A method and apparatus for an x-ray tube having an emitter and a
differentially biased emitter-cup cathode configured to provide an
electron beam of substantially greater perveance and beam compression
ratio than otherwise obtainable with conventional cathode designs is
disclosed. The method and apparatus include a cathode assembly opposing an
anode and spaced apart therefrom. The cathode is maintained during
operation of the x-ray tube at a negative potential with respect to the
anode. The cathode assembly includes an emitter for emitting an electron
beam to a focal spot on the anode during operation of the x-ray tube and a
cathode front member having an aperture defined by the cathode front
member on a first side of the emitter. A backing is disposed on a second
side of the emitter and is operably connected to the cathode front member
via a backing insulator. The cathode further includes a means for applying
a differential bias in the cathode assembly to variably change the focal
spot size.
