Space charge insensitive electron gun designs

An electron gun system comprising:

• an electron source that directs a primary beam of electrons;
• a plate defining a beam limiting aperture, wherein the primary beam of electrons is projected through the beam limiting aperture;
• a magnetic field source that generates a magnetic field;
• a channel that the primary beam of electrons is projected through after being projected through the beam limiting aperture;
• and
• a sweep electrode that generates an electric field in a drift region, wherein the electric field increases kinetic energy of secondary electrons in the channel and is configured to sweep the secondary electrons away from the primary beam thereby removing secondary electrons from the primary beam, wherein the sweep electrode is a biased plate that is disposed across the channel with an aperture that the primary beam of electrons passes through.

A method comprising:

• generating a primary beam of electrons with an electron source;
• projecting the primary beam of electrons through a beam limiting aperture;
• projecting the primary beam of electrons through a channel downstream of the beam limiting aperture, wherein secondary electrons in the channel have a Larmor radius and a Larmor time, wherein the channel has an inner width dimension perpendicular to a direction the primary beam of electrons is projected, wherein the inner width dimension is less than or equal to twice a value of the Larmor radius, and wherein the Larmor time for the secondary electrons is greater than 1 ns;
• and
• limiting the secondary electrons in the channel to one orbit or less.