Emittance growth due to multipole transverse magnetic modes in an rf gun

Abstract

Cylindrical asymmetry of an rf gun cavity excites the multipole rf field which will result in emittance growth of electron beams. With demanding of electron beams of ultralow emittance for the x-ray free electron laser, it is necessary to study the effect of the multipole rf field to the emittance growth and to eliminate the multipole field effect on electron beam emittance in an rf gun. In this paper, we present both theoretical analysis and numerical simulations of the multipole field effect to the emittance growth. We proposed a four-hole scheme to minimize the emittance growth from the multipole field.