plasmapy.formulary.collisions.coupling_parameter(T: Unit("K"), n_e: Unit("1 / m3"), species, z_mean: Unit(dimensionless) = <Quantity nan>, V: Unit("m / s") = <Quantity nan m / s>, method='classical') -> Unit(dimensionless)

Ratio of the Coulomb energy to the kinetic (usually thermal) energy.

Classical plasmas are weakly coupled (\(Γ ≪ 1\), where \(Γ\) is the coupling parameter). Dense plasmas tend to have significant to strong coupling (\(Γ ≥ 1\)). For more details, see the notes section below.

  • T (Quantity) – Temperature in units of temperature or energy per particle, which is assumed to be equal for both the test particle and the target particle.

  • n_e (Quantity) – The electron number density in units convertible to per cubic meter.

  • species (tuple) – A tuple containing string representations of the test particle (listed first) and the target particle (listed second).

  • z_mean (Quantity, optional) – The average ionization (arithmetic mean) of a plasma for which a macroscopic description is valid. This parameter is used to compute the average ion density (given the average ionization and electron density) for calculating the ion sphere radius for non-classical impact parameters. z_mean is a required parameter if method is "ls_full_interp", "hls_max_interp", or "hls_full_interp".

  • V (Quantity, optional) – The relative velocity between particles. If not provided, thermal velocity is assumed: \(μ V^2 \sim 2 k_B T\) where \(μ\) is the reduced mass.

  • method (str, optional) – The method by which to compute the Coulomb logarithm. The default method is the classical straight-line Landau-Spitzer method ("classical" or "ls"). The other 6 supported methods are "ls_min_interp", "ls_full_interp", "ls_clamp_mininterp", "hls_min_interp", "hls_max_interp", and "hls_full_interp". Please refer to the docstring of Coulomb_logarithm for more information about these methods.


coupling – The coupling parameter for a plasma.

Return type

float or ndarray

  • ValueError – If the mass or charge of either particle cannot be found, or any of the inputs contain incorrect values.

  • UnitConversionError – If the units on any of the inputs are incorrect.

  • TypeError – If any of n_e, T, or V is not a Quantity.

  • RelativityError – If the input velocity is same or greater than the speed of light.

  • UnitsWarning – If units are not provided, SI units are assumed.

  • RelativityWarning – If the input velocity is greater than 5% of the speed of light.


The coupling parameter is given by

\[Γ = \frac{E_{Coulomb}}{E_{Kinetic}}\]

The Coulomb energy is given by

\[E_{Coulomb} = \frac{Z_1 Z_2 q_e^2}{4 π \epsilon_0 r}\]

where \(r\) is the Wigner-Seitz radius, and 1 and 2 refer to particle species 1 and 2 between which we want to determine the coupling.

In the classical case the kinetic energy is the thermal energy:

\[E_{kinetic} = k_B T_e\]

The quantum case is more complex. The kinetic energy is dominated by the Fermi energy, modulated by a correction factor based on the ideal chemical potential. This is obtained more precisely by taking the the thermal kinetic energy and dividing by the degeneracy parameter, modulated by the Fermi integral 1

\[E_{kinetic} = 2 k_B T_e / χ f_{3/2} (μ_{ideal} / k_B T_e)\]

where \(χ\) is the degeneracy parameter, \(f_{3/2}\) is the Fermi integral, and \(μ_{ideal}\) is the ideal chemical potential.

The degeneracy parameter is given by

\[χ = n_e Λ_{de Broglie} ^ 3\]

where \(n_e\) is the electron density and \(Λ_{de Broglie}\) is the thermal de Broglie wavelength.

See equations 1.2, 1.3 and footnote 5 in 2 for details on the ideal chemical potential.


>>> import astropy.units as u
>>> n = 1e19*u.m**-3
>>> T = 1e6*u.K
>>> species = ('e', 'p')
>>> coupling_parameter(T, n, species)
<Quantity 5.8033...e-05>
>>> coupling_parameter(T, n, species, V=1e6 * u.m / u.s)
<Quantity 5.8033...e-05>



Dense plasma temperature equilibration in the binary collision approximation. D. O. Gericke et. al. PRE, 65, 036418 (2002). DOI: 10.1103/PhysRevE.65.036418


Bonitz, Michael. Quantum kinetic theory. Stuttgart: Teubner, 1998.