Intense soft X-ray source having an enclosure filled with gas and having two electrodes connected to a high voltage source, one of the electrodes having an opening. A device is provided for producing a photoionizing radiation directed through the opening. The radiation traversing the gas in the direction of the other electrode and during its passage producing a plasma microchannel, an electrical discharge supplied by the source then occurring in the microchannel, wherein the device for producing the photoionizing radiation is an auxiliary soft X-ray source.