Terahertz-Integrated Structures for Novel Accelerators

We have developed dielectric-lined rectangular waveguide structures for terahertz-driven acceleration of electron beams, and the high-power THz sources for powering them. Our structures and sources achieve THz phase velocity matching with co-propagating electron bunches at relativistic energies. An approach which can facilitate accelerating gradients far beyond the 100 MV/m breakdown threshold typically limiting RF accelerators, with THz source development now targeting the 10 GV/m regime. The benefit of laser-driven THz approach within the field of accelerator science is however not limited solely to acceleration. Laser-generated THz pulses also offer routes to femtosecond control of electron beams and have demonstrated their ability to compress high energy electron beams [L. Zhao et al. Phys. Rev. Lett. 122, 144801 (2019) and E. C. Snively et al. Phys. Rev. Lett. 124, 054801 (2020)]. The demonstration of THz-driven compression may enable few-femtosecond duration electron beams with the femtosecond-level synchronisation control needed for external injection into other novel acceleration schemes, such as the plasma wakefield acceleration, and is the focus of the H3+Beams STFC consortia grant proposal. There are, therefore, a plethora of opportunities for exploiting laser-driven THz sources to enhance accelerators. The current obstacle is the inability to generate high-THz fields within the required dielectric-lined waveguide (DLW) mode used to facilitate the electron/THz field interaction. This stems from several compounding factors including: a low laser to THz energy conversion; a poor transport and coupling efficiency into the DLW; and losses associated with conversion of the THz field into the desired waveguide mode.

This project aims to maximise the THz radiation field in the desired accelerating mode inside a dielectric-lined waveguide by addressing the 3 key issues of generation efficiency, coupling and mode conversion. Solving these issues will allow us to drive our current programmes utilising THz-driven acceleration, compression and diagnostics to a world-leading level. The project will achieve this by developing an integrated device where generation and mode conversion occur within the waveguide structure, thereby eliminating the transport and coupling losses.

Proposed Scheme of Work

This 3.5 year PhD will start in October 2025. The 1st year of the PhD will focus on training in experimental laser physics, THz generation, and CST simulations. The experimental training will be done via hands-on use of the sub-10 mJ regenerative amplifier laser system in the THz bunker. This will be in addition to gaining a broad understanding of accelerator physics by attending the CI graduate lecture series. Initially, the student will continue to develop and optimise our periodically poled lithium niobate (PPLN) sources that generate high-pulse energies with narrow-bandwidths.

We have a deep understanding of the periodic pulse lithium niobate capabilities, with a key observation being the inter-wafer phase shifts on the on the frequency of the source. We will utilise this intra-wafer effect to develop a process for precise frequency tuning of the source. This frequency tuning is required for precision matching to the waveguide structures in presence of source and structure manufacturing imperfections, and varying laser and environmental conditions. In parallel, the student will start to design efficient mode-conversion and coupling from the non-linear THz sources to the integrated waveguide structures. For acceleration electromagnetic modes we will seek to use concepts of in-coupler conversion through coupler bends, and seek to incorporate the source within the structure assembly. The 2nd year will focus on the THz-integrated structure design and production, and later in the year the characterisation of the produced integrated-THz structures. The characterisation will be undertaken in the THz bunker where they can measure the electromagnetic coupling to the structure through time-domain THz spectroscopy. They will also test the structures using higher energy laser systems such as the 100mJ energy upgrade to the THz bunker laser system at Daresbury (expected to be fully operational from mid-2026), and also via applications to high-energy laser facilities such as ELI-ALPS. In the 3rd year, as the THz integrated-structures develop, the student will contribute to the novel acceleration programmes at Cockcroft, with key contributions envisaged to the proposed H3+Beams grant. Here THz integrated waveguide structures will be exploited for proof-of-concept demonstrations of bunch compression, and streaking. The 4th year will be focused on writing up the thesis and journal papers.