# Knudsen_number¶

plasmapy.physics.transport.collisions.Knudsen_number(characteristic_length, T, n_e, particles, z_mean=<Quantity nan>, V=<Quantity nan m / s>, method='classical')

Knudsen number (dimless)

Parameters: characteristic_length (Quantity) – Rough order-of-magnitude estimate of the relevant size of the system. 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 non-classical 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. knudsen_param – The dimensionless Knudsen number. 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. ~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 Knudsen number is given by [1]

$Kn = \frac{\lambda_{mfp}}{L}$

where $$\lambda_{mfp}$$ is the collisional mean free path for particles in a plasma and :mathL is the characteristic scale length of interest.

Typically the characteristic scale length is the plasma size or the size of a diagnostic (such a the length or radius of a Langmuir probe tip). The Knudsen number tells us whether collisional effects are important on this scale length.

Examples

>>> from astropy import units as u
>>> L = 1e-3 * u.m
>>> n = 1e19*u.m**-3
>>> T = 1e6*u.K
>>> particles = ('e', 'p')
>>> Knudsen_number(L, T, n, particles)
<Quantity 7839.36310417>
>>> Knudsen_number(L, T, n, particles, V=1e6 * u.m / u.s)
<Quantity 8.52931736>


References