Yong Seok Hwang

Abstract Discharge with an external magnetic field is promising for various applications of low-temperature plasmas from electric propulsion to semiconductor processes owing to high plasma density. It is essential to understand plasma transport across the magnetic field because plasma confinement under the field is based on strong magnetization of light electrons, maintaining quasi-neutrality through the inertial response of unmagnetized ions. In such a partially magnetized plasma, different deg

Abstract An efficient and robust ECH (electron cyclotron heating)-assisted plasma start-up scheme with a low loop voltage and low volt-second consumption utilizing the trapped particle configuration (TPC) has been developed in the versatile experiment spherical torus (VEST). The TPC is a mirror-like magnetic field configuration providing a vertical magnetic field in the same direction as the equilibrium field. It significantly enhances ECH pre-ionization with enhanced particle confinement due to

Characteristics of helicon plasmas are examined as a candidate for accelerator ion sources. Helicon plasmas have much higher density with very high power efficiency in a relatively lower magnetic field than electron cyclotron resonance plasmas do. High electron density and high electron temperature are beneficial to extract high ion current density. Low magnetic field favors for low-emittance accelerators. Taking advantage of placing antenna outside of the plasma chamber, source contamination ca

Abstract Composition, thermodynamic properties, and electrical conductivity of water plasma are calculated under the assumption of thermodynamic equilibrium. The plasma composition is calculated by stoichiometric approach in which chemical reactions and multiple ionizations are formulated by following mass action law and Saha's equation, respectively. Correction for weakly non‐ideal behavior at high density regime is conducted using the Debye‐Hückel theory. Based on the calculated composition, t

Abstract Internal reconnection events (IREs), one of the relaxation events driven by internal magnetohydrodynamic (MHD) instabilities in fusion plasmas, are accompanied by a strongly MHD-correlated blob at the edge in the Versatile Experiment Spherical Torus spherical tokamak. The MHD-correlated blob plays a significant role in the onset and the strength of IREs. Various techniques analyzing visible camera images show correlated waveforms between blobs and magnetic fluctuations, and they produce

In time-resolved measurement for triton burnup in Korea Superconducting Tokamak Advanced Research (KSTAR) deuterium plasmas, an NE213 liquid scintillation detector was installed and operated during the 2017 KSTAR campaign. The detector is composed of an NE213 scintillator (50 mm in diameter and 10 mm in thickness) and a photomultiplier tube (PMT). The PMT anode signal was processed under a data acquisition system which contains a field programmable gate array circuit and pulse processing softwar

Reconstruction and modeling of the plasma current profiles in a fully pressure driven tokamak have been performed in the Current Drive Experiment-Upgrade. The reconstructed experimental current profile has a significant deviation from that of the calculated neoclassical currents. Satisfactory agreement between the measured and calculated model profiles was obtained by including a helicity conserving current diffusion term in the modeling which created the required self-generated ``seed'' current

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

At TRIUMF, H− ion sources have been characterized on a teststand to improve the understanding of their performance. Measured beam characteristics such as current, emittance, and e∕H− ratio were correlated with source conditions as a function of relevant plasma parameters. Plasma densities, temperatures, and plasma potentials were measured with a Langmuir probe and correlated with beam properties for different confining magnetic fields and different values of arc power and gas pressure. The mecha

A Thomson scattering (TS) system has been utilized to measure the electron temperature and density of the core region of Versatile Experiment Spherical Torus (VEST). Recently, the laser injection system is successfully upgraded adopting the burst laser with the repetition rate of 1 kHz and the energy of 2 J. Furthermore, improved collection optics with additional polychromators and a 32-channel fast digitizer are prepared to observe the fast time evolution of radial profiles. This improvement is

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