Energy modulation of electron bunches using a terahertz-driven dielectric-lined waveguide

Healy, Alisa and Burt, Graeme (2020) Energy modulation of electron bunches using a terahertz-driven dielectric-lined waveguide. PhD thesis, Lancaster University.

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Abstract

In this thesis, the use of a rectangular dielectric-lined waveguide for energy modulation of electron bunches is presented. The choice of waveguide allows for guided THz pulse propagation with phase velocity matched to the electron velocity. The effect of dispersion, in particular group velocity slippage, has been explored and the choice of a narrow bandwidth THz pulse discussed with regards to the increase in interaction length and minimised group velocity dispersion. A coupler was designed for maximising transmission into the accelerating mode of the waveguide. A non-conventional THz source design was required to generate the correct mode. Modelling of the interaction was performed with different methods and tools so as to investigate the required accuracy of simulations. The use of the Time-Domain (TD) and Particle-in-Cell (PIC) solver in CST Microwave Studio (CST-MWS) was compared with purpose-built simulations in Mathematica. It was established that for narrowband THz pulses the interaction as a function of time delay between THz and the bunch is well approximated by a sinusoidal energy modulation. PIC simulations were used to verify the THz bandwidth and centre frequency for which this approximation was valid. A full structure was designed, manufactured and analysed. THz time domain spectroscopy allowed for measurement of the dispersion relation to compare to the model. Dimensional analysis gave the dimensions of the apertures of the structure. The dimensional analysis showed that, due to a manufacturing error, the waveguide dimensions were larger than designed. Experimental work performed using the CLARA beam at Daresbury Laboratory demonstrated energy modulation of a long, chirped electron bunch. This has potential for use as a bunch diagnostic to assess the time-dependent properties. The THz source was of limited energy, showing that only small laser power is required for such a scheme. An energy spread increase of approximately 8 keV was verified, but full bunch acceleration was not observed