R. Behrisch

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.

Implantation Profiles of Low-Energy Helium in Niobium and the Blistering Mechanism

The depth profiles of 1.5–15‐keV 3He ions implanted into a Nb single crystal at doses of 5×1016–7×1018/cm2 have been measured using the 3He  (d,p)  4He reaction. A comparison of the results with theoretical predictions for the range and the damage distribution of 3He in amorphous material shows reasonable agreement. Furthermore, the Deckeldicke (i.e., thickness of the covers of the blisters) was determined by Rutherford backscattering in double alignment. The results indicate that stress release rather than explosion of gas bubbles is the dominant mechanism in blister formation.

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.

The Sputtering Mechanism for Low-Energy Light Ions

The computer simulation program MARLOWE which follows the trajectories of energetic ions and recoiling target atoms in solids has been used to calculate sputtering yields for low energy (0.1–10keV) light ions (H, D, T,4He). Recoil energy densities were calculated for comparison with analytical theories. The sputtering yields obtained for amorphous Fe agree within a factor of two with experimentally measured values for polycrystalline stainless steel, while the calculated yields for protons on amorphous molybdenum are more than twice the experimental values on polycrystalline material. The calculations show that in the parameter range investigated, ions backscattered in the solid contribute a major part to sputtering. This result confirms earlier calculations of the threshold energy for sputtering which are in agreement with recent measurements.

Material erosion at the vessel walls of future fusion devices

Abstract For a next-step fusion device the erosion at the vessel walls during normal operation caused by the bombardment with different species from the plasma is estimated. The wall fluxes are calculated for a given ‘burning’ plasma with the computer code B2-EIRENE. With the known sputtering yields the erosion rates for vessel walls made out of Be, C, Fe, Mo or W, are calculated. The erosion by ions is found to be of the same order of magnitude as the erosion by energetic neutrals. The erosion has minima around the area of gas feed and maxima close by and at the bottom near the baffles of the divertor. The calculated total erosion is of the order of 1021 to 1022 atom/s for low Z materials, and a factor 10–20 lower for high Z materials, such as W, resulting in about 5 kg per full-day of operation or 1 t/y.

Depth Profiling of Deuterium Implanted into Stainless Steel at Room Temperature

Abstract The trapping of 7 keV deuterium in 316 stainless steel at room temperatures has been measured as a function of current density, fluence and time. The depth distribution of the deuterium which diffuses into the stainless steel is obtained using the D( 3 He, p) 4 He reaction. The measured yields have been deconvoluted to obtain the concentration profile.

Blistering of Niobium Due to 0.5_keV to 9_keV Helium and Hydrogen Bombardement

Abstract Blistering of well-annealed niobium single crystals due to 0.5 to 9 keV helium and hydrogen ion bombardment at temperatures between −110°C and 1000°C has been investigated by Rutherford backscattering (RIBS) in double alignment and with a scanning electron microscope (SEM). For He bombardment blistering was observed by RIBS in the temperature range investigated for all energies above 1 keV. The critical dose at which blisters first appear is about 1 to 2 × 10 17 incident He ions per cm 2 . It increases slightly with increasing ion energy and with decreasing target temperature. Blisters of 500 to 5000 A in diameter were found. The depth at which the blisters develop increases from ≈ 180 A for 1 keV to 1100 A for 9 keV He ions. It is a factor of ≈ 3 larger than the theoretical mean range of the ions in amorphous material. Above ≈ 600°C grain boundaries develop extending also into the unbombarded region. For hydrogen bombardment no blistering could be observed at room temperature up to doses of 2 × 10 19 ions per cm 2 .

Codeposition of hydrogen with beryllium, carbon and tungsten

Abstract The trapping of energetic deuterium codeposited with beryllium, carbon and tungsten has been measured on a silicon collector probe at room temperature. The subsequent release of the trapped deuterium at elevated temperatures was determined by thermal desorption spectroscopy. At room temperature, deuterium codeposits both with carbon and BeO with a ration of 0.41 D-atorns/C-atom and 0.38 D-atoms/BeO, respectively. No codeposition of deuterium with tungsten is observed. The thermal release of codeposited hydrogen from BeO begins at 400 K. All hydrogen is released at temperatures above 800 K.

Depth Profiling by Ion-Beam Spectrometry

A procedure is developed for determining the stoichiometry of a sample as function of depth from ion-beam analysis energy spectra. The approach is in principle equally applicable to back scattering experiments, experiments involving nuclear reactions with known cross sections and experiments combining these techniques. The procedure is especially suitable for routine computer evaluation of energy spectra. It is more straightforward and/or involves less rigid assumptions and approximations than alternative approaches. If all elemental signals are measured, an accurate beam dose is not needed. The limitations are basically those inherent to ion beam analysis in general.

Trapping and Replacement of 1_keV - 14_keV Hydrogen and Deuterium in 316 Stainless Steel

Abstract The trapping of deuterium with energies between 1 and 14 keV implanted into 316 stainless steel at 150 K has been studied using the D( 3 He, p) 4 He nuclear reaction. Initially the trapping is 100% up to fluences of about 2 × 10 17 –1 × 10 18 D / cm 2 , depending on the ion energy. Saturation is reached at fluences of 5 × 10 17 –3.5 × 10 18 D / cm 2 . The depth profiles obtained from the α-energy spectra show that at low temperature a deuterium to metal atom ratio of 0.9 to 1.0 is reached at saturation. There is no obvious diffusion of the implanted D into the bulk. Subsequent bombardment of the deuterium saturated surface by protons of the same energy results in an exponential decrease of the trapped deuterium. The opposite process, i.e. the release of protons previously implanted up to saturation by deuterium bombardment, shows the same behavior. The replacement may be described by a two-term inverse exponential function, which represents an easily replaced component of trapped deuterium as well as a replacement resistant component. Depth profiles in the advanced stages of replacement show that near surface deuterium is replaced with greater efficiency than deuterium trapped beyond the mean projected range. The replacement mechanism is found to be independent of isotopic mass and insensitive to implant energy in the range 1–14 keV. The replacement process is believed to involve detrapping as the rate limiting step.