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Plasma parameters



 

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]

Additional recommended knowledge

Contents

Fundamental plasma parameters

All quantities are in Gaussian cgs units except temperature expressed in eV and ion mass expressed in units of the proton mass μ = mi / mp; Z is charge state; k is Boltzmann's constant; K is wavelength; γ is the adiabatic index; ln Λ is the Coulomb logarithm.

Frequencies

  • electron gyrofrequency, the angular frequency of the circular motion of an electron in the plane perpendicular to the magnetic field:
\omega_{ce} = eB/m_ec = 1.76 \times 10^7 B \mbox{rad/s} \,
  • ion gyrofrequency, the angular frequency of the circular motion of an ion in the plane perpendicular to the magnetic field:
\omega_{ci} = eB/m_ic = 9.58 \times 10^3 Z \mu^{-1} B \mbox{rad/s} \,
  • electron plasma frequency, the frequency with which electrons oscillate when their charge density is not equal to the ion charge density (plasma oscillation):
\omega_{pe} = (4\pi n_ee^2/m_e)^{1/2} = 5.64 \times 10^4 n_e^{1/2} \mbox{rad/s}
  • ion plasma frequency:
\omega_{pi} = (4\pi n_iZ^2e^2/m_i)^{1/2} = 1.32 \times 10^3 Z \mu^{-1/2} n_i^{1/2} \mbox{rad/s}
  • electron trapping rate
\nu_{Te} = (eKE/m_e)^{1/2} = 7.26 \times 10^8 K^{1/2} E^{1/2} \mbox{s}^{-1} \,
  • ion trapping rate
\nu_{Ti} = (ZeKE/m_i)^{1/2} = 1.69 \times 10^7 Z^{1/2} K^{1/2} E^{1/2} \mu^{-1/2} \mbox{s}^{-1} \,
  • electron collision rate
\nu_e = 2.91 \times 10^{-6} n_e\,\ln\Lambda\,T_e^{-3/2} \mbox{s}^{-1}
  • ion collision rate
\nu_i = 4.80 \times 10^{-8} Z^4 \mu^{-1/2} n_i\,\ln\Lambda\,T_i^{-3/2} \mbox{s}^{-1}

Lengths

 

\Lambda_e= \sqrt{\frac{h^2}{2\pi m_ekT_e}}= 6.919\times 10^{-8}\,T_e^{-1/2}\,\mbox{cm}
  • 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 have a velocity typical of the temperature, ignoring quantum-mechanical effects:
e^2/kT=1.44\times10^{-7}\,T^{-1}\,\mbox{cm}
  • electron gyroradius, the radius of the circular motion of an electron in the plane perpendicular to the magnetic field:
r_e = v_{Te}/\omega_{ce} = 2.38\,T_e^{1/2}B^{-1}\,\mbox{cm}
  • ion gyroradius, the radius of the circular motion of an ion in the plane perpendicular to the magnetic field:
r_i = v_{Ti}/\omega_{ci} = 1.02\times10^2\,\mu^{1/2}Z^{-1}T_i^{1/2}B^{-1}\,\mbox{cm}
  • plasma skin depth, the depth in a plasma to which electromagnetic radiation can penetrate:
c/\omega_{pe} = 5.31\times10^5\,n_e^{-1/2}\,\mbox{cm}
  • Debye length, the scale over which electric fields are screened out by a redistribution of the electrons:
\lambda_D = (kT/4\pi ne^2)^{1/2} = 7.43\times10^2\,T^{1/2}n^{-1/2}\,\mbox{cm}

Velocities

v_{Te} = (kT_e/m_e)^{1/2} = 4.19\times10^7\,T_e^{1/2}\,\mbox{cm/s}
v_{Ti} = (kT_i/m_i)^{1/2} = 9.79\times10^5\,\mu^{-1/2}T_i^{1/2}\,\mbox{cm/s}
  • ion sound velocity, the speed of the longitudinal waves resulting from the mass of the ions and the pressure of the electrons:
c_s = (\gamma ZkT_e/m_i)^{1/2} = 9.79\times10^5\,(\gamma ZT_e/\mu)^{1/2}\,\mbox{cm/s}
  • Alfven velocity, the speed of the waves resulting from the mass of the ions and the restoring force of the magnetic field:
v_A = B/(4\pi n_im_i)^{1/2} = 2.18\times10^{11}\,\mu^{-1/2}n_i^{-1/2}B\,\mbox{cm/s}

Dimensionless

 

  • square root of electron/proton mass ratio
(m_e/m_p)^{1/2} = 2.33\times10^{-2} = 1/42.9 \,
  • number of particles in a Debye sphere
(4\pi/3)n\lambda_D^3 = 1.72\times10^9\,T^{3/2}n^{-1/2}
  • Alven velocity/speed of light
v_A/c = 7.28\,\mu^{-1/2}n_i^{-1/2}B
  • electron plasma/gyrofrequency ratio
\omega_{pe}/\omega_{ce} = 3.21\times10^{-3}\,n_e^{1/2}B^{-1}
  • ion plasma/gyrofrequency ratio
\omega_{pi}/\omega_{ci} = 0.137\,\mu^{1/2}n_i^{1/2}B^{-1}
  • thermal/magnetic pressure ratio ("beta")
\beta = 8\pi nkT/B^2 = 4.03\times10^{-11}\,nTB^{-2}
  • magnetic/ion rest energy ratio
B^2/8\pi n_im_ic^2 = 26.5\,\mu^{-1}n_i^{-1}B^2
 
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Plasma_parameters". A list of authors is available in Wikipedia.
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