Microwave measurements of the BNL/SLAC/UCLA 1.6 cell photocathode RF gun

Abstract

The longitudinal accelerating field E, has been measured as a function of azimuthal angle in the full cell of the cold test model for the 1.6 cell BNL/SLAC/UCLA #3 S-band RF Gun using a needle rotation / frequency perturbation technique [l]. These measurements were conducted before and after symmetrizing the full cell with a vacuum pump out port and an adjustable short. Two different waveguide to full cell coupling schemes were studied. The dipole mode of the full cell is an order of magnitude less severe before symmetrization for the &coupling scheme. The multi-pole contribution to the longitudinal field asymmetry are calculated using standard Fourier series techniques. The Panofsky-Wenzel theorem [2] is used in estimating the transverse emittance due to the multipole components of E t .

Tailored electron bunches with smooth current profiles for enhanced transformer ratios in beam-driven acceleration

“…The setup we consider throughout this section is depicted in Fig. 4 and consists of a typical 1 þ 1 2 -cell BNL/SLAC/UCLA S-band rf-gun operating at 2.856 GHz surrounded by a solenoidal lens [22]. The large (∼140 MV=m) acceleration gradients in the gun help preserve larger charge densities compared with, e.g., L-band guns.…”

Transverse-emittance measurements on an S-band photocathode RF electron gun

“…The RF gun installed at the GTF is a 1.6 cell, S-band, photocathode, rf gun with cavity symmetrization to remove dipole fields and the associated emittance growth [4].…”

“…The YAG screen is 1.2 m downstream of the last quadrupole and is installed at normal incidence to the e-beam. 4 The light is extracted through a standard viewport and sent to an analog CCD camera.…”

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.…”

“…However, the one-sided coupling scheme causes asymmetry in the electromagnetic field inside the cavity. This asymmetric field can be expanded into multipole fields [4,7]. In particular, the dipole and quadrupole fields are the main causes of emittance growth.…”