The advent of technology has enabled the development of metamaterials. Essentially, these are materials comprised of a synthetic composite with a structure such that they exhibit electromagnetic properties that are not found in natural materials, especially a negative refractive index. Consequently, they have been used in designing passive microwave devices such as filters, perfect lenses, antennas and phase shifters. In recent times, there has been an increasing interest in investigating the use of metamaterials for designing coherent sources of high power microwave radiation. Presently, Backward Wave Oscillators (BWOs) and Traveling Wave Tubes (TWTs) are the popular and most widely used vacuum electron devices for generating and amplifying microwaves, respectively. They operate based on Cherenkov radiation, which generates coherent output power from an electron beam passing through a slow wave structure with phase velocity less than the speed of light. The aforementioned slow wave structure system is quite expensive to fabricate. To address this, 2D planar metamaterials have been proposed for designing 2D slow wave structure vacuum electron devices. Nonetheless, a thorough review of existing literature reveals that further research is still necessary.
There is increasing interest in designing a BWO which does not have a large volume and at the same time can increase electron beam interaction with slow wave structure loads. In this view, University of New Mexico researchers from the Department of Electrical and Computer Engineering: Hamide Seidfaraji (Ph.D. student leading the work), Dr. Ahmed Elfrgani, Professor Christos Christodoulou and Professor Edl Schamiloglu developed a fast growth response, efficient BWO which could optimize electron beam coupling to a metamaterial slow wave structure through a multibeam interaction. The researchers proposed to study the interaction of a multibeam cathode with a set of metamaterials inside a cylindrical waveguide. Their work is currently published in the research journal, Physics of Plasmas.
Metamaterials possess interesting properties such as the negative refractive index, low group velocity, and below cutoff propagation, among others. In their approach, a specific structure was developed for the purpose of addressing the subject matter. The developed structure comprised of a number of metamaterial metallic plates that were periodic in the axial direction and were repeated in the azimuthal coordinate. A unit cell of their structure comprised four pairs of rectangular plates that were oriented 90 degrees apart in the azimuth. One of the pair of plates provided negative permittivity while the other negative permeability. As a consequence, the developed system was not based on a cutoff waveguide and can support higher power levels than a system based on a cutoff waveguide.
The authors reported that simulations of their designed four-beam BWO with an electron beam of 440 kV and a current of 250 A per beam (1 kA in total) generated 105 MW with a rise time of about 50 nanoseconds. Additionally, by using the eigenmode simulations, the researchers were able to obtain negative dispersion around the operating frequency where the group velocity was reported to be negative and extremely small.
In summary, University of New Mexico scientists demonstrated a novel multibeam metamaterial BWO for the first time. The proposed metamaterial was an all-metallic structure with a thickness of more than 2 mm, which would be robust in a high-power microwave environment. Overall, the study described a multibeam metamaterial slow wave structure for high power microwave generation as an oscillator.
Hamide Seidfaraji, Ahmed Elfrgani, Christos Christodoulou, Edl Schamiloglu. A multibeam metamaterial backward wave oscillator. Physics of Plasmas, volume 26, 073105 (2019).Go To Physics of Plasmas