This block contains information about particle collisions. See EPOCH input deck for more information on the input deck.

EPOCH has a particle collision routine with scattering algorithms based on the model presented by Sentoku and Kemp¹ or the model presented by Pérez et al ², which in turn was based on the work of Nanbu and Yonemura³. This adds a new output block named “collisions” which accepts the following four parameters.

• use_collisions - This is a logical flag which determines whether or not to call the collision routine. If omitted, the default is “T” if any of the frequency factors are non-zero (see below) and “F” otherwise.
• use_nanbu - This logical flag determines whether the scattering angle of Pérez/Nanbu will be used. The default is “T”. If “F”, the Sentoku-Kemp algorithm will be used.
• coulomb_log - This may either be set to a real value, specifying the Coulomb logarithm to use when scattering the particles or to the special value “auto”. If “auto” is used then the routine will calculate a value based on the local temperature and density of the particle species being scattered, along with the two particle charges. If omitted, the default value is “auto”.
• collide - This sets up a symmetric square matrix of size $nspecies,\times,nspecies$ containing the collision frequency factors to use between particle species. The element (s1,s2) gives the frequency factor used when colliding species s1 with species s2. If the factor is less than zero, no collisions are performed. If it is equal to one, collisions are performed normally. For any value between zero and one, the collisions are performed using a frequency multiplied by the given factor.

If “collide” has a value of “all” then all elements of the matrix are set to one. If it has a value of “none” then all elements are set to minus one. If the syntax “species1 species2 ” is used, then the (species1,species2) element of the matrix is set to the factor “”. This may either be a real number, or the special value “on” or “off”. The “collide” parameter may be used multiple times. The default value is “all” (ie. all elements of the matrix are set to one).

• collisional_ionisation - If this logical flag is set to “T” then the collisional ionisation model is enabled. This process is independent of field_ionisation (see here). However, in order to set up collisional_ionisation you must also specify ionisation energies and electrons in a species block (see here). The default value is “F”. For example:

begin:collisions
   use_collisions = T
   use_nanbu = T
   coulomb_log = auto
   collide = all
   collide = spec1 spec2 off
   collide = spec2 spec3 0.1
end:collisions

With this block, collisions are turned on, the Nanbu-Pérez scattering algorithm is used and the Coulomb logarithm is automatically calculated. All values of the frequency array are set to one except (spec1,spec2) is set to minus one (and also (spec2,spec1)) and (spec2,spec3) is set to 0.1

References