Terahertz-driven acceleration of subrelativistic electron beams using tapered rectangular dielectric-lined waveguides

We investigate the use of tapered rectangular dielectric-lined waveguides (DLWs) for the acceleration of low-energy, subrelativistic, electron bunches by the interaction with multicycle narrowband terahertz (THz) pulses. A key challenge exists in this subrelativistic regime; the electron velocity changes significantly as energy is gained. To keep electrons in the accelerating phase, the phase velocity must also be increased to match. We present simulations which demonstrate that the dielectric thickness can be kept constant and the width of the dielectric lining can be tapered along the direction of travel to vary the phase velocity, an approach only possible by the use of a rectangular waveguide geometry. The properties of tapered DLWs are discussed and following this, a design process is presented to demonstrate that the way this tapering can be optimized for different pulse and beam parameters. The minimum accelerating gradient for electron bunch capture is derived and compared to simulations. As examples of this design process, designs are considered based on considerations of the THz source, incoming electron beam, and manufacturing tolerances. A maximum THz pulse energy of $22.5\mathrm{\mu }\mathrm{J}$ in the DLW was considered, which represents what is readily achievable using mJ-level regenerative amplifier laser systems together with optical-to-terahertz conversion in lithium niobate crystals. This will be more than double the energy of a 100 keV electron beam, increasing it to 205 keV. We describe the optimization process and present a detailed exploration of the beam dynamics, discussing how the performance will further improve with compressed bunches.