Plasma parameters

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The complex self-constricting magnetic field lines and current paths in a Birkeland current that may develop in a plasma (Evolution of the Solar System, 1976)

Plasma parameters define various characteristics of a plasma, an electrically conductive collection of charged particles that responds collectively to electromagnetic forces. Plasma typically takes the form of neutral gas-like clouds or charged ion beams, but may also include dust and grains.[1] The behaviour of such particle systems can be studied statistically.[2]

Fundamental plasma parameters[edit]

All quantities are in Gaussian (cgs) units except energy and temperature expressed in eV and ion mass expressed in units of the proton mass ; is charge state; is Boltzmann's constant; is wavenumber; is the Coulomb logarithm.

Frequencies[edit]

  • electron gyrofrequency, the angular frequency of the circular motion of an electron in the plane perpendicular to the magnetic field:
  • ion gyrofrequency, the angular frequency of the circular motion of an ion in the plane perpendicular to the magnetic field:
  • electron plasma frequency, the frequency with which electrons oscillate (plasma oscillation):
  • ion plasma frequency:
  • electron trapping rate:
  • ion trapping rate:
  • electron collision rate in completely ionized plasmas:
  • ion collision rate in completely ionized plasmas:

Lengths[edit]

  • classical distance of closest approach, the closest that two particles with the elementary charge come to each other if they approach head-on and each has a velocity typical of the temperature, ignoring quantum-mechanical effects:
  • electron gyroradius, the radius of the circular motion of an electron in the plane perpendicular to the magnetic field:
  • ion gyroradius, the radius of the circular motion of an ion in the plane perpendicular to the magnetic field:
  • plasma skin depth (also called the electron inertial length), the depth in a plasma to which electromagnetic radiation can penetrate:
  • Debye length, the scale over which electric fields are screened out by a redistribution of the electrons:
  • ion inertial length, the scale at which ions decouple from electrons and the magnetic field becomes frozen into the electron fluid rather than the bulk plasma:
  • mean free path, the average distance between two subsequent collisions of the electron (ion) with plasma components:
,
where is an average velocity of the electron (ion) and is the electron or ion collision rate.

Velocities[edit]

  • ion speed of sound, the speed of the longitudinal waves resulting from the mass of the ions and the pressure of the electrons:
,
where is the adiabatic index
  • Alfvén velocity, the speed of the waves resulting from the mass of the ions and the restoring force of the magnetic field:

Dimensionless[edit]

A 'sun in a test tube'. The Farnsworth-Hirsch Fusor during operation in so called "star mode" characterized by "rays" of glowing plasma which appear to emanate from the gaps in the inner grid.
  • number of particles in a Debye sphere
  • Alfvén velocity/speed of light
  • electron plasma/gyrofrequency ratio
  • ion plasma/gyrofrequency ratio
  • thermal/magnetic pressure ratio ("beta")
  • magnetic/ion rest energy ratio

Collisionality[edit]

In the study of tokamaks, collisionality is a dimensionless parameter which expresses the ratio of the electron-ion collision frequency to the banana orbit frequency.

The plasma collisionality is defined as[3][4]

where denotes the electron-ion collision frequency, is the major radius of the plasma, is the inverse aspect-ratio, and is the safety factor. The plasma parameters and denote, respectively, the mass and temperature of the ions, and is the Boltzmann constant.

See also[edit]

References[edit]

  1. ^ Peratt, Anthony, Physics of the Plasma Universe (1992);
  2. ^ Parks, George K., Physics of Space Plasmas (2004, 2nd Ed.)
  3. ^ Nucl. Fusion, Vol. 39, No. 12 (1999)
  4. ^ Wenzel, K and Sigmar, D.. Nucl. Fusion 30, 1117 (1990)