Study of geometrical tolerances of an electron gun for THz vacuum tube

In linear vacuum tubes (klystron, TWT) as the operation frequency increases up to THz range, the required beam current density into the interaction structure drastically increases up to 100-1000 A/cm² [1], leading to a required beam dimension in the order of few tenths of micron [2]. As consequence of these requirements, an intense effort has to be made to shrink the electron gun dimensions. When the geometrical size of the electron gun parts, such as the focusing grid or the anode, are in the range of few millimeter or less, the realization and assembly tolerances, in the order of some tenths of micron, could become unacceptable.