Y.D. Joo

High order mode formation of externally coupled hybrid photonic-band-gap cavity

The electromagnetic field distribution obtained from a finite-difference-time-domain simulation shows that a hybrid photonic-band-gap (PBG) cavity enveloped by a dielectric lattice and three metallic walls provides a better field uniformity of a high order mode, TMmn0, than a conventional one does under an external coupling with the maintenance of a high quality factor of the metallic cavity. Experimentally measured reflection and transmission scattering matrices of a TM550 mode show that the hybrid PBG structure improves the field uniformity to within 10% compared with a larger variation reaching a few tens of a percent with the conventional cavity under critical coupling.

Theory of high-power wide-band traveling-wave tube using coaxial inverted helical groove slow-wave structure

A novel slow-wave structure (SWS), the coaxial inverted helical groove structure, is presented and those of its properties used for wide-band traveling-wave tube (TWT) are investigated. The first part of the paper concerns the wave properties of this structure in the case of a vacuum. The influence of the geometrical dimensions on dispersion characteristics and interaction impedance are investigated. The theoretical results reveal a very weak dispersion for the fundamental wave in the structure. The negative dispersion can be realized by a suitable selection of the structural parameters. The interaction impedance of the fundamental wave is about 10 /spl Omega/. The interaction impedance of the -1 space harmonic wave is much lower than that of the fundamental wave. Thus, the risk of backward wave oscillation is reduced. The software high frequency structure simulator (HFSS) is also used to calculate the dispersion property of the SWS. The simulation results from HFSS and the theoretical results agree well, which supports the theory. In the second part, a self-consistent linear theory of a coaxial inverted helical groove TWT is presented. The typical small signal gain per period is about 0.5 dB and the 3-dB small-signal gain bandwidth can exceed 25% with a 33-dB gain of tube.

Electromagnetic Wave Propagation through an Azimuthally Asymmetric Helix Slow Wave Structure

In the fabrication of the practical helix slow wave structure (SWS), the misalignment or the dielectric constant variation in its support is not avoidable and, thus, causing azimuthal asymmetry in the structure. Therefore, the analysis of the asymmetric structure becomes an important issue. In this study, rigorous field analysis of a general asymmetric helical SWS was done by considering the structure as a helix supported by n wedge shaped dielectric supports, enclosed in a metal envelope. To consider the angular harmonic effect, the dielectric was not smoothed out. Using the present theory, the dispersion characteristics of the asymmetric helical structure were obtained. The study of the dispersion characteristics of the asymmetric structure revealed the stopband characteristics near the π-phase-shift point. This result was compared with that obtained from 3D FEM code, high frequency structure simulator (HFSS) and found to be in good agreement. It was also found that the asymmetry in the structure lowered the phase velocity and the cut-off frequency but increased the stopband width.

Dispersion characteristics of an asymmetric helical slow wave structure

In a practical traveling tube (TWT), helix slow wave structure (SWS) consists of a helix loaded by a number of dielectric supports arranged at fixed angular intervals, enclosed by a metal shell. In this paper we analysis the general asymmetric helical SWS estimates the dispersion characteristics through dispersion relation.

Simplified approach to the nonlinear analysis in helix slow-wave-structure for a traveling wave tube

A stationary 1-D nonlinear code based on Lagrangian disk model is developed on the basis of a simple set of analytical expressions to study nonlinear dynamics in the helix slow-wave structure used in a traveling wave tube. The loss profiles such as triangular and Gaussian types are modeled as stairsteps and a simple formula is developed to found the loss at a plane for such loss profile. In contrast to the earlier works in nonlinear theory, at present, no numerical method is used at any stage. The method, introduced in this work, is general in nature because it can handle (a) multi-section structure with sever, (b) different loss profiles, namely, center (Gaussian) and tip (triangular: increasing or decreasing), (c) space charge effect on the electrons, (d) backward waves arising due to reflections, etc. Accuracy of the theory and code is verified with comparison of the computed present results with the results from simulation code MAGIC and published elsewhere and found to be in good agreement. The generation and suppression of the harmonic power are studied for a typical structure. It is found that the introduction of resynchronization section of the reduced pitch enhances the fundamental power with the reduction of the second harmonic power. In addition, the method can be used for any helix slow-wave-structure consisting of homogeneous/inhomogeneous dielectric support rods in isotropic/anisotropic overall metallic enclosure, because the axial propagation constant and interaction impedance obtained for any structure and model such as sheath and tape helix approximations or from any simulation codes can be used as the input in the program to make the code more general.

Design of a Third-Harmonic Gyrotron Oscillator Using a Photonic Crystal Cavity

A photonic crystal cavity of the transverse electric (TE) mode is designed to support the third harmonic interaction with a 30 keV–0.7 A axis-encircling electron beam at the operating frequency of the Ka-band, where a magnetic field of 0.38 T is used. The lattice parameters of a square lattice of alumina rods are determined to fit the TE301-like mode in the photonic-band-gap spectral region, and the output signal of a fundamental waveguide mode is conveniently coupled to a conventional waveguide. A particle-in-cell code simulation predicts the single mode operation with the output power of concerning 2.9 kW, corresponding with an efficiency rating of 14%. This scheme can be extended to a n-th harmonic gyrotron using the rectangular cavity of a TEm01-like mode.

Analysis of pi-point instability in an asymmetric helical slow-wave structure in helix traveling wave tubes

The energy exchange takes place between the linear electron beam and the electromagnetic wave propagating in the helix (E.I. Lien, IEDM Tech. Digest, pp. 412-415, 1979; T. Onodera, IEEE Trans. ED, vol. 35, no 10, 1988; B. Levush, Int. Vacuum Electron. Conf., pp. 71-72, 2002; B. N. Basu, Electromagnetic Theory and Applications in Beam-Wave Electronics, World Publ. Co. Inc. 1996; J. R. Pierce, Traveling Wave Tubes, D Van Nostrand, New York, 1950; L. N. Loshakov et al., Radio Eng. Electron Phys., vol. 13, 1968). For the efficient operation of the device, the beam velocity should be nearly synchronous with the axial phase velocity of the wave over the frequency range. Thus, the dispersion characteristics of helix SWS become of great importance. In this study, rigorous field analysis (B.N. Basu, et al., J. Appl. Phys., vol. 58, no. 9, 1985) of a general asymmetric helical SWS was done.

SINCOHET: simple nonlinear analysis code for helix traveling wave tube

In this study, a stationary 1-D nonlinear code based on Lagrangian disk model is developed on the basis of a simple set of analytical expressions to study nonlinear dynamics in the helix slow-wave structure used in a traveling wave tube.

Simple nonlinear analysis for a helix travelling wave tube

Helix travelling wave tube was analyzed by nonlinear theory and the results taken from this analysis were compared with the results obtained from 2D MAGIC code.

CW Terahertz imaging of paraffin-embedded epithelia cell of rat

CW Terahertz imaging system is being constructed to investigate the response of cell to the terahertz wave. This system consists of a 0.2 THz radiation source and some off-axis-parabolic mirrors (OAPs) with 2 dimensional (2D) translation stage. Detection is achieved with a pyroelectric sensor operated at room temperature. Biological terahertz imaging of a ratpsilas paraffin-embedded epithelial cell with tumor is presented.