivity Integrals Advantages: Simple Only end point temperatures are important. The actual temperature distribution is not. This is quite useful for heat leak calculations
Introduction to Cryostat Design J. G. 7/7/14 39 Weisend II ICEC 2014 From Handbook of Cryogenic Engineering, J. Weisend II (Ed)
Introduction to Cryostat Design 7/7/14 40 J. G. Weisend II ICEC 2014 From Lakeshore Cryotronics
Introduction to Cryostat Design 7/7/14 41 J. G. Weisend II ICEC 2014 Electrical Resistivity
Ohm’s Law V=IR
R=L/A where is the electrical resistivity Conduction electrons carry the current & there are 2 scattering mechanisms
Scattering of electrons off phonons Scattering of electrons off impurities or defects (e.g. dislocations)
Introduction to Cryostat Design 7/7/14 42 J. G. Weisend II ICEC 2014 Electrical Resistivity of Metals
For T ~ phonon scattering dominates is proportional to T For T<< impurity scattering dominates is constant Between these two regions (T~ /3) is proportional to T5 for metals RRR = (300 K)/ (4.2K) an indication of metal purity
Introduction to Cryostat Design 7/7/14 43 J. G. Weisend II ICEC 2014 Electrical Resistivity of Copper From Handbook of Materials for Superconducting Machinery (1974)
Introduction to Cryostat Design 7/7/14 44 J. G. Weisend II ICEC 2014 Electrical Resistivity of Other Materials
Amorphous materials & semiconductors have very different resistivity characteristics than metals The resistivity of semiconductors is very non linear & typically increases with decreasing T due to fewer electrons in the conduction band Superconductivity –another course
Introduction to Cryostat Design 7/7/14 45 J. G. Weisend II ICEC 2014 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
Introduction to Cryostat Design 7/7/14 46 J. G. Weisend II ICEC 2014 Typical Properties of 304 Stainless Steel From Cryogenic Materials Data Handbook (Revised) Schwartzberg et al ( 1970)
Typical Properties of 304 Stainless Steel From Cryogenic Materials Data Handbook (Revised) Schwartzberg et al ( 1970)
Introduction to Cryostat Design 7/7/14 J. G. Weisend II ICEC 2014 47 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/
Introduction to Cryostat Design J. G. 7/7/14 48 Weisend II ICEC 2014 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 loads Number 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
Introduction to Cryostat Design 7/7/14 49 J. G. Weisend II ICEC 2014 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 design 10‐6 torr or better is best for isolation vacuum Vacuum pressure is frequently much better than due to cryopumping of cold surface
Introduction to Cryostat Design 7/7/14 50 J. G. Weisend II ICEC 2014 Superconducting RF Test Cryostat
Contains 3 isolation vacuum spaces
Introduction to Cryostat Design 7/7/14 51 J. G. Weisend II ICEC 2014 Radiation Heat Transfer
A major source of cryostat heat leak “Grey body” approximation
4 4 q 2 T H T L
Introduction to Cryostat Design J. G. 7/7/14 52 Weisend II ICEC 2014 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)
Introduction to Cryostat Design 7/7/14 53 J. G. Weisend II ICEC 2014 Radiation Heat Transfer
Use Multilayer Insulation (MLI) or “superinsulation” inside the vacuum space to reduce heat leak
4 4 q N 1 2 T H T L
Introduction to Cryostat Design 7/7/14 54 J. G. Weisend II ICEC 2014 Multilayer Insulation
Used in almost all cryostats Consists of highly reflective thin sheets with poor thermal contact between sheets Don’t pack MLI too tightly. Optimal value is ~ 20 layers / inch Great care must be taken with seams, penetrations and ends. Problems with these can dominate the heat leak
Introduction to Cryostat Design 7/7/14 55 J. G. Weisend II ICEC 2014 Multilayer Insulation
Introduction to Cryostat Design 7/7/14 56 J. G. Weisend II ICEC 2014 Examples of Proper Seams & Penetrations in MLI Systems
Introduction to Cryostat Design J. G. 7/7/14 57 Weisend II ICEC 2014 “SERIES‐PRODUCED HELIUM II CRYOSTATS FOR THE LHC MAGNETS: TECHNICAL CHOICES, INDUSTRIALISATION, COSTS“”SERIES” ‐PRODUCED HELIUM II CRYOSTATS A. Poncet and V. ParmaFOR THE LHC MAGNETS: TECHNICAL CHOICES, Adv. Cryo. Engr. Vol 53 INDUSTRIALISATION, COSTS” A. Poncet and V. Parma Adv. Cryo. Engr. Vol 53
Introduction to Cryostat Design 7/7/14 58 J. G. Weisend II ICEC 2014 Conduction Heat Leak
Reduce conduction heat leak by: Using lower conductivity materials for supports e.g. G‐10 Increasing length and decreasing cross sectional area of connections between room temperature & cryogenic temperatures Reducing as much as possible amount of instrumentation wiring Using heat intercepts at intermediate temperatures (less heat leak to coldest temperatures)
Introduction to Cryostat Design 7/7/14 59 J. G. Weisend II ICEC 2014 Recall Use of Conductivity Integrals
Q =– G ( 2 – 1 )
Introduction to Cryostat Design 7/7/14 60 J. G. Weisend II ICEC 2014 Introduction to Cryostat Design 7/7/14 61 J. G. Weisend II ICEC 2014 Structural Supports
Solution is highly dependent on cryostat requirements Choose materials carefully Acceptable for cryogenic temperatures Low heat leak Don’t over constrain supports: allow for thermal contraction Does solution meet alignment and vibration requirements? Must alignment be changed while cold?
Introduction to Cryostat Design J. G. 7/7/14 62 Weisend II ICEC 2014 Structural Support Example #1 ILC Cryostat FRP support between 300 K support and Cryo temps
2.2 K forward Cavity assemblies tied to 2 K return 4.5 K forward 300 mm pipe backbone 80 K return 40 K forward All other connections to 300 mm 300 K have flex line or bellows in line 4.5 K return
cool down/ 2 K 2-phase Meets alignment specs warmup
cavity coupler
Introduction to Cryostat Design J. G. 7/7/14 63 Weisend II ICEC 2014 Structural Support Example #2 JLab 12 GeV Upgrade Cryomodule
All components are tied to space frame which rolls into vacuum vessel Connections to 300 K done via flex lines and bellows Similar system planned for ESS
Introduction to Cryostat Design 7/7/14 64 J. G. Weisend II ICEC 2014 Structural Support Example #3 Simple 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
Introduction to Cryostat Design J. G. 7/7/14 65 Weisend II ICEC 2014 Safety Issues
Warning: This is NOT a complete safety class
Safety considerations should be designed in from the beginning. Retrofits are time consuming & expensive
Introduction to Cryostat Design 7/7/14 66 J. G. Weisend II ICEC 2014 Over Pressurization Hazards
A principal hazard results from the large volume expansion that occurs when a cryogenic liquid is converted to 300 K gas As a result large pressure rises are possible This could easily result in material failures, and explosive bursting
Introduction to Cryostat Design J. G. 7/7/14 67 Weisend II ICEC 2014 Change in volume from liquid at normal boiling point to gas at 300 K and 1 atm
Substance Vgas/Vliquid
Helium 701 Parahydrogen 788 Neon 1341 Nitrogen 646 Argon 779
CO2 762 Oxygen 797
Introduction to Cryostat Design 7/7/14 68 J. G. Weisend II ICEC 2014 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)
Introduction to Cryostat Design 7/7/14 69 J. G. Weisend II ICEC 2014 Relief Devices Continued
Do not put shut off valves between system and relief valves Ask What If ? questions
Introduction to Cryostat Design 7/7/14 70 J. G. Weisend II ICEC 2014