Six-dimensional phase space preservation in a terahertz-driven multistage dielectric-lined rectangular waveguide accelerator

Staged acceleration, driven by terahertz (THz) frequency radiation pulses in a lattice with alternating orientation dielectric-lined waveguides and intervening matching optics, is shown to mitigate transverse emittance and energy spread growth, opening a route to multistage THz linacs. Decomposition of the longitudinal THz field into the multipolar components reveals a quadrupole field component with strong radial dependence. As such, it induces a transverse energy correlation in the beam during acceleration due to the large variation in the electric field with radius and azimuthal position of the electrons. An alternating orientation of stages separated by a matching section provides a compensation of transverse energy spread correlation induced in the beam during its interaction with the THz field. Furthermore, the monopolar component of the accelerating LSM11 mode was found to be constant with respect to transverse position, entailing zero monopolar transverse voltage and preventing emittance growth, unlike conventional radiofrequency structures. We demonstrate in a rectangular dielectric-lined waveguide structure that, when used for the acceleration of relativistic electrons, the slice transverse emittance is conserved and the growth in the slice energy spread is reduced by 70%-80% simultaneously over a system of two stages, each providing an interaction length of 4 mm and an energy gain of up to 2 MeV.