A two-Frequency RF Photocathode Gun

Abstract

In this paper we resurrect an idea originally proposed by Serafini[1] in 1992 for an RF photocathode gun capable of operating simultaneously at the fundamental frequency and a higher frequency harmonic. Driving the gun at two frequencies with the proper field ratio and relative phase produces a beam with essentially no rf emittance and a linear longitudinal phase space distribution. Such a gun allows a completely new range of operating parameters for controlling space charge emittance growth. In addition, the linear longitudinal phase space distribution aids in bunch compression. This paper will compare results of simulations for the two-frequency gun with the standard rf gun, and the unique properties of the two-frequency gun will be discussed.

Field-emission cathode gating for rf electron guns

“…The fields in the cavity will then ''beat'' in phase with each other. (Some work has been performed using such a technique to generate an improved field for photoinjector rf guns [11][12][13].) Figure 5 illustrates the effect of adding or subtracting a 3rd-harmonic component to the fundamental, at a particular point in the rf cavity, as a function of the phase of the fundamental field.…”

“…1 is the angular frequency of the fundamental field, E 13 is the amplitude of the fundamental (3rd-harmonic) field, 13 is the phase of the fundamental (3rd-harmonic) field, and t is time. We can choose to set 1 0, and we can also write E 3 E 1 , where is simply a proportionality constant.…”

“…Some linear accelerator designs include harmonic rf systems to help linearize the energy spread on the beam; they typically operate as in case (a) to remove the energy-versus-time curvature imparted by accelerating a beam on the crest of a rf wave [11,[13][14][15].…”

A multi-frequency RF photocathode gun

“…It has been known that addition of a field harmonic of the fundamental rf field could reduce the emittance, allowing improved freedom to manipulate beam size and length to control space charge forces [2,3]. As was shown recently, the use of many modes (N ≥ 2) in an RF gun is appealing due to several additional reasons [8], described below.…”

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

“…Nonaxisymmetric structure excites multipole field components, such as the dipole, quadrupole, and octopole, which kick the beam time dependently, leading to the emittance growth [1]. While removal of the kick from the fundamental mode is difficult [3], minimization of the multipole rf field has been investigated and implemented in the design of the state-ofthe-art photocathode rf gun [4,5], which aims to produce high-brightness electron beams.…”