Effect of auto-focusing of the electron beam in the relativistic vacuum diode

A hypothesis of the beam auto-focusing in the relativistic vacuum diode with a needle anode is proposed. A physical model of the effect is elaborated. As is shown, auto-focusing of a high current beam can be caused by ions emitted from the anode. The initial (pre-focusing) stage of focusing is provided with ionization of the gas molecules absorbed in the anode surface. Then (at a quasi-steady stage), the focusing is driven by the metal ions being evaporated from the anode heated by the beam. Simulation of the process show that the duration of the initial pre-focusing stage would be within 0.1-0.2 nsec