What is a Plasma?

Plasma is overwhemingly the dominant constituent of the universe as a whole. Yet most people are ignorant of plasmas. In daily life on the surface of planet Earth, perhaps the plasma to which people are most commonly exposed is the one that produces the cool efficient glow from fluorescent lights. Neither solid, nor liquid, nor gas, a plasma most closely resembles the latter, but unlike gases whose components are electrically neutral, plasma is composed of the building blocks of all matter: electrically charged particles at high energy.

Plasma is so energetic or "hot" that in space it consists soley of ions and electrons. It is only when plasma is cooled that the atoms or molecules that are so predominant in forming gases, liquids, and solids that we are so accustomed to on Earth, is possible. So, in space, plasma remains electrically charged. Thus plasmas carry electric currents and are more influenced by electromagnetic forces than by gravitational forces. Outside the Earth's atmosphere, the dominant form of matter is plasma, and "empty" space has been found to be quite "alive" with a constant flow of plasma.

Plasmas are conductive assemblies of charged particles, neutrals and fields that exhibit collective effects. Further, plasmas carry electrical currents and generate magnetic fields. Plasmas are the most common form of matter, comprising more than 99% of the visible universe.

Plasma is by far the most common form of matter known. Plasma in the stars and in the tenuous space between them make up over 99% of the visible universe and perhaps most of that which is not visible. On earth we live upon an island of "ordinary" matter. The different states of matter found on earth are solid, liquid, and gas. We have learned to work, play, and rest using these states of matter. Sir William Crookes, an English physicist, identified another, more fundamental, state of matter in 1879. In 1929, Nobel Laureate Irving Langmuir gave this state a name, plasma. He borrowed the term from medical science because the matter with which he worked resembled life itself. It formed cells through bifurcation and often acted in a complicated and unpredictable manner. Plasma is defined as an assemblage of charged particles called electrons and ions that react collectively to forces exerted by electric and magnetic fields.

Given its nature, the plasma state is characterized by a complexity that vastly exceeds that exhibited in the solid, liquid, and gaseous states. Correspondingly, the study of the physical and especially the electrodynamical properties of plasma forms one of the most far ranging and difficult research areas in physics today. From spiral galaxies to controlled fusion, this little-known state of matter, the fundamental state, is proving to be of ever greater significance in explaining the dynamics of the universe and in harnessing the material world for the greatest technological result.

*There are only four dominant naturally-occurring states of matter although many other states of matter exist when considered broadly (see A. Barton, States of Matter, States of Mind, IOP Press, 1997).

*The Navier-Stokes equations are basic equations for studies of fluids and neutral gas systems.
The Maxwell equations for electromagnetism and the plasma Boltzmann equation are the basic equations for studies of electromagnetic systems of which plasmas are a prime example

Plasma consists of a collection of free-moving electrons and ions - atoms that have lost electrons. Energy is needed to strip electrons from atoms to make plasma. The energy can be of various origins: thermal, electrical, or light (ultraviolet light or intense visible light from a laser). With insufficient sustaining power, plasmas recombine into neutral gas. Plasma can be accelerated and steered by electric and magnetic fields which allows it to be controlled and applied. Plasma research is yielding a greater understanding of the universe. It also provides many practical uses: new manufacturing techniques, consumer products, and the prospect of abundant energy. Courtesy of T. Eastman

In analysis, plasmas are far harder to model than solids, liquids, and gases because they act in a self-consistent manner. The separation of electrons and ions produce electric fields and the motion of electrons and ions produce both electric and magnetic fields. The electric fields then tend to accelerate plasmas to very high energies while the magnetic fields tend to guide the electrons. Both of these mechanisms, the accelerated (or fast) electrons and the magnetic fields produce what is called sychrotron radiation, so called because it was first discovered in large magnetized containers of electrons beams in laboratories on earth.

Because of their self-consistent motions, plasma are rampant with instabilities, chaosity, and nonlinearities. These also produce electric and magnetic fields but also electromagnetic radiation. For example, all beams of electrons produce microwaves. Plasma science has, in turn, spawned new avenues of basic science. Most notably, plasma physicists were among the first to open up and develop the new and profound science of chaos and nonlinear dynamics. Plasma physicists have also contributed greatly to studies of turbulence, important for safe air travel and other applications. Basic plasma science continues to be a vibrant research area. Recent new discoveries have occurred in understanding extremely cold plasmas which condense to crystalline states, the study of high-intensity laser interactions, new highly-efficient lighting systems, and plasma-surface interactions important for computer manufacturing.

The term fundamental is used to denote plasma because the constituent components of plasmas, electrons and ions, are the longest lived particles know. Their lifetimes far exceed that of any other known particle. Thus long after other forms of matter and radiation have ceased to exist, it will have reverted back into the plasma state.

Where Are Plasmas?

The figure here illustrates where many plasma systems occur in terms of typical density and temperature conditions. Plasma temperatures and densities range from relatively cool and tenuous (like aurora) to very hot and dense (like the central core of a star). Ordinary solids, liquids, and gases are both electrically neutral and too cool or dense to be in a plasma state.

Plasmas are common in nature and found nearly everywhere. For instance, stars are predominantly plasma as are most space and astrophysical objects. However, plasmas are also found on Earth where they find a wide range of uses.

All of the following are examples where plasmas are to be found:

• Lightning!
• The Sun—from Core to Corona
• Fluorescent Lights and Neon Signs
• Nebulae - Luminous Clouds in Space
• The Solar Wind
• Primordial Fusion during the evolution of the Universe
• Magnetic Confinement Fusion Plasmas
• Inertially Confined Fusion Plasmas
• Flames as Plasmas*
• Auroras - the Northern and Southern Lights
• Interstellar Space - it's not empty, it's a plasma!
• Quasars, Radiogalaxies, and Galaxies—they emit plasma radiation and microwaves
• Large Scale Structures of Galaxies—their filamentary and magnetized!
• Dense Solid State Matter—when shocked by nuclear explosion or earthquakes, emit both light and radio emission.

*Ordinary flames and fire is a plasma, albeit a strongly interacting, collision-dominated plasma with diminished collective effects. These are examples of "strongly interacting plasmas" where the Coulomb interaction energy (distance between particles) is larger than the thermal energy (temperature). This leads to a small (often less than one) number of particles in a Debye sphere. This changes the physics of the beast, but it is still called a plasma. For example, instead of small angle collisions dominating transport that can be modeled with a Fokker-Planck equation, one must use the full Boltzman equation description. For example, a metal is in many respects a plasma, yet conventional definitions breakdown.

The full range of possible plasma density, energy(temperature) and spatial scales go far beyond these illustrations. For example, some space plasmas have been measured to be less than 10-10 /m3 (13 orders of magnitude less than the scale shown in the figure!). On one extreme, quark-gluon plasmas (although mediated via the strong force field versus the electromagnetic field) are extremely dense nuclear states of matter. For temperature (or energy), some plasma crystal states produced in the laboratory have temperatures close to absolute zero. On the other extreme, space plasmas have been measured with thermal temperatures above 10+9 degrees Kelvin and cosmic rays (a type of plasma with very large gyroradii) are observed at energies well above those produced in any man-made accelerator laboratory.

Understanding Plasmas

While all matter is subject to gravitational forces, the positively charged nuclei, or ions, and the negatively charged electrons react strongly to electromagnetic forces, as formulated by James Clerk Maxwell (1831-1879) and Hendrik Antoon Lorentz (1853-1928). because of this strong interaction with electromagnetism, plasmas display a compexity in structure that far exceeds that found in matter in the gaseous, liquid, or solid states. In addition to the cellular structure, most visible to us on the Sun, plasmas most often display a filamentary structure. This structure drives from the fact that plasma, becaue ot its free electrons, is an excellent conductor of electricity, far exceeding the conducting properties of metals such as copper or gold. For example, the ballast resistor in a fluorescent lighting system is included for good reason. The florescent gas, as weakly ionized as it is, would completely short circuit the electrical main supply without the resistor. Wherever charged particles flow in a neutralizing medium, such as free electrons in a background of ions, the charged particle flow or current produces a ring of magnetic field around the current, pinching the plasma into multi-filamentary strands of conduction currents.

Beyond the filamentation, by far the most distinguishing characteristic of energetic plasma in comparison with the states of matter on the crustal regions of planets is that plasma are prodigious producers of electromagnetic radiation.

Gases, liquids and solids can be ionized, by intense beams of laser light, intense electromagnetic pulses, and nuclear explosions. In each case, these states can be made to produce electromagnetic radiation but the phenomenon is weak and short lived and the degree of ionization weak compared to plasma. Errors in perception have also been made, especially in the case of 'Ionized Gases,' a topic studied intensely in the early 1900's. However, gases and plasmas are distinct states of matter. The fluids states of gas and liquid are treated with the Navier-Stokes equation whereas plasmas are treated with the Boltzmann and Maxwell equations. The term 'plasma' is for everyone and not just for specialists. Sometimes the solar wind is described as a "vast stream of ions" but this leads to an incomplete description of the physics of the wind as electrons and electromagnetic fields are not included., In spite of their mathematical complexity, the acknowledgement of their existence throught space and utilization in industrial processes (80% of the manufacture of computing chips requires a plasma) it is time to acknowledge that 'plasmas' are for everyone.