An x-ray source has an evacuated tube. An anode is disposed in the tube
and includes a material configured to produce x-rays in response to
impact of electrons. A cathode is disposed in the tube opposing the anode
configured to produce electrons accelerated towards the anode in response
to an electric field between the anode and the cathode. A flange extends
from the cathode toward the anode, and has a smaller diameter than the
evacuated tube. The flange extends closer to the anode than an interface
between the cathode and the tube thus forming a reduced-field region
between the evacuated tube and the flange.