Sun-Shin Jung

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.

Wide-band semivane and heavily dielectric loaded helix traveling-wave tubes

Theoretical simulation of the dispersion and interaction impedance characteristics of a semivane loaded negative dispersion helical slow-wave structure, which is a slight variant of a conventional vane-loaded helix of a wide-band traveling-wave tube (TWT), was validated for the nonresonant perturbation measurement. While the experimental and theoretical results agreed well with respect to interaction impedance for both the semivane structure and the structure without vanes, there was disagreement found with respect to the dispersion characteristics of the semivane structure, which was explained quantitatively by the deformation that had taken place in assembling the structure. The output performance of a TWT using the semivane structure fairly agreed with the prediction by an in-house one-dimensional nonlinear Lagrangian code, with respect to saturated output power (/spl sim/50 W), saturated gain (/spl sim/40 dB), and lower-band-edge second harmonic level (/spl sim/-5 dBc), in the operating frequency range of 6-18 GHz, except at the low frequency end of the operating band, where the saturated power was less and the second harmonic level higher than predicted, a finding that may be attributed to the departure of the deformed structure from exhibiting a negative dispersion at the lower band edge.

Positive phase-velocity tapering of broadband helix traveling-wave tubes for efficiency enhancement

A positive phase-velocity tapering of 1.5 octave broadband helix traveling-wave tubes for efficiency enhancement, where the phase velocity is linearly increased in the output section, was studied by using the one-dimensional nonlinear theory. At high frequencies, the electromagnetic wave in the positively tapered section traps the fastest electrons in the decelerating electric field, extracting more energy from the electron beam. At low frequencies, a decreased velocity difference between the electron beam and the electromagnetic wave destroys the phase condition for second-harmonic generation, retaining fundamental wave efficiency as well as reducing its second-harmonic power.

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.

Effect of conductive perturber diameter on nonresonant measurement of interaction impedance for helical slow-wave structures

The measurement accuracy of the interaction impedance for a helical slow-wave structure (SWS) using the nonresonant perturbation method has been studied using conductive wire perturbers with different diameters. Data obtained by the measurement were compared with a rigorous numerical analysis. It is shown that the measured values of the interaction impedance for the helical SWS converge to those obtained by using a three-dimensional finite-element computational method when the diameter of the perturber is reduced to less than 10% of the helix diameter.

1.5 octave wideband traveling-wave tube with heavily-loaded helical slow-wave structure

Summary form only given. A 1.5 octave wideband traveling wave tube (TWT) with a helical structure loaded by the thick dielectric support rods has been designed and fabricated for the frequency range of 6-18 GHz. Helical slow-wave structure (SWS) was modeled using three-dimensional HFSS code. The nonresonant perturbation measurement using a thin copper wire with 20 mm diameter was performed to verify the phase velocity and interaction impedance of the helical structure. The performance of TWT was predicted using one-dimensional (1-D) nonlinear theory involving a macro particle beam model. The harmonic effect was considered in this calculation. The measured performance of TWT using a beam voltage 4 kV and a beam current of 120 mA was shown. These results were compared with a 1-D nonlinear theory. The comparison showed that the measured power and gain were less than the predicted one but had a similar trend over the operating frequency range. The 2nd harmonic levels at the low frequency range of 6-8 GHz were nearly 0 dBc. This relatively high 2nd harmonic level might be attributed to the positive dispersion at the low frequency range due to the deformation of a barrel during the assembly process.

Method for measuring interaction impedance in helix TWT

A nonresonant method for measuring the interaction impedance in a helix TWT using a hairline conductive wire is studied. The theory of the interaction impedance measurements using a dielectric and/or a conductive perturbing rod with a finite diameter, becomes complex when the upper space harmonics of a slow wave structure are taken into account. This problem may be overcome if one uses a hairline conductive wire instead of a dielectric rod with a finite diameter. The diameter of this conductive wire placed precisely along the helix axis, is about 1-2% of the helix diameter. It is shown that in this case the change of the propagation constant characterises the interaction impedance of the fundamental space harmonic independently of the upper harmonics.

Investigation of folded waveguide TWT

The folded waveguide TWT using a linear electron beam is being investigated both theoretically and experimentally. A nonlinear theory is studied. The cold test is performed comparing with the numerical prediction. The experimental work is now underway and will be presented.

Improvement of Two-Dimensional Terahertz Image by Digital Image Processing

Two-dimensional (2D) images that are produced by terahertz (THz) irradiation we presented. It is possible to obtain 2D image of various materials by observing the amplitude and the phase of the THz signals which go through them. Better images are produced by combining the amplitude and phase of the signal rather than using only one of these. Homomorphic filtering that is one elf the well-known technique of digital image signal processing is effective to reduce the noise signal and can provide better quality images. The results can be applied to real-time imaging afterwards.

Pulsed Terahertz Emission and Detection Properties from ZnTe Crystal

Pulsed terahertz (THz) radiation was generated by optical rectification and detected by a fee space electro-optic sampling (FS-EOS) method. We used ZnTe (110) crystals for both generation and detection. By coating dielectric anti-reflection film on the ZnTe crystal surface, we can reduce the reflectance of a pump laser beam from to , and the terahertz pulse amplitude increased compared with an uncoated crystal. A wider bandwidth of THz radiation was obtained by using a thinner crystal but the signal intensity was decreased in this case. And variations of THz radiation by changing orientation of the ZnTe crystal with respect to the pump (or probe) laser polarization, and by changing the power of the pump laser have also been investigated and discussed.