Matter is traditionally understood to exist in three forms: solid, liquid, and gas. These common states represent different energy levels of atoms and molecules, but they do not account for the majority of the observable universe. When enough energy is added to push matter beyond the gaseous state, a fourth state emerges. This state, known as plasma, is an electrically charged medium that possesses unique properties and is now being harnessed for technological applications.
The Foundation: Solid, Liquid, and Gas
The distinction between the three familiar states of matter is governed by the thermal energy contained within a substance’s atoms or molecules. In a solid, particles are locked into fixed positions by strong forces, allowing only for vibrational movement. Adding thermal energy increases this vibration until the particles overcome these forces, leading to the transition to a liquid.
Once in the liquid state, particles can move past one another, allowing the substance to flow and take the shape of its container. Introducing more heat raises the kinetic energy further, causing the particles to move faster and farther apart until the attractive forces are completely overcome. This results in the formation of a gas, where particles are highly energetic, dispersed, and occupy the entire volume of their container. The addition of an even greater amount of energy to this gas facilitates the jump to the fourth state of matter.
Defining Plasma: The Ionized State
Plasma is an ionized gas, formed when atoms absorb such a high level of energy that electrons are stripped from their orbits, a process called ionization. This separates the neutral gas into a highly energetic, electrically conducting medium composed of free, negatively charged electrons and positively charged ions.
This resulting medium of charged particles is electrically quasi-neutral, meaning the total positive charge roughly balances the total negative charge. Unlike a standard gas, which is an electrical insulator, plasma is a highly efficient electrical conductor due to the presence of these mobile free electrons. The charged nature of the particles also causes plasma to respond strongly to electric and magnetic fields, a property utilized in many technological applications.
The degree of ionization varies significantly. A partially ionized plasma, such as that found in a neon sign, has only a small fraction of its atoms ionized. Conversely, a fully ionized plasma, like the interior of the Sun, is composed almost entirely of ions and electrons. This collective behavior of charged particles allows long-range electromagnetic forces to dominate the substance’s dynamics, making plasma fundamentally different from neutral states of matter.
Natural Occurrence of Plasma
Plasma is the most abundant state of matter in the observable universe, accounting for an estimated 99.9% of all ordinary matter. This is because the conditions for its creation—extremely high energy and temperature—are common in space. Every star, including our Sun, is a massive, superheated sphere of plasma powered by nuclear fusion reactions.
Vast stretches of interstellar and intergalactic space are also filled with low-density plasma, including the solar wind streaming outward from the Sun. On Earth, plasma occurs naturally in high-energy phenomena. The aurora borealis and aurora australis are created when plasma particles from the solar wind collide with gases in the Earth’s upper atmosphere, causing them to glow. Lightning is another terrestrial example, created by a massive electrical discharge that superheats the air molecules along its path.
Engineering Plasma for Practical Use
The unique electrical and magnetic properties of plasma make it a versatile tool for various technological and industrial applications.
Low-temperature, or “cold,” plasmas are used when the bulk gas must remain near room temperature. These applications include everyday lighting, such as fluorescent light bulbs and neon signs, created by passing an electrical current through a gas. Plasma display panels (TVs) also use tiny cells containing gas converted to plasma to emit light. In manufacturing, plasma etching is a standard process in the semiconductor industry for fabricating microchips.
High-temperature, or “thermal,” plasmas are employed in industrial applications like plasma welding and cutting torches. These tools use an ionized gas jet heated to tens of thousands of degrees Celsius to melt and cut materials. Plasma is also the required medium for nuclear fusion research. To achieve controlled fusion on Earth, researchers must heat hydrogen isotopes to temperatures exceeding 100 million degrees Celsius. This superheated plasma is then confined using strong magnetic fields within specialized reactors called tokamaks.