# Collisionality

In a plasma, the collision time τcoll is defined as the time in which the trajectory of a (charged) particle undergoes a change of direction of 90 degrees. Due to the long range of the Coulomb force, Coulomb interactions are typically small angle scattering events, so that this direction change typically requires a large number of interactions.

Consider a test particle with charge q, mass m, and velocity v colliding with bulk particles with charge q*, mass m*, and thermal velocity v*. Then the collision frequency ν = 1/τcoll is given by 

$\nu = \left ( \frac{qq^*n^*}{\varepsilon_0 m}\right )^2 \frac{\ln \Lambda}{4\pi (m_r/m)v^3n^*}$

assuming v > v*, where mr = mm*/(m+m*) is the reduced mass and n* the bulk particle density. The factor ln Λ appears due to the accumulation of many small-angle collisions within a Debye sphere.

## Dimensionless collisionality

The dimensionless collisionality ν* is defined as 

$\nu^* = \frac{\rm connection~length}{\rm trapped~particle~mean~free~path}$

See Connection length. The mean free path is estimated by the thermal velocity divided by the collision frequency, vth / ν.