A supercritical fluid is defined as a substance whose temperature and pressure exceed those of its critical point. Every pure substance has a critical point that is defined in thermodynamic space by a critical temperature and a corresponding critical pressure. For example, the critical point for helium has a critical temperature of 5.2 K and a critical pressure of 227.46 kPa, while hydrogen has a critical temperature of 32.9 K and a critical pressure of 1283.8 kPa. A substance that is above its critical pressure and temperature is said to be a supercritical fluid or in the supercritical state or region. One can also define supercritical mixtures, whose components are all above respective critical points, but here we will stay with pure substances such as helium, hydrogen or nitrogen that are significant in cryogenics.

The physical meaning of a supercritical fluid is that as long as a fluid is in the supercritical region, it will not undergo a phase transition between liquid and gas and no meniscus separating liquid and gas will be observed. Technically speaking, a substance in its supercritical state is neither a liquid nor a gas, but rather a fluid.

The technological advantage of a supercritical fluid is that it cannot turn into a two-phase mixture of liquid and gas. This eliminates the possibility of increased pressure drops, flow instabilities or possible vibrations associated with two-phase systems. As a result, many cryogenic systems are designed to operate with supercritical fluids.

A typical example of this is the use of supercritical helium operating around 50 K above its critical pressure to cool thermal radiation shields in accelerator magnet cryostats such as those in the Large Hadron Collider (LHC) or in superconducting radiofrequency cryomodules such as those used in the European XFEL machine.

Notice however, that the critical temperatures for helium and hydrogen (5.2 K and 32.9 K) are higher than required for many cryogenic applications, particularly in the area of superconductivity. Thus, cryogenic systems in many cases use helium or hydrogen flows that are below critical temperatures but above critical pressures. Such fluids are also frequently referred to as supercritical, though strictly speaking they are subcooled liquids or liquids operating above their critical pressure. While not truly supercritical, these fluids are a far way from the liquid-vapor transition line and will remain single phase as long as low temperatures and high pressures are maintained. As a result, the fluids also avoid issues associated with two-phase systems.
There are many examples of such systems. The ITER toroidal field magnets use a cable-in-conduit style conductor that is cooled by helium operating at nominally 4.6 K and 490 kPa. A similar design is used in the JT-60SA Tokamak magnets operating at 4.4 K and 500 kPa. The European Spallation Source (ESS) hydrogen moderator system circulates hydrogen at 17 K and 1500 kPa.

Another common use of single-phase fluids operating above critical pressure but below critical temperature occurs in cryogenic distribution systems or transfer lines (see Defining Cryogenics in Cold Facts, Winter 2011). Systems distributing cryogens over long distances thus avoid problems associated with two-phase flows. The end user of such a system may then use the fluid in its current single-phase state, as in the case of ITER, or expand the fluid into a two-phase mixture for local cooling, as in the case of the cryomodules for the European Spallation Source and for the Facility for Rare Isotope Beams (FRIB).

There are disadvantages to supercritical or single-phase flows. Any heat deposited into the fluid will result in a temperature rise since there are no phase transitions and thus no latent heat involved in these systems. Cryogenic distribution systems have to be designed to allow for sufficient flow and low enough heat leak so that the temperature at the end user of the system meets requirements. There is an additional challenge of operating near the critical point. Many cryogenic fluid properties change rapidly near the critical point. One must be very careful not to extrapolate fluid properties near the critical point and even well designed computer models of fluid properties such as NIST-12 are less accurate in this regime. Figure 1 (page 21)shows the thermal expansivity of oxygen as a function of temperature and pressure—both normalized by critical values. Note the rapid changes near the critical point.

Information on the properties of cryogenic fluids may be found in: The Cryogenic Fluids Data Book, P. Cook & B.A. Hands (Ed), British Cryoengineering Society (2002); Thermodynamic Properties of Cryogenic Fluids, R.T. Jacobsen et al., Springer (1997); The Handbook of Cryogenic Engineering, J.G. Weisend II (Ed), Taylor and Francis (1998) and at the NIST website http://webbook.nist.gov/chemistry/fluid/

Further details on cooling of the ITER and JT-60SA magnets may be found in “Challenges for Cryogenics at ITER,” L. Serio, Adv. Cryo. Engr., Vol. 55A (2010) and “Cryogenic Requirements for the JT-60SA Tokamak,” F. Michel et al., Adv. Cryo. Engr., Vol. 57A (2012). A description of the complicated ITER cryogenic distribution system is given in “Progress and Present Status of the ITER Cryoline System,” S. Badgujar et al. Adv. Cryo. Engr., Vol. 59A (2014).