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The mechanisms of the beginning of depth oscillations of the vapour-gas channel are examined in beam welding. It is assumed that the reasons of the root peaks are the evaporation and condensation of metal vapour in the channel. To test this assumption, a system of the non-stationary equations of energy and balance of pressure at the channel surfaces has been solved. The channel configuration is determined by the boiling temperature isotherm. The heat, spent for the production of the vapour, defined as the difference between the beam power and the heat sink power in metal. The heat distribution along the channel length is determined by the temperature difference between the actual value at a given point of the channel and the value required to maintain the pressure equilibrium. Solving this system of equations shows that the periodic collapse of the channel occurs in its neck when the pressure therein vanishes, and the vapour from the bottom of the whole channel completely condenses at its walls.