Plasma, often called the fourth state of matter, is a superheated, ionized gas. It forms when a gas is heated so intensely that electrons are stripped from their atoms, creating a soup of charged particles, including free electrons and positive ions. While rare on Earth’s surface, plasma is the most prevalent form of matter in the cosmos, making up an estimated 99% of the visible universe. This electrified gas powers stars, but replicating its extreme conditions on Earth is a monumental challenge, driving research from astrophysics to fusion science.
Understanding Temperature in Plasma
Measuring plasma heat is complicated because “temperature” refers to the average kinetic energy of the particles. Unlike gases or liquids, where all particles share a single temperature, plasma often exhibits a distinction between its components.
In laboratory plasmas, the much heavier ions and the electrons often do not possess the same average energy, resulting in a “two-temperature” or non-thermal plasma. Lighter electrons can reach tens of thousands of Kelvin while the bulk ions remain near room temperature.
Fusion research requires a fully thermalized plasma where ions and electrons are in equilibrium and share the same, extremely high kinetic energy. This unified temperature is necessary because energetic ion collisions drive the nuclear fusion reaction. Fusion reactor heat records thus cite the ion temperature, which is the condition required for atomic nuclei to merge.
Plasma in the Natural Universe
The most familiar natural plasma is the Sun, with a core temperature of approximately 15 million Kelvin. This heat, coupled with gravitational pressure, forces hydrogen nuclei to fuse into helium, releasing energy. Larger stars achieve higher core temperatures as they fuse progressively heavier elements.
The highest temperatures occur during the final moments of a massive star’s life in a core-collapse supernova. As the core implodes under gravity, the sudden compression generates temperatures vastly exceeding those in stable stars. The dense, inner core can momentarily reach temperatures estimated up to 100 billion Kelvin.
This heat spike occurs just before the core rebounds in a shockwave, causing the star’s catastrophic explosion. Even more energetic phenomena, such as accretion disks around black holes or gamma-ray bursts, involve plasma temperatures that momentarily surpass this figure.
Reaching Extreme Temperatures on Earth
Scientists pursue controlled nuclear fusion by heating hydrogen plasma far hotter than any star. This extreme heating compensates for the lack of a star’s immense gravitational pressure. Atomic nuclei must be given enormous kinetic energy to overcome their natural electrical repulsion, known as the Coulomb barrier.
Sustained fusion requires heating plasma to 100 million to 300 million degrees Celsius, seven to 20 times the Sun’s core temperature. This is achieved in experimental reactors like tokamaks and stellarators. The Korea Superconducting Tokamak Advanced Research (KSTAR) device sustained plasma at 100 million degrees Celsius for a record 48 seconds.
The goal is achieving the triple product: a combination of temperature, density, and confinement time necessary for a self-sustaining reaction. The French WEST tokamak demonstrated progress toward continuous operation by sustaining plasma at fusion-relevant temperatures for over 22 minutes.
The Physics of Containment and Measurement
Since no physical material can withstand plasma exceeding 100 million degrees, the hot gas must be suspended without touching the reactor walls. This is achieved through magnetic confinement, where powerful superconducting magnets shape and contain the plasma within a vacuum chamber. Devices like the tokamak use a toroidal magnetic field to keep the charged particles spiraling along field lines and away from the vessel material.
Measuring Plasma Temperature
The intense heat means plasma temperature cannot be measured directly with a physical probe. Scientists rely on non-invasive diagnostic techniques that analyze the light and radiation emitted or scattered by the plasma particles.
One of the most accurate methods is Thomson scattering, which involves firing a high-powered laser beam into the plasma. As the laser light interacts with electrons, a small fraction is scattered. By analyzing the wavelength shift—related to the Doppler shift—researchers determine the electron temperature and density. Collective Thomson scattering is a variation used to determine the ion temperature, the crucial parameter for assessing fusion readiness.