From FusionWiki
Jump to navigation Jump to search
The printable version is no longer supported and may have rendering errors. Please update your browser bookmarks and please use the default browser print function instead.

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 [1]

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 [2]

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


  1. K. Miyamoto, Plasma Physics and Controlled Nuclear Fusion, Springer-Verlag (2005) ISBN 3540242171
  2. ITER Physics Basis, Nucl. Fusion 39 (1999) 2137