Multicell accelerating structure driven by a lens-focused THz pulse

Recently, gradients on the order of $1\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ have been obtained with single-cycle ($\ensuremath{\sim}1\text{ }\text{ }\mathrm{ps}$) THz pulses produced by the conversion of high peak power laser radiation, in nonlinear crystals ($\ensuremath{\sim}1\text{ }\text{ }\mathrm{mJ}$, 1 ps, up to 3% conversion efficiency). Such high intensity radiation can be utilized for charged particle acceleration. For efficient acceleration, a large number of accelerating cells have to be stacked together with delay lines in order to provide proper timing between accelerating fields and moving electrons. Additionally, THz pulses in individual cells need to be focused transversely to maximize the accelerating gradient. This focusing is currently done by the tapering of the waveguide. In this paper, we propose instead to use dielectric lenses for the robust focusing of THz pulses in accelerating cells. Each lens is made from the same material as the delay line in a monolithic unit. We have fabricated prototype units from quartz and silicon. Such an approach simplifies the fabrication and alignment of the multicell accelerating structure. We present a design in which a 100 microJoule THz pulse produces a 600 keV energy gain in a 5-mm long 10-cell accelerating structure for an ultrarelativistic electron. This approach can be extended to nonrelativistic particles.