EunMi Choi

Nano-CNC Machining of Sub-THz Vacuum Electron Devices

Nano-computer numerical control (CNC) machining technology is employed for the fabrication of sub-THz (100-1000 GHz) vacuum electron devices. Submicron feature tolerances and placement accuracy have been achieved and surface roughness of a few tens of nanometers has been demonstrated providing high-quality radio frequency (RF) transmission and reflection parameters on the tested circuit structures. Details of the manufacturing approach are reported for the following devices: W-band sheet beam (SB) klystron, two designs of a 220-GHz SB double-staggered grating traveling wave tube (TWT), 263-GHz SB TWT amplifier for an electron paramagnetic resonance spectrometer, 346-GHz SB backward wave oscillator for fusion plasma diagnostics, 346-GHz pencil beam backward wave oscillator, and 270-GHz pencil beam folded waveguide TWT self-driving amplifier. Application of the nano-CNC machining to nanocomposite scandate tungsten cathodes as well as to passive RF components is also discussed.

Direct Real-Time Power Measurement of a High-Power Electron Cyclotron Maser by a Simple One-Point Schottky Detector Signal

Measurements of the absolute power of high-power millimeter/THz wave generators have been considered to be highly variable and to occasionally produce faulty results owing to experimental difficulty. In this study, we propose a calibration-free method for real-time power measurement of an electron cyclotron maser system, a so-called gyrotron. We demonstrate the feasibility of the method by using a recently developed W-band gyrotron with a peak power of tens of kilowatts. The technique is based on transmitter antenna power measurement by using the Friis transmission equation with a defined gyrotron antenna gain. The new technique will allow extremely simple and accurate real-time measurement of output power over a wide dynamic range ( dB) for continuous wave sources to high-power millimeter/THz wave sources with nanosecond pulse lengths.

A comprehensive analysis of a TE11 to HE11 mode converter for an oversized F-band corrugated waveguide

We have investigated the mode transition behavior from approximately half- to quarter-wavelength in typical F-band (90–140 GHz) TE11 to HE11 converters of varying corrugation depths. Simulations in which the number of slots in the corrugation is varied indicate that the optimal number of slots should be greater than two, which is in excellent agreement with the experimental results. The mode content was also analyzed with and without a quarter-wavelength depth-corrugated tapered section. The electric field pattern at the exit of a quarter-wavelength tapered transition connected to a half-wavelength to quarter-wavelength TE11 to HE11 mode converter is compared with that without a converter. Electric field patterns both with and without a half- to quarter-wavelength TE11 to HE11 mode converter show a highly Gaussian field pattern with less than 35 dB of cross-polarization. Simulated field patterns are in good agreement with the experimentally measured field patterns, with the measured and simulated electric field distributions matching approximately 96.5 and 97.5%, respectively, to a pure Gaussian distribution.

Orbital Angular Momentum (OAM) of Rotating Modes Driven by Electrons in Electron Cyclotron Masers

The well-defined orbital angular momentum (OAM) of rotating cavity modes operating near the cutoff frequency excited by gyrating electrons in a high-power electron cyclotron maser (ECM)—a gyrotron—has been derived by photonic and electromagnetic wave approaches. A mode generator was built with a high-precision 3D printing technique to mimic the rotating gyrotron modes for precise low-power measurements and shows clear natural production of higher-order OAM modes. Cold-test measurements of higher-order OAM mode generation promise the realization towards wireless long-range communications using high-power ECMs.

In situ endoscopic observation of higher-order mode conversion in a microwave mode converter based on an electro-optic probe system.

Visualizing the electromagnetic field transformation inside a microwave mode conversion region has been considered to be only realizable by simulation studies. For the first time, we present a comprehensive experimental observation of the electric field transformation occurring inside a metallic waveguide TE(01)-to-TE(02) mode converter. An efficient electro-optic (EO) probe and its associated probing system were used for measuring the electric field pattern in the external near-field region as well as in the internal and penetrated region of the mode converter. Utilizing the optically measured field patterns at the aperture of the mode converter, the conversion performance from the TE(01) mode to the TE(02) mode can be also evaluated. Experimentally measured field patterns near the apertures show excellent agreement with simulation data. The mode conversion to the next higher-order mode (TE(01) to TE(02)) was experimentally demonstrated with phase-stabilized and field-animated post processing. The presented in situ endoscopic photonic measurement technique for the field evolution inside a semi-enclosed structure could be used for visually inspecting manufacturing errors in fabricated structures, and could be of great interest for research on higher-order mode formation and transmission.

Near-Field Pattern of Large Aperture Higher Order Mode Generator Using Backpropagated Fields in Free Space

This paper reports the estimation of the electric field from a large aperture cavity that generates a higher order mode by backpropagation of fields measured in free space. A higher order mode of a TE6,2 mode at 94 GHz is designed and fabricated using a quasi-optical manner for the analysis. The backpropagating field at the aperture is compared with the measured field. The field estimated at the aperture is used to analyze the mode information, such as the amount of mode mixture of two different rotating fields and the phase difference between two modes. This paper provides a quantitative way to analyze the rotating cylindrical modes generated from an oversized cavity.

Study on statistical breakdown delay time in argon gas using a W-band millimeter-wave gyrotron

In this study, we investigated plasma initiation delay times for argon volume breakdown at the W-band frequency regime. The threshold electric field is defined as the minimum electric field amplitude needed for plasma breakdown at various pressures. The measured statistical delay time showed an excellent agreement with the theoretical Gaussian distribution and the theoretically estimated formative delay time. Also, we demonstrated that the normalized effective electric field as a function of the product of pressure and formative time shows an outstanding agreement to that of 1D particle-in-cell simulation coupled with a Monte Carlo collision model [H. C. Kim and J. P. Verboncoeur, Phys. Plasmas 13, 123506 (2006)].

Cold test of gyrotron cavity modes using a 3D CFDTD method

Modeling a gyrotron cavity in 3D and time-domain is very challenging due to the open-end features of cavity structure and higher-order mode excitation employed to achieve high efficiency. In this work, a 3D conformal finite-difference time-domain (CFDTD) method has been developed to perform a cold test of a gyrotron cavity. Arbitrarily high order modes can be launched in a gyrotron cavity in time domain to determine the open cavity resonant characteristics. Our preliminary results show that the 3D CFDTD method could provide an alternative modeling tool for designing and validating a gyrotron cavity configuration for further hot tests.

Design study of a 0.4 THz 100 kW pulsed gyrotron

We present a status of development of a 0.4 THz, 100 kW pulsed gyrotron at UNIST. 0.4 THz, 100 kW gyrotron is currently under design for a remote radioactive material detection. A magnetic injection gun (MIG) is used for the electron gun with a beam voltage of 70 kV and beam current of 10 A with a pulse duration of 10 usec. A second harmonic cavity for the gyrotron interaction is considered for the high power THz gyrotron. Numerical optimization of the electron gun design and the cavity is performed in the study. In this paper, we briefly report the design study of the gyrotron.

Novosphingobium humi sp. nov., isolated from soil of a military shooting range

A Gram-stain-negative, strictly aerobic bacterium, designated R1-4T, was isolated from soil from a military shooting range in the Republic of Korea. Cells were non-motile short rods, oxidase-positive and catalase-negative. Growth of R1-4T was observed at 15-45 °C (optimum, 30 °C) and pH 6.0-9.0 (optimum, pH 7.0). R1-4T contained summed feature 8 (comprising C18 : 1ω7c/C18 : 1ω6c), summed feature 3 (comprising C16 : 1ω7c/C16 : 1ω6c), cyclo-C19 : 0ω8c and C16 : 0 as the major fatty acids and ubiquinone-10 as the sole isoprenoid quinone. Phosphatidylglycerol, phosphatidylethanolamine, diphosphatidylglycerol, sphingoglycolipid, phosphatidylcholine, an unknown glycolipid and four unknown lipids were detected as polar lipids. The major polyamine was spermidine. The G+C content of the genomic DNA was 64.4 mol%. The results of phylogenetic analysis based on 16S rRNA gene sequences indicated that R1-4T formed a tight phylogenetic lineage with Novosphingobium sediminicola HU1-AH51T within the genus Novosphingobium. R1-4T was most closely related to N. sediminicola HU1-AH51T with a 98.8 % 16S rRNA gene sequence similarity. The DNA-DNA relatedness between R1-4T and the type strain of N. sediminicola was 37.8±4.2 %. On the basis of phenotypic, chemotaxonomic and molecular properties, it is clear that R1-4T represents a novel species of the genus Novosphingobium, for which the name Novosphingobium humi sp. nov. is proposed. The type strain is R1-4T (=KACC 19094T=JCM 31879T).