title: Intensity Effects in Dielectric Laser Accelerator Structures
Creators: name: Egenolf, Thilo
Publisher: name: UNSPECIFIED
Year: 2020

description: Electron accelerators have a wide range of applications in basic research, medicine and industry. In order to reduce costs and size of future devices and facilities, advanced compact accelerator concepts are subject to extensive research and development efforts in accelerator physics. A promising approach is the dielectric laser accelerator (DLA). The electromagnetic near-fields of an ultra-short laser pulse scattered by a dielectric structure are used to accelerate electrons. Dielectrics increases the achievable accelerating gradient by a factor of at least 10 in comparison to metallic structures. Recent progress in the development of femtosecond lasers and lithographic techniques in semiconductor fabrication has paved the way to experimental demonstration of DLAs in 2013. Possible applications of DLAs are in attosecond science, in ultrafast electron-microscopy and -diffraction, and in providing relativistic electrons for lithography and radiotherapy. The use of near-fields in structures with period lengths in the order of micrometers leads to complicated particle dynamics during acceleration. Theoretical descriptions of the electromagnetic fields and the resulting particle dynamics are presented in this thesis. In order to confine the particles in the accelerating structures, a focusing scheme is introduced which is adapted from ion accelerators. This allows to transport particles through a DLA structure of arbitrary length. Due to non-linear dynamics of the accelerated electrons, the particle trajectories are calculated numerically. A simulation tool based on the analytical description of the electromagnetic fields in the structures is implemented. Since DLA structures are quasiperiodic, the description simplifies to one complex number per structure period, i.e., the spatial harmonic of the synchronous accelerating mode. The required computational effort is thus significantly reduced as compared to the full electromagnetic fields. Many applications of DLAs require particle distributions of high charge. The beam dynamics are additionally affected by wake fields, which are generated by the interaction of the charge distribution ensemble with the dielectric structures. Detailed studies of wake fields in DLA structures are discussed in this thesis. Moreover, first measurements of energy spectrum modulations caused by wake fields are presented. The wake field effects can lead to instabilities. For the analysis of possible instabilities, wake fields are integrated in the aforementioned numerical simulation tool. This extended tool is applied to compare particle dynamics with simplified analytical models of longitudinal and transverse wake effects and instabilities. These studies provide estimates for the maximum attainable charges of an accelerated particle bunch. For higher bunch charges, the effects from the wake fields exceed the accelerating and focusing laser fields and thus lead to beam loss. Finally, a possible damping mechanism is presented in order to counteract the instabilities and increase the bunch charge limits.