Liquid helium (LHe) is the chemical element helium in its liquid state, achieved only at incredibly low temperatures. It possesses the lowest boiling point of all known substances, approximately 4.2 Kelvin (K), which is equivalent to about -269 degrees Celsius. This substance is the only stable cryogen cold enough to enable a wide range of unique physical phenomena and technological applications. The existence of a stable, easily handled medium so close to absolute zero is fundamental to modern science and high-tech industry.
The Role of Extreme Cold
The primary function of liquid helium is to create and maintain an environment of extreme cold, which is necessary for two major scientific principles to operate. The first is achieving superconductivity, a state where certain materials conduct electricity with zero resistance. Superconducting materials have a critical temperature (Tc) they must be cooled below to lose all electrical resistance, and for most high-field applications, this temperature requires the use of liquid helium.
Many widely used low-temperature superconductors, such as Niobium-Titanium alloys, have a critical temperature near 9 K. Liquid helium, boiling at 4.2 K, provides the necessary temperature margin to ensure the material remains superconducting even when carrying extremely high currents. Cooling the material below 2.2 K causes the liquid helium to enter its superfluid state, which possesses exceptional thermal conductivity. This allows for highly efficient heat removal, which is important for stabilizing powerful, high-energy magnets.
The second principle is maintaining ultra-low noise environments for sensitive scientific instruments. At ambient temperatures, the thermal motion of atoms generates background noise that can obscure faint measurements. Cooling detectors to near absolute zero with liquid helium effectively eliminates this thermal noise. This reduction in random energy allows instruments to achieve maximum sensitivity, enabling the detection of extremely weak signals in laboratory experiments and astronomical observations.
Medical Imaging and Diagnosis
Liquid helium’s most recognizable application is in the medical field, where it is indispensable for operating Magnetic Resonance Imaging (MRI) scanners. These machines rely on powerful, stable magnetic fields to align the protons in the body’s water molecules. To generate the necessary field strengths, which typically range from 1.5 Tesla to 7 Tesla, the magnet coils must be made from superconducting wire.
The liquid helium is continuously circulated around the massive superconducting coils to maintain their temperature at 4.2 K. This ultra-low temperature ensures the wire remains in its zero-resistance state, allowing a persistent, high electrical current to flow. If the helium level drops too low, the magnet can “quench,” causing the superconductor to rapidly heat up and lose superconductivity, which releases a large volume of helium gas and temporarily disables the machine.
A closely related application is Nuclear Magnetic Resonance (NMR) spectroscopy, used extensively in chemistry and pharmaceutical research to determine the molecular structure of compounds. Like MRI, high-resolution NMR spectrometers also employ powerful superconducting magnets that must be immersed in liquid helium for stable operation. In both applications, the cooling is aimed solely at the magnet components.
The high magnetic field strength is directly correlated with the sensitivity and resolution of the resulting images or spectra. By enabling these high-field superconducting magnets, liquid helium facilitates the detailed diagnostic imaging and structural analysis foundational to modern medicine and drug discovery. The reliability of these instruments depends on the consistent maintenance of the liquid helium cryogen.
Fundamental Physics and Space Exploration
Liquid helium is a foundational tool for high-energy physics and advanced research, serving as the cooling medium for the world’s largest scientific instruments. The Large Hadron Collider (LHC) at CERN uses thousands of superconducting magnets to steer and focus particle beams, all cooled by a massive liquid helium distribution system. The magnets operate using superfluid helium at 1.9 K to achieve the highest possible magnetic fields required to accelerate particles to near the speed of light.
The element is also integral to quantum computing research. Many quantum computing architectures, particularly those using superconducting circuits or electron spins as qubits, require temperatures near absolute zero to function. Liquid helium is used to achieve the millikelvin temperatures necessary to isolate the quantum systems from thermal interference. Research is exploring the use of electrons floating on the surface of liquid helium as a pure system for hosting qubits.
In space exploration, liquid helium has been used to chill the detectors of infrared space telescopes to eliminate self-generated thermal noise. Observatories like the Herschel Space Telescope carried large, Dewar-like vessels of liquid helium to cool their infrared sensors. This cooling is essential because the detectors must be cooled far below the temperature of the radiation they are trying to measure.
Cooling the instruments to temperatures as low as 5.5 K allows them to detect the faint thermal signatures of distant celestial objects without being blinded by the telescope’s own heat. While newer instruments use sophisticated mechanical cryocoolers, the underlying principle remains the necessity of helium’s unparalleled cooling capacity. This cooling enables the highest sensitivity observations of the cold, dark universe.
Storage, Conservation, and Supply
Handling liquid helium presents significant logistical challenges due to its extremely low boiling point. The substance must be stored in highly specialized, vacuum-insulated containers known as cryogenic dewars. Even with advanced insulation, ambient heat inevitably leaks in, causing the liquid to slowly vaporize, a process called “boil-off.”
The global supply of helium is a complex issue because it is a non-renewable resource, primarily extracted as a byproduct of natural gas processing. Once released into the atmosphere, helium is light enough to escape Earth’s gravitational pull and is lost to space. This finite supply, coupled with increasing demand, leads to market volatility and intermittent shortages.
Consequently, conservation efforts have become a major focus for large-scale users and suppliers. Many modern MRI and NMR systems now incorporate cryocooler technology to reliquefy the helium gas boiled off the magnet, significantly reducing the frequency of refilling. Large-scale users in research often employ recovery systems to capture, purify, and reliquefy the vaporized gas for reuse, mitigating the waste of this valuable element.