Terahertz driven dielectric linacs

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In particle physics, future linear colliders such as CLIC or ILC require ultra-short, sub-picosecond, bunches to obtain the luminosity necessary for the particle physics science exploration. In accelerator based X-Ray sources, sub 10fs bunches are highly sought after, opening new windows on material science. Going beyond these massive accelerators will require higher gradient structures to shrink the size and cost of future high energy colliders. Current metallic microwave structures are limited by electric breakdown to around 100 MV/m.In the drive to obtain every shorter particle bunches, the synchronization of the bunches, both to each other and to facility infrastructure (RF, drive lasers, facility clocks, diagnostic systems) is crucial for the facility optimization, and yet continues to present unsolved problems. Both generation and synchronization of such ultra-short particle beams presents a subsequent problem for the measurement of the ultra-short bunches; only two technologies exist that approach the requirements of future linear colliders and accelerator light sources; laser based 'electro-optic' detection and transverse deflecting cavities. However, EO detection is not currently able to address the few-fs regime, while the gradient requirements make RF (Transverse deflecting cavity) TDC's infeasible for sub-ps measurements on high energy particle physics machines. A high gradient, high frequency (>300 GHz) TDC would allow the same technique to be applied to ultra-short, high energy colliders.Here we propose to develop a blue-skies approach to these problems, using laser generated THz radiation to be coupled to dielectric lined waveguide for acceleration and manipulation of those beams. In extending the GHz-RF techniques into the picosecond/THz regime we gain in time resolution for manipulation and detection; we gain orders of magnitude in the temporal gradient necessary for manipulation on the sub-picosecond time scale; we gain orders of magnitude in energy efficiency, through only generating the accelerating and deflecting fields over a time window matched to the beams to be accelerated (rather than matched to microsecond cavity filling times). Similar high profile research at Stanford has demonstrated gradients in excess of 250 MV/m using laser pulses at much shorter wavelengths. The short timescales involved and the lack of metallic-vacuum transitions prevents the breakdown in the structure up to the dielectric strength of the lining or grating. However in contrast to a wide body of research into optical acceleration methods, the THz approach allows complete trapping of 'conventional' picosecond bunches rather than the high frequency optical smearing associated with the <3fs periods of laser-acceleration concepts. By using THz the electron bunch can occupy a small range of phases producing a far higher quality bunch consistent with requirements from accelerator applications such as colliders and FELs. One of the first steps towards this goal is the demonstration of laser-based schemes for the acceleration and deflection of relativistic electron beams. We propose to take preliminary simulations and experiments to the next stage of proof-of-concept demonstration on STFC's test accelerator, VELA. Our approach is a melding of concepts in electro-optic detection and THz generation (areas of expertise for Jamison and Graham) and particle physics cavity design (expertise of Burt). By using an integrated team the structure and source can be developed in tandem to provide a technology ideally suited to accelerator development. While other experiments have taken place with sub-relativistic particles this would be the first acceleration of fully relativistic particles by a THz structure and would be the first transverse deflecting cavity operating in the THz regime.