mean_free_path¶

plasmapy.transport.collisions.
mean_free_path
(T, n_e, particles, z_mean=<Quantity nan>, V=<Quantity nan m / s>, method='classical')¶ 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.
particles (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 incorrect
TypeError – 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 >>> particles = ('e', 'p') >>> mean_free_path(T, n, particles) <Quantity 7.83950983 m> >>> mean_free_path(T, n, particles, V=1e6 * u.m / u.s) <Quantity 0.01091773 m>
References
 1
Francis, F. Chen. Introduction to plasma physics and controlled fusion 3rd edition. Ch 5 (Springer 2015).