Plasma, often called the fourth state of matter, is a highly energized, electrically charged gas that exists beyond the familiar states of solid, liquid, and gas. While it is rarely encountered in a stable form on Earth’s surface, plasma dominates the cosmos, accounting for more than 99% of the visible matter in the universe. Scientists and engineers have successfully developed several methods to create this energetic state artificially, harnessing its unique properties for various technological purposes.
The Physics of Plasma Formation
Creating plasma involves ionization, the energetic transition of a gas into an electrically conductive state. Unlike an ordinary gas, where atoms are electrically neutral, plasma is a “soup” of charged particles. This mixture consists of free-moving electrons and positively charged ions—atoms that have lost one or more electrons.
The energy required to strip an electron from its atom must exceed the electron’s binding energy. When a gas receives sufficient energy input, electrons are knocked free, generating the ionized particles that define plasma. The resulting plasma is electrically neutral overall because the total negative charge of the free electrons balances the total positive charge of the ions. This state must be maintained with a continuous supply of energy to prevent the charged particles from recombining back into neutral atoms.
Creating Plasma Using High Voltage Discharge
One common way to generate plasma uses a strong electric field to induce electrical breakdown in a gas. This method, known as gas discharge, is the principle behind everyday items like neon signs and fluorescent lighting. A high voltage is applied across a gas-filled chamber, accelerating the free electrons already present in the gas.
As these accelerated electrons collide with neutral gas atoms, they transfer enough kinetic energy to knock loose other electrons, initiating a chain reaction known as the Townsend avalanche. This rapid multiplication of charged particles quickly ionizes the gas, forming a glowing plasma. To reduce the voltage needed, the gas is often held at a low pressure within a vacuum chamber. At lower pressures, electrons travel longer distances between collisions, gaining more speed and energy before impacting an atom, which makes ionization easier. This low-pressure environment produces the familiar steady glow found in fluorescent tubes and plasma globes.
Creating Plasma Using Extreme Thermal Energy
Another method for producing plasma relies on extreme heat, where the kinetic energy of the gas particles is sufficient to cause ionization. This process is called thermal ionization and is the mechanism at work in the cores of stars and in industrial tools like plasma cutters. When a gas is heated to temperatures typically exceeding 10,000 degrees Celsius, the atoms move so rapidly that their violent collisions strip electrons from their orbits.
The intense heat directly provides the energy needed to overcome the electron’s binding forces, creating a thermal plasma. In industrial applications, such as arc welding or plasma cutting, an electric arc is used to superheat a gas stream to these extreme temperatures. Unlike the high-voltage discharge method, which can produce “cold” plasma where electrons are hot but the bulk gas remains near ambient temperature, this thermal method produces plasma where all particles—electrons, ions, and neutral atoms—are at a uniformly high temperature.
Practical Uses of Artificial Plasma
Artificially generated plasma is integral to a wide range of modern industrial and technological applications. In the semiconductor industry, plasma etching is a precise technique used to manufacture microchips by selectively removing material from wafers. Plasma-enhanced chemical vapor deposition (PECVD) uses plasma to deposit thin, protective films on electronic components.
Plasma is utilized for modifying material surfaces to enhance properties like adhesion, wettability, or corrosion resistance. Plasma cleaning, for example, removes microscopic contaminants from sensitive surfaces, ensuring stronger bonds for coatings in aerospace and medical components. Plasma technology is also used in healthcare for sterilizing heat-sensitive medical instruments and is being researched for advanced medical treatments. Finally, the pursuit of controlled nuclear fusion aims to harness the energy of extremely hot plasma, replicating the power source of the Sun to create a potentially limitless energy source.