Liquid helium is the liquid form of the element helium, existing only at extremely low temperatures. Its unique characteristics at the edge of absolute zero are foundational to a range of modern scientific and technological advancements, making it a substance of significant interest.
Production and Properties at Extreme Cold
Producing liquid helium is an energy-intensive process. Gaseous helium, recovered from natural gas deposits, undergoes a cycle of compression, cooling, and expansion. This procedure uses the Joule-Thomson effect, where a gas cools as it expands, to eventually lower the helium’s temperature to its liquefaction point.
Helium liquefies at a low temperature of 4.2 Kelvin (K), equivalent to -269.15°C or -452.47°F. At this point, it is a colorless, odorless, and non-corrosive fluid. It also has a very low density of about 125 grams per liter, roughly one-eighth that of water. Unlike almost every other substance, helium remains liquid under standard atmospheric pressure down to absolute zero, a behavior rooted in quantum mechanics.
The Phenomenon of Superfluidity
When liquid helium is cooled further, it transforms below a temperature known as the “lambda point.” This transition occurs at 2.17 K (-271°C or -455.8°F), where the normal liquid (Helium I) becomes a superfluid (Helium II). The name “lambda point” comes from the characteristic shape of the specific heat curve at this temperature. This change is a rare instance of quantum effects becoming observable on a macroscopic scale.
The properties of Helium II are counter-intuitive and defy the behavior of ordinary fluids. Its most defining characteristic is zero viscosity, meaning it can flow without internal friction or energy loss. This allows it to move through microscopic pores that would be impermeable to normal liquids. Another manifestation is the “Rollin film” effect, where a thin film of the superfluid will creep up the inner walls of its container and flow over the edge. Helium II also exhibits very high thermal conductivity, transferring heat with high efficiency.
These behaviors are explained by the two-fluid model, which posits that Helium II is composed of two interpenetrating components. The first is a normal fluid component, and the second is a superfluid component with zero viscosity and zero entropy. The superfluid component consists of helium atoms condensed into the lowest quantum energy state, a phenomenon related to Bose-Einstein condensation. As the temperature is lowered toward absolute zero, the proportion of the superfluid component increases.
Cooling Superconducting Magnets
A primary use of liquid helium is as a refrigerant for superconducting magnets. Superconductivity is a state where certain materials exhibit zero electrical resistance when cooled below a specific temperature. This allows electromagnets made from superconducting wires to generate powerful, stable magnetic fields without overheating. Liquid helium is the only coolant cold enough to maintain these magnets at their operating temperature of 4.2 K.
This technology is central to Magnetic Resonance Imaging (MRI) and Nuclear Magnetic Resonance (NMR) spectroscopy. In an MRI machine, a superconducting magnet cooled by liquid helium creates the strong magnetic field necessary for detailed images of the human body. An MRI scanner can contain about 1,700 liters of liquid helium to keep its magnet coils in a superconducting state. Without this cooling, the resistance in the coils would generate immense heat, making the technology unfeasible.
The same principle applies to particle accelerators like the Large Hadron Collider (LHC) at CERN. The LHC uses thousands of superconducting magnets to bend and focus beams of high-energy particles. These magnets must be kept at a temperature of 1.9 K, even colder than the lambda point, requiring an extensive liquid helium cryogenic system. This application showcases the substance’s role in physics research.
Broader Scientific and Industrial Applications
Beyond its primary role in cooling superconducting magnets, liquid helium has a range of other specialized applications. Its extreme cold is necessary for cooling highly sensitive infrared detectors used in space telescopes. For instance, the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) is cooled to below 7 K by a helium cryocooler, which allows it to detect faint infrared signals from distant cosmic objects.
Liquid helium is also indispensable in cryogenics research, where scientists study the behavior of materials at temperatures approaching absolute zero. It is used to cool superconducting quantum interference devices (SQUIDs), which are extremely sensitive magnetometers capable of measuring subtle magnetic fields. This technology is applied in magnetoencephalography (MEG), a non-invasive neuroimaging technique that maps brain activity. Additionally, helium’s inertness and high thermal conductivity make it useful in manufacturing processes, such as for optical fibers and semiconductors.
Handling Safety and Global Supply Concerns
Working with liquid helium requires strict safety protocols. Direct contact with the liquid or uninsulated equipment can cause severe frostbite. A significant hazard is asphyxiation, as liquid helium has a large expansion ratio when it vaporizes. One liter of liquid can expand to about 760 liters of gas, which can displace oxygen in a poorly ventilated area and become fatal.
Compounding these handling challenges is the issue of global supply. Helium is a finite, non-renewable resource primarily extracted as a byproduct from natural gas deposits. This makes the supply chain vulnerable to disruptions from geopolitical tensions and the operational status of key processing facilities worldwide. Recent shortages have been exacerbated by shutdowns at major plants and increased demand from the healthcare and technology sectors. This scarcity poses a significant risk to the scientific and medical communities that rely on this unique substance.