Advances in Cryogenic Engineering pp 489-499 | Cite as

# Techniques for Reducing Radiation Heat Transfer between 77 and 4.2 K

## Abstract

The present-day applied superconductivity is a liquid-helium-based technology. The efficiency of a cryogenic or superconducting device is determined by its rate of consumption of the cryogen. Liquid helium has an extremely small heat of vaporization; thus, its storage almost always requires a state-of-the-art insulating method. Radiation heat leak becomes significantly more important as larger superconducting devices are built, such as huge high-energy physics analysis magnets, fusion reactor systems, and energy storage facilities. For large superconducting magnets,[^{1–3}] it is common practice to surround the liquid helium vessel with a nitrogen shield (at 77 K) and wrap multilayer insulation around both the liquid helium vessel and the nitrogen shield in an attempt to further reduce the heat leaks (Fig. 1a). Multilayer insulation is inexpensive and generally effective; yet, it is a rather difficult material to apply because its performance depends on a few hard-to-control parameters such as the layer density, the compressive loading, and the lateral heat transfer effect. The effective insulation capability obtained in practice is at least a factor of 2 worse than carefully measured laboratory values (or those claimed by manufacturers). Careless and/or inexperienced application can easily generate heat leak values a few times higher than the predicted value, especially when dealing with peculiarly shaped cryostats.

## Keywords

Liquid Helium Heat Leak Residual Resistivity Ratio Multilayer Insulation Aluminum Tape## Abbreviations

## Notation

*A*= cold surface area

*c*= velocity of light

*C*_{s}= liquid specific heat at saturation

*dm*_{t}/*dt*= rate of change of mass flow rate

*dp*/*dt*= rate of change of atmospheric pressure

*E*_{g}= energy gap (threshold)

- ER
= evaporation rate owing to change in atmospheric pressure

*h*= Planck’s constant

*h*_{v}= heat of vaporization

*H*_{c}= critical field of a superconductor

*k*= Boltzmann’s constant

*I*= (

*C*_{ s }/*V*_{ L }*h*_{ v }), (*∂T/∂p*)_{ sat }*k*= thermal conductivity

*k*ρ= thermal conductivity x mass density

*P*= barometric pressure

- \(\dot Q\)
= heat transfer rate

- (RRR)
_{T} = residual resistivity ratio, i.e., ratio of electrical resistivity of a material at room temperature,

*ρ*_{300 K}, to electrical resistivity of a material at temperature*T*,*ρ*_{ T }.*t*= time

*T*= temperature

*T*_{1}= 4.2 K (liquid helium temperature)

*T*_{2}= 77.4 K (liquid nitrogen temperature)

*T*_{c}= critical temperature of a superconductor

*V*= container volume (liters)

*V*_{L}= volume of liquid involved

## Greek symbols

*ε*_{1}= surface emissivity of the cold surface

*ε*_{2}= surface emissivity of the hot surface

*λ*= frequency

*v*= wavelength

*ρ*= electrical resistivity

- (
*∂T/∂ρ*)_{sat} = slope of the temperature vapor curves

*σ*= Stefan-Boltzmann constant

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## References

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