What Are Plasmas?

Plasmas are ionized gases in which a significant fraction of atoms have been stripped of one or more electrons, producing a mixture of free electrons and positive ions alongside any remaining neutral particles. This ionized state is sometimes called the fourth state of matter, distinct from solids, liquids, and gases. Plasmas respond collectively to electromagnetic fields because the free charges they contain can sustain electric currents and support wave modes that have no analog in neutral gases. They are the most abundant form of visible matter in the universe, composing stars, stellar winds, and much of the interstellar medium. On Earth, plasmas are produced artificially for applications spanning semiconductor manufacturing, lighting, medicine, and fusion energy research. The fundamental plasma parameters, including electron density, electron temperature, and Debye length, are described in the NRL Plasma Formulary, a standard reference published by the Naval Research Laboratory.

Plasma Sources and Classification

Plasmas are generated by supplying enough energy to a gas to ionize it, through electrical discharge, electromagnetic radiation, or heating. The resulting plasma is characterized primarily by temperature and density. Low-temperature plasmas, in which electron temperatures range from roughly 0.1 to 10 electronvolts while ions and neutrals remain near room temperature, are the workhorses of industrial plasma processing. Atmospheric-pressure plasmas operate at or near ambient pressure and include dielectric barrier discharges, plasma jets, and corona discharges. These are distinct from the low-pressure glow discharges used in semiconductor fabrication, which require vacuum chambers but achieve more uniform and controllable chemistry. High-temperature plasmas, reaching millions of degrees, are the province of fusion research and astrophysical study.

Low-Temperature Plasmas and Atmospheric-Pressure Applications

Low-temperature plasmas generate reactive neutral species, energetic photons, and charged particles that enable surface modification, sterilization, and synthesis at energy levels far below those needed for bulk thermal processing. In semiconductor manufacturing, plasma-enhanced chemical vapor deposition (PECVD) and reactive ion etching (RIE) are essential steps in building integrated circuits at nanometer feature scales. The precise control of plasma chemistry and ion energy flux that these processes require has driven decades of research into plasma sources and diagnostics. Atmospheric-pressure plasma jets, operating without vacuum infrastructure, have emerged as tools for surface activation, food decontamination, and wound healing. Research on plasma medicine, an active area connecting plasma physics and biology, is documented in publications such as Clinical Plasma Medicine.

Plasma Diagnostics

Characterizing a plasma, measuring its temperature, density, composition, and spatial uniformity, requires specialized diagnostic techniques because direct material probes perturb or are destroyed by the plasma. Langmuir probes, small metal electrodes inserted into the plasma, yield local electron density and temperature from current-voltage characteristics. Optical emission spectroscopy identifies excited species by their spectral lines and can be applied non-intrusively through a viewport. Thomson scattering, which measures the Doppler broadening of laser light scattered by electrons, provides absolute electron temperature and density with high spatial and temporal resolution. Mass spectrometry of neutral and ion fluxes at plasma boundaries characterizes the chemistry reaching a substrate or wall surface. These techniques are reviewed in resources maintained by IEEE's Nuclear and Plasma Sciences Society.

Plasma Confinement and Fusion

At the high-temperature extreme, plasmas are the active medium in fusion energy research. Both magnetic confinement devices such as tokamaks and inertial confinement systems must keep extremely hot plasmas isolated long enough for fusion reactions to occur at useful rates. Plasma instabilities, turbulent transport, and wall interactions are central research problems at this frontier, addressed by large international programs including ITER.

Applications

• Reactive ion etching and PECVD deposition in integrated circuit fabrication
• Plasma sterilization of medical instruments and packaging
• Neon, fluorescent, and plasma display lighting
• Plasma-assisted combustion and fuel reforming
• Thermonuclear fusion energy research
• Plasma thrusters for satellite and deep-space propulsion