WHAT IS PLASMA?

In this page you will find:
      What is plasma?
      Degree of ionization in plasma
      Collisions
Chapter 4


What is plasma?

The definition teaches us: plasma is a set of quasi-neutral particles with free electric charge carriers, which behave collectively. Let us analyze each part of this definition. The most important part is that free electric charge carriers are found in the plasma state of matter. Atoms are at least partially ionized. The degree of ionization does not have to be too large, if the size of the plasma formation is big enough. Precisely a plasma is different from a gas in that there are free carriers of charge in the former. A plasma is conductive and reacts strongly to electric and magnetic fields. The second quality is its quasi-neutrality. Let us assume a certain volume, which microscopically shows in average the same quantity of positive and negative particles. Seen from the outside, the plasma behaves as if it were a fluid without charge (liquid or gas). The demanding of quasi-neutrality excludes the beams of charged particles from the definition of plasma. The last part of the definition of plasma is its collective behaviour. With this it is understood that plasma as a whole is capable of processes that generate electric and magnetic fields to which plasma can react in turn. The plasma definition does not include the beams of charged particles since they do not fulfill the requirement of quasi-neutrality. Neither are included the very weekly ionized gases, like the flame of a candle (they do not fulfill the requirement of collective behaviour). The plasma concept was used for the first time by Irwing Langmuir (1881-1957).

The plasma state of matter can be further subdivided into a few more groups:

  • Common Plasma: the atomic electron orbitals are partially deteriorated (due to high pressure, temperature or both). The free electrons are responsible for the plasma characteristics of the substance.
  • Thermonuclear Plasma: the electron orbitals no longer exist; the substance is a mixture of nuclei and free electrons. Plasma of the nuclei of the stars where thermonuclear synthesis takes place is found in this state.
  • Nucleon Plasma: Due to very high temperatures or pressures, the atomic nuclei themselves are shattered. Matter is a mixture of electrons, protons and neutrons. The nucleonic plasma appeared at 10−5 s after the beginning of the Universe, when the quarks created the first protons and neutrons. We find also this type of plasma in the exterior layers of an exploding supernova, where a shock wave presses gas. Bustling thermonuclear reactions which generate heavy elements of the periodic table, take place in this layer for a short time.
  • Quark-Gluon Plasma: When energies are top high, nucleons themselves are shattered into their parts: quarks and gluons. It was in that state that matter was probable being found maybe up to the first tenth of microsecond after the beginning of the Universe. Such state of matter was possible to be artificially reproduced at CERN in the year 2000.

By plasma, however, some authors also understand some parts of the ionosphere, specially the layer F, which reflects the radio waves and allows the radio communication through the reflection of the ionosphere. Plasma is found in the van Allen radiation belts. The solar wind, an uninterrupted current of particles coming from our Sun, inside which our Earth is found, is also a plasma. In the plasma state are found the nuclei and atmospheres of the stars, the nucleus of our galaxy, the nebulae and most of the objects in the Universe. On Earth we encounter ourselves with plasma inside the channels of the lightnings, in different electric discharges and the plasma is also artificially created and researched in laboratories.

Fibrous structure

Fibrous structure of the remnants of the explosion of a supernova in the Vela constellation (in optical wavelength).
Photograph by David Malin - UK Schmidt Telescope, copyright: Anglo Australian Telescope Board, 1996.

What are the basic phenomena in a plasma? A plasma has the tendency of creating surface or filament formations – the plasma fiber or pinch and the current surfaces or pinched walls. The plasma projects phenomena which are collectively known as drifts – movement of particles perpendicular to a magnetic field or other force fields. Through a plasma can be expanded an enormous quantity of different kinds of waves – the magnetoacoustic waves, to which belong for example the well-known Alfvn waves which are the analogy of the acoustic waves in gases, and the electromagnetic waves of many different modes. These waves are also very easily generated in plasma. A plasma can reach a whole range of instabilities, which have as a consequence, for example, the short-time radiation of certain energy quantities leading to certain characteristic structures. To plasma belongs without doubt radiation (due to the electron-atom recombination, bremsstrahlung radiation and synchrotron radioation), the creation of electric double layers, the acceleration of charged particles to high energies, the magnetohydrodynamic dynamo which produces the magnetic field inside our Sun and the planets, and many more interesting phenomena.

Nowadays, people can create plasma very easily also in the laboratory. The most typical examples are:

  • Laser plasma – life time: 10−12 ÷ 10−9 s
  • Pulsed plasma – life time:  10−9 ÷ 10−6 s
  • Tokamak – life time: 1 s
  • Cold plasma – life time: hours, days, years


Degree of ionization in plasma

The degree of ionization in plasma (the rate of ionized particles against the total present particles) is one of the most important parameters which allows to precise the plasma behavior. It depends mainly on temperature and is possible to be deduced as the first approximation from Saha's equation for single ionized plasma in a thermodynamical equilibrium

ni2/nn = C T 3/2 exp[−Ui/kT]  ;      C ~  2,4×1021 m−3 .

where ni is the ion concentration which have lost one electron, nn is the neutral particles concentration, Ui is the ionization potential and T is the plasma temperature. Saha's equation is usable for gases. Sometimes, even certain solid substances (for example metals) are considered to be a kind of plasma, since they have free carriers of charge and exhibit collective behavior. Here, however, the quantity of free carriers of charge can not be determined from Saha's equation.


Collisions

In plasma there are also collisions between charged particles. The character of the collisions and their mechanism is different from the collisions of neutral particles. During the collision of neutral particles there are abrupt changes in the direction of the movement, while in plasma the changes in direction, caused mostly by the interaction with the electric field (~ 1/r2), are smoother.

Collisions

Collisions in a neutral gas (left) and in plasma (right)

  • The Mean Free Path can be defined for example, as the average distance during which the particles turn about 90 from the original direction. With the increase of temperature the effective cross section is diminished – the charged particles at high temperatures acquire high velocities, which imply that they interact only a short period of time and the deviations from the original direction are small.
  • The Electric Conductivity in plasma is given by the character of the collisions. The conductivity depends mainly on temperature (σ ~ T 3/2) and minimally on plasma concentration. The circulating current prevents, during small concentrations, a small number of charge carriers, and during high concentrations, prevents a large number of collisions. With the increase of temperature, the plasma conductivity increases (in metals this is the other way around), because the collision effective cross section is diminished.
  • The Optical Thickness of plasma depends on the mean free path of the photons in plasma. By optically diffuse is known the plasma of such dimensions which are comparable with the mean free path of the electromagnetic radiation passing through the plasma. Optically thick is such plasma, which dimensions are much larger than the mean free path of the photons which brilliancy intensely interacts with the plasma.

Translation: Arturo Ortiz Tapia, 2005