Transverse dynamics of the relativistic electron beams passing through coaxial cylindrical dielectric waveguides

Abstract

Transverse beam dynamics is a critical issue for the wakefield accelerating structure development. The passing length of the driving high current low energy bunch, which generates wakefields, is dominated by strong deflecting field of HEM (Hybrid Electro Magnetic) modes. Properly adjusted FODO focusing system allows to keep the bunch off the dielectric walls along the waveguide axis. We present here our investigation results on the high order (m>1) HEM modes effect on the transverse beam dynamics of the driving 20-30 nC beam. Our analysis was carried out with the help of the in-house code BBU'3000 especially developed for the dielectric loaded wakefield accelerator . The calculation results will be used for the newly updated Argonne Wakefield Accelerator for experiments with the 75 MeV electron drive beams.Wakefield in dielectric waveguides is used to achieve high acceleration rates of charged particles [1]. This scheme includes at least two bunches passing through the vacuum channel: driver bunch with high charge and low energy and witness bunch with low charge and high energy. The delay between these bunches is chosen to correspond witness to an accelerating phase of the wakefield. In dielectric waveguide with cylindrical geometry 100nC driver bunch creates accelerating field up to 100 MV/m. Transverse beam dynamics of diver bunch is a critical issue for the wakefield accelerating structure development [2]. The passing length of the driver dominated by strong deflecting field of HEM (Hybrid Electro Magnetic) modes. The tail of driver bunch is deflected by strong transverse force.Properly adjusted focusing-defocusing (FODO) quadruples system [3] allows to keep the bunch off the dielectric walls along the waveguide axis (Fig.1). Transverse structure of quadrupole focusing field F t is presented on Fig.2. We investigated the influence of transverse offset of driver bunch on the propagation length.We present here our investigation results on the high order (m>1) HEM modes effect on the transverse beam dynamics of the driving 20-30 nC beam. Our analysis was carried out with the help of the in-house code BBU'3000 [4] especially developed for the dielectric loaded wakefield accelerator [2,3]. The calculation results will be used for the newly updated Argonne Wakefield Accelerator for experiments with the 75 MeV electron drive beams [5]. Figure 1. The driver bunch is passing through dielectric waveguide and sequence of FODO cells Figure 2. The transverse field structure Ft of quadrupole FODO focusing system in vacuum channel