Mean transverse energy of electrons generated from mixed thermionic, multiphoton, and field emissions

The brightness of an electron emitter is fundamentally limited by the intrinsic emittance of the electrons born at a cathode. Different electron-emission mechanisms from a cathode give different values of mean transverse energy for calculating the intrinsic emittance of emitted electrons. This study reports a theory for calculating the mean transverse energy of electrons generated at a cathode via mixed thermionic, multiphoton, and field emissions. The theory successfully reveals the details of thermionically assisted multiphoton emission from a copper cathode excited by an intense infrared laser. An extension of the theory to calculate the intrinsic emittance and brightness of an electron emitter is also presented.