Actively cooled thermal radiation shields are a common feature of cryostats whose lowest temperature is less than 77K. These shields, which typically operate at temperatures between that of LN2 and 40K, block thermal radiation from higher temperatures from reaching lower temperature cryogenic components or fluids. Since the heat radiated from a surface scales as the fourth power of the surface’s temperature, the effect of these shields on heat leak can be quite impressive.
Consider two parallel plates: A 300K surface will deposit approximately 46 W/m2 via thermal radiation to a 4.2K surface. However, if the warm surface is at 77K, the thermal radiation heat leak to the 4.2K surface is only about 0.2 W/m2. Actively cooled thermal shields are generally enclosed in a vacuum space to eliminate convection heat transfer. Thermal radiation shields are frequently combined with multilayer insulation (see Cold Facts, Summer 2010) to further reduce radiation heat transfer.
Given their advantages, thermal radiation shields are found in almost all sophisticated helium temperature cryostats, including the Large Hadron Collider magnets at CERN; superconducting radiofrequency cryomodules at the European Spallation Source, the Spallation Neutron Source, Thomas Jefferson National Accelerator Facility and the Facility for Rare Isotope Beams; space cryogenic systems and helium storage and transport dewars.
In the case of the proposed International Linear Collider (ILC), the cryomodule design contains two nested thermal shields, one at approximately 80K and one at approximately 5K surrounding the 1.8K helium space. This multiple shield solution is driven by the large number of cryomodules (~ 2000) required for the ILC and the subsequent need to reduce the total heat leak to the 1.8K space. However, a recent paper (“Study of Thermal Radiation Shields for the ILC Cryomodule” by Ohuchi et al. in Adv. Cryo. Engr. Vol. 57A, 2010) suggests that by optimizing the design, the 5K shield may be eliminated. Another example of multiple nested thermal shields is found in dilution refrigerators (see Cold Facts, Winter 2012) which frequently have four or more actively cooled thermal shields at successively lower temperatures to shield the lowest temperature (tens of millikelvin) space.
Thermal radiation shields typically take the form of relatively thin sheets of good thermal conductors (aluminum and copper are generally used) that surround but don’t directly touch the components being shielded. Design requirements include the shields being nearly isothermal at the shield temperature and completely surrounding the lower temperature space without gaps or holes. This frequently results in complicated shapes for the shields. A challenge with these shields is that they must be able to be thermally cycled from room temperature to cryogenic temperatures without deforming, buckling or failing.
Cooling of these shields can be accomplished in a number of ways, including forced cooling from a cryogenic refrigeration plant; conduction cooling from a cryogenic liquid bath; cooling by cold gas boiled off from a liquid bath (this approach is very common in helium storage dewars) and cooling by a small cryocooler attached to the shield. Proper connection of the cold source to the shields is vital to successful shield operation. Thermal shields should have uniform temperatures and their design must include the proper material, thickness and connections between sections to result in a nearly isothermal shield.
A good introduction to thermal radiation heat transfer in cryogenics may be found in Helium Cryogenics by S.W. Van Sciver (2012). The use of thermal radiation shields in cryostats is covered in Cryogenic Systems by R. Barron (1966) and in the chapter “Cryostat Design” by G. McIntosh in The Handbook of Cryogenic Engineering (1998). “Further Improvements of the TESLA Test Facility (TTF) Cryomodule in View of the TESLA Collider” by C. Pagani et al. in Adv. Cryo. Engr. Vol. 45A, (2000) provides a description of the ILC thermal shields, including the solution to allow flexibility during thermal cycling.
Two other useful papers on thermal shield design are: “Numerical Analysis on Shields and Struts Cooling Using Cold Helium Gas in MECO Magnets” by H.M. Tang et al., Adv. Cryo. Engr. Vol. 51A, (2006) and “Heat Shields: Materials and Cost Considerations by M.L. Seely et al., Adv. Cryo. Engr. Vol. 57B, (2010). A description of a very complicated thermal shield designed for use in a fusion experiment is given in “Design and Trial Manufacturing of the Thermal Shield for JT-60SA” by K. Kamiya et al., Proc. ICEC 23 (2011).
