L. Fisher

Surface wave Cherenkov maser based on a periodic lattice

The excitation of a surface wave cavity based on a two-dimensional periodic structure by an energy source, such as relativistic electrons, results in stimulated, single frequency coherent emission. A high-Q cavity has been achieved via a resonant coupling between surface waves and volume waves. The concept of a Cherenkov maser based on the surface wave cavity is discussed and results of numerical studies presented. Links between the model described and the concept of a surface plasmon amplifier, which has been recently introduced are described.

Wave propagation and tunneling through periodic structures

The phenomenon of tunneling manifests itself in nearly every field of physics. The ability to distinguish a wave tunneling through a barrier from one propagating is important for a number of applications. Here we explore the properties of the wave traveling through the band gap created by a lattice, either as a consequence of tunneling through the barrier or due to the presence of a pass band inside the gap. To observe the pass band for studying tunneling and propagating waves simultaneously, a localized lattice defect was introduced. The differences between the two phenomena are highlighted via waves’ dispersion characteristics.

Surface-field cavity based on a two-dimensional cylindrical lattice

A Ka-band cavity based on a two-dimensional periodic lattice is studied. The apparatus designed to excite the cavity is discussed. Coupling between the surface and volume fields is demonstrated and pulse propagation through the cavity is investigated. The measurements agree well with simulations.

Excitation of surface field cavity and coherence of electromagnetic field scattering on two-dimensional cylindrical lattice

The excitation of a surface field cavity based on a large area two-dimensional cylindrical lattice and surface field scattering within the cavity are investigated. It is shown that the interaction between surface and volume fields via distributed scatterers becomes coherent and the cavity excitation takes place only when it is irradiated with a near cut-off transverse-magnetic polarized field. The coherence of the radiation observed from the surface field scattering is investigated.

Mechanism of azimuthal mode selection in two-dimensional coaxial Bragg resonators

The analysis of electrodynamic properties of two-dimensional (2D) Bragg resonators of coaxial geometry realizing 2D distributed feedback was carried out using a quasioptical approach of coupled-wave theory and three-dimensional (3D) simulations. It is shown that the high selectivity of a 2D Bragg resonator over the azimuthal index originates from the topological difference in the dispersion diagrams of the normal symmetrical and nonsymmetrical waves near the Bragg resonance frequency in a double-periodic corrugated unbounded waveguide. For a symmetrical mode near the Bragg frequency it was found that the group velocity tends to zero as well as its first derivative. This peculiarity of the dispersion characteristic provides the conditions for the formation of an eigenmode with a Q-factor essentially exceeding the Q-factors of other modes. The results of the theoretical analysis coincide well with results of 3D simulations using the CST code “MICROWAVE STUDIO” and confirm the high azimuthal selectivity of t...

High-Current Electron Beams for High-Power Free-Electron Masers Based on Two-Dimensional Periodic Lattices

High-power gigawatt-level radiation can be generated by the interaction of an electromagnetic wave and an annular electron beam with a transverse dimension much larger than the operating wavelength. The use of such a large-circumference annular beam allows the generation of high beam currents while also maintaining low space charge and RF power densities inside the interaction region. This circumvents the problems associated with potential depression in the beam channel and RF breakdown inside the oscillator. In this paper, we present the studies of high-current magnetically confined annular electron beams and discuss their production and transportation through a coaxial beam channel which formed the interaction region of a free-electron maser (FEM). The results from numerical simulations, using the software packages KARAT and MAGIC, are compared with the experimental measurements. The operation of a FEM, driven by a high-current annular electron beam, is presented, and the tunability of the maser, inside a frequency range defined by an input 2-D Bragg mirror, is demonstrated.

Two-dimensional periodic lattice cherenkov maser: Scalability from 37.5GHz TO 350GHz

Masers capable of producing high-power output radiation in the spectral range from 100s of GHz to low-THz, to address a range of user needs will be presented. Future applications include remote sensing and biological/medical imaging (to reduce the exposure time during large area mapping). Compact masers that are capable of producing the required output power at these frequencies presently are not widely available. Many current techniques require high-voltage (sometimes up to 500kV) high-current electron beams giving multi-MW output powers. However at low-THz (0.3THz–1THz) frequencies many applications require an output power of 1kW-10kW but need compact transportable sources of coherent radiation that use a relatively low voltage (from 50kV-100kV) and low current (up to 100 A) electron beam.

Surface field cavity based on a two-dimensional cylindrical lattice

A Ka-band cavity based on a two-dimensional periodic lattice is studied. The apparatus designed to excite the cavity is discussed. Coupling between the surface and volume fields is demonstrated and pulse propagation through the cavity is investigated. The measurements agree well with simulations.

Cherenkov maser based on a twodimensional periodic lattice

The recent results from the study of periodic structures have made a significant impact in many branches of physics and led to a number of breakthroughs in photonics, optics, electronics and communications. We discuss the concept1 and the results of studies of a Cherenkov maser based on a 2D lattice. The maser considered is capable of producing high-power output radiation in the high-GHz - low-THz frequency range, which is very attractive for a number of applications. Although the ability to generate high-power output radiation is necessary for remote sensing, imaging, biological and medical science (for instance to reduce the exposure time during large area mapping), masers that are capable of producing the required output power in the high GHz to low THz frequency range are still not available. One notes that, the Fresnel number of a typical maser based on a closed cavity and driven by a mildly-relativistic electron beam (Lorentz factor γ ≥ 1.2) is usually much smaller than 1, limiting the masers interaction region aperture, while keeping the interaction length large as compared with the operating wavelength, which acts to maintain single mode steady state lasing. Problems arise as one seeks to reduce the operating wavelength resulting in a reduction in the transverse aperture which leads to lower output power, high energy losses and maser miniaturization. To maintain the coherence of the radiation the use of a twodimensional periodic lattice to form the interaction region is considered. We present the results of study of a Cherenkov maser based on a 2D periodic lattice used to achieve the desired level of output power while still maintaining the coherence of the output radiation. In this case the large interaction region formed by the periodic lattice allows a moderate power density inside the beam-wave interaction space whilst simultaneously avoiding the formation of electron beam instabilities associated with high beam charge density. We demonstrate that the structure also provides effective mode selection over the azimuthal and radial wave numbers. This ensures a single mode, steady state operation of the maser with an interaction space defined by a large diameter (a/λ ∼7) 2D periodic cylindrical structure. The results of the studies of the operation of a Cherenkov maser based on a 2D periodic lattice are presented and we show that a single mode, steady state operation at a frequency of ∼90GHz was achieved. The conditions required to observe effective interaction between the electron beam and an EM wave are also discussed.

Design of a high power W-band aser based on a two dimensional periodic structure

High power millimetre-wave sources operating in the W-band (75 GHz-110 GHz) frequency range are important for a number of applications. This work will focus on the design and construction of a high power maser operating in the W-band frequency range, which can be capable of generating spatially and temporally coherent radiation at a power of at least 10 MW.