L.G. Haggmark

Monte Carlo studies of sputtering

Abstract A computer program has been developed to simulate the sputtering process using the Monte Carlo method and the binary collision approximation. This program is a result of the generalization of the TRIM computer program such that the target atom trajectories are followed in addition to those of the incident particles. This program, which includes electronic energy loss, uses an analytic formula which is based on realistic interatomic potentials for determining particle scattering angles and the energy transfer to target atoms. A model of the sputtering process has been developed for physically defining the surface and bulk binding energies necessary for calculations. A number of sputtering yield calculations have been performed for H, D, T, and 4He ions incident on C, Ni, Mo, and Au targets for energies less than 10 keV. The validity of the Monte Carlo model is demonstrated by the good agreement between the calculated results and the most recent experiments.

Alpha transport and blistering in tokamaks

The particle flux and angular distribution of 3.5 MeV alpha particles impinging on the first wall from uncontained banana orbits in an axisymmetric tokamak reactor have been calculated. The resulting helium concentration profiles in the first wall can give rise to surface exfoliation under specified conditions. The major mitigating factor is the simultaneous surface recession due to sputtering by the D-T charge-exchange neutral flux. For the parameters used in these calculations blistering in high-sputtering-rate materials such as beryllium is unlikely, whereas in low-sputtering-rate materials such as niobium helium-induced surface deformation is quite probable.

Retention and recycling of plasma edge hydrogen isotopes in C and TiC

Abstract The retention, release, and isotopic exchange of hydrogen, deuterium, and tritium incident on room temperature C and TiC with Maxwellian velocity and isotropic angular distributions have been calculated using the Local Mixing Model. The calculations are based on the hydrogen saturation concentration, range distributions and isotopic exchange behavior, all of which have been verified experimentally by monoenergetic implants. Particle fluences (1016 to 1020/cm2) and characteristic Maxwellian energies (50 eV to 3200 eV) were chosen in the range of interest for tokamak plasma edge conditions. The results of these calculations are used to estimate tritium inventory buildup and to predict tritium changeout rates by isotopic exchange in tokamaks and other fusion devices. Tritium inventories in the TiC components (which comprise ≳ 20% of the TFTR first-wall surface) are anticipated to remain at reasonable levels in TFTR, and isotopic exchange is shown to be a feasible method to reduce these tritium inventories.

Ion impact desorption and hydrogen release

Abstract Desorption cross sections have been measured for the removal of deuterium from stainless steel by hydrogen ion impact. Using hydrogen ions with energies similar to those in the charge-exchange neutral flux striking the first-wall in tokamaks, the desorption cross section was determined for substrate temperatures

Monte Carlo calculations of light-ion sputtering as a function of the incident angle☆

Abstract The sputtering of metal surfaces by light ions has been studied as a function of the incident angle using an extension of the TRIM Monte Carlo computer program. Sputtering yields were calculated at both normal and oblique angles of incidence for H, D, T, and + He impinging on Ni, Mo, and Au targets with energies ≤10 keV. Direct comparisons are made with the most recent experimental and theoretical results. There is generally good agreement with the experimental data although our calculated maximum in the yield usually occurs at a smaller incident angle, measured from the surface normal. The enhancement of the yield at large incident angles over that at normal incidence is observed to be a complex function of the incident ions energy amd mass and the targets atomic weight and surface binding energy.

Deuterium profiles in titanium and alloys

Abstract Deuterium concentration profiles in Ti, T1-6A1, and Ti-6A1-4V have been investigated after 10 keV D3+ implantation at room temperature. Profiles were determined using the D(3He,α)p nuclear reaction and also by secondary ion mass spectrometry (SIMS). The amount of retained D in the near-surface region (~.5 μm) was monitored as a function of time and was found to be strongly microstructure dependent. Annealed Ti retained the most D while the Ti-6A1-4V alloy retained the least D in this region. The D remained in the bulk of the sample during implant and subsequent aging at room temperature.

Helium induced blistering during simultaneous sputtering

Abstract The results of helium implantation during sputtering of stainless steel are described in this paper. These experiments simulate the simultaneous bombardment of unconfined 3.5 MeV alpha particles and erosion due to sputtering primarily from the charge exchange neutral (D, T) flux. The results indicate that helium induced blister formation can take place, with simultaneous sputtering, under conditions appropriate to a fusion reactor. The critical experimental parameters of helium flux and surface velocity are in excellent agreement with simple calculations that predict the conditions under which helium induced surface deformation takes place.

Monte Carlo studies of light ion reflection from metal surfaces

Abstract The reflection of D and T from amorphous targets of Be, Fe, Mo, and W has been studied with the TRIM Monte Carlo, computer program. Results for the particle and energy reflection coefficients are presented for the various combinations of monoenergetic and Maxwell-Boltzmann energy distributions and normal, cosine, and isotropic angular distributions. The incident particle energies, E 0 , for the monoenergetic cases and characteristic energies, kT, for the Maxwell-Boltzmann distributions ranged from 0.1 to 10 keV. For a given incident angular distribution, the energy dependent Maxwell-Boltzmann results scale well in most cases with that of the monoenergetic results when plotted at kT and (3/2)kT for the particle and energy reflection coefficients, respectively. The particle reflection coefficients at normal incidence agree well with previous monoenergetic calculations, but they are greater than recent experimental measurements.

Hydrogen ion impact desorption of adsorbed deuterium from stainless steel

Abstract The removal of adsorbed deuterium from stainless steel by energetic molecular hydrogen ions (H 2 + and H 3 + ) has been studied with secondary ion mass spectrometry. Desorption cross sections for this process range from 1.2 × 10 −16 cm 2 to 5 × 10 −17 cm 2 for incident ion energies 0.3 to 1.5 keV/atom. The cross sections increase as the incident ion energy is lowered. At low energies, direct interactions between incident particles and adsorbate atoms appear to be important in causing desorption. Dissociation of molecular hydrogen ions occurs prior to desorption. The implications of these findings with respect to the mechanisms for ion impact desorption and to fuel recycling phenomena in tokamaks are discussed.

Sputtering yield calculations for neutral beam particle energies

Abstract Sputtering yield calculations have been performed for 50, 100, and 200 keV D incident on Fe, Mo, and W at both normal and oblique angles of incidence. The calculations were performed with an extension of the TRIM Monte Carlo computer program. The calculated yields at these high energies decrease with the incident energy approximately as the high energy, nuclear stopping power for both normal and oblique incident angles. The increases in the yield as a function of the incident angle α relative to yield at normal incidence (α = 0°) are quite large and cannot be represented by a simple cos−f α dependence, where f is a constant. These relative increases as a function of angle are found to be very similar for the three materials considered in this study.