Issue 76

Beam Dynamics Newsletter

5.6 Dielectrically Lined Rectangular Waveguides as Wakefield Based Dechirper Structures

CANDIDATE: Franziska Reimann, Rostock University. Germany
SUPERVISOR: Prof. Dr. Ursula van Rienen
GRADUATION: December 2018

In the physics of particle accelerators, light sources and compact accelerators are gaining more and more importance for various fields of science, medicine and industry. This is coupled to increasing demands on the beam quality, like, e.g., the need for ever higher charge densities and pulses in the range of sub-picoseconds.

Another requirement for particle beams, especially in a linear accelerator (linac) or linac-based X-ray Free Electron Laser (FEL), is a small energy width. In fact, large energy spreads have been present in accelerator science for a long time. In the last years, however, a new concept has been proposed as a counter measure to this problem: the so-called ‘dechirper’ or ‘silencer’, a passive, geometrically simple structure based on the use of the interaction between the particle beam and its own wakefield.

The aim of this thesis is to provide a comprehensive study of the general properties of a specific type of dechirper: a rectangular waveguide coated with dielectrics. For this purpose, a semi-analytical model is developed which calculates the wake function inside the dechirper using an eigenmode expansion. As a second step, this model is implemented in the programming language Python and benchmarked against simulations with CST Particle Studio® (CST PS). These studies show that the geometrical simplicity of the chosen dechirper type makes an analytical treatment of the wakefield possible, even though the final computation of the wake function has to be performed numerically. This method has the advantage of delivering the wakefield of a point charge, the so called wake function, as a result. The wake function can be considered a Green’s function and enables the calculation of wake potentials of arbitrary bunches by a simple convolution. The programme, named Wakefield Calculation In Rectangular Waveguides Lined with Dielectrics (WIzaRD), is then used to perform parameter studies to provide an overview over the influence of variations of the geometrical and dielectric properties on the magnitude of the wakefield. Here it is shown that the most powerful parameters to adjust the magnitude of the wakefield are the length of the dechirper, the total charge of the particle beam that generates the wakefield, and the distance between the upper and lower dielectric in the dechirper, the gap width. Especially the latter enables a tuning of the wakefield after the construction of the dechirper and during experiments, which also makes it possible to turn off the dechirper without removing it from the beamline.

The underlying project work of this thesis has been performed in collaboration with the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The ultimate goal of the simulation phase has been providing simulations for the later test of a dechirper prototype at the Electron Linac for beams with high Brilliance and low Emittance (ELBE) facility at the HZDR. Consequently, the geometry of a prototype dechirper is introduced in this thesis. Following this, the results of experiments carried out with this prototype are presented here and compared to simulations performed with the aforementioned semi-analytical programme algorithm. The experiments underline the tuning possibilities of the wakefield that the gap width provides. Comparisons with semi-analytic results show a qualitative agreement between theory and experiments.