Patent ID: 8716925

Claim:
An electron gun assembly comprising: a gun header assembly comprising: a cathode adapted to emit an electron beam, wherein the cathode is coupled to a cathode lead connection permitting a cathode voltage bias to be applied to the cathode; a focus electrode fixed in position adjacent to the cathode but electrically isolated from the cathode, wherein the focus electrode is further coupled to a focus electrode lead connection permitting a focus electrode voltage bias to be applied to the focus electrode; and a plurality of ceramic isolating rings fixed in position such that at least one of the plurality of ceramic isolating rings provides electrical isolation between the cathode lead connection and the focus electrode lead connection; and an input body assembly comprising: an anode configured to be adjustably held in place with respect to and in proximity to the cathode and further configured such that an anode voltage potential can be applied to the anode; a magnetic gun polepiece fixed with respect to the anode but adjustably held in place with respect to the cathode; wherein the input body assembly and the gun header assembly are mechanically coupled using a flexible bellows that enables the input body to be translated axially with respect to the gun header assembly such that a distance between the anode and the cathode and a distance between the magnetic gun polepiece and the cathode can be adjusted; and wherein a magnetic flux adjustment shield having a substantially cylindrical shape is located outside of the gun header assembly and configured to slide along an axial direction, wherein a magnetic field inside of the gun header assembly is modified depending on an axial position of the magnetic flux adjustment shield.