J. Bohdansky

A universal relation for the sputtering yield of monatomic solids at normal ion incidence

Abstract The sputtering yield of polycrystalline monatomic solids at normal incidence show a universal behaviour for light and heavy ions in the threshold regime. For heavy ions at higher energies a similar behaviour has been explained, based on first principles. In this work a relation is derived valid for both of these energy regimes. The relation is based on reasonable assumptions for the momentum distribution of the recoil target atoms and on the usual emission condition (planar surface potential). A “surface correction” — important for light ion sputtering — is also introduced. It is shown that experimental data for light ion sputtering at higher energy are also in good agreement with this relation.

An analytical formula and important parameters for low‐energy ion sputtering

Sputtering yields for different ions and materials at low ion energies have a similar energy dependence. Due to this similarity, yield data can be characterized by a normalized energy function and two parameters for each ion target combination. One of these parameters is the threshold energy. An energy scaling can be based on this parameter. The other parameter is a multiplication factor. Both parameters depend mainly on the ion and target mass M1 and M2 and on the surface binding energy EB. An analytic expression for the normalized functions and both the parameters is given. This empirical relation also allows an estimate of unknown sputtering data, if M1, M2, and EB are noted. A physical interpretation of the empirical relation is given for the case M1≪M2, as in this case special collision processes which dominate the sputtering can be identified.

Trapping of deuterium implanted in carbon and silicon: A calibration for particle-energy measurements in the plasma boundary of tokamaks

Measurements have been made of the number of deuterons retained in carbon and silicon as a function of fluence, for incident energies between 50 eV and 1 keV. Three independent techniques were used for measuring the retained D: (1) by probing the trapped deuterons with 790 kV 3He+, and counting the total proton yield from the D(3He, H)4He nuclear reaction, (2) by measuring the re-emitted deuterium during and after implantation using mass spectrometry, (3) by thermal desorption mass spectrometry. Initially, the amount of trapped hydrogen increases proportionally to the fluence, while at high doses a saturation value is reached. The quantity of hydrogen trapped at saturation as a function of particle energy follows a power law. The data have particular significance in the estimate of the mean energy of particles to the near-wall region in tokamaks by observing the build-up of trapped hydrogen in a carbon probe during cumulative discharges. As an aid to the interpretation of such data, a multi-energy implant simulating a Maxwellian energy distribution has been made, and the trapping characteristics have been investigated as a function of incident ion fluence.

Data Compendium for Plasma-Surface Interactions

A review of particle-solid processes pertinent to modelling plasma-wall interactions is presented, and sets of recommended data are given. Analytic formulas are used where possible; otherwise, data are presented in the form of tables and graphs. The incident particles considered are e−, H, D, T, He, C, O, and selfions. The materials include the metals aluminum, beryllium, copper, molybdenum, stainless steel, titanium, and tungsten and the nonmetals carbon and TiC. The processes covered are light ion reflection, hydrogen and helium trapping and detrapping, desorption, evaporation, sputtering, chemical effects in sputtering, blistering caused by implantation of helium and hydrogen, secondary electron emission by electrons and particles, and arcing.

Erosion of Carbon Due to Bombardment with Energetic Hydrogen at Temperatures up to 2000_K

Abstract The erosion of carbon in the form of pyrolytic graphite discs and PAPYEX strips due to the bombardment with 0.4 to 7 keV hydrogen and deuterium ions has been measured between room temperature and 2000 K. Both weight loss measurements and residual gas analysis (RGA) have been applied to determine chemical sputtering yields and reaction products. At temperatures around 900 K the erosion yield for H and D shows a maximum with methane as the dominant reaction product. The reaction yield and the temperature of maximum reaction yield vary with ion flux and energy as predicted by an empirical model [1]. At temperatures above 1100 K the erosion yield increases again monotonically reaching a value of 3× 10−1 atoms/ion at 2000 K for 1 keV H+ bombardment. No hydrocarbon production could be found. The dependence of this high temperature erosion process on ion mass, energy and angle of incidence is presented. A simple model relating the temperature dependence of the erosion yield to the formation and annealing of active surface states is proposed.

Physical and chemical sputtering of graphite and SiC by hydrogen and helium in the energy range of 600 to 7500 eV

The erosion of pyrolytic graphite and silicon carbide due to the bombardment with monoenergetic hydrogen ions with energies of 600 to 7500 eV has been investigated in the temperature range of near room temperature to 750°C. The erosion yield of SiC is about 10−2 and shows no pronounced temperature dependence. In contrast to SiC the erosion yield of pyrolytic graphite shows a maximum at a temperature of about 600°C. The ratio of the maximum erosion yield to that at room temperature depends on the energy of the hydrogen ions and increases from about 11 at 3000 eV to 32 at 670 eV. The production of CH4 during the bombardment of the graphite has been found proportional to the erosion yield. When graphite was bombarded with He ions no hydrocarbon production and no temperature dependence of the erosion yield could be observed. The results are compared with values for the erosion yields of carbon by thermal atomic hydrogen taken from literature.

Measurements of the Erosion of Stainless Steel, Carbon, and SiC by Hydrogen Bombardement in the Energy Range of 0.5_keV to 7.5_keV

Total erosion yields by sputtering and blistering for 1 to 15 keV H2+ bombardment at normal incidence have been measured by weight loss of 304 stainless steel, pyrolytic graphite, carbon fibres, glassy carbon and SiC. The erosion yields are in the range of 3 × 10−3 to 2.6 × 10−2 atoms per incident hydrogen atom. Observation in the scanning electron microscope shows that blisters occur in stainless steel and SiC at doses of 5 × 1018 particles/cm2, but disappear at doses of 5 × 10 particles/cm2 . The surface roughening observed depends largely on grain orientation. On carbon no blistering could be found. After bombardment the carbon surfaces are generally more smooth than before.

Sputtering yields of graphite and carbides and their potential use as first wall materials

Abstract The sputtering yields of graphite, B4 C, SiC and TiC have been measured at normal incidence for H, D and 4He ions in the energy range between 100 and 8 keV. The yields are energy dependent and display a maximum for all target-ion combinations in the energy range investigated. The maximum yield values are strongly dependent on the mass of the bombarding ions, but show only a weak influence on the target materials. The consequences of this behaviour for the potential use of these materials as a first wall are discussed under the assumption that plasma will be generated mainly by sputtering. Special emphasis in this discussion is given to the importance of the plasma edge temperature with respect to the choice of a first wall material and the relative advantage of the materials investigated compared to several metals (stainless steel, inconel, molybdenum).

The Sputtering Yield of Typical Impurity Ions for Different Fusion Reactor Materials

Abstract Sputtering yields of prospective materials for the first wall in fusion devices such as TiC, SiC, C, W, and stainless steel with typical impurity ions as O + and C + have been measured in the fusion-relevant energy range from 100 eV to 10 keV. In the case of oxygen-bombardment sputtering is reduced compared to sputtering with noble gas ions of similar atomic masses for all materials investigated but graphite. In the case of C + -bombardment a collection of carbon was observed.

Sputtering of graphite with light ions at energies between 20 and 1000 eV

Abstract Measurements of the physical and chemical sputtering yield of different sorts of graphite with H, D, and He ions have been extended down to energies of 20 eV. H + 3 , D + 3 , and He + ions of 3 keV have been decelerated in front of the target to energies of 60 eV. The target temperature could be varied between room temperature and 1000 K by simultaneous electron bombardment. The sputtering yield was obtained from the weight loss determined in situ using a vacuum balance, and the reaction products were analyzed using a liquid nitrogen-cooled quadrupole mass analyzer. At room temperature, the yield curve for He ions decreases at low energies as predicted by physical sputtering theory. For H + and D + , however, below 100 eV the sputtering yield increases with decreasing ion energy. It could be shown that this increase is due to the formation of CH 4 and CD 4 , respectively. For ion energies of 500 eV and higher, chemical sputtering occurs only in a narrow temperature region between 700 and 900 K. This temperature region for chemical sputtering broadens at low ion energies and extends below 100 eV down to room temperature. Simultaneously, the maximum sputtering yield at 825 K decreases with decreasing energy and the temperature variation of the yield at energies below 100 eV is not very pronounced. For 50 eV, D + sputtering yield values vary between 2 × 10 −2 atoms/ion and 5 × 10 −2 atoms/ion between room temperature and 900 K.