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 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) for a plasma where the a macroscopic description is valid. This is used to recover the average ion density (given the average ionization and electron density) for calculating the ion sphere radius for nonclassical impact parameters.
 V (Quantity, optional) – The relative velocity between particles. If not provided,
thermal velocity is assumed: \(\mu V^2 \sim 2 k_B T\)
where
mu
is the reduced mass.  method (str, optional) – Selects which theory to use when calculating the Coulomb logarithm. Defaults to classical method.
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 incorrectTypeError
– If the n_e, T, or V are not Quantities.RelativityError
– If the input velocity is same or greater than the speed of light.
Warns:  ~astropy.units.UnitsWarning – If units are not provided, SI units are assumed
 ~plasmapy.utils.RelativityWarning – If the input velocity is greater than 5% of the speed of light.
Notes
The collisional mean free path is given by [1]
\[\lambda_{mfp} = \frac{v}{\nu}\]where \(v\) is the interparticle velocity (typically taken to be the thermal velocity) and \(\nu\) 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).