M. Oberkofler

Retention mechanisms and binding states of deuterium implanted into beryllium

The retention of 1 keV D+ ions implanted into clean and oxidized single crystalline Be at room and elevated temperatures is investigated by a combination of in situ analytical techniques including temperature programmed desorption (TPD), nuclear reaction analysis, low-energy ion spectroscopy (LEIS) and x-ray photoelectron spectroscopy. For the first time, the whole temperature regime for deuterium release and the influence of thin oxide films on the release processes are clarified. The cleaned and annealed Be sample has residual oxygen concentration equivalent to 0.2 monolayer (ML) BeO in the near-surface region as the only contamination. LEIS shows that Be from the volume covers thin BeO surface layers above an annealing temperature of 1000 K by segregation, forming a pure Be-terminated surface, which is stable at lower temperatures until again oxidized by residual gas. No deuterium is retained in the sample above 950 K. By analyzing TPD spectra, active retention mechanisms and six energetically different binding states are identified. Activation energies (EA) for the release of D from binding states in Be are obtained by modelling the experimental data. Two ion-induced trap sites with release temperatures between 770 and 840 K (EA= 1.88 and 2.05 eV, respectively) and two trap sites (release between 440 and 470 K) due to supersaturation of the bulk above the steady state fluence of 2×1017 cm-2 are identified. None of the release steps shows a surface recombination limit. A thin BeO surface layer introduces an additional binding state with a release temperature of 680 K. Implantation at elevated temperatures (up to 530 K) changes the retention mechanism above the saturation limit and populates a binding state with a release temperature of 570 K.

Interaction of high flux deuterium/nitrogen plasmas with beryllium

Before nitrogen can be used as a radiator for edge plasma temperature control in experiments with beryllium as the wall material, the compatibility of nitrogen-containing plasma with beryllium has to be tested. Therefore beryllium samples were exposed to a variety of mixed N2/D2 plasmas in PISCES-B and codeposits from the sputtered material were collected. It was found that introducing N2 to a D2 plasma reduces Be erosion significantly but recovery to the pre-N2 levels is possible in pure D2 plasma. Berylliated vessel walls can be a reservoir for N2 and chemical processes probably play a significant role during nitriding and N2 removal. Nitrided target samples remain conductive and do not lead to additional arcing, but codeposits are insulating. Thermal desorption measurements of nitrided and un-nitrided target samples are comparable, while codeposits show a slightly reduced D retention. However, D release for both target and codeposits is shifted ≈100 K to higher temperatures, above 510 K.

Gas analyses of the first complete JET cryopump regeneration with ITER-like wall

Analytical results of a complete JET cryopump regeneration, including the nitrogen panel, following the first ITER-like wall campaign are presented along with the in situ analyses of residual gas. H/D mixtures and impurities such as nitrogen and neon were injected during plasma operation in the vessel to study radiation cooling in the scrape-off-layer and divertor region. The global gas inventory over the campaign is incomplete, suggesting residual volatile impurities are remaining on the cryogenic panel. This paper presents results on (i) residual deuterium on the panel which is very loosely related to the campaign, (ii) impurities like nitrogen which stick on the panel, and (iii) the ammonia production which can be observed by mass spectrometry.

Gross and net erosion of tungsten in the outer strike-point region of ASDEX Upgrade

We have investigated net and gross erosion of W in the outer strike-point (OSP) region of ASDEX Upgrade with the help of marker probes during low-density/high-temperature L-mode discharges. Post mortem analyses indicate net-erosion rates of 0.04–0.13 nm s−1, with the highest rates measured close to the OSP. Re-deposition was some 30–40% of gross erosion, which is lower than what has earlier been obtained spectroscopically (~50–60%), possibly due to the special plasma conditions of our experiment and intense flux of W atoms originating from the main chamber. Gross erosion was also estimated by passive emission spectroscopy, and around the OSP the results matched with post mortem data. However, the spectroscopic erosion profile in the poloidal direction was much steeper than the post mortem one. Preliminary ERO simulations have predicted net erosion of the same order of magnitude as experimental results but reproducing the poloidal erosion/re-deposition profiles requires further work.

Nitrogen retention mechanisms in tokamaks with beryllium and tungsten plasma-facing surfaces

Global gas balance experiments at ASDEX Upgrade (AUG) and JET have shown that a considerable fraction of nitrogen injected for radiative cooling is not recovered as N-2 upon regeneration of the liq ...

Hydrogen retention in beryllium: concentration effect and nanocrystalline growth

We herein report on the formation of BeD2 nanocrystalline domes on the surface of a beryllium sample exposed to energetic deuterium ions. A polycrystalline beryllium sample was exposed to D ions at 2 keV/atom leading to laterally averaged deuterium areal densities up to 3.5 10(17) D cm(-2), and studied using nuclear reaction analysis, Raman microscopy, atomic force microscopy, optical microscopy and quantum calculations. Incorporating D in beryllium generates a tensile stress that reaches a plateau at  ≈1.5 10(17) D cm(-2). For values higher than 2.0 10(17) cm(-2), we observed the growth of  ≈90 nm high dendrites, covering up to 10% of the surface in some zones of the sample when the deuterium concentration was 3  ×  10(17) D cm(-2). These dendrites are composed of crystalline BeD2, as evidenced by Raman microscopy and quantum calculations. They are candidates to explain low temperature thermal desorption spectroscopy peaks observed when bombarding Be samples with D ions with fluencies higher than 1.2 10(17) D cm(-2).

Towards a detailed understanding of the mechanisms of hydrogen retention in beryllium

In this paper, we present new temperature-programmed desorption measurements from implanted single and polycrystalline beryllium as well as new computational results on the diffusion of interstitials through an undisturbed beryllium lattice. The threshold fluence for the appearance of a low-temperature desorption region around 460 K after implantation of 1 keV D ions into polycrystalline beryllium is determined to be 1.2×1021 m−2. In the high-temperature release region, an implantation energy-dependent shift in desorption temperatures is observed after implantation to very low fluences. It is explained by the variation of the mean distance from the surface for atoms implanted at different energies rather than by changes in the activation energy for detrapping. Morphological investigations of the surface of polycrystalline beryllium after several implantation and annealing cycles show a strong dependence of the damage on the surface orientation of grains. This dependence correlates with the preferred diffusion of interstitials parallel to basal planes of the beryllium lattice as calculated within the formalism of density functional theory.

Experimental analysis and WallDYN simulations of the global nitrogen migration in ASDEX Upgrade L-mode discharges

This work presents ASDEX Upgrade experiments, where the nitrogen deposition and re-erosion on divertor manipulator samples and the effect of its transport through the plasma were studied. These results are compared to WallDYN-DIVIMP simulations based on SOLPS plasma backgrounds and employing an improved WallDYN model, which includes the effusion of nitrogen from saturated surfaces. On one hand, this allows the WallDYN code and the new saturation model with a comprehensive data set to be benchmarked, on the other hand the simulations help in the interpretation of the experimental results. Both, experimental results and simulations, show that the N content in the region of the outer strike line reaches its steady-state value within one discharge. The simulations also reproduce the experimentally observed nitrogen content in samples exposed to N2-seeded discharges. With respect to the boron deposition, the nitrogen deposition in a non-seeded discharge and the re-erosion of nitrogen discrepancies to the WallDYN-DIVIMP simulations are observed. Based on SDTrimSP simulations, these are attributed to the missing depth resolution of the WallDYN surface model. A detailed comparison of spectroscopic measurements to WallDYN simulations, based on a novel synthetic spectroscopy diagnostic for WallDYN, shows that the nitrogen fluxes in the plasma are well described by the simulations. From a comparison of several WallDYN-DIVIMP simulations employing customized onion-skin model plasma backgrounds the physical processes controlling the nitrogen concentration in the core plasma and the applicability of onion-skin model plasma backgrounds are discussed. From these simulations the private flux zone with the gas valve, the outer baffle and the high field side main wall are identified as the main sources for the nitrogen content of the core plasma.

Simulating the nitrogen migration in Be/W tokamaks with WallDYN

The migration of wall material or seeding impurities plays an important role in the formation of mixed materials, the impurity contamination of the plasma and tritium retention. First, this work pr ...

Optimized analysis of deuterium depth profiles in beryllium

Various accelerator-based ion beam analysis techniques are reviewed in simulations as well as experimentally with emphasis on their depth resolution and sensitivity for detecting deuterium in beryllium. The depth information contained in the ? particles that originate from the nuclear reaction analysis (NRA) with 3He is compared to elastic recoil detection analysis (ERDA) with various projectiles. The best depth resolution for D implanted up to approximately 150?nm below the Be surface is obtained with medium-heavy ion ERDA: for ERDA with 10?MeV 28Si ions a resolution of 47?nm (2?) is experimentally attained. ERDA with 4He and 28Si as well as NRA are applied to analyse a Be sample implanted with 3?1022?D?m?2 at an energy of 3?keV per atom. The resulting depth profile can only be resolved with medium-heavy ion ERDA. The depth profile of the D concentration features a plateau close to the sample surface with a constant D atomic fraction of 0.1. It drops to zero around a depth of 150?nm, corresponding to the calculated ion range.