Gun-Sik Park

Vacuum Electronic High Power Terahertz Sources

Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from <; 0.1 to >; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

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.

Broadband operation of a Ka-band tapered gyro-traveling wave amplifier

A wideband low-voltage millimeter-wave gyro-traveling wave tube (gyro-TWT) amplifier operating in the TE/sub 10/ rectangular waveguide mode at the fundamental cyclotron frequency is under investigation, The device incorporates precise axial tapering of both the magnetic field and the interaction circuit for broadband operation. Experimental results of a wide (33%) instantaneous bandwidth with a small signal gain in excess of 20 dB and saturated efficiency of /spl sim/10% were achieved and shown to be in good agreement with the theory. Reflective instability due to multi-pass effects by mismatches was observed and characterized. Gain and efficiency have been limited by this reflective instability rather than by absolute instabilities which limit the performance of gyro-TWTs with uniform cross-section. The start-oscillation current in terms of the relevant experimental parameters such as the beam velocity ratio (/spl alpha/), magnetic field detuning and reflection coefficient has been measured and compared with theory. Measurements of the phase variation in terms of the RF frequency have shown that the phase varies /spl plusmn/30/spl deg/ from fitted linear phase line. >

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.

Theoretical Study for Folded Waveguide Traveling Wave Tube

A wideband folded waveguide traveling-wave tube (TWT) amplifier has advantages of simpler coupling structures and robust structure over the conventional helix TWT. The phase velocity of waves in folded waveguide is slowed down to the velocity of electron beam. Slow-wave interaction with the electron beam in folded waveguide is studied in a linear fashion. For a cold beam, the linear theory predicts a gain of 2 dB/cm and a bandwidth of 37% at the center frequency of 14 GHz. A closed algebraic dispersion relation for the frequency and the axial phase shift per period is obtained using an equivalent circuit model. Numerical solution calculated from the dispersion relation and three-dimensional electromagnetic code, HFSS simulations predict a mode coalescing in the folded waveguide. And a theoretical phase velocity prediction of the electromagnetic wave in this circuit is verified by HFSS simulations.

Three-dimensional simulation of super-radiant Smith-Purcell radiation

A simulation of coherent and super-radiant Smith-Purcell radiation is performed in the gigahertz regime using a three-dimensional particle-in-cell code. The simulation model supposes a rectangular grating to be driven by a single electron bunch and a train of periodic bunches, respectively. The true Smith-Purcell radiation is distinguished from the evanescent wave, which has an angle independent frequency lower than the minimum allowed Smith-Purcell frequency. We also find that the super-radiant radiations excited by periodic bunches are emitted at higher harmonics of the bunching frequency and at the corresponding Smith-Purcell angles.

Novel coupled-cavity TWT structure using two-step LIGA fabrication

In order to make millimeter-wave sources practical, improved fabrication methods are required. The use of Lithographie, Galvanoformung, Abformung; German acronym (LIGA) to fabricate a coupled-cavity traveling-wave tube (TWT) is presented. The proposed circuit design will focus on two-step LIGA fabrication and dual in-line ladder structure with a rectangular beam tunnel. The fabrication sequence of two-step LIGA is illustrated and the advantages and tradeoff of this approach are described. Three electromagnetic codes-HFSS, GdfidL, and MAGIC3D-are employed to analyze the circuit characteristics in cold and hot test simulations. Finally, the results from small and large signal analyses are presented to verify the validity of the circuit design.