About:

Amy MacLachlan is a Chancellor’s Fellow / Lecturer in the Atoms, Beams and Plasmas group at the University of Strathclyde.  Her research involves the study of collective non-linear relativistic electrodynamics, and in particular, the development of Cherenkov sources based on novel multi-dimensional phase control lattices to address the long-standing terahertz gap.

In 2022-2023, Amy was seconded to the Culham Centre for Fusion Energy (CCFE) to explore the potential for novel microwave sources for plasma heating and current drive in fusion tokamaks.  

Amy obtained an integrated master’s degree in physics from the University of Strathclyde in 2011 and was awarded her PhD on the control and manipulation of complex electrodynamic structures in 2016.

Cherenkov oscillators based on multi-dimensional interaction cavities hold strong promise to address the long-standing “terahertz gap”.  Sources with the potential to deliver kilowatts or megawatts of power in the THz spectral range are urgently needed for applications in fusion energy (microwave sources for plasma heating and current drive in tokamaks, turbulence diagnostics for fusion plasma), enhancing Nuclear Magnetic Resonance using Dynamic Nuclear Polarisation (DNP-NMR) for biochemical spectroscopy and drug discovery, radar, remote sensing, and security.

In traditional backward wave oscillators, the transverse dimension of the interaction region is comparable to the free-space radiation wavelength (λ) ensuing adequate phase and spectral coherence, but restricting the output power (P~1/f2) at high frequencies. The interaction volume can be increased, while avoiding the excitation of parasitic modes, through the use of a two-dimensional (2D) surface lattice interaction cavity. The 2D corrugations mediate the formation of a cavity eigenmode, composed of coupled volume (high radial order) and surface (high azimuthal order) partial modes. The excitation of this eigenmode provides the mode selection required for stable operation. Coherent radiation is generated by the Cherenkov interaction between the cavity eigenmode and a thin annular electron beam. 

Maximum energy extraction has been achieved by optimizing the non-linear dynamics of the system over a wide range of control parameters and modifying the electromagnetic dispersion near the point of interaction. Numerical simulations show good agreement with the theoretical predictions. We present the detailed design of powerful, efficient Cherenkov sources, scalable both in frequency and in transverse size1-4, with potential to deliver  high-frequency megawatt (MW) pulses, or continuous-wave MW radiation for RF heating and current drive in fusion tokamaks. 

Our results show that slow-wave, 2D surface lattice Cherenkov sources can operate efficiently in the mm to sub-mm wave part of the spectrum, with the modest beam and wave power density required for high average power CW operation with a cylindrical cavity diameter exceeding 9 times  λ.

The authors gratefully acknowledge support from the Air Force Office of Scientific Research (AFOSR) through grants FA8655-13-1-2132, FA9550-17-1-0095 UK EPSRC - University of Strathclyde Impact Acceleration Account and the UKAEA STEP project.

References
[1] A. J. MacLachlan, C. W. Robertson, A. W. Cross et al., IEEE Trans. Electron Dev., 69,11, 2022
[2] A. J. MacLachlan, C. W. Robertson, A. W. Cross et al., IEEE Trans. Electron Dev., 70,6, 2023
[3] A. J. MacLachlan, C. W. Robertson, I.V. Konoplev et al., Phys. Rev. Appl., 11(3), 034034, 2019
[4] A. R. Phipps, A. J. MacLachlan, C. W. Robertson, et al., Nuclear Instrum. and Methods in Phys. Res. B, 402, pp.202-205, 2017