NOTE

It has to deal with the usage of directly heated cathodes in the electron guns. The typical cathode is a W or W 80% - Re 20% wire, 150 micrometers in diameter and ~ 9 mm in length; the voltage applied is ~1.6 V. The primary beam is formed by the electrons emitted into the modulator's aperture from the part of the cathode, comprising only 5% of its length. The voltage drop present on this cathode's section contributes to the non-monoenergetic distribution of the electrons. The corresponding error can be estimated as 0.05 eV.