Update of user-manual

The user manual has been updated with a simple explanation on how to use
Coulomb Scattering.

doc/user-manual/bibliography.bib

@@ -62,4 +62,27 @@
   publisher = {Taylor \& Francis},
 }
 
+@Article{sherlock2008monte,
+  author    = {Sherlock, Mark},
+  journal   = {Journal of Computational Physics},
+  title     = {A Monte-Carlo method for Coulomb collisions in hybrid plasma models},
+  year      = {2008},
+  number    = {4},
+  pages     = {2286--2292},
+  volume    = {227},
+  groups    = {Particle-in-cell},
+  publisher = {Elsevier},
+}
+
+@article{lemons2009small,
+  title={Small-angle Coulomb collision model for particle-in-cell simulations},
+  author={Lemons, Don S and Winske, Dan and Daughton, William and Albright, Brian},
+  journal={Journal of Computational Physics},
+  volume={228},
+  number={5},
+  pages={1391--1403},
+  year={2009},
+  publisher={Elsevier}
+}
+
 @Comment{jabref-meta: databaseType:bibtex;}

doc/user-manual/fpakc_UserManual.tex

@@ -223,8 +223,15 @@
         \end{itemize}
       
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%      \subsection{\acrlong{cs}}
-%        Although not yet implement, a first approach will be soon implemented using Ref.~\cite{higginson2020corrected} as a guideline.
+     \subsection{\acrlong{cs}}
+       A simple linearization of the Coulomb operator based on Ref.~\cite{sherlock2008monte} is implemented.
+       This method assumes that the species involved in the scattering process have a Maxwellian distribution.
+       The method is made to conserve momentum and kinetic energy based on the approach in Ref.~\cite{lemons2009small} for self (same species) and intra (different species) collisions.
+       
+       The user must specify the charged species that will interact together.
+       The Coulomb logarithm involved in these processes is currently set to a fix value of $10$.
+       
+       This method is not valid for situations in which the distribution functions are far from Maxwellian.
         
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
     \section{Reset of particle array}
@@ -759,6 +766,16 @@ make
                                                                                          Only valid for \textbf{ionization} and \textbf{recombination} processes.
                                                               \end{itemize}
                                      \end{itemize}
+          \item \textbf{Coulomb}: Array of objects.
+                                  Contains the information about which species must use the Coulomb linear scattering.
+                                  This method assumes a Maxwellian distribution for all species involved.
+                                  Each object in the array is defined by:
+                                  \begin{itemize}
+                                    \item \textbf{species\_i}, \textbf{species\_j}: Character.
+                                                                                    Define the two species involved in the collision processes.
+                                                                                    Order is indiferent.
+                                  
+                                  \end{itemize}
         \end{itemize}
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
       \subsection{parallel}