  Terahertz (THz)-based electron acceleration and manipulation has recently
been shown to be feasible and to hold tremendous promise as a technology for
the development of next-generation, compact electron sources. Previous work has
concentrated on structures powered transversely by short, single-cycle THz
pulses, with mm-scale, segmented interaction regions that are ideal for
acceleration of electrons in the sub- to few-MeV range where electron
velocities vary significantly. However, in order to extend this technology to
the multi-MeV range, investigation of approaches supporting longer interaction
lengths is needed. Here, we demonstrate first steps in electron acceleration
and manipulation using dielectrically-lined waveguides powered by temporally
long, narrowband, multi-cycle THz pulses that co-propagate with the electrons.
This geometry offers centimeter-scale single-stage interaction lengths and
offers the opportunity to further increase interaction lengths by cascading
acceleration stages that recycle the THz energy and rephase the interaction. We
prove the feasibility of THz-energy recycling for the first time by
demonstrating acceleration, compression and focusing in two sequential
Al2O3-based dielectric capillary stages powered by the same multi-cycle THz
pulse. Since the multi-cycle energy achievable using laser-based sources is
currently a limiting factor for the maximum electron acceleration, THz energy
recycling provides a key enabling factor for reaching relativistic energies
with existing sources.
