Investigating higher order modes effects on thermionic RF gun transverse emittance

As the excitation of higher order modes in high gradient accelerating cavities of the RF gun negatively influences electron beam quality, in the present work a theory is obtained based on generalizing Panofsky-Wenzel theorem to study the effect of transverse magnetic modes on transverse emittance growth of the RF gun. Based on this theory, the impact of higher order modes on transverse momentum is investigated. Based on analysis and simulation results, it is shown that different RF modes result in divergence or convergence effects on beam transverse dynamics. The presence of dipole and quadrupole modes can enhance the transverse emittance by ~320% and ~450%, respectively. The compound effect of the presence of two higher order modes results in ~470% transverse emittance growth.