# Collisions (plasmapy.formulary.collisions)¶

Functions to calculate transport coefficients.

This module includes a number of functions for handling Coulomb collisions spanning weakly coupled (low density) to strongly coupled (high density) regimes.

## Coulomb collisions¶

Coulomb collisions are collisions where the interaction force is conveyed via the electric field, instead of any kind of contact force. They usually result in relatively small deflections of particle trajectories. However, given that there are many charged particles in a plasma, one has to take into account the cumulative effects of many such collisions.

## Coulomb logarithms¶

Please see the documentation for the Coulomb_logarithm for a review of the many ways in which one can define and calculate that quantity.

## Collision rates¶

The module gathers a few functions helpful for calculating collision rates between particles. The most general of these is collision_frequency, while if you need average values for a Maxwellian distribution, try out collision_rate_electron_ion and collision_rate_ion_ion. These use collision_frequency under the hood.

These include:

## Functions¶

 Coulomb_logarithm(T, n_e, species, …[, method]) Estimates the Coulomb logarithm. impact_parameter_perp(T, species, ), V) Distance of closest approach for a 90 degree Coulomb collision. impact_parameter(T, n_e, species, z_mean, V) Impact parameters for classical and quantum Coulomb collision collision_frequency(T, n, species, z_mean, V) Collision frequency of particles in a plasma. Coulomb_cross_section(impact_param) Cross section for a large angle Coulomb collision. fundamental_electron_collision_freq(T_e, …) Average momentum relaxation rate for a slowly flowing Maxwellian distribution of electrons. fundamental_ion_collision_freq(T_i, n_i, ion) Average momentum relaxation rate for a slowly flowing Maxwellian distribution of ions. mean_free_path(T, n_e, species, z_mean, V[, …]) Collisional mean free path (m) Spitzer_resistivity(T, n, species, z_mean, V) Spitzer resistivity of a plasma mobility(T, n_e, species, z_mean, V[, method]) Electrical mobility (m^2/(V s)) Knudsen_number(characteristic_length, T, …) Knudsen number (dimensionless) coupling_parameter(T, n_e, species, z_mean, V) Coupling parameter.