Abstract
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 ${\mathrm{LSM}}_{11}$ mode was found to be constant with respect to transverse position, entailing zero monopolar transverse voltage and preventing emittance growth, unlike conventional radio-frequency 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.
Highlights
The generation of free electron lasers and particle colliders will require injectors that can deliver high-quality beams with bunch durations far shorter than what are currently achievable with higher accelerating gradients than currently available
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
We demonstrate that by employing an orthogonal multistaging scheme, the transversely correlated slice energy spread introduced by the THz field can be corrected up to 82%, alongside the transverse emittance, conserving the six-dimensional phase space for the studied regime of up to 0.5 GV=m accelerating fields
Summary
The generation of free electron lasers and particle colliders will require injectors that can deliver high-quality beams with bunch durations far shorter than what are currently achievable with higher accelerating gradients than currently available. This experiment used a dielectric-lined rectangular waveguide structure driven by narrow-band, frequency-tunable, polarization-tailored THz pulses and demonstrated up to 10 keV acceleration of a 2 ps (subcycle) bunch with an initial energy of 35 MeV It marked a key milestone on the path to whole-bunch linear acceleration of subpicosecond electron beams with multistaged concepts capable of preserving beam quality. It is important to have all stages synchronous with the electron beam and ideally at the same frequency to simplify the THz sources This requires some degree of tunability in the accelerating structure, which is achieved with rectangular dielectric lined waveguides as the gap can be adjusted.
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