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Normalized ion distribution function in expanding sheaths of 2D grid electrodes
Yi, Changho,Namkung, Won,Cho, Moohyun IOP 2016 PLASMA SOURCES SCIENCE AND TECHNOLOGY Vol.25 No.2
<P>Ion distributions in expanding collisionless sheaths of two-dimensional (2D) grid electrodes were studied by using XOOPIC (particle-in-cell) simulations when short pulses of negative high-voltage were applied to electrodes immersed in plasmas. 2D grid electrodes consist of a periodic array of cylindrical electrodes, and the opening ratio of the grid electrodes is defined by the ratio of the spacing between cylindrical electrodes to the periodic length of the grid electrodes. In this paper, we introduce a normalized ion distribution function in normalized coordinates, and it is shown by simulation that the normalized ion distribution function depends only on the opening ratio of the grid electrodes. When the opening ratio of the grid electrodes is fixed, the ion distribution in expanding sheaths can be easily found in various conditions using only a single run of a PIC simulation, and the computation time can be significantly reduced.</P>
Particle-in-Cell Simulation on Power Coupling of Grill of KSTAR 5.0-GHz LHCD Launcher
Young-soon Bae,Moohyun Cho,Won Namkung 한국물리학회 2004 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.44 No.52
The 5.0-GHz, 2.0-MW KSTAR Lower-Hybrid Current Drive (LHCD) system is under design for steady-state operation of the KSTAR tokamak. The launcher structure is designed as a grill (4 rows by 32 columns; phased-array waveguide structure) that provides asymmetric power spectra. The KSTAR tokamak plasma is a high-density and high-temperature plasma with an edge density of 1018 m
Quench Protection System for the Superconducting Coil of the KSTAR Tokamak
Song, Inho,Choi, Changho,Cho, Moohyun Institute of Electrical and Electronics Engineers 2007 IEEE transactions on applied superconductivity Vol.17 No.1
<P>This paper presents two-stage high-power mechanical-thyristor switches developed as 40-kA direct current (dc) circuit breakers for the superconducting coil energy dump system of Korea superconducting Tokamak advanced research (KSTAR). An advanced switching system used in the quench protection should allow multishot mode of operation without maintenance and long lifetime with arcless dc current commutation. The system is designed to combine the advantages of the both mechanical and solid-state power switches. The mechanical stage is to conduct dc current for normal superconducting coil operation and the thyristor stage is to provide fast arcless dc current interruption for quench protection (QP). Compared with the all solid-state switches and vacuum interrupters of equal capabilities the proposed switch is much cheaper than the dc breakers and has lower power dissipation in current conducting state 'Kuchinski in Proc. Power Ele., 899-903, 1995;' 'Benfatto IEEE Power Del., Vol. 3, No. 4, pp. 1372-1376, Oct. 1998.' Also, it is much faster than the vacuum interrupters. The system is designed to break 40-kA dc current within 200 ms. Also, KSTAR device, a configuration of the energy dump system and coil power supplies are presented</P>
Design of a High-Eciency S-band Klystron with a Multi-cell Output Cavity
Jihyun Hwang,Sung-Ju Park,Won Namkung,Moohyun Cho 한국물리학회 2018 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.73 No.8
As large-scale scientific facilities develop, the klystron eciency becomes an important issue to reduce high operating costs. We are developing high-eciency klystrons for the Pohang Light Source-II (PLS-II) and the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) at the Pohang Accelerator Laboratory (PAL). The newly developed klystrons should have the same specifications, including perveance and overall length, to replace existing ones. Under these limited conditions, we designed the klystrons based on a multi-cell output cavity to improve the power-conversion eciency significatly. We simulated the beam-dynamics using the Field Charge Interaction (FCI) code with a MATLAB script to find the optimum design parameters for high eciency. We have determined the design parameters, such as cell tuning frequencies, inter-cell distances, couplings between cells and the Qe of the last cell, of the multi-cell output cavity .
Stability Analysis of a C-band 500-kW Klystron with a Multi-Cell Output Cavity
Jihyun Hwang,Sung-Ju Park,Won Namkung,Moohyun Cho 한국물리학회 2016 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.69 No.6
A prototype 5-GHz 500-kW CW klystron (model E3762 provided by Toshiba Electron Tubes & Devices Co. Ltd.) has been operating as the RF source for the lower hybrid current drive (LHCD) system in the KSTAR tokamak. A cavity design study is being carried out with a simulation code based on the main klystron’s operation parameters in order to investigate how the efficiency of the 5-GHz 500-kW CW klystron prototype can be enhanced. This is being done by simulating the klystron’s performances for various cavity parameters, including the number of cavities, the inter-cavity distance, and the cavity’s tuning frequencies. The simulation has been done with the FCI (field charge interaction) code aided by a matlab script for scanning input parameters. The initial set of scan parameters was obtained by benchmarking the E3762 klystron, and we are able to obtain optimized design parameters for a cavity system with better efficiency by adopting a multi-cell output cavity. However, the multi-cell output cavity is prone to self-oscillations due to the prolonged (several half RF periods) beam-field interaction along its multiple gaps. We have checked the feasibility of the optimization by evaluating the stability of the output cavity system. The stability is given by the ratio of a beam-loading conductance to the circuit conductance.
Study on rectangular-to-circular waveguide mode converter for KSTAR LHCD transmission line
Kim, Jeehyun,Seong, Taesik,Namkung, Won,Cho, Moohyun Elsevier 2018 Fusion engineering and design Vol.135 No.1
<P><B>Abstract</B></P> <P>A major upgrade of KSTAR LHCD system to 4 MW is planned for 2021. The system will be composed of eight 5-GHz 500-kW klystrons, low loss transmission lines with oversized circular waveguides, and a passive active multijunction launcher. Circular TE<SUB>01</SUB> mode will be adopted for transmission lines longer than 50 m because it has extremely low loss in oversized waveguide, but mode converters from rectangular TE<SUB>10</SUB> to circular TE<SUB>01</SUB> mode are not easy to design. Mock-ups of two types of mode converters were developed. One is serpentine type, and the other is symmetric sidewall coupling type. The TE<SUB>01</SUB> mode purities were demonstrated by measuring the transmission efficiencies between two identical mode converters at various angles between symmetric planes. A sidewall coupling mode converter with a refined design, which showed better circular symmetry and higher transmission efficiency than serpentine type, is under fabrication for prototype. The prototype mode converter is calculated to have 99.5% transmission efficiency, with 0.16% diffraction loss and 0.35% resistive loss, according to the simulation.</P>