Predictive Numerical Simulations on the Formation of Internal Transport Barriers in the Reversed Shear Regime of KSTAR Tokamak

Numerical simulations have been carried out to predict the formation of internal transport barriers (ITBs) in the reversed shear (RS) regime of Korea Superconducting Tokamak Advanced Research (KSTAR) tokamak by using the ASTRA-1.5D transport code coupled with a simplified neutral beam injection (SINBI) code developed for this work. The present simulations employ a multi-mode transport model, MMM95, for anomalous transport calculations and use neutral beam injection for additional plasma heating and current drive. The simulations for the KSTAR plasmas reveal that ITBs are formed in the RS regime by control of flow shear and by very low magnetic shear, and steep gradient regions apparently reside in both the ion and the electron temperature profiles. Suppression of plasma transport has been considered in this simulation work in two ways: control of the coefficient deciding the strength of flow shearing rate and addition of the effect of magnetic shear to the flow shear.Supported by the Korea Basic Science Institute

Susceptibility to Oxidative Stress is Greater in Korean Patients with Coronary Artery Disease than Healthy Subjects

There are some evidences that the increased oxidative stress and thus increased oxidizability of lipoproteins and DNA can contribute to the development of certain human diseases, such as cardiovascular disease. To confirm the association of DNA damage with cardiovascular disease, we investigated susceptibility of DNA to oxidation in lymphocytes and oxidative stress related parameters in blood of patients with coronary artery disease (CAD). Subjects were consisted of 42 patients (27 men, 15 women) with documented CAD and 49 apparently healthy subjects (33 men, 16 women) as controls. Cellular DNA damage induced by 100 µM H2O2 was measured using Comet assay and quantified by TL. There were no differences in age (61.4 ± 1.7 years vs 62.0 ± 2.2 years) between the two groups. All the findings were shown to be independent of either sex or smoking habit. The patients showed significantly higher TL (87.3 ± 1.6 µm) compared to the control (79.3 ± 1.7 µm, p<0.01). Plasma TRAP, vitamin C, γ-tocopherol, and α-carotene levels in patients group were lower than those of control groups, while erythrocytic catalase activity increased in patients group. In conclusion, we observed that reduced overall antioxidant status was closely connected to higher susceptibility of DNA damage in CAD patients

Numerical Simulation on Mode Transition of Atmospheric Dielectric Barrier Discharge in Helium-Oxygen Mixture

A one-dimensional numerical simulation of a homogeneous dielectric barrier discharge (DBD) has been carried out for a nonequilibrium helium-oxygen mixture plasma to understand the influences of oxygen additive on its discharge characteristics at atmospheric pressure. The numerical results obtained by solving continuity equations for plasma species and Poisson equation show that, depending on the amount of oxygen added, the homogenous barrier discharge turns out to have two fundamental modes: glow and Townsend. When oxygen is rare, the discharge has similar characteristics to the dc glow discharge at low pressure. As the oxygen additive increases, the discharge characteristics of the glow mode are destroyed and changed into the Townsend mode. The reason for this mode transition is due to the fact that oxygen plays an important role both in quenching helium metastables and in attaching electrons on it in the plasma. As a practical method of sustaining the glow mode even with high oxygen concentration in the discharge, adjustment of the frequency of applied driving voltage is introduced. The numerical simulation reveals that the glow mode recovers from the Townsend mode by increasing the frequency while the amount of oxygen is highly contained. Finally, discharge operation regimes for the glow and Townsend modes are numerically obtained, which are dependent on both oxygen additive and applied frequency

Geometrical Effects of the Discharge System on the Corona Discharge and Chemically Active Species Generated in Wire-Cylinder and Wire-Plate Reactors

Time-dependent one- and two-dimensional numerical models are developed to analyze the in uence of the design and the operating conditions on streamer propagation and active-species generation in wire-cylinder and wire-plate reactors. One-dimensional calculations for the corona model of a wire-cylinder reactor showed that fixed ratios of the wire to the cylinder radii, a/b, and the applied voltage to the cylinder radius, Va/b, were the key design parameters for controlling the discharge characteristics. In addition, shape of pulse voltage externally applied to the electrodes is newly suggested for generating a near-uniform electric field over the entire discharge region. Two-dimensional calculations for a wire-plate reactor found that the wire-to-wire spacing, c, along the parallel-plate direction should be at least twice the wire-to-plate distance, d, to produce non-equilibrium plasmas effectively for generating a large volume of resultant chemically active species (c/d > 2)

Thermal plasma flow and equivalent circuit analyses on the electrical coupling of a DC-RF hybrid plasma torch

Numerical analyses on the electrical coupling of a DC-RF (direct current – radio frequency) hybrid plasma torch are conducted on the basis of magneto-hydrodynamic flow and equivalent circuit models to find the dependency of coupling efficiency on RF frequency and confinement tube radius. Computations are also carried out for the inductively coupled RF plasma torch to make a comparison between their calculated results. Numerical results reveal that the electrical coupling efficiencies of the RF and DC-RF hybrid plasma torches have a similar dependency on RF frequency with an almost constant difference of slightly higher efficiencies for the hybrid plasma, due to the relatively linear frequency dependency of equivalent circuit parameters as well as the resultant radially expanded DC-RF hybrid plasma toward the confinement tube wall compared with the RF plasma. But it is found that the reduction in the confinement tube radius less than some critical value, for instance 22 mm in this numerical work, possibly causes the coupling efficiency of the hybrid plasma to drastically deteriorate compared with that of the RF plasma. Such poor efficiency of the hybrid torch with relatively small radius is attributed to a significant diminution of the high temperature region upstream between the DC torch exit and the first induction coil segment, which means that the reduced tube radius may lead to an ineffective superposition of DC arc jet and RF plasma. As a result of the reduced high temperature region, the magnetic flux linkage is decreased for the smaller confinement tube, which leads to a drastic decrease in the electrical coupling. As the confinement tube radius becomes smaller, the re-circulation eddies under the DC torch are almost destroyed by a DC arc jet and a stagnation region formed is contracted to the central region. This contracted stagnation region prohibits the convection heat transfer by re-circulation of sheath gas flow from the coil zone to the upper part of the confinement tube, which ultimately results in a significant diminution of the high temperature region in the upstream. The present numerical analyses indicate that a special focus need to be brought into the influences of the DC arc jet on the electrical and thermal flow characteristics of the DC-RF hybrid plasma in determining the torch dimensions for effective conversion of RF power into plasma