Plasma (physics)

• 1. Introduction

  ›

  Plasma makes up an estimated 99.9% of all ordinary matter in the universe, yet you will rarely encounter it on Earth. Every star is almost a pure ball of it. It fills the rarefied space between galaxies and the thin medium drifting between clusters. And yet on the surface of our planet, where solids, liquids, and gases rule, plasma is the stranger.

  It is called the fourth state of matter, the one that comes after solid, liquid, and gas. But unlike those three, there is no clean line you cross to reach it. The transition to plasma is not well defined. Whether a substance has ionized enough to earn the name depends entirely on the phenomenon you are studying.

  So what actually changes when a gas becomes plasma? Why did a scientist studying glowing tubes reach for a word borrowed from human blood? And how does something so dominant across the cosmos remain so elusive in the room around you? This documentary follows the charged particles, the magnetic fields, and the laboratory sparks that answer those questions.

• 2. A Word From Blood

  ›

  Irving Langmuir introduced the term plasma to describe ionized gas in 1928. He drew a careful boundary in his definition. Except near the electrodes, where thin sheaths held very few electrons, the ionized gas held ions and electrons in roughly equal numbers, so the net space charge stayed very small. He chose the name plasma for that balanced region.

  Lewi Tonks and Harold Mott-Smith both worked alongside Langmuir in the 1920s. They recall that he first reached for the word by analogy with blood plasma. Mott-Smith remembered the specific image that prompted it. The way electrons traveled from thermionic filaments reminded Langmuir of the way blood plasma carries red and white corpuscles and germs.

  The word itself runs deeper than the laboratory. Plasma in the medical sense ultimately derives from the Greek word for the result of forming or moulding. Langmuir gave that ancient term a new home among ions and electrons, but the science he named had roots stretching back more than a century before him.

• 3. From Arc To Fourth State

  ›

  The electric arc was discovered independently by Vasily Petrov and Humphry Davy in 1803. The early understanding of plasma grows out of the history of electricity itself, not from any single sudden insight. In 1831, Michael Faraday systematically investigated electric glow discharge in rarefied gases.

  Electrical conduction through gases was first thought to resemble conduction in liquids. Sir William Crookes, working with low-pressure discharges, proposed a different model. In a well-known paper in 1879, he presented the idea of a fourth state of matter. The modern science of plasma derives from Langmuir's work in the 1920s, but these earlier questions framed everything that followed.

  Describing a low-density plasma as merely an ionized gas is wrong and misleading. The two share something. Both assume no definite shape or volume. But where gases interact through short-range binary collisions, plasma is dominated by long-range collective motion, producing waves and other collective phenomena throughout.

• 4. Charge And Conductivity

  ›

  Gases are excellent insulators, holding firm up to electric field strengths of tens of kilovolts per centimetre. Plasma is the opposite. For many purposes its conductivity may be treated as infinite. That single property reshapes everything about how it behaves.

  Moving charged particles generate electric currents, and any motion of a charged particle affects and is affected by the fields the other charges create. In a gas, all particles behave alike. In a plasma, electrons and ions carry different charges and vastly different masses, so they respond differently to the same conditions. Out of that difference come plasma-specific waves and instabilities.

  Plasma is typically quasineutral, meaning its overall charge sits roughly at zero across large volumes. But this is not absolute. On the scale of the Debye length there can be charge imbalance. When double layers form, that charge separation can stretch across some tens of Debye lengths. A plasma with a real excess of charge, or built from a single species, is called non-neutral. Charged particle beams, an electron cloud in a Penning trap, and positron plasmas all belong to that category.

• 5. Heat And Magnetism

  ›

  Plasma temperature is commonly measured in kelvin or electronvolts, a measure of the thermal kinetic energy per particle. High temperatures are usually needed to sustain the ionization that defines a plasma. In thermal equilibrium, the degree of ionization follows the Saha equation, set by the electron temperature relative to the ionization energy and, more weakly, by density.

  Electrons and the heavy particles, the ions and neutral atoms, each tend to hold a relatively well-defined temperature, close to a Maxwellian distribution even under strong fields. Because electrons and ions differ so greatly in mass, those temperatures can diverge sharply. In weakly ionized technological plasmas, the ions often sit near ambient temperature while electrons reach thousands of kelvin. The z-pinch plasma reverses this, with ion temperature exceeding that of the electrons.

  Charged particles cause plasma to generate and respond to magnetic fields. A plasma is magnetized when its field is strong enough to steer the charged particles, the common criterion being that a particle completes at least one gyration around a field line before colliding. Often the electrons are magnetized while the ions are not. Magnetized plasmas are anisotropic, behaving differently parallel and perpendicular to the field.

• 6. Modeling The Unwieldy

  ›

  Writing down every particle location and velocity in a plasma would fully describe its state, but it is neither practical nor necessary. Plasma physicists instead reach for less detailed descriptions, and two main families dominate the field.

  Fluid models describe plasma through smoothed quantities like density and averaged velocity at each position. The simplest, magnetohydrodynamics, treats the plasma as a single fluid governed by Maxwell's equations combined with the Navier-Stokes equations. A two-fluid model goes further, describing ions and electrons separately. These models work well when collisionality keeps the velocity distribution close to Maxwell-Boltzmann, but they cannot capture beams, double layers, or wave-particle effects.

  Kinetic models track the particle velocity distribution function at each point, so they need not assume a Maxwell-Boltzmann distribution. They become necessary for collisionless plasmas. One approach places the smoothed distribution on a grid in velocity and position. The other, the particle-in-cell technique, follows the trajectories of a large number of individual particles. The Vlasov equation describes charged particles interacting with an electromagnetic field, and in magnetized plasmas a gyrokinetic approach can sharply cut the cost of a fully kinetic simulation.

• 7. Plasma In The Cosmos

  ›

  Plasma constitutes more than 99% of the matter observed in the visible universe, much of it at very low densities. Above the Earth's surface the ionosphere is a plasma, and the magnetosphere holds plasma in the space environment around the planet. The polar aurorae and lightning are terrestrial plasmas too, alongside St. Elmo's fire and upper-atmospheric phenomena like sprites, blue jets, and gigantic jets.

  The solar wind fills interplanetary space with plasma expelled from the Sun, reaching out to the heliopause. Beyond the Solar System, distant stars and much of interstellar and intergalactic space are filled with plasma. Astrophysical plasmas also appear in accretion disks around white dwarfs, neutron stars, and black holes in close binary star systems.

  Plasma drives the ejection of material in astrophysical jets, observed with accreting black holes and in active galaxies. The jet from M87 is one such example, possibly extending out to 5,000 light-years. Stars themselves are plasmas heated by nuclear fusion, which is why plasma research is also bound up with the pursuit of fusion energy on Earth.

• 8. Sparks Made To Order

  ›

  Most artificial plasmas are generated by applying electric or magnetic fields through a gas, and the principle behind them is simple. There must be energy input to produce and sustain the plasma. In a discharge tube, current applied across a dielectric gas pulls bound electrons toward the anode while the cathode pulls the nucleus.

  As the voltage rises, the material is stressed beyond its dielectric strength into electrical breakdown, marked by a spark, as it shifts from insulator to conductor. The Townsend avalanche drives this. The first impact of an electron on an atom yields one ion and two electrons, and after about 20 successive sets of collisions the number of charged particles climbs into the millions. With ample current density the discharge forms a luminous arc, similar to lightning, passing through stages of saturation, breakdown, glow, transition, and thermal arc.

  These engineered plasmas reach far into industry. Arc discharge at roughly 10,000 K is used to smelt minerals containing Al2O3 to produce aluminium. Capacitively coupled plasma, typically driven at 13.56 MHz, serves microfabrication and integrated circuit manufacturing through plasma etching and deposition. In the mid-1990s, the dielectric barrier discharge was shown to inactivate bacterial cells, and later work with mammalian cells opened a field known as plasma medicine.