Kaoru Ohya

Kinetic modelling of impurity transport in detached plasma for integrated divertor simulation with SONIC (SOLDOR/NEUT2D/IMPMC/EDDY)

A Monte Carlo (MC) impurity code IMPMC has been coupled self-consistently with a divertor code SOLDOR/NEUT2D by overcoming the intrinsic problems of MC modelling for impurity transport. MC modelling is required for impurity transport in order to take into account the kinetic effect and the complex dissociation processes of hydrocarbons. The integrated divertor code SONIC enables us to investigate the details of impurity transport including erosion/redeposition processes on the divertor plates by further coupling of an 3D plasma–surface interaction MC code EDDY. The dynamic evolution of X-point MARFE observed in JT-60U is investigated. The simulation results indicate that the hydrocarbons sputtered from the dome contribute to the enhanced radiation near the X-point. The kinetic effect of thermal force on the He transport is investigated for JT-60SA detached plasmas. Without the recycling, the kinetic effect improves the helium compression, compared with the conventional (fluid) evaluation. This effect is, however, masked by the recycling at the divertor targets.

Experiments with tungsten limiters in TEXTOR-94

Abstract The release of tungsten and light impurities from tungsten limiters exposed into the plasma edge of TEXTOR-94 has been measured by spectroscopic methods. Absolute effective tungsten sputtering yields are compared with model calculations on physical sputtering. The agreement is reasonable: however the observed strong decrease of tungsten release with increasing density cannot be fully explained. Erosion areas are clearly separated from carbon deposition zones. Surface analysis found neither carbon nor deuterium on the shiny metallic areas: A very sharp transition from “clean” metallic areas to carbon deposition zones within about 2–4 mm is found, instead. The carbon deposit is about 200–300 nm thick and contains deuterium with a D/C ratio of 0.05–0.1.

A Modified EDDY Code to Simulate Erosion/Redeposition of Carbon Target in an ITER-FEAT Divertor

Modification of a Monte Carlo simulation code, Erosion and Deposition based on DYnamic model (EDDY), for plasma-surface interactions in a designed tokamak, International Thermonuclear Experimental Reactor-Fusion Energy Advanced Tokamak (ITER-FEAT), and its application for erosion and redeposition of a carbon target in the divertor are presented. The modified EDDY code allows us to treat the deposition of plasma impurities and the prompt redeposition of sputtered atoms and molecules on the target surface. At elevated temperatures, furthermore, the impurity diffusion inside the target and chemical sputtering of carbon are taken into account. In the ITER-FEAT, physical sputtering of the divertor target is very small in the scrape-off layer (SOL) region, and chemical sputtering dominates the erosion near the strike point and in the private flux region. Prompt redeposition strongly suppresses the sputtering of the target and plasma carbon impurity deposits on it. As a result, no erosion is calculated in the SOL region and a thick deposition layer is produced near the strike point. A narrow erosion zone remains only in the private flux region. Furthermore, radial distributions of each particle species released in the plasma and their redeposition profiles on the surface are discussed.

Dynamic simulation of erosion and redeposition on plasma-facing materials

A dynamic Monte Carlo simulation code, erosion and deposition based on a dynamic model (EDDY), which combines ion reflection and sputtering of plasma-facing materials with transport of released particles in plasma, is developed. The surface composition changes due to impurity deposition, collisional mixing, and thermal diffusion of the deposited impurities at elevated temperatures are taken into account. Ionization and dissociation of released atoms and molecules and their gyromotion in the plasma are calculated, so that prompt and local redepositions are simulated. This enables the simulation of erosion and redeposition patterns, and the impurity depth profiles of plasma-facing materials. Recent applications to analyses of plasma-wall interactions in existing fusion devices are presented.

Comparative study of target atomic number dependence of ion induced and electron induced secondary electron emission

Abstract Target atomic number, Z 2 , dependence of ion-induced electron emission is investigated by means of a Monte Carlo simulation for 17 species ( Z 2 =4–79) of solids bombarded with light (He, Ne) and heavy (Kr, Xe) ions with the energy of 50 keV. The simulation derives the contributions of electron excitation by a projectile ion, recoiled target atoms and electron cascades from the total electron yield. With increasing Z 2 the electron yield decreases while showing a detailed structure. The Z 2 dependence is different from that for electron bombardment, where the secondary electron yield increases with increasing Z 2 . The decreasing electron yield for heavy ions with increasing Z 2 is due to the decrease in the number and energy of electrons excited by the projectile ion, resulting from a large energy transfer from the projectile ion to the recoiled atoms. Furthermore, the decreasing component from the recoiled atoms contributes it as well.

Monte Carlo simulation of heavy ion induced kinetic electron emission from an Al surface

Abstract A Monte Carlo simulation is performed in order to study heavy ion induced kinetic electron emission from an Al surface. In the simulation, excitation of conduction band electrons by the projectile ion and recoiling target atoms is treated on the basis of the partial wave expansion method, and the cascade multiplication process of the excited electrons is simulated as well as collision cascade of the recoiling target atoms. Experimental electron yields near conventional threshold energies of heavy ions are simulated by an assumption of a lowering in the apparent surface barrier for the electrons. The present calculation derives components for electron excitations by the projectile ion, the recoiling target atoms and the electron cascades, from the calculated total electron yield. The component from the recoiling target atoms increases with increasing projectile mass, whereas the component from the electron cascade decreases. Although the components from the projectile ion and the electron cascade increase with increasing impact energy, the component from the recoiling target atoms is less influenced by the energy in the energy range of tens of keV. The escape depth of the excited electrons is independent of the excitation processes; however, lateral distributions of the excited electrons are clearly different between the excitations by the projectile ion, the recoiling target atoms and the electron cascades.

Comparative study of secondary electron emission from solids under positron and electron impacts

Abstract Secondary electron emission from gold caused by the impacts of positrons and electrons at keV or less energy is investigated by means of a Monte Carlo simulation of elastic and inelastic collision processes of the projectiles and cascade electrons inside the solid. The cross-section for the elastic collision of each particle with an atom in the solid is calculated using the partial wave expansion technique, whereas the cross-section for the inelastic collision or for the electron excitation is calculated from the Ashley’s optical-data model. Because of less large-angle elastic scattering of the positron, the calculated backscattering coefficient is smaller for the positron than the electron. At low impact energies (

Transport studies of high-Z elements in neon edge radiation cooled discharges in TEXTOR-94

High-Z materials as tungsten are intended to be used in future fusion reactors due to their low sputtering rates and high melting points. In this context the important question is whether the use of high-Z materials is compatible with the concept of a cold radiative boundary. To investigate the local release and transport behaviour of the high-Z impurities, Mo and W test limiters were used in auxiliary heated discharges under different radiation scenarios with neon seeding. In addition, laser blow-off of tungsten as well as xenon gas puffing were performed. In some particular discharge conditions impurity accumulation was observed in the plasma core, which in the case of ohmic discharges led to minor disruptions. The connection between the accumulation and the evolution of the current density profiles is discussed. A transport analysis is made, in order to compare the observations with the neoclassical theory. It is shown that with neon the impurity - impurity driven fluxes enhance the high-Z concentration in the plasma core. However, if the source of the high-Z elements is significantly reduced, by increasing the plasma density, a development of an accumulation instability can be avoided.

Target material dependence of secondary electron images induced by focused ion beams

Monte Carlo calculation of ion-induced kinetic secondary electron emission was performed to study the target material dependence of secondary electron images in scanning ion microscopy (SIM) by using focused ion beams, which is different from that in scanning electron microscopy (SEM). In the calculation, the electron excitation by projectile ions is treated using the partial wave scattering cross-section of conduction electrons by the ions, and the cascade multiplication process of the excited electrons is simulated as well as the elastic collision cascade of recoiling target atoms. The calculated secondary electron yields of Al (Z s13) ,C u(Z s29) and Au (Z s79) for 30 and 10 keV Ga ion impacts decrease with increasing atomic number, Z , q 22 2 2 of the materials, whereas those for 1 keV H ions and 10 keV electrons increase with Z . This is consistent with the observed q 2 opposite trend between the SIM and SEM in the Z dependence of secondary electron images. Simultaneous calculation of 2 individual elastic and inelastic collisions of the projectile ion, the recoiling target atoms and the excited electrons suggests that the Z dependence of the secondary electron yield for heavy-ion impact is related to large elastic energy loss of the projectile ion 2 and low energy of the excited electrons in the solid. 2002 Elsevier Science B.V. All rights reserved.

Simulation of secondary electron emission from rough surfaces

Abstract The effect of surface roughness on the secondary electron emission from a beryllium surface under low-energy (≤1 keV) electron bombardment is investigated using a Monte Carlo simulation combined with the model of bowl-structured surface. With increasing aspect ratio H / W of the bowl structure, the secondary electron yield becomes greater than that for a flat surface, whereas for large H / W the yield is smaller; where H and W are the depth and width of the bowl structure, respectively. The former is due to emission of electrons, which cannot escape from the flat surface, from an inclined plane; it increases the low-energy component in the energy distribution. The latter is due to re-entrance of once-emitted electrons into the next part of the topographic surface; it decreases the number of electrons emitted with oblique angles.