황용석 교수

AbstractThe mission of the National Spherical Torus Experiment (NSTX) is to prove the principles of spherical torus physics by producing high-βt plasmas that are noninductively sustained and whose current profiles are in steady state. The NSTX will be one of the first ultralow-aspect-ratio tori (R/a ≤ 1.3) to operate at high power (Pinput up to 11 MW) to produce high-βt (25 to 40%), low-collisionality, high-bootstrap-fraction (≤70%) discharges. Both radio-frequency and neutral beam heating and c

A global model was developed to investigate the densities of negative ions and the other species in a low-pressure inductively coupled hydrogen plasma with a bi-Maxwellian electron energy distribution. Compared to a Maxwellian plasma, bi-Maxwellian plasmas have higher populations of low-energy electrons and highly vibrationally excited hydrogen molecules that are generated efficiently by high-energy electrons. This leads to a higher reaction rate of the dissociative electron attachment responsib

Although gas breakdown phenomena have been intensively studied over 100 years, the breakdown mechanism in a strongly magnetized system, such as tokamak, has been still obscured due to complex electromagnetic topologies. There has been a widespread misconception that the conventional breakdown model of the unmagnetized system can be directly applied to the strongly magnetized system. However, we found clear evidence that existing theories cannot explain the experimental results. Here, we demonstr

A conventional 20-MeV drift tube linac (DTL) for the Proton Engineering Frontier Project (PEFP) has been developed as part of the low-energy section of the 100-MeV proton accelerator. The 20-MeV DTL consists of four tanks, and the RF power generated by a klystron is coupled to the DTL tanks through RF power couplers. By introducing analytical and numerical methods to correlate the coupling coefficient with the coupling hole size in a dumbbell shaped iris coupler, we optimized the coupling coeffi

Measured spectra in a high count rate environment are difficult to analyze because of the spectral distortions caused by the pulse pile-up effect. This study proposes a deep learning-based method for separating and predicting the true pulse height of a signal with pulse pile-up events for application to radiation measurement and spectroscopy with a scintillation detector. To train the deep learning model, pulse signals simulating scintillation pulses were prepared by using a predefined mathemati

An integral imaging system enabling extended depth of field was proposed and demonstrated based on a birefringence lens array (BLA) whose focal length was switched via the light polarization. The lens array system was constructed by combining two different liquid crystal(LC) embedded lens arrays, BLA I and II, which were fabricated by injecting a ZLI-4119 LC and an E-7 LC in between a lens array substrate and an ITO (indium-tin-oxide) glass plate respectively. The BLA I played a role as a convex

This paper presents a simulation model for the generation of strong pressure wave by means of pulsed spark discharge in water and its application to well cleaning. In the simulation model, one-dimensional time-dependent magnetohydrodynamic equations are coupled to a capacitive discharge circuit equation. A cylindrical conducting spark channel formed by electrical breakdown of water gap between a pair of electrodes is treated as a load of which resistance and inductance are allowed to change with

This research investigates structural change made by Naver's online news section and how it has impacted the overall traffic flow of Korea's online news. This paper examined 45 websites in 2008 and 2010, and the total number of pages viewed within these sites was considered in the analysis. Social network analysis was applied to study the relationships between the news sites. The analysis through degree centrality and Bonacich powers shows that there has been a shift in market leadership. In 200

In this work, integral photography (IP) using variable focusing lens arrays is proposed. To achieve both fast response time and short focal length of the lens array for IP, we designed a biconvex lens structure. We fabricated two kinds of liquid crystal (LC) lens array cell: one using the cohesion of a UV curable polymer, and the other using photolithography. From the results of measuring electrooptic properties of the fabricated cell, we discovered that synchronized elemental image array for th

A folded RF front-end with merged LNA and mixer is designed in 0.25 mum CMOS process for low power applications. The designed low power RF front-end uses the folded and current reuse techniques. It consumes 13.8 mW from 1.7 V supply voltage, while 5.2 dB NF and 25.7 dB conversion gain are achieved. By using PMOS in the switching stage of the folded mixer, low flicker noise of 7 dB at 10 kHz is also achieved. The proposed RF front-end can be used in direct conversion receiver for a relatively nar

A variable gain LNA with large and continuous gain variation is proposed. By adding a simple gain-control loop at the second stage of a cascade LNA, a continuous gain tuning range of 28 dB is achieved. The maximum gain and NF of the LNA are 19 dB and 2.1 dB, respectively, while the DC power consumption is 13.5 mW.

A dual-band direct conversion receiver front-end is designed in 0.18 mum CMOS process. It is composed of two LNAs and a dual-band mixer. A dual-band mixer uses the switched inductor matching network to select the bands such as 2 GHz and 5 GHz band. Two LNAs are switched according to the operation frequency, and the gain is controlled for multi-standard capability; thus it can be applied to a multi-standard transceiver that covers WCDMA and 802.11a/b/g

In the low-aspect ratio tokamak regime, a lower q(a) regime (i.e. q(a)<=5, A =R/a =1.5) has been explored in CDX-U, and the ultra-low-aspect-ratio tokamak regime (1.05 <= A <= 1.5) has been explored in TS-3. Using a relatively low toroidal magnetic field, plasma discharges with Ip <= 53 kA and q(a) >= 4[qcy(a) >= 1] have been obtained in CDX-U. Low q(a), ohmic plasmas in CDX-U show increasing MHD activity as the edge safety factor ir lowered. These models appear to reduce the c