mean_free_path¶

plasmapy.formulary.
mean_free_path
(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("m")¶ Collisional mean free path (m)
 Parameters
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 nonclassical impact parameters.z_mean
is a required parameter ifmethod
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 straightline LandauSpitzer 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 ofCoulomb_logarithm
for more information about these methods.
 Returns
mfp – The collisional mean free path for particles in a plasma.
 Return type
 Raises
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.
RelativityError – If the input velocity is same or greater than the speed of light.
 Warns
UnitsWarning
– If units are not provided, SI units are assumed.RelativityWarning
– If the input velocity is greater than 5% of the speed of light.
Notes
The collisional mean free path is given by 1
\[λ_{mfp} = \frac{v}{ν}\]where \(v\) is the interparticle velocity (typically taken to be the thermal velocity) and \(ν\) is the collision frequency.
Examples
>>> from astropy import units as u >>> n = 1e19 * u.m ** 3 >>> T = 1e6 * u.K >>> mean_free_path(T, n, ('e', 'p+')) <Quantity 7.839... m> >>> mean_free_path(T, n, ('e', 'p+'), V=1e6 * u.m / u.s) <Quantity 0.0109... m>
References
 1
Francis, F. Chen. Introduction to plasma physics and controlled fusion 3rd edition. Ch 5 (Springer 2015).