Plasma Physics
Plasma physics is concerned with the study of the physical characteristics, properties, and applications of plasmas. A plasma is usually defined as the "fourth state of matter." A more precise definition is that a plasma is a quasineutral gas in which many of the atoms or molecules are ionized and exhibit collective behavior. Quasineutral refers to the uniform density (n) of particles within the plasma and collective behavior means that a plasma can undergo motions which not only depend on local conditions but also on the state of the plasma in more remote regions. To be considered a plasma, a gas must satisfy the three following conditions: &lgr;<<L; N>>1 and t>1. In these expressions, &lgr; is the Debye length, or the characteristic distance over which charges are shielded in a plasma, and is equal to 6.9 (T/n)½ cm; N is the number of particles contained in a Debye sphere, a sphere around a charged test particle whose radius is equal to the Debye length, and is the natural collective oscillation frequency of the charged species (such as electrons, ions, etc.) present in the plasma in the absence of a magnetic field and is equal to 9000n½ x 2sec-1 ; and t is the average time elapsed between collisions with neutral atoms. Much of our understanding of the physics of plasmas is due to the work of the Swedish physicist H. Alfven (Nobel Prize in physics, 1971) on the theory of magneto-hydrodynamics and its plasma applications.
Plasmas are found first of all in fusion plasmas, and also in much of interplanetary, interstellar, and intergalactic space, in the Sun's corona, in solar wind, in Earth's magnetosphere and ionosphere, in parts of the atmosphere around lightning discharges and also in fluorescent light bulbs and other gas-discharge tubes, as well as in very hot flames. Plasmas are accordingly classified as follows: astrophysical, collisionless, cylindrical, electrostatically neutral, inhomogeneous, intergalactic, interstellar, magnetized, nonthermal, partially ionized, relativistic, solid state, strongly coupled, thermal, unmagnetized, and vlasov.
Plasmas are produced in fusion reactions. Fusion combines the nuclei of light elements to form a heavier element. This is a nuclear reaction and results in the release of very large amounts of energy. In fusion, the total mass of the resultant nuclei is slightly less than the total mass of the original particles. For fusion reactions to occur, temperatures must be high enough to heat the particles and the particles must be present in sufficient number and well contained. These conditions are all well met by plasmas. A plasma contains electrons that are stripped from their nuclei and so the plasma consists of charged particles, ions and electrons. Four particle-containment techniques are presently used to confine these hot plasmas: magnetic, inertial, electrostatic and gravity confinement. Magnetic confinement makes use of strong magnetic fields, typically 100,000 times greater than the earth's magnetic field, which are arranged in a configuration that prevents the charged particles from leaking out; this is referred to as a magnetic bottle. Inertial confinement uses powerful lasers or high energy particle beams to compress the fusion fuel. The plasma is imploded so quickly and the inertia of the converging particles is so high that many fuse before they disperse. This is the method used in a hydrogen bomb. Inertial confinement designs for power production usually use small pellets of fuel in an attempt to make "miniature" H-bomb type explosions that can be controlled. The electrostatic approach confines the charged particles by means of electric fields, rather than the magnetic fields used in magnetic confinement. Gravity is strong enough to confine plasmas in the Sun and other stars. An example of a fusion confinement device is the tokamak. The word tokamak is an acronym derived from Russian words meaning "toroidal chamber and magnetic coil." The tokamak has a doughnut-shaped configuration through which a current is introduced up to several million amperes and flows through the plasma, heating it to temperatures exceeding a hundred million degrees centigrade with the use of high-energy particle beams or radio-frequency waves.
The present goals of plasma physics research are focused in three main directions: first, achieve the demonstration of a steady-state, high-gain fusion plasma capable of producing reactor-level fusion power; second, improve the present understanding of the underlying physics and advance the state-of-the-art critical enabling technologies. This requires investigating the transport of heat particles from plasma, the contribution of magnetohydrodynamic factors and the effects of large populations of energetic alpha particles. The third goal is to find high strength materials that do not become excessively activated from the fusion neutrons or weakened by the extreme heat for the structural design of functional reactors, as well as improved, high field superconducting magnets to provide the required steady-state confinement of fusion plasmas.
Plasma research has also been successfully applied to the study of the physics of the earth's magnetosphere and its interactions with the solar wind and the ionosphere. Because the important physical processes in the earth's space environment occur on a wide range of spatial and temporal scales, a variety of simulation techniques are being used to investigate them. For example, magnetohydrodynamic (MHD) models are used to study the large scale circulation of plasma in the magnetosphere-ionosphere system which is in many ways similar to the global atmosphere-ocean circulation models used for climate research.
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