From Ionized Gas to Collective Behavior
Matter exists in four fundamental states: solid, liquid, gas, and plasma. Plasma represents the most prevalent form of baryonic matter in the visible universe, constituting the substance of stars and interstellar space. Despite its cosmic abundance, plasma requires special conditions to be generated and studied in laboratory settings.
While often described as an ionized gas, this definition is incomplete. The defining characteristic of plasma is the ability of free charged particles—electrons and ions—to exhibit collective behavior through long-range electromagnetic interactions.
The Principle of Quasi-Neutrality
A foundational property of plasma is quasi-neutrality. While composed of charged particles, plasma appears electrically neutral on macroscopic scales larger than the Debye length. The number density of positive ions (ni) approximately equals that of negative electrons (ne).