title: Electron Beam Dynamics in Dielectric Laser Accelerators
Creators: name: Niedermayer, Uwe
Publisher: name: UNSPECIFIED
Year: 2022

description: Dielectric Laser Acceleration (DLA) is a nascent scheme of electron acceleration, which is particularly promising due to its high acceleration gradients. Although these gradients are lower than what is obtained in plasma-based schemes, they are the highest in structure based schemes, which are limited by material breakdown. DLAs can be implemented on microchips, leveraging on the nano-technology available in the semiconductor industry. This work aims to tackle the electron beam dynamics in DLAs systematically, with the goal to turn the already experimentally demonstrated record gradients into large energy gain. In other words, the goal is to increase the length of the acceleration channels while keeping a full 6D (3 coordinates and 3 momenta) confinement of the electron beam. This is particularly challenging, since DLAs are based on optical near-fields, requiring the transversal size of the channel to be tiny, down to a tenth of the laser wavelength at subrelativistic electron energies. In order to keep the electron beam in this nanophotonic channel, enormous focusing strengths are required. Conventional techniques, usually involving solenoid- or quadrupole magnets, are too weak, since their aperture cannot be de-magnified in the same ratio as the DLA cells are de-magnified compared to conventional radiofrequency (RF) accelerator cavities. The solution to this problem is brought up in this work. It borrows from the Alternating Phase Focusing (APF) scheme as introduced for heavy ion accelerators in the 1950’. APF uses the laser fields themselves to focus the electron beam and thereby enables to omit external focusing devices entirely. While only a small amount of the large available acceleration gradient is sacrificed, full 6D confinement is obtained in length scalable strucures. Thus in principle arbitrary high energy can be obtained provided the required laser parameters are available. This work comprises two parts: A theoretical one introducing the DLA structures and a semi-analytic highly numerically efficient simulation approach named DLAtrack6D. From this approach, the Hamiltonian and the entire dynamics in DLAs is derived. This leads to the recipe to design scalable APF DLA structures, especially suitable for fabrication on Silicon-On-Insulator (SOI) wafers, which are very common in commercial nanophotonics. More conventional structures are also created on the basis of pure silicon technology. These devices are also experimentally investigated in the second part of this work, where simulations and experimental results are matched. The requirements and experimental achievements of subrelativistic DLAs in ultralow-emittance injector chambers are discussed. While low energy DLAs mostly aim at ultrafast (attosecond!) dynamics, high energy DLAs particularly exploit the available high acceleration gradient, in order to provide high energy electrons in small scale facilites. Furthermore DLA devices can also be used as a versatile bunch-shaping tool in large-scale, high-energy conventional accelerator facilities. For that purpose, the beam current limit as being imposed by wakefields due to the structure surfaces that come very close to the beam is investigated. Our semianalytic tracking code DLAtrack6D is supplemented with a wakefield module to assess collective effects and coherent beam instabilities. Moreover, the wakefields of DLAs can also be used in beneficial ways to shape the longitudinal phase space in high energy conventional accelerator facilites. Application goals for DLA are Ultrafast Electron-Microscopy and -Diffraction (UEM/ UED) at boosted energy and on a longer time scale the high acceleration gradients can be exploited for a high energy electron-positron collider for elementary particle physics. High energy ultrashort electron pulses can also be used for radiation generation, potentially in DLA-based microchip undulators. Another imaginable goal would be to accumulate electrons from a continuously running DLA injector in a storage ring. All these applications require a length scalable DLA and stable 6D-confined electron beam dynamics therein.