Kyu-Ha Jang

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.

Microfabrication of millimeter wave vacuum electron devices by two-step deep-etch x-ray lithography

The circuits for millimeter wave vacuum electron devices with all circuit elements including an electron beam tunnel are microfabricated by two-step deep-etch x-ray lithography (x-ray LIGA). The discrepancies of eigenfrequency between experiment and simulation are within 1.1% in a coupled-cavity structure and 1.4% in a folded waveguide structure when the operating frequency is about 100GHz. Furthermore, a measured tolerance of below 2μm, and a measured surface roughness of 20–70nm, of LIGA-fabricated circuits implies the two-step LIGA microfabrication has potential applications up to the submillimeter wave region.

Low-voltage operation of Ka-band folded waveguide traveling-wave tube

Low-voltage operation of millimeter-wave folded waveguide traveling-wave tube (TWT) was investigated using a 12 kV linear electron beam. Backward wave oscillation operated at second space harmonic was observed with output power of 20 W, linear tunability of 6% within 0.01 dB/MHz, and voltage-frequency stability of 0.56 MHz/V. The measured frequency and output power are in a good agreement with the predicted values using a particle-in-cell (PIC) code. In addition, backward-wave interaction at second-space harmonic as an amplifier was observed with a measured linear gain of 15 dB and a bandwidth of 0.3%. For forward wave interaction, a linear gain of 25 dB, bandwidth of 17%, and efficiency of 7% were predicted for fundamental space harmonic using a PIC code. Improvement in efficiency was predicted for the forward-wave amplifier where it was operated as an oscillator employing a delayed feedback. Output power was increased by 10 dB in the delayed feedback oscillator comparing with the backward-wave oscillator. The effect of period doubling due to potential fabrication inaccuracies on the stopband was studied experimentally. The folded waveguide TWT fabricated using the Lighographie, Galvanoformung, Abformung (LIGA) process operated at much higher frequencies is discussed.

Investigations on a microfabricated FWTWT oscillator

A vacuum tube oscillator fabricated by the deep etch X-ray lithography: lithographie, galvanoformung, abformung (LIGA) process was successfully developed for the first time. For the proof-of-concept experiment involving a delayed feedback oscillator, a folding-waveguide traveling-wave tube (FWTWT) was fabricated by computer numerical control milling in advance. With a 12.4-kV and 150-mA electron beam, a Ka-band FWTWT amplifier shows a linear gain of 25 dB and a bandwidth of 10%. Applying a delayed feedback scheme, the threshold feedback strengths for the onset of oscillation and for self-modulation were measured to be -30 dB and -16 dB, respectively. The optimum value of the feedback strength for the single-frequency oscillation at 32.5 GHz was about -18 dB with a net electronic efficiency of 6%. The LIGA-fabricated FWTWT circuit was constructed by a lithographic process using the synchrotron X-ray source at the Pohang Light Source. The resulting accuracy and average surface roughness were less than 10 and 1 /spl mu/m, respectively. The LIGA-fabricated Ka-band amplifier shows a linear gain of 15 dB and bandwidth of 1.7% with a 12.4-kV, 47-mA electron beam. The threshold for the onset of oscillation was about -11 dB and the optimum value of feedback strength for a single frequency oscillation at 35 GHz with a net electronic efficiency of 3.5% was about -8 dB.

Experimental investigations on miniaturized high-frequency vacuum electron devices

We investigated the foundations for high-frequency vacuum electron devices experimentally, with emphasis on deep etch X-ray lithography: lithographie, galvanoformung, abformung (LIGA) to fabricate a miniaturized interaction circuit and a photonic crystal (PC) resonator to excite a stable high-order mode. The successful operation of a LIGA-fabricated folded-waveguide traveling-wave tube was reported. From such physical considerations as Debye length and photonic band gap, we proposed a reflex klystron adopting a cold cathode and a PC resonator.

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.

Nonstationary behavior in a delayed feedback traveling wave tube folded waveguide oscillator

Folded waveguide traveling-wave tubes (FW TWT) are among the most promising candidates for powerful compact amplifiers and oscillators in millimeter and submillimeter wave bands. In this paper, the nonstationary behavior of a FW TWT oscillator with delayed feedback is investigated. Starting conditions of the oscillations are derived analytically. Results of numerical simulation of single-frequency, self-modulation (multifrequency) and chaotic generation regimes are presented. Mode competition phenomena, multistability and hysteresis are discussed.

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.

Improvement of Field-Emission Characteristics of Carbon Nanotubes by Post Electrical Treatment

The field-emission characteristics of carbon nanotubes (CNTs), such as uniformity and brightness, were improved by electrical treatment using nonstationary electric fields between the cathode of screen-printed CNT emitters and the anode of a phosphor-coated indium-tin-oxide glass substrate in diode configuration. Dead or weak emission spots, where almost no emission of electrons was observed, started to emit electrons by applying an alternating-current voltage to the cathode electrode and a constant voltage to the anode electrode. The nonstationary electrical treatment was more effective than the direct-current (dc) and the square-pulsed electrical treatments for the emission uniformity and brightness. It was found that the nonstationary electrical treatment not only activated CNT emitters but also suppressed abnormally high emission spots without the drawbacks of electrical breakdown. Consequently, more than 1.8 and 1.3 times improvements of emission uniformity and brightness, respectively, were obtained after the treatment, when compared with the dc electrical treatment for the same amount of emission currents and the same duration of the treatments. Therefore, the method can be effectively applied to field-emission devices based on CNTs for the enhancement of emission properties.