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Introduction to Cryostat Design

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Introduction to Cryostat Design 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.
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