The Rayleigh number is a figure of merit for convection. Supercold gaseous helium (i.e., helium just above its liquid-gas critical point) can be used in a structure known as a Bénard cell to study convection phenomena. This is possible because buoyancy forces dominate viscous forces in low-viscosity helium gas, leading to high Rayleigh numbers and convective behavior.
Superconducting electronics systems are being developed for use in radar and electronic warfare. Various switches and analog-to-digital and digital-to-analog converter systems are also being developed for communications applications. Superconducting systems using Rapid Single-Flux Quantum Logic are currently the leading technology under consideration for petaflop computing.
With a limited number of exceptions, liquid helium or cryocoolers using helium are required for all of the systems that are currently commercial or under development. Closed-cycle refrigeration is an alternative to current practices in which helium boil-off gas is expended. After the discovery of high-temperature superconductors, their use in tunneling junctions appeared to be very promising. It was believed that they could lead to a superconducting electronics technology that would not require helium. This may ultimately be possible, but probably not in large computer systems and other applications, such as ultrahigh-speed analog-to-digital and digital-to-analog converters that require large numbers of tunneling junctions on a single chip. The junctions in these systems must have tightly controlled parameters to meet operating margins. It is likely to be a very long time before chips using high-temperature superconductors can be made, and these complicated systems, if they are ever built, will probably require liquid-helium temperatures.