Patent ID: 7826595

Claim:
A method for generating x-ray radiation from a micro-focus spot with electronically adjustable spot size and x-ray tube current, the method comprising: providing a device comprising: a field emission cathode comprising a film with a layer of electron field emitting materials patterned on a conducting surface, wherein the electron field emitting materials comprise first and second portions; a gate electrode for extracting field emitted electrons from the cathode when a bias electrical field is applied between the gate electrode and the cathode; an anode; and an electrostatic focusing unit between the gate electrode and the anode, wherein the unit comprises multiple focusing electrodes including a pre-focusing electrode and a focusing electrode that are electrically separated from each other and that are positioned in series between the cathode and the anode, and wherein each of the, electrodes has an independently adjustable electrical potential; and adjusting at least one of the electrical potentials of the focusing electrodes and adjusting a size of the cathode for setting an x-ray focal spot size of the emitted electrons on the anode; wherein the method comprises activating the second portion of the electron field emitting materials when a maximum current density of the first portion of the electron field emitting materials is reached before a predetermined electron beam current is reached.