A plasma model of relaxation of a medium in heavy ion tracks in condensed matter is proposed. The model is based on three assumptions: the Maxwell distribution of plasma electrons, localization of plasma inside the track nanochannel and constant values of the plasma electron density and temperature during the x-ray irradiation. It is demonstrated that the plasma relaxation model adequately describes the x-ray spectra observed upon interaction of a fast ion with condensed target. Assumptions of plasma relaxation model are validated by the molecular dynamics modeling and simulation.