Aufbau und Test eines flexiblen Laserpuls-Strahlformungssystems für den FLUTE Photoinjektor zur Kontrolle von Elektronenpaketen = Setup and test of a flexible laser pulse shaping system for the FLUTE photoinjector towards the control of electron bunches

This work was performed at the Ferninfrarot Linac- und Test-Experiment (FLUTE) at the Karlsruhe Institute of Technology (KIT). FLUTE is designed as a test facility for accelerator physics, as well as a source of high-intensity coherent THz radiation [1].
Very short electron bunches are needed to generate the coherent THz radiation, which requires a high degree of electron beam control. Since the electron source of FLUTE is a photocathode, the laser pulse hitting it, represents the first adjustable parameter that determines the electron beam shape and thus significantly affects the beam characteristics.
Good control of the laser pulse is thus essential for these challenging conditions. For example, an elliptical laser pulse shape leads to a reduction in emittance [2].
In this work, the first step is taken toward a system that provides transverse and longitudinal/ temporal laser pulse shaping based on spatial light modulators (SLM). This can be integrated into a feedback system based on machine learning for machine optimization, or make individual adjustments to meet experimental requirements. Transverse laser pulse shaping was initially tested in a test setup on a visible alignment laser.
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A variety of different application scenarios were run through using SLM control software specially programmed for this purpose, before the functionality was demonstrated in a side arm of the actual FLUTE photoinjector laser by generating arbitrary patterns. Special emphasis was placed on the reduction of unmodulated light components that inevitably occur in the process, and various methods for this were compared.
The longitudinal laser pulse shaping follows the scheme of a 4F pulse compressor built directly in the side arm of the FLUTE photinjector laser. The use of this scheme became necessary due to the ultrashort fs laser pulses and is based on a spectrometer setup in which an SLM was integrated into the spectrometer plane. An inverted spectrometer setup after modulation by the SLM recomposes the spectral components. Due to the correlation between spectral and temporal domain, the chirp of the laser pulse, a spectral modulation thus leads to a temporal/longitudinal laser pulse shaping. This makes it possible to shape even fs laser pulses longitudinally. For this purpose, a concept was developed and the design was verified by means of a simulation. The concept was then implemented and adjusted accordingly. Subsequent calibration made it possible to determine the relationship between the pixel position on the SLM and the corresponding wavelength of the spectrum imaged on it. This allowed the functionality of the setup to be demonstrated using a spectral top-hat. This represents the first step towards temporal modulation.
This work is thus an important building block on the way to better control over the generated electron bunches and therefore the generated THz pulses.