Transient beam loading effects due to back-streaming electrons onto a thermionic cathode in an RF gun

Adverse effect of back-streaming electrons onto a thermionic cathode in an S-band 4.5-cell RF gun was studied. A numerical model is presented based on an equivalent circuit of the RF gun, taking beam-loading effect into account through 2-dimensional particle-in-cell simulations. Experimentally observed time evolutions of RF power reflected from the RF gun, and of output beam energy were both well reproduced by the present numerical model which takes a cathode temperature rise during the RF macro-pulse. In conclusion, observed undesirable decrease of beam energy during the macro-pulse is due to an increase of beam loading caused by a considerable increase of the cathode temperature due to back-streaming electrons hitting the cathode.