Seok Won Hwang

Non-Thermal Atmospheric Pressure Plasma Preferentially Induces Apoptosis in p53-Mutated Cancer Cells by Activating ROS Stress-Response Pathways

Non-thermal atmospheric pressure plasma (NTAPP) is an ionized gas at room temperature and has potential as a new apoptosis-promoting cancer therapy that acts by generating reactive oxygen species (ROS). However, it is imperative to determine its selectivity and standardize the components and composition of NTAPP. Here, we designed an NTAPP-generating apparatus combined with a He gas feeding system and demonstrated its high selectivity toward p53-mutated cancer cells. We first determined the proper conditions for NTAPP exposure to selectively induce apoptosis in cancer cells. The apoptotic effect of NTAPP was greater for p53-mutated cancer cells; artificial p53 expression in p53-negative HT29 cells decreased the pro-apoptotic effect of NTAPP. We also examined extra- and intracellular ROS levels in NTAPP-treated cells to deduce the mechanism of NTAPP action. While NTAPP-mediated increases in extracellular nitric oxide (NO) did not affect cell viability, intracellular ROS increased under NTAPP exposure and induced apoptotic cell death. This effect was dose-dependently reduced following treatment with ROS scavengers. NTAPP induced apoptosis even in doxorubicin-resistant cancer cell lines, demonstrating the feasibility of NTAPP as a potent cancer therapy. Collectively, these results strongly support the potential of NTAPP as a selective anticancer treatment, especially for p53-mutated cancer cells.

The Effects of Electrode Structures on the Luminous Efficacy of Micro Dielectric Barrier Discharges

A high-efficacy alternating-current plasma display panel utilizing various electrode structures is simulated using a 3-D fluid code for the investigation of the effects of cell structures on the discharge characteristics of micro dielectric barrier discharges. For the modification of the conventional coplanar electrode structure, three suggested electrode structures were simulated, namely, a patterned bus electrode, a multilayer bus electrode, and a two-electrode facial discharge structure. The time evolutions of infrared radiation images for the three structures compare well with the experimental results of test panels. The ultraviolet and visible-light emission profiles, as well as the time evolution of wall charge profiles, are analyzed in order to explain the role of the electrode structure for the improvement of luminous efficacy.

Effect of electron Monte Carlo collisions on a hybrid simulation of a low-pressure capacitively coupled plasma

Fluid models have been widely used and conducted successfully in high pressure plasma simulations where the drift–diffusion and the local-field approximation are valid. However, fluid models are not able to demonstrate non-local effects related to large electron energy relaxation mean free path in low pressure plasmas. To overcome this weakness, a hybrid model coupling electron Monte Carlo collision (EMCC) method with the fluid model is introduced to obtain precise electron energy distribution functions using pseudo-particles. Steady state simulation results by a one-dimensional hybrid model which includes EMCC method for the collisional reactions but uses drift–diffusion approximation for electron transport in a fluid model are compared with those of a conventional particle-in-cell (PIC) and a fluid model for low pressure capacitively coupled plasmas. At a wide range of pressure, the hybrid model agrees well with the PIC simulation with a reduced calculation time while the fluid model shows discrepancy in the results of the plasma density and the electron temperature.

Ray-Trace Calculation of Visible Lights Emitted From a Display Device Utilizing Ultraviolet Lights From Gaseous Discharges

A ray-trace model has been developed for the visible lights coming out from phosphors excited by vacuum ultraviolet (UV) lights in gaseous discharges. The spatial and angular light distributions are calculated numerically by considering multiple reflections of the visible lights within a plasma-display-panel (PDP) cell. Luminance of the emitting visible lights is illustrated for two different electrode shapes of an ac-type PDP cell. This calculation can be used for the improvement of optical properties of display devices utilizing phosphors and UV lights from plasma discharges, such as flat fluorescent lamps as well as PDP.

Discharge Characteristics of a Gas-Filled Capillary Plasma for Laser Wakefield Acceleration

A gas-filled capillary plasma source for potential use in conjunction with laser wakefield acceleration is developed, and its discharge characteristics are presented here. In the capillary, helium or hydrogen gas (~150 torr in pressure) is injected, and a pulsed high voltage (~25 kV) is applied for electrical discharge and plasma production. The discharge properties of the helium and hydrogen gases are compared. The plasma densities inside the capillary hole are measured with a Mach-Zehnder interferometer and are obtained separately by a simple analysis using the diffusion equation and measured current profiles.

Betatron Radiation of an Off-axis Injected Electron in a Laser Wakefield Accelerator

The electrons injected into a laser wakefield undergo betatron oscillation and give rise to the emission of intense X-ray radiation. To investigate the generation conditions of the X-rays, the relativistic motion of an electron injected in an off-axis position has been simulated with wakefield profiles which are pre-calculated with a two-dimensional particle-in-cell code. The wakefield with a plasma density of 1.78×10 18 ㎝ -3 is generated by the laser with an intensity of 1.37×10 18 W/㎠ and a pulse width of 30 fs. From the calculation of the single particle motion, the characteristics of the betatron radiation are investigated in the time domain. As the transverse injection position increases, the power and the duration time of the radiation increase, but the width of each pulse decreases.

Parallelized two-dimensional particle-in-cell simulation for capacitively coupled plasmas using graphic processing units

Summary form only given. A large size chamber with 450 mm and plasma wall-interaction for the nano-scale feature are needed in semiconductor manufacturing industry recently. However, there are difficulties to experiment and simulate a large scale reactor chamber and nano-scale features. In the simulation case, plasma simulation using particle-in-cell (PIC) method shows very high accuracy compared with fluid simulations. However, PIC method has a disadvantage of slow speed caused by individual calculation of lots of computational particles. Recently, new computing method using graphic processing units (GPUs) enables to make a low-cost and low-power personal super computer and there is a study to utilize the GPUs for the parallelization of PIC plasma simulators [1,2]. To overcome the heavy computation problem of a PIC method, we have developed a parallelized PIC code using GPUs. In this work, two-dimensional axisymmetric simulator for a capacitively coupled plasma (CCP) sources is presented with the performance improvement using GPUs. Also, investigated were plasma phenomena with frequency variation as well as dual frequency with phase variation for CCP.

The effect of electron cyclotron resonance heating on breakdown for start-up of a tokamak

Summary form only given. Although various experiments have been conducted routinely in many tokamak devices, the detailed physics background of so-called, start-up including processes of the plasma breakdown, burn-through, and current ramp-up has not been identified because too many parameters are involved together non-linearly and diagnostic tools are restricted in this phase. Plasma breakdown using electron cyclotron resonance heating (ECRH) has been proposed in the Korean superconducting tokamak advanced research (KSTAR) device because the loop voltage is limited due to thick vacuum liners and engineering limits of superconducting poloidal field coils. Particle-in-cell (PIC) simulation is a very effective tool to research the plasma phenomena but difficult to be applied to three dimensional, whole-size tokamak devices because of its large computation time. We developed a three-dimensional PIC code for investigating the breakdown of the tokamak start-up with ECRH. A simplified toroidal geometry is adopted for easy calculations with a prescribed magnetic field profile calculated for the KSTAR device. This PIC code includes Monte Carlo collision (MCC) routine for hydrogen atoms. In this work, we investigated the discharge characteristics and the effect of ECRH when the breakdown occurs in a tokamak. The influence of the magnitude of the toroidal magnetic field, loop voltage, gas pressure, and incident angle and intensity of ECRH is studied for breakdown in a tokamak.

Development of a parallelized two-dimensional axisymmetric capacitively coupled plasma simulator using graphics processing units

As a result of the development of the plasma-assisted nano-electronics processing, there are lots of efforts that diagnose and analyze the plasma phenomena using computer simulation for the plasma sources as well as the plasma-surface interactions for the nano-scale feature. However, increasing wafer size as well as minimized feature size makes it hard to simulate the whole system. Therefore, there is a study to utilize the graphic processing units (GPUs) for the parallelization of particle-in-cell (PIC) plasma simulators [1]. PIC simulation is accurate but needs heavy calculation as it takes quite long time to calculate the dynamics and collision processes of all the computational particles. Especially, a larger size chamber with 450 mm wafer is more difficult to simulate using PIC method. Recently, heterogeneous computing, which uses graphics processing units or other device with or without CPU for computing, has gradually distributed to various computing fields including physics and fluid dynamics. In this presentation, a two-dimensional axisymmetric simulator for capacitively coupled plasma (CCP) sources is presented with the performance improvement using GPUs. The improvement of speed compared with that of single CPU calculation.

Trace of Self-Injected Electron Bunches in a Laser Wakefield

The motion of self-injected electrons into a laser wakefield is traced using a 2-D electromagnetic and relativistic particle-in-cell simulation. It gives important information about where the injected electrons were located initially in the background plasma and how those electrons are injected into a laser wakefield. After self-injection, the electron bunches show betatron oscillations while being accelerated.