The state of matter known as plasma is often called the fourth state, distinct from the more familiar solid, liquid, and gas phases. This highly energized medium is essentially an electrically charged gas that consists of unbound positive ions and negative electrons. The transition to plasma occurs when enough energy is added to a gas to strip electrons from their atoms. This creates a soup of charged particles that can conduct electricity and respond to electromagnetic fields. This article explores the physical process of creating this unique state of matter, which is the ionized gas studied in physics and engineering, not blood plasma.
The Physical Process of Ionization
Ionization is the mechanism for generating plasma, where a neutral gas atom gains enough energy to lose one or more orbiting electrons. The resulting mixture is a quasi-neutral medium of positively charged ions and the negatively charged free electrons that were stripped away. This collection of charged particles gives plasma its unique properties, especially its high electrical conductivity.
Energy input is the fundamental requirement for this state change. This energy can be supplied as intense heat, causing vigorous collisions between gas molecules, or through strong electromagnetic fields, such as high-voltage electricity. Extreme heat increases the kinetic energy of particles, making collisions forceful enough to knock electrons completely out of their atomic orbits.
Applying a strong electric field accelerates free electrons already present in the gas, causing them to collide with neutral atoms. If these electrons gain sufficient energy between collisions, they strip an electron from an atom, creating a new ion and another free electron. This electron avalanche, or cascade process, quickly transforms the gas into a conductive plasma.
The required energy level depends on the gas’s ionization potential. The resulting plasma is characterized by its temperature and density. High-temperature plasmas, found in stars, are fully ionized. Many artificial plasmas on Earth are only partially ionized, meaning a significant number of neutral atoms remain mixed with the charged particles. For plasma to be sustained, there must be a continuous supply of energy to counteract the tendency of ions and electrons to recombine back into neutral gas atoms.
Creating Plasma in Controlled High-Energy Systems
In industrial and research settings, specialized, high-power equipment is used to create and maintain plasma for specific applications. These systems often require precise control over temperature and pressure to achieve either thermal (hot) or non-thermal (cold) plasma states.
Plasma Torches
Plasma torches, used for cutting and welding metals, exemplify a high-temperature method of plasma generation. A torch forces a gas, like air or argon, through a small nozzle while an electric arc is generated between an electrode and the workpiece. The immense heat from the arc, reaching temperatures up to 22,000°C (40,000°F), rapidly ionizes the gas, creating a high-velocity jet of thermal plasma.
Fusion Reactors
Fusion energy research utilizes highly controlled, long-duration plasma, often in devices like the Tokamak. Hydrogen fuel is heated to extreme temperatures, often between 150 and 300 million degrees Celsius, far hotter than the sun’s core. This superheated plasma is necessary for light elements to fuse and release energy. It is confined within a donut-shaped vacuum chamber by immensely powerful magnetic fields. The magnetic fields manipulate the charged plasma particles, preventing the scorching-hot material from touching the vessel walls.
Semiconductor Manufacturing
The semiconductor industry relies on a different type of controlled plasma, often referred to as “cold plasma” or non-thermal plasma, for manufacturing microchips. These processes use strong radio frequency (RF) fields or microwave energy to ionize gases at relatively low temperatures and pressures. This plasma is used for precise operations like etching, which involves removing material from a silicon wafer, and deposition, which is used for applying thin films. The lower temperature of this plasma is beneficial because it prevents damage to the delicate, nanometer-scale structures being fabricated on the semiconductor surface.
Simple Ways to Observe Plasma
The general public encounters plasma regularly through common household and natural phenomena that involve low-pressure or transient ionization.
Plasma Globes
Plasma globes, a popular novelty item, demonstrate how a high-frequency, high-voltage electric current can excite noble gases within a sealed glass sphere. A central electrode emits an oscillating electric field that ionizes the low-pressure gas mixture, typically neon and argon, into visible, glowing filaments of plasma. These filaments illuminate as electrons move from higher energy levels back down, releasing light in the process.
Gas-Discharge Lighting
Gas-discharge lighting, such as neon signs and fluorescent tubes, operates on a similar principle of electrical excitation. Applying a high voltage across a glass tube filled with low-pressure gas causes the gas atoms to ionize and form plasma. The color of the light depends on the gas used; for instance, pure neon gas emits a red-orange glow, while other noble gases or mixtures produce different colors. Fluorescent lights use this process to generate ultraviolet light, which then strikes a phosphor coating on the inside of the tube to create visible light.
Lightning
The most dramatic natural example of plasma is lightning, a massive electrical discharge that occurs in the atmosphere. The intense electric field that builds up between a cloud and the ground, or within a cloud, ionizes the air, creating a highly conductive plasma channel. The resulting lightning bolt is a column of plasma that can reach temperatures of approximately 28,000°C (50,000°F).