Patent ID: 8278813

Claim:
An apparatus for generating electron beam, the apparatus comprising: a cathode, including a transmission window in the cathode to allow incident laser radiation to pass therethrough; an anode, including an opening in the anode positioned to receive at least a portion of an electron beam generated from a material that includes metal between the transmission window and the anode; and a focusing unit in electrical contact with the cathode and shaped for generating an electric field between the cathode and the anode, at least a region of the electric field in the vicinity of the electron beam having a longitudinal component parallel to a propagation axis of the electron beam and having a radial component perpendicular to the propagation axis, with the region of the electric field configured to apply a force to electrons in the electron beam towards the propagation axis to concentrate the electron beam as the electron beam propagates towards the opening, and wherein the electric field generated by the focusing unit includes a first portion having a longitudinal component and a radial component with the magnitude of the longitudinal component at least 10 times the magnitude of the radial component, and includes a second portion having a longitudinal component and a radial component with the magnitude of the radial component at least 0.2 times the magnitude of the longitudinal component.