Jun Kawata

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 (

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.

Ion backscattering and sputtering of plasma-irradiated carbon and tungsten surfaces in an oblique magnetic field

A Monte Carlo simulation model which combines ion backscattering and sputtering of solids with transport of emitted particles in the scrape-off layer plasma is developed for the study of the plasma-wall interaction in magnetic confinement fusion devices. The emphasis is put on the precise understanding of ionization and redeposition processes of backscattered and sputtered particles. In the model, Maxwellian ion-solid interaction, ionization of backscattered and sputtered neutral particles in the plasma, and gyromotion of the ionized particles in an oblique magnetic field are included. The heavier masses (lower charge states and lower energies) the particles have, the more they are redeposited. Due to impact of carbon ions on carbon and tungsten, therefore, less backscattered carbon is redeposited, whereas most of the sputtered tungsten atoms are redeposited. The redeposition causes substantially lower effective sputtering yields and a decrease in low-energy component of the sputtered atoms.

Surface roughness effect on secondary electron emission from beryllium under electron bombardment

A direct Monte Carlo model is developed to simulate secondary electron emission from beryllium with a flat surface and Gaussian-ripple surfaces. The calculated electron yield and energy distribution of secondary electrons are in reasonable agreement with the experimental data. The emphasis is in this study put on the effect of surface roughness on secondary electron emission. The number of secondary electrons emitted largely depends on the position of bombardment of primary electrons on the ripple surface. The energy distribution of secondary electrons emitted from the ripple surface shifts towards low-energy side in comparison with the distribution for the flat surface. The over-cosine and gourd-shaped angular distributions, depending on the position of bombardment, are calculated for emission angle of electrons from the ripple surface; the distribution for the flat surface agrees quite well with the cosine distribution.

Simultaneous Calculation of Reflection, Physical Sputtering and Secondary Electron Emission from a Metal Surface due to Impact of Low-Energy Ions

A computer simulation code which treats elastic and inelastic collision processes of low-energy ions in solids is presented. In the code, the direct excitation of electrons by a penetrating ion and recoiling atoms is simulated using the Monte Carlo technique, in addition to the simulation of elastic collisions of the moving particles with solid atoms. Electron cascades of the excited electrons and collision cascades of the recoil atoms are also taken into account, and as a result, the code allows us to simulate ion-solid interactions such as ion reflection, physical sputtering and secondary electron emission from the solids. This code is applied to calculations of the energy and angular distributions of emitted particles and the total particle yields of aluminum by impact of ions with the atomic numbers Z1 of 1 to 17 and energies Ei of 10 eV to 10 keV at normal incidence. The calculated sputtering yield and ion reflection coefficient are in reasonable agreement with empirical formulae which have been recently presented. The calculated electron yield shows the clear dependence on Z1 and Ei, but the Ei-dependence is different from that of the electronic stopping power at such low impact energies. The energy and angular distributions of emitted particles indicate the similarities of the secondary electron emission and the physical sputtering, as observed in recent experiments.

Monte Carlo Simulation of Secondary Electron Emission from Rough Surface

The surface roughness effect on the secondary electron yield, as well as the energy and angular distributions of emitted electrons, is investigated using a direct Monte Carlo simulation of the secondary electron emission from aluminum with a sinusoidal ripple surface. By introducing the roughness into the calculation, the electron yield for normal incidence increases. A low-energy shift of the energy distribution and an angular distribution, being different from the cosine distribution are calculated.

Comparative study of kinetic electron emission from solids under ion and electron impacts

Abstract Electron emission from beryllium due to impacts of protons and electrons at keV energy is studied using Monte Carlo simulation techniques. The emphasis is placed on the similarities and differences between the electron emissions by ion and electron impacts at the same energy. The energy distributions of emitted electrons for both particles peak at 1–4 eV. The distributions broaden towards the high-energy side, as the proton energy is increased or as the primary electron energy is decreased. The dominant emission of the cascade electrons makes significant increases of total electron yield and the low-energy component in the energy distribution for both particles with decreasing the surface work function. With increasing the work function, the high-energy component is decreased and increased for proton and electron impacts, respectively.

Monte Carlo Simulation of Yield and Energy Distribution of Secondary Electrons Emitted from Metal Surfaces

A Monte Carlo simulation of secondary electron emission from Be (atomic number Z=4), Mg (Z=12), Al (Z=13), Mo (Z=42) and W (Z=74) due to 100 eV–4 keV electron impacts is performed in order to understand the primary energy (E p) dependence of the yield and the energy distribution of secondary electrons. The E p-dependence of the secondary electron yield calculated for the metals is characterized in terms of the maximum yield and E p at which it occurs, which are in good ageement with Kollaths empirical formula for E p where the formula can be applied. At high E p (>1 keV), the calculated energy distribution of the secondary electrons, except for Mg which has a low surface potential barrier, approaches the E p-independent theoretical curve derived by Chung and Everhart. At low E p, however, the energy distribution largely depends on E p, in particular for high-Z metal.

Monte Carlo study of incident-angle dependence of ion-induced kinetic electron emission from solids

Abstract The incident angle dependence of ion-induced kinetic electron emission (KEE) from solids is calculated using a Monte Carlo simulation of the transport of incident ions and recoiling target atoms, and a semi-empirical theory of KEE. The emphasis is put on the origin of the deviation from the inverse cosine law. The effect of the high-energy recoiling target atoms on the deviation is much greater than that of the trajectory distribution and backscattering of the incident ions, except for light and low-energy ion impact. The positive (negative) contribution of the recoiling target atoms to the incident angle dependence is dominant for high (low) impact energy. The deviation can be explained by the increase in the electron excitation by recoiling target atoms localized near the surface and the increase in the emission of the atoms, i.e., sputtering.

Ion reflection and sputtering at tungsten surface exposed to edge plasmas in TEXTOR

Abstract Ion reflection and sputtering at a W test limiter under simultaneous bombardment with C and O ions, as well as D ions, in TEXTOR edge-plasmas are investigated using a Monte Carlo simulation model which combines dynamic composition change in the surface layer with transport of emitted particles in the plasma. The main aim of this work is to discuss the prompt redeposition process on the limiter and the impurity transport in the edge plasma. With increasing plasma density, the effective sputtering yields of W and deposited C considerably decrease due to the increase in the number of redeposited particles, in addition to the decrease in the yields due to the simultaneous decrease in the plasma temperature. Due to the prompt redeposition, the energy distribution of sputtered W, depending on the plasma density (temperature), substantially deviates from the well-known Thompson distribution. The observation in TEXTOR that both the decrease in the intensity of WI line and the increase in the intensity of CII line with increasing plasma density are reproduced by our simulation. Nevertheless, dominant contribution of much low-energy thermalized particles is found in the observed distributions of Dγ and OII line spectra emissions.