A new method is proposed to reduce non-physical effects in the PIC method. The method is demonstrated on an example of simulation of recombination of positive and negative ions in the radio frequency (RF) discharge.
In discharges in electro-negative gases, the negative ions are generated by dissociative attachment and lost by recombination with the positive ions. The forward modeling of the recombination in Particle-in-Cell simulation by removing a positive and a negative super-particles gives rise to essential numerical fluctuations. These fluctuations lead to non-physical electric fields and heating of electrons. In this paper, a method is proposed that enables the simulation of the recombination of positive and negative ions in an accurate way, even at low densities. The recombination in a cell is done by changing the mass of super-particles in this cell. Thus, it is possible to recombine any desired amount of real particles even if it appears to be less than one super-particle. Moreover, the number of super-particles in each cell is kept nearly constant by adding zero-mass super-particles (if a cell contains too few particles) or by removing a super-particle while increasing the mass for each of the rest (if a cell contains too many). It is shown that this simulation of ion recombination results in a lower level of non-physical fluctuations. The lower level of computational noise is shown to result in a lower ionization rate and lower ion fluxes as compared to the PIC/MCC simulations without adaptive mass alteration.