Non-linear Electrodynamics Mediated by Multidimensional Surfaces

The project will be concerned with the development of high-power sub-terahertz and terahertz (THz) microwave sources to address the long-standing "THz gap". The THz gap refers to the dearth of powerful and moderately powerful sources of electromagnetic (EM) radiation in the 0.1-10THz range. The challenges associated with the generation of radiation at THz frequencies, using either electronic or lasing techniques, are well-documented.

Despite these challenges, research in this area is stimulated by growing demand from applications. Novel THz sources operating in different regimes are urgently needed to meet the requirements for biochemical spectroscopy (enhancing Nuclear Magnetic Resonance using Dynamic Nuclear Polarisation), current drive and plasma heating in tokamaks (for the eventual realization of clean fusion energy), communications, radar, non-destructive testing and plasma diagnostics capable of operating in inherently noisy environments.

One device which holds strong promise to deliver kilowatts, or even megawatts, of power in the THz domain, is the 'overmoded' surface wave oscillator (SWO). In conventional microwave oscillators based on cylindrical interaction cavities, the cavity diameter D scales with the source wavelength (lambda) to maintain the phase and spectral coherence of the output radiation.

At shorter wavelengths, the reduced interaction volume greatly impacts on the power handling capabilities of the device. When D is increased in relation to lambda (D/ lambda >>1), the system is said to be "oversized", and therefore overmoded.

The suppression of parasitic modes is pertinent to all overmoded systems. Shallow periodic corrugations can be machined onto the inner interaction cavity wall to reduce the number of permitted eigenmodes (through the introduction of complex boundary conditions and phase-matching constraints). The excitation of a single eigenmode, and therefore single-frequency radiation, can be achieved through the resonant coupling of volume and surface waves.

However, this problem becomes increasingly challenging as D/ lambda is increased and research is needed to determine what limiting factors might exist. This could allow game-changing mode control and therefore substantial uplift in peak and average power from microwave sources based on novel interaction structures.

Studies of SWOs based on complex surface structures have focused primarily on steady state wave-beam interactions. As such, the potential for new and diverse regimes remains relatively unexplored. This project will develop understanding of the highly non-linear electrodynamics in the stationary (steady-state) and non-stationary (transient) regimes.

In the latter, slippage of the wave relative to the particles (due to a difference in the group velocity of the EM wave and drift velocity of the driving electron beam) compresses all the free electron energy into a short duration superradiant pulse, with the potential for exceptionally high instantaneous power. By developing understanding of the complex underpinning physics, a new generation of THz sources can be envisioned.