title: Design and optimisation of a dielectric focusing structure for relativistic electron beams
Creators: name: Hasenbichler, Jan Anton
Publisher: name: TU Wien
Year: 2018
Subjects: subject: Laser
subject: Beschleuniger
subject: Dielektrikum
subject: Fokusierung
subject: laser
subject: acceleration
subject: dielectric
subject: focusing

description: Particle accelerators are used in a vast amount of ﬁelds in research and industry, everywhere where high energy particles are needed to probe materials, produce radiation for experiments or treatment of patients or simply to test the current understanding of Physics. They typically consist of a huge amount of diﬀerent components, but most prominently of RF-units to accelerate and dipole and quadrupole magnets to deﬂect and focus the particles. Though, as conventional particle accelerators, which achieve high energies, are huge and costly to built and operate, a cheap and compact particle accelerator option would be great to revolutionise several ﬁelds of research as it would grant access to high energy particles for nearly any laboratory. [3] One idea to solve this problem is the DLA (dielectric laser acceleration). This new type of accelerator design uses the electromagnetic ﬁelds of modern lasers in clever combination of dielectric materials to accelerate and focus the particles [4, 5, 6]. This master’s thesis focused on optimisation of focusing structures for relativistic electrons in such dielectric laser accelerators within the ACHIP experiment. As accelerators not only need accelerating structures, but also focusing structures to keep the beam well collimated, the idea of this thesis was to model a laser based focusing structure, do electromagnetic simulations with it and use the discretised electromagnetic ﬁelds [7] to simulate the particles propagating through the simulation volume to track their path and deﬂection. The design of the focusing structure was proposed by Joshua McNeur, a Postoctoral Researcher at the Chair of Laser Physics at the University of Erlangen-Nuremberg who conducted research on a similar design [8]. The construction of the design was done in Autodesk Inventor [9], a computer aided design application for 3D design. The electromagnetic simulations were performed with Lumerical FDTD Solutions [10], a ﬁnite diﬀerence time domain solver for electromagnetic problems using the Maxwell’s equations, because sample ﬁles and some experience were already available. The particle tracking code was written from scratch during this thesis in Matlab [11], a numerical computing environment, using a discretised Lorentz force equation and the Boris algorithm for time propagation [7]. The main work of this thesis was about setting up the electromagnetic simulations and the particle tracker for ultra-relativistic electrons, as lower velocities would lead to further diﬃculties in fabrication, to optimise said focusing structure to ﬁgure out the most appropriate design, featuring the highest deﬂection, the best combined quadrupole like deﬂection behaviour in both transversal directions as well as not too fragile setup for fabrication. Apart from the computational work, some time was also invested in fabrication possibilities and their limits, possible experiment setups to verify the computational results and theoretical description of the deﬂection behaviour. The results of this work include several points. The deﬂection at highest oﬀsets from the centre increased by up to a factor 60. The so called parallel eﬀect, which shifts the deﬂection curves independently of the oﬀset depending on the laser phase the particle enters the structure at was strongly decreased. The behaviour of the structure at two diﬀerent wavelengths to investigate scaling possibilities of parameters was investigated.Complications when using diﬀerent refractive indices were described. A mathematical model to ﬁgure out the crucial aspects of the electric and magnetic ﬁelds was created. Also how deﬂection in one direction contributes to deﬂection in the other direction was investigated. Finally ﬁrst fabrication results and one possible experiment to verify the simulated results were elaborated.