Developing Terahertz Frequency Drivers for Novel Accelerators

Novel acceleration schemes, such as THz-driven acceleration, aim to drastically shrink the size, and cost of future particle accelerators compared to conventional radio-frequency (RF) technology [1,2]. The high frequency and ultrashort picosecond duration of laser-generated THz pulses 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 sources 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. 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. Furthermore, THz pulses can provide longitudinal beam diagnostics via THz-driven electron beam streaking. There are, therefore, a plethora of opportunities for exploiting laser-driven THz sources to enhance accelerators. The current obstacle is the lack of laser-driven narrowband, frequency-tuneable, high-energy THz sources.

This project aims to develop a state-of-the-art laser-driven high-energy THz source at the Cockcroft Institute that can drive our current programmes utilising THz-driven acceleration, compression and diagnostics to a world-leading level. The studentship is available in the THz acceleration group at the Cockcroft Institute, and the researcher will join the Department of Physics and Astronomy at the University of Manchester. The project is experimental in nature, involving a number of high-power ultrafast lasers, including state-of-the-art femtosecond laser systems in Dr Graham’s lab at the Photon Science Institute, a Terawatt laser system at the Cockcroft Institute, and particle accelerators at STFC Daresbury Laboratory. The applicant will be expected to have a first or upper second-class degree in physics or other appropriate qualification. Hands-on experience in the use of lasers and optical components is not essential, but the student is expected to have a keen interest in experimental physics. A full graduate programme of training and development is provided by the Cockcroft Institute. The student will be based primarily at the Institute at Daresbury, with some work at the University of Manchester. A willingness to travel to international conferences and research facilities is desirable.

Plan for the 3.5 years:

This 3.5 year PhD will start in October 2023. The 1^st year of the PhD will focus on training in experimental laser physics, cryogenics, and vacuum systems. This will be done via hands-on use of the sub-10 mJ regenerative amplifier laser system and a closed-cycle helium cryostat. This will be in addition to gaining a broad understanding of accelerator physics by attending the CI graduate lecture series. Initially, the student will investigate the optical quality and damage threshold of lithium niobate (LN) and MgO-doped LN wafers from different suppliers. They will then optimise the construction of PPLN wafer stack sources [3]. The 2^nd year will focus on a detailed characterisation of the PPLN wafer stack sources, including investigating the THz conversion efficiency dependence on pump pulse energy, fluence, intensity, wavelength, and number of wafers in the PPLN stack. The experimental work will be supported with the development of THz generation simulations. As the THz sources develop, the student will contribute to the novel acceleration programmes at Cockcroft, with key contributions envisaged to the programme in the THz bunker. Here THz sources will be need to be exploited for bunch compression, streaking, and to achieve MeV acceleration. Terahertz pulse shaping via pre-engineered domain structures will also be explored by varying the wafer thicknesses used to construct the poled lithium niobate stacks. Only with such an integrated approach between THz source development and exploitation can the right source of THz be developed. In the 3^rd year the student will explore cryogenic cooling of the PPLN wafer stacks, where reduced temperatures should reduce absorption and refractive index within the lithium niobate wafers, leading to higher output energies and the ability to frequency tune the THz source to match the acceptance bandwidth of THz/electron beam mediating waveguide structures. However, as the wafers in the stack are not optically bonded, with typically small ≤ 10 𝜇m gaps between wafers (dictated by the wafer total thickness variation), there are none negligible phase-shifts. It is therefore essential to experimentally measure the frequency tuning effect of cooling a PPLN wafer stack to optimise the source performance. Towards the end of the project we will seek to test the sources on high-energy laser systems such as the FEBE laser system at Daresbury (expected to be fully operational from 2025), and also via applications to international high-energy laser facilities such as ELI-ALPS. The 4^th year will be focused on writing up the thesis and journal papers.

For more information, please contact Dr Darren Graham ([Email Address Removed])

References

[1] M. T. Hibberd et al., Acceleration of relativistic beams using laser-generated terahertz pulses, Nature Photonics, 14, 755 (2020).

[2] D. A. Walsh et al., Demonstration of sub-luminal propagation of single-cycle terahertz pulses for particle acceleration, Nature Communications 8, 421 (2017).

[3] F. Lemery et al., Highly scalable multicycle THz production with a homemade periodically poled macrocrystal, Communications Physics 3, 150 (2020).