The Particle–Field Interaction, Nonrelativistic Interface combines charged particle tracing with electrostatics to model nonrelativistic beams of charged particles that can create significant space charge density distributions. Optionally, the particle trajectory calculation and the electric potential calculation can be bidirectionally coupled: the charged particles perturb the electric potential distribution, which in turn exerts a force on the particles and changes their trajectories.

Although individual ions or electrons usually move through the system at high speed, for some setups the charge and current density may appear stationary from an Eulerian perspective. That is, if a point in space is observed over time, particles may enter and leave the vicinity of that point at the same rate. In this case, it may be possible to couple a stationary analysis of the electric potential with a transient analysis of the particle trajectories through the system, and iterate between these two analysis types until a self-consistent solution is reached. This type of iteration between stationary and transient analyses is automatically set up by the Bidirectionally Coupled Particle Tracing study step, described in the COMSOL Multiphysics Reference Manual.