Particle simulation of plasmas, employed since the 1960s, provides a selfconsistent, fully kinetic representation of general plasmas. Early incarnations looked for fundamental plasma effects in one-dimensional systems with ∼10 2-10 3 particles in periodic electrostatic systems on computers with 100 kB memory. Recent advances model boundary conditions, such as external circuits to wave launchers, collisions and effects of particle-surface impact, all in fully relativistic three-dimensional electromagnetic systems using ∼10 6-10 10 particles on massively parallel computers. While particle codes still enjoy prominance in a number of basic physics areas, they are now often used for engineering devices as well.

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In high-power microwave systems, the transition of window breakdown from single surface vacuum multipactor discharge to rf plasma with increasing gas pressure is investigated using particle-in-cell simulations. An intermediate pressure regime where multipactor discharge and rf plasma coexist was found. The pressure range where the multipactor can be maintained is summarized in the plot of the secondary electron emission yield as a function of the gas pressure. As the gas pressure increases, electron-neutral collisions prevail against secondary electron emissions and the electron energy probability function changes from the bi-Maxwellian at low pressures to Druyvesteyn at high pressures as a result of the change in electron heating and cooling processes. The discharge formation time in argon, neon, and xenon is shown for different gas pressures. Different scaling laws in the discharge formation time are presented at low and high pressures, respectively.