Duck-Hee Kwon

Bio: Duck-Hee Kwon is an academic researcher from Columbia University. The author has contributed to research in topics: Ion & Ionization. The author has an hindex of 8, co-authored 35 publications receiving 177 citations. Previous affiliations of Duck-Hee Kwon include KAIST.

Optical emission spectroscopy and collisional-radiative modeling for low temperature Ar plasmas

Abstract: Reliability of electron temperature and density diagnostics by an optical emission spectroscopy and a collisional radiative modeling is investigated by being compared with a Langmuir probe diagnostics for capacitively-coupled and inductively-coupled Ar plasmas in the ranges of electron temperature 1.2–2.2 eV and density 4.0 × 10 9 − 8.0 × 10 11 cm − 3 . Particular attention has been paid to radiation trapping and electron energy distribution function (EEDF) regarding the modeling. It is found that the two-temperature EEDF model gives better agreement of electron temperature diagnostics with the Langmuir probe measurement for non-Maxwellian plasma. The radiation trapping with an escape factor for finite size cylinder geometry reduces the discrepancy between the modeled and measured spectral line intensities compared to an escape factor for plane parallel geometry, specially for the optically thick 811.5 nm transition to the metastable level whose population depends on the diffusion rate.

Theoretical electron-impact-ionization cross section for Fe 11 + forming Fe 12 +

Abstract: We have calculated cross sections for electron impact ionization (EII) of P-like Fe${}^{11+}$ forming Si-like Fe${}^{12+}$. We have used the flexible atomic code (FAC) and a distorted-wave (DW) approximation. Particular attention has been paid to the ionization through the $3l\ensuremath{\rightarrow}n{l}^{\ensuremath{'}}$ and $2l\ensuremath{\rightarrow}n{l}^{\ensuremath{'}}$ excitation autoionization (EA) channels. We compare our results to previously published FAC DW results and recent experimental results. We find that the previous discrepancy between theory and experiment at the EII threshold can be accounted for by the $3l\ensuremath{\rightarrow}n{l}^{\ensuremath{'}}$ EA channels which were not included in the earlier calculations. At higher energies the discrepancy previously seen between theory and experiment for the magnitude of the $2l\ensuremath{\rightarrow}n{l}^{\ensuremath{'}}\phantom{\rule{4pt}{0ex}}(n\ensuremath{\ge}4)$ EA remains, though the difference has been reduced by our newer results. The resulting Maxwellian rate coefficient derived from our calculations lies within 11$%$ of the experimentally derived rate coefficient in the temperature range where Fe${}^{11+}$ forms in collisional ionization equilibrium.

Stable Isotope Production of 168Yb and 176Yb for Industrial and Medical Applications

Abstract: We have developed a laser isotope separation technology for the production of the 168Yb and 176Yb isotopes 168Yb is very useful for the generation of a non destructive testing source, 169Yb 176Yb can be used to produce 177Lu which is known to be a promising radioisotope for a medical application For these applications, the abundances of 168Yb and 176Yb isotopes should be enriched to more than 15% and 97%, respectively Our developed system consists of three dye lasers pumped by a diode-pumped solid-state laser, a Yb evaporator, and a photo-ion extractor Up to now, we could enrich 168Yb to more than 31% with a productivity of 05 mg/h Also, we succeeded in enriching 176Yb to more than 97% with a productivity of 27 mg/h

Laboratory spectroscopy of silicon plasmas photoionized by mimic astrophysical compact objects

Abstract: Photoionized plasmas are encountered in astrophysics wherever low-temperature gas/plasma is bathed in a strong radiation field. X-ray line emissions in the several kiloelectronvolts spectral range were observed from accreting clouds of binary systems, such as CYGNUS X-3 and VELA X-1, in which high-intensity x-ray continua from compact objects (neutron stars, black holes or white dwarfs) irradiate the cold and rarefied clouds. X-ray continuum- induced line emission accurately describes the accreting clouds, but experimental verification of this photoionized plasma model is scarce. Here we report the generation of photoionized plasmas in the laboratory under well-characterized conditions using a high-power laser. A blackbody radiator at a temperature of 500 eV, corresponding to a compact object, was created by means of a laser-driven implosion. The emerging x-rays irradiate a low-density (n(e) < 10(20) cm(-3)) and low- temperature (T(e) < 30 eV) silicon plasma. Line emissions from lithium- and helium-like silicon ions were observed from a thermally cold silicon plasma in the 1.8-1.9 keV spectral region, far from equilibrium conditions. This result reveals the laboratory generation of a photoionizing plasma. Atomic kinetic calculations imply the importance of direct K-shell photoionization by incoming hard x-rays.

Energetic Proton Generation in Ultra-Intense Laser-solid Interactions

Abstract: An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Cloudy: Numerical simulation of plasmas and their spectra

Abstract: ABSTRACT CLOUDY is a large‐scale spectral synthesis code designed to simulate fully physical conditions within an astronomical plasma and then predict the emitted spectrum. Here we describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84. The computational methods and uncertainties are outlined together with the direction future development will take. The code is freely available and is widely used in the analysis and interpretation of emission‐line spectra. Web access to the Fortran source for CLOUDY, its documentation Hazy, and an independent electronic form of the atomic database is also described.

Plasma wall interaction and its implication in an all tungsten divertor tokamak. Invited Paper

Abstract: ASDEX Upgrade has recently finished its transition towards an all-W divertor tokamak, by the exchange of the last remaining graphite tiles to W-coated ones. The plasma start-up was performed without prior boronization. It was found that the large He content in the plasma, resulting from DC glow discharges for conditioning, leads to a confinement reduction. After the change to D glow for inter-shot conditioning, the He content quickly dropped and, in parallel, the usual H-Mode confinement with H factors close to one was achieved. After the initial conditioning phase, oxygen concentrations similar to that in previous campaigns with boronizations could be achieved. Despite the removal of all macroscopic carbon sources, no strong change in C influxes and C content could be observed so far. The W concentrations are similar to the ones measured previously in discharges with old boronization and only partial coverage of the surfaces with W. Concomitantly it is found that although the W erosion flux in the divertor is larger than the W sources in the main chamber in most of the scenarios, it plays only a minor role for the W content in the main plasma. For large antenna distances and strong gas puffing, ICRH power coupling could be optimized to reduce the W influxes. This allowed a similar increase of stored energy as yielded with comparable beam power. However, a strong increase of radiated power and a loss of H-Mode was observed for conditions with high temperature edge plasma close to the antennas. The use of ECRH allowed keeping the central peaking of the W concentration low and even phases of improved H-modes have already been achieved.

Recent progress in spectroscopy of tungsten11This review is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

Abstract: This contribution reviews experimental and theoretical work on spectroscopy of tungsten published since the last critical compilation of the energy levels and spectral lines of highly ionized tungs...

High-Mach number collisionless shock and photo-ionized non-LTE plasma for laboratory astrophysics with intense lasers

Abstract: We propose that most of the collisionless shocks in the Universe, for example, supernova remnant shocks, are produced because of the magnetic field generated by Weibel instability and its nonlinear process. In order to verify and validate the computational result confirming this theory, we are carrying out model experiments with intense lasers. We are going to make a collisionless counter-streaming plasma with intense laser ablation based on the scaling law to laser plasma with the particle-in-cell simulation resulting in Weibel-mediated shock formation. Preliminary experimental data are shown. The photo-ionization and resultant non-LTE plasma physics are also very important subjects in astrophysics related to mainly compact objects, for example, black hole, neutron star and white dwarf. Planckian radiation with its temperature 80–100 eV has been produced in gold cavity with irradiation of intense lasers inside the cavity. The sample materials are irradiated by the radiation inside the cavity and absorption and self-emission spectra are observed and analyzed theoretically. It is demonstrated how the effect of non-LTE is essential to reproduce the experimental spectra with the use of a precision computational code.