A method for operating a device having an anode and a cathode for generating microwave radiation with an accelerating voltage is provided. A chronological sequence of the accelerating voltage is provided by a series of voltage pulses. A first voltage pulse with an operating amplitude is applied between the anode and the cathode in order to determine whether an electric flashover occurs during the applied first voltage pulse. A second voltage pulse is applied following the first voltage pulse with a deionization amplitude that is smaller than the operating amplitude when the electric flashover occurs during the applied first voltage pulse.