Electron Beam Manipulation in Terahertz Frequency Radiation-Driven Structures

Finlay, Oliver and Jamison, Steven and Bailey, Ian (2022) Electron Beam Manipulation in Terahertz Frequency Radiation-Driven Structures. PhD thesis, Lancaster University.

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Abstract

Electron beam manipulation using THz frequency radiation has been considered in two regimes, fully relativistic and sub-relativistic. For the fully relativistic regime, experiments were undertaken at 35 MeV, while the sub-relativistic experiments were undertaken with 100 keV electron bunches. Acceleration of relativistic electron beams in a THz-driven rectangular dielectric-lined waveguide was demonstrated at the Compact Linear Accelerator for Research and Applications (CLARA) facility. To the author’s knowledge, this experiment was the first demonstration of THz-driven linear acceleration of a fully relativistic beam. Here, factors effecting efficiency of acceleration, including THz transport, mode-conversion, THz dispersion, and THz-beam velocity matching, are examined. Experimental verification of THz phase velocity to particle velocity matching is provided through the frequency response of the interaction strength and the observed modulation period. Velocity matching is key to achieving long interaction lengths between particle bunches and laser fields. This is the first conclusive demonstration of its achievement in a THz-driven linear accelerator. The sub-relativistic regime was explored by THz-driven deflection and compression of 100 keV electrons. Deflection has been performed experimentally to diagnose the electron bunch duration with two different structures: a dielectric-lined waveguide for extending the interaction length by velocity matching, and a sub-wavelength aperture in a metal foil for breaking the free-space propagation symmetry. The streaking speed achieved with the dielectric-lined waveguide compares favourably with previously reported experiments, which is indicative of a bunch duration diagnostic with high temporal resolution. Finite difference and semi-analytical calculations have been performed with these two structures to characterise the compression interaction. Simulation results show that the THz-driven dielectric waveguide is capable of achieving bunch compression within the interaction vacuum chamber using only a fraction of the available THz pulse energy. The results bode well for future experimentation where compression and deflection of subrelativistic electrons will be performed in conjunction.