C. Garcia-Rosales

Revised formulae for sputtering data

Abstract A revision and improvement of analytic formulae for calculating sputtering yields is performed based on the large number of experimental and calculated sputtering yield data accumulated at IPP in the last three decades. The Bohdansky formula for calculating sputtering yields as a function of energy is revised by introducing a nuclear stopping cross-section based on an analytic fit to the KrC potential. New analytic expressions for the two fit parameters Q and E th of the Bohdansky formula are deduced. Yamamuras formula for the angular dependence of the sputtering yield is shown not to be valid for self-sputtering and for heavy projectiles at energies near the threshold.

Re-emission and thermal desorption of deuterium from plasma sprayed tungsten coatings for application in ASDEX-upgrade

Abstract The trapping and release of deuterium implanted with an energy of 100 eV in wrought and in plasma sprayed tungsten of different manufacture and structure has been investigated by means of re-emission as well as thermal and isothermal desorption spectroscopy. The experimental data for wrought tungsten are compared with model calculations with the PIDAT code in order to estimate the parameters governing diffusion, surface recombination and trapping in tungsten. The amount of retained deuterium in tungsten is of the same order of magnitude as in graphite for the implantation parameters used in this work. The mobile hydrogen concentration in tungsten during the implantation is of the same order of magnitude than the trapped one, being released after the termination of the implantation. The fraction of deuterium trapped to defects increases strongly with the porosity of the samples. The temperature needed for the release of the trapped deuterium (∼ 600 K) are considerably lower than for graphite, due to the smaller trapping energy (≤ 1.5 eV).

Erosion processes in plasma-wall interactions

During plasma-wall interactions in fusion experiments large energy and particle fluxes are deposited predominantly onto limiter and divertor structures. Besides good thermomechanical properties, plasma facing materials must exhibit low erosion yields. Under normal operation conditions, the main erosion mechanisms are physical sputtering, chemical sputtering, radiation enhanced sublimation and thermal sublimation. High-Z materials, such as W, show low effective sputtering yields at low plasma temperatures and a favourable redeposition behaviour, and may be suited for divertor plates for these plasma conditions. For higher plasma temperatures low-Z materials such as CFC materials and Ti-doped graphites, which enable high power removal, are preferable. In the other wall areas a low-Z material with reduced chemical erosion, e.g. B4C coatings, could be appropriate.

Threshold energy for sputtering and its dependence on angle of incidence

Abstract The sputtering yield at low energies and at various angles of incidence is investigated by computer simulation with the program TRIM.SP and compared to available experimental data. The question of the sputtering threshold energy is discussed in detail, and the processes responsible for sputtering at these low energies are studied. It is shown that for heavy ion sputtering the threshold depends on the angle of incidence. Analytic equations are derived which demonstrate that the inelastic energy loss plays an important role for the threshold energy for heavy projectiles.

Temperature-dependent sputtering of doped graphites and boron carbide

Abstract The temperature dependence of the sputtering behaviour of various low-Z materials is presented. The hydrocarbon production under deuterium bombardment is investigated as a function of the target temperature. Carbon doped with beryllium or boron, as well as boron carbide show a strong reduction in the chemical sputtering yield. The reduction of the radiation-enhanced sublimation yield (RES) above 1200 K is small and not uniquely correlated with the dopant concentration. A variety of doped graphites is compared for 1 keV D+ sputtering at 300, 800 and 1470 K. The resulting erosion yields are discussed in terms of dopant concentration and the microstructure of the materials.

Chemical sputtering of pyrolytic graphite and boron doped graphite USB15 at energies between 10 and 1000 eV

The chemical sputtering of pyrolytic graphite and boron doped graphite USB15 at ion energies down to 10 eV is presented in this paper. Hydrocarbon formation under deuterium bombardment as a function of target temperature up to 1000 K as well as thermal desorption of D2 and CD4 after room temperature implantation have been measured for different energies. The results are compared for pyrolytic graphite and boron doped graphite. The presence of boron in graphite results in a complete suppression of chemical reactivity in the bulk. Only in a shallow surface layer hydrocarbon formation persists within the first 1 nm, probably due to boron depletion.

Manufacturing and high heat flux loading of tungsten coatings on fine grain graphite for the ASDEX-Upgrade divertor

Abstract Fine grain graphite tiles coated with tungsten layers by plasma spray (PS, thickness 100–550 μm) and physical vapour deposition (PVD, 30–200 μm), respectively, were subjected to thermal loads up to 17 MW/m2 and 2 s pulse duration. The damage limit was evaluated by increasing the heat flux and the pulse length stepwise. The results proved that PS coatings are capable of withstanding heat loads up to 15 MW/m2 at 2 s pulse length without any structural changes, and cyclic loading with 1000 cycles at 10 MW/m2. The highly dense PVD coatings suffered damage by crack formation at slightly lower heat loads, and thin PVD layers failed under cyclic loading with 1000 cycles at 10 MW/m2 due to thermal fatigue and melting. The good performance of PS coatings is related to their porosity, which provides a crack arresting mechanism, and to their mechanical strength, depending on the density of the PS layer.

Molecular impurities in ASDEX UPGRADE plasma discharges

Abstract The introduction presents a historical review of the role of molecules in tokamak research starting from the first installations at the Kurchatov Institute. Molecular impurities were mostly considered as a transient conditioning problem, but with the use of carbon for wall and limiter elements, it was perpetuated. New results about the elementary processes involved in hydrogenic carbon erosion are reported and the existing data base is briefly discussed. Results from mass spectrometry are presented as well as data from optical spectroscopy including determination of CD 4 and CD fluxes from molecular band intensities. A typical yield of about 5% for hydrogenic chemical erosion is obtained. In combination, all these results show the impact of hydrogenic carbon erosion. They strongly suggest that with boronized walls it remains as the dominating process for the carbon fluxes in the SOL and even dominates the carbon concentration in the central plasma in spite of a high SOL screening action for hydrocarbons.

Quantitative AMS depth profiling of the hydrogen isotopes collected in graphite divertor and wall tiles of the tokamak ASDEX-Upgrade

The accelerator mass spectrometry (AMS) facility at the 3 MV Tandetron in Rossendorf has been applied for quantitative depth profiling of deuterium and tritium in samples cut from graphite protection tiles at the vessel walls of the fusion experiment ASDEX-Upgrade at the Max-Planck-Institut fur Plasmaphysik in Garching. The tritium originates from D(d,p)T fusion reactions in the plasma and it is implanted in the vessel walls together with deuterium atoms and ions from the plasma. The T concentrations in the surface layers down to the analyzing depth of about 25 μm are in the range of 1011 to 5 × 1015 T-atoms/cm3 corresponding to a tritium retention of 3 × 1010 to 3.5 × 1012 T-atoms/cm2. The much higher deuterium concentrations in the samples were simultaneously measured by calibrated conventional SIMS. In the surface layers down to the analyzing depth of about 25 μm the deuterium concentrations are between 3 × 1010 and 8 × 1021 atoms/cm3, corresponding to a deuterium retention of 2.5 × 1016 to 2.5 × 1018 atoms/cm2 The estimated total amount of tritium in the vessel walls is of the same order of magnitude as the total number of neutrons produced in D(d,n)3He reactions.

Composition of the plasma facing material Tokamakium

Abstract In experiments with magnetically confined hot plasmas in respect to controlled thermonuclear fusion, such as tokamaks or stellerators, the surface layers of the vessel walls are modified by the plasma by erosion, redeposition, hydrogen isotope implantation and heating due to the power load from the plasma. Thus, the composition and structure of the surface layers are finally different to those of the material initially installed. This new material at the surface layers of tokamak experiments is sometimes also named ‘Tokamakium’. Generally, all elements ever introduced into the vessel can be found in the surface layers of the plasma-facing wall tiles. At all areas of the plasma-facing components both erosion and deposition have been observed including the deposition of metal droplets. Some areas are erosion dominated, while at others deposition dominates with atomic depositions of up to several μm, which partly flake. The elements of dust particles introduced into the vessel by in-vessel works get mostly incorporated as impurities into the surface layers of the plasma facing material.