First commissioning of the ARES focusing system and its possible upgrade for accelerator R&D experiment

Abstract

The SINBAD (Short INnovative Bunches and Accelerators at DESY) facility at DESY is a dedicated accelerator research and development project for the production of ultra-short electron bunches and the development of novel high gradient accelerator technology. The ARES linac (Accelerator Research Experiment at SINBAD) is the first infrastructure implemented at SINBAD and has a focus on the generation and characterization of ultrashort, small emittance electron pulses, ultimately aiming at the few femto-second regime. Those ultra-short, low emittance electron bunches are required, amongst others, for testing laser based novel external injection concepts such as laser driven dielectric acceleration structures or plasma wakefields. This thesis focuses on the optimization of the required high quality beam transport and focusing from the electron source to the end of the linac or into the experimental area. As part of the thesis, an algorithm has been developed to optimize the beam transport from the photo-injector and through the travelling wave structures by a beam based alignment of the ARES focusing solenoids. The algorithm was validated against simulations with the ASTRA computer code. A user interface was developed using MATLAB for beam commissioning and data taking at ARES. In various experimental campaigns, offsets of solenoids were determined and a realignment was implemented into the machine. In the end, a very good alignment of the beam trajectory with the magnetic axis along the beamline has been achieved, a major step in the successful commissioning of the ARES linac. The measurement results, the outcome of the data analysis and the alignment results are presented. As part of the thesis, the design of a focusing system was developed. The system would allow for injection into a laser wakefield acceleration experiment at ARES. For such an experiment, ultra-short bunches from ARES, ranging from sub-fs to few fs, would be injected into a plasma cell. The injection of short bunches requires strong transverse focusing to match the beam into small accelerating channels of the plasma. Transverse matching of the electron beam to the plasma channel prevents emittance growth in the finite energy spread beams. The conceptual design of a final focus system for injection into a laser plasma experiment at ARES is presented. The performance of the design is evaluated by means of ASTRA simulations of the beamline. The design includes sensitivity studies on relevant sources of error. The performed error analysis, which could be generalized for any quadrupole triplet, gives a useful estimate about the performance of the final focus system and suggests critical parameters in the implementation of the experiment.