Electron-beam diagnostics for a laser-driven plasma wakefield accelerator in the framework of FLASHForward

Abstract

Plasma wakefields are a promising approach for the acceleration of electrons with ultrahigh (10 to 100 GV/m) electric fields. Nowadays, high-intensity laser pulses are routinely utilized to excite these large-amplitude plasma waves. Particle beams from conventional radio-frequency accelerators can also excite such waves and therefore may work as an energy booster. In addition, this long-existing technology is quite well understood such that the particle beams can be operated as probes to gain an insight into the plasma waves and the acceleration process within them. For this purpose, among others, a new facility is being set up: FLASHForward. Owing to the particle beam planned to drive the plasma wakefield not being powerful enough to ionize the applied gas and generate a plasma, a high-power laser is available, which by itself is capable of driving plasma waves. Thus, a laser-driven plasmawakefield accelerator could be built and commissioned to support the operation of FLASHForward as well as for stand-alone experiments. This thesis reports on the implemented infrastructure, including the laser and the accelerator with the appertaining diagnostics, which can be used to prepare FLASHForward covering target investigations, diagnostics or experiments. Especially, this thesis is focused on diagnostics for the electron beam. Since plasma accelerators can produce short bunches to gain insight into the acceleration process as well as allow a comparison of the beam quality with respect to conventional acceleration their detection is of interest. A basic detection method is the application of scintillating screens. For the low charges which are typical of plasma acceleration, a high-sensitivity phosphor screen was investigated and calibrated. For longitudinal profiling based on common spectroscopic investigations, a spectral phase measurement technique is suggested to overcome ambiguities based on the phase retrieval. The emittance is a key parameter in accelerator science and a measure of the beamquality important for particle physics as well as light sources. Typically, the emittance is measured after the plasma interaction where the plasma transition to the vacuum might have affected it.Here, betatron radiation is accessed to get an insightinto the emittance inside the plasma. Beams both from self and ionization injection are compared, the emitted betatron radiation is investigated and the micron-level emittance deduced as well as related to numerically analyzed PIC simulations.