[en] The invention concerns direct-voltage electron accelerators, more exactly - the design of vacuum chambers in which an extended electron beam is accelerated. The aim of the invention is to increase beam power and reliability of the accelerator. The aim is achieved by providing the vacuum chamber (behind the escape slit) with a device for the beam extension in the direction of the narrow side of the exit opening. In this case the width of the escape slit is much less (at least 3-5 times) than the width of the exit opening and the gap between the high voltage electrode and the chamber wall is increased by 1/3 and more in the place of beam passage. This design of a vacuum chamber can be also used for the acceleration of heavy particles. In this case a heavy particle source should be installed instead of a cathode