Multipacting is a resonant phenomenon in which charged particles (usually electrons) traverse stable orbits between one or more impact points on the structure surface. In energetically favorable conditions an impact will, on average, yield multiple particles, leading to a net gain of particles after each orbit. This situation leads to a resonant cascade that can drain power from the cavity and/or lead to surface damage.
A multipacting simulation is used to determine field levels for which the structure supports resonant electron orbits, and also to identify those resonances that may lead to damage. The general strategy used by the solver is to initiate particles with random velocities either on the surface or in the volume, and then track these particles in the RF fields of the structure. Most of these particles will impact the structure walls and be absorbed, but typically a small fraction of the particles will impact a surface such that they are either reflected or yield true secondary particles. These new particles, and subsequent generations, are tracked and within a few RF cycles the only particles that remain are those that are in a resonance. The number of prior impacts in each particle history are maintained, and when a user-specified number of impacts is reached, then the particle is tagged as "resonant" and statistics about the orbit are accumulated for presentation to you.