Jong-Hyo Won

Superradiant terahertz Smith-Purcell radiation from surface plasmon excited by counterstreaming electron beams

The authors show that evanescent tunneling transmission of effective surface plasmon polaritons between two counterstreaming electron beams noticeably increases Smith-Purcell radiation (SPR) intensity by about two orders of magnitude as well as lower its transition threshold from a spontaneous emission to a stimulated one. An emission mechanism of the superradiant SPR is theoreticallyanalyzed by the dielectric conversion of the structured metal surface and the boundary matching condition of Maxwell’sequations in comparison with numerical simulations.

Cerenkov radiation in metallic metamaterials

The electromagnetic response of a metallic metamaterial to fast-moving electrons is studied by numerical simulations. The considered metamaterial is a one-dimensional array of slits perforated on a metallic film and is found to generate Cerenkov wakes when the electron beam travels near its surface. There is no energy threshold for the generation of such wakes, which would be promising to lower the operation energy of the electron beam in compact Cerenkov free-electron-lasers. Moreover, by analyzing the spectral dependence of the Cerenkov light, it is possible to map the dispersion relation of the guided modes supported by the metamaterial.

High order mode oscillation in a terahertz photonic-band-gap multibeam reflex klystron

TM330-like higher order mode was excited in a multibeam reflex klystron oscillator employing a hybrid photonic-band-gap (PBG) cavity using a three-dimensional particle-in-cell simulation. One side of a conventional metal cavity was replaced with a dielectric photonic crystal lattice to form a hybrid PBG resonator that uses lattice band-gap effects resulting in a more uniform field of a higher order mode as well as the exclusion of some conventional-cavity-type modes, thereby reducing mode competition. Simulated reflex klystron in the hybrid PBG cavity produced an output power much higher than could be delivered in a conventional metal cavity.

Frequency-dependent refractive index of one-dimensionally structured thick metal film

The authors have studied the frequency-dependent dielectric response of thick metal film one-dimensionally perforated by a subwavelength slit waveguide array. Our theoretical approach based on a three-dimensional homogenization of the periodic metallic structure shows that an effective refractive index chromatically varying due to TE waveguide mode formation gives rise to dense Fabry-Perot resonance population near the cutoff, which leads to an energy level continuum in the transmission spectrum. Finite difference time domain simulation numerically demonstrates the transmission spectrum and spatial field distributions of even and odd resonance modes.

Theoretical analysis of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator

The basic theory of cross-talking signals between counter-streaming electron beams in a vacuum tube oscillator consisting of two two-cavity klystron amplifiers reversely coupled through input/output slots is theoretically investigated. Application of Kirchhoff’s laws to the coupled equivalent RLC circuit model of the device provides four nonlinear coupled equations, which are the first-order time-delayed differential equations. Analytical solutions obtained through linearization of the equations provide oscillation frequencies and thresholds of four fundamental eigenstates, symmetric/antisymmetric 0∕π modes. Time-dependent output signals are numerically analyzed with variation of the beam current, and a self-modulation mechanism and transition to chaos scenario are examined. The oscillator shows a much stronger multistability compared to a delayed feedback klystron oscillator owing to the competitions among more diverse eigenmodes. A fully developed chaos region also appears at a relatively lower beam cur...

Three-dimensional particle-in-cell simulation of 10-vane strapped magnetron oscillator

The performance of a 10-vane strapped magnetron oscillator used in microwave ovens was benchmarked using the three-dimensional particle-in-cell (PIC) code MAGIC3D. The formation of the five electron spokes in the oscillation region confirms the /spl pi/-mode oscillation of a 10-vane strapped resonator showing its mode separation with the adjacent mode to be 82%. The measured operating frequency of 2.465 GHz and the saturated output power of 1.04 kW are in good agreement with the simulated values of 2.470 GHz and 1.07 kW, respectively. The magnetron with an efficiency of 75% is operated at the beam voltage of 4.3 kV, the anode current of 0.33 A, and the cathode current of 1.08 A when the external axial magnetic field of 0.19 T is applied. In addition, the measured harmonic components of the radiated output are compared with the simulated one estimated by Fourier transformation of an induced radio-frequency voltage signal, showing good agreement.

Thermal Analysis of a Strapped Magnetron

Thermal analysis of an L-band 60-kW (continuous-wave) double-ring strapped magnetron for optimizing the design of a cathode support structure and cooling system is presented. This magnetron consists of an oxygen-free copper anode with ten built-in cooling channels and a directly heated tungsten cathode. The operating temperature of the cathode is greater than 2000°C; therefore, proper thermal design of the cathode support structure is essential for reliable operation. The heat convection coefficient for the forced air is estimated by simulations comparing the simulated temperatures for different heat convection coefficients with the measured one. It is observed that the comparison between simulations with measurement is in good agreement for various temperatures from 1000°C to 2000°C when the estimated heat convection coefficient of the forced air is used.

Experimental Investigation of 95GHz Folded Waveguide Backward Wave Oscillator Fabricated by Two-Step LIGA

Most recently, 95GHz three-dimensional folded waveguide structure including the electron beam-tunnel is completely microfabricated by two-step deep-etch X-ray lithography (X-ray LIGA). The precision and positional tolerance of the circuit elements are respectively 2mum and 2-3 mum. We experimentally measured its dispersive and attenuative properties through scattering matrices and compare them to analytical and numerical calculations. The MAGIC3D simulation numerically predicted the folded waveguide BWO (FWBWO) has start-oscillation current of 30mA and produces output power of 5 Watts at the beam condition of about 12kV and 50mA. Currently, we prepare experiment of the FWBWO equipped with the electron gun

Experimental Investigation of Micro-fabricated Folded Waveguide Backward Wave Oscillator for Submillimeter Application

Successful two-step X-ray lithography (LIGA) process is demonstrated for the first time for the application to millimeter and submillimeter wave vacuum devices. The experiment of the two-step LIGA-fabricated folded waveguide BWO with a 12 kV electron gun using thermionic cathodes and cold cathodes using carbon nanotubes are being investigated at the frequency of 0.1 THz.

Efficient terahertz oscillation using a half-period staggered grating resonator

A highly efficient terahertz (THz) radiation mechanism is studied with the use of strong interaction between convection electrons and a half-period phase-shifted grating resonator. The large fraction of the fundamental TE mode is longitudinally polarized, and it excites the intense plasma–terahertz wave coupling at the shallow grating, which enables highly efficient RF generation at a relatively low operating voltage. A particle-in-cell (PIC) simulation predicts that the half-period phase-staggered grating resonator generates 0.22 THz wave with output power exceeding 100 W and interaction efficiency of more than 15% at a low beam acceleration voltage of 5.2 kV.