M. Thumm

Observation of the high-Q modes inside the resonance zone of two-dimensional Bragg structures

Electrodynamical properties of two-dimensional Bragg resonators of planar geometry are studied in the frame of the geometrical-optical approach and in three-dimensional simulations. Results of the theoretical analysis are corroborated by data obtained in “cold” microwave measurements. Formation of the high-Q eigenmodes in the vicinity of the Bragg resonance frequency has been demonstrated.

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...

Short-wavelength tunable Bragg reflectors based on coupling of propagating and cutoff waves: Modeling and experimental studies

To advance relativistic masers into the short wavelength region of the millimeter and sub-millimeter wavelength bands, advanced Bragg reflectors based on coupling of propagating and quasi-cutoff waves were proposed in Ginzburg et al., Appl. Phys. Lett. 95, 043504 (2009). In this paper, we present experimental examination of the reflectors of such type, which was carried out in the 4-mm wavelength band. A power reflection coefficient of ∼0.8 was measured in the bandwidth of ∼200–300 MHz. Tunability of the reflection zone by varying the distance between the plates in the frequency band of 3%–5% was demonstrated. Experimental data agree well with the result of 3D simulations.

Production of Powerful Spatially Coherent Radiation in Planar and Coaxial FEM Exploiting Two-Dimensional Distributed Feedback

Two-dimensional distributed feedback is an effective method of producing ultrahigh-power spatially coherent radiation from an active medium, that is spatially extended along two coordinates, including relativistic electron beams with sheet and annular geometry. This paper describes the progress in the investigations of planar and coaxial free-electron masers (FEMs) based on a novel feedback mechanism. The theoretical analysis of these FEM schemes was conducted in the frame of the coupled-wave approach and 3-D simulations and agrees well with the experimental data obtained in ldquocoldrdquo and ldquohotrdquo tests. As a result, the effective transverse (azimuthal) mode selection has been demonstrated under a transverse size of about 20-25 wavelengths, and narrow-frequency multimegawatt microwave pulses have been generated in the Ka- and W-bands.

High selective two-dimensional Bragg resonators of planar geometry: Theoretical, computational, and experimental study

Electrodynamic properties of two-dimensional (2D) planar Bragg resonators realising 2D distributed feedback were studied. It is shown that high selectivity of a 2D Bragg resonator originates from the specificity of the dispersion diagrams of normal waves in a double-periodic corrugated unbounded planar waveguide resulting in formation of high-Q eigenmodes in the vicinity of the Bragg resonance frequency. Results of theoretical analysis in the frame of geometric-optical approach of coupled-wave theory coincide well with 3D simulations using the code CST “Microwave Studio” and are corroborated by data obtained in “cold” microwave tests. Frequency dependences of scattering coefficients were measured for incident wave-beams of different structures and excitation of the high-Q eigenmodes has been demonstrated.

Selective Properties of Planar Bragg Reflectors and Their Application for Multichannel Masers

Abstract In This Paper we Discuss Selective Properties of Planar Reflectors Composed by a Pair of 1-D or 2-D Bragg Gratings with Various Types of their Surfaces Corrugation, Which can Realize One- or Two-Dimensional Distributed Feedback in a Maser. The Properties of Reflectors are Investigated by Using Theory, Computer Simulations and “cold” Measurements. Obtained Results Allow One to Create a Superpower Generator by Combining Several Identical Planar Fem-Oscillators in Common Multichannel Device. PACS Codes: 41.60. Cr, 42.25. Fx, 84.40.1k