The time evolution of an initially uniform slightly underdense (&ohgr;e≲&ohgr;) plasma in a resonant cavity has been studied using two‐dimensional electromagnetic simulations. The ponderomotive force compresses plasma into the nodes of the applied radio frequency field. The plasma buildup at the nodes terminates either when the plasma pressure increases to equal the field pressure or when phase space breaking occurs. Explanations of the density buildup and tail formation of the particle distribution functions are provided.