Beryllium Surface Research Articles

Modelling of massive gas injection for ITER disruption mitigation

Abstract In ITER the plasma impact on the wall after disruptions can be mitigated by a preventive massive gas injection (MGI) of a noble gas into the confined plasma. The gas ionises in the core and the contamination of plasma leads to a fast loss of thermal energy by photon radiation. For the MGI modelling a tokamak code TOKES is applied. Two-dimensional simulations for argon MGI into ITER deuterium plasma assuming toroidally symmetric model for a horizontal mid-plane injector are performed. Plasma cooling time and maximal temperatures of a beryllium and tungsten first wall are assessed. One result is that in cases of substantial erosion the beryllium surface can melt. The obtained cooling time is not sensitive in respect to the electron thermal cross-conductivity based on a magnetic field braiding.

Influence of surface electronic states on electron emission from metallic targets

We study the electron emission produced by swift protons impinging grazingly on a Beryllium (0001) surface. This material presents a technological interest in modern fusion reactors. We found that surface states play an important role in the double differential energy- and angle- resolved electron emission probabilities, but the effect disappears when angular distributions are integrated to obtain electron energy spectra.

Carbon monoxide adsorption on beryllium surfaces

Density functional calculations are here carried out to study the carbon monoxide molecule adsorption on pristine, hydrogenated and hydroxylated beryllium Be (0001) surfaces. The adsorption energies and structures, the activation barriers to molecular adsorption and dissociation are calculated. These reactions are described in terms of potential energy surfaces and electronic density of states. The quantum results are discussed along two directions: the beryllium surface reactivity in the domain of nuclear fusion devices and the possible usage of beryllium as a catalyst of Fischer–Tropsch-type synthesis.

Measuring the difference between gross and net erosion

A series of systematic experimental investigations are reported, using a beryllium-containing oven to seed a controlled and adjustable amount of impurities into a steady-state plasma. Beryllium impurity seeding of the plasma is used to simulate redeposition of eroded material to investigate the relationship between gross and net erosion. The measured net erosion rates from beryllium targets do not change when the influx of beryllium ions is increased unless the Be-seeding rate exceeds, by an order of magnitude, the beryllium surface loss rate, which was determined without beryllium seeding of the plasma. Measurements of the penetration distance of eroding beryllium atoms axially into the plasma column reveal that the reflection coefficient of the incident beryllium ions is unchanged, but the sputtering yield of the plasma-deposited surface beryllium is increased with respect to the bulk beryllium material by a factor of ∼10.

Survival probability of slow ions colliding with room-temperature and heated surfaces of beryllium

Survival probabilities of slow ions colliding with room-temperature and heated (600°C) surfaces of beryllium have been determined. Earlier data on the survival probabilities of ions interacting with surfaces of carbon and tungsten are complemented by measurements of the survival probability of selected C1, C2, and C3 hydrocarbon ions and some non-hydrocarbon ions (, ) on the surfaces of beryllium. For the incident angle of 30° (with respect to the surface) and incident energy of 31 eV, the absolute survival probability (S a) and the ionization energy of the ion in question (IE) can be correlated with the use of the emprirical relation log S a = (2.9 ± 0.6)–(0.31 ± 0.06)IE for both room-temperature (hydrocarbon-covered) and heated (600°C) surfaces. The similar dependence for both non-heated and heated surfaces may presumably be due to a similar work function of the heated samples, containing substantial amounts of beryllium oxide and carbides on the surface, and of hydrocarbon-covered non-heated beryllium surfaces.

Effects of Hypervapotron Geometry on Thermalhydraulic Performance

Plasma disruptions and edge localized modes can result in transient heat fluxes as high as 5 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> on portions of a tokamak reactor first wall (FW). To accommodate these heat loads, the FW will likely use water-cooled hypervapotron heatsinks to enhance the heat transfer. In this article, we present the results of a computational fluid dynamics (CFD) study using 70 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C inlet water at 2.7 MPa to investigate the tooth height and backchannel depth of 50-mm-wide hypervapotrons with 6-mm-pitch and 3-mm side slots. We compare a popular design with 4-mm-high teeth and a 5-mm backchannel to a more optimal case with 2-mm-high teeth and a 3-mm backchannel under nominal heat loads (0.5 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) on a 100-mm-heated length and under single-phase flow conditions. Better heat transfer in the latter case and the smaller backchannel permit a factor of two reduction in the required mass flow while maintaining the same beryllium armor surface temperatures near 130 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C. The shallow teeth and smaller backchannel allow the 40 fingers in a typical panel to flow in parallel and simplify the water circuit. A comparison of the two hypervapotron designs during off-normal loading (5.0 MW/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) and two-phase flow then follows. The design with 2-mm teeth has a 3.5% higher beryllium surface temperature of 648 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C and reduces the critical heat flux (CHF) by ~2%. Hypervapotron width also plays a role in heat transfer and CHF. CFD results for 36 and 70 mm wide hypervapotrons compared to the 50-mm case reveal similar thermal performance at low heat flux, but a reduction in CHF with increasing width. This study highlights the necessary compromise between design margin during transient events, effective heat transfer under nominal conditions, limitations on finger width, and the simplicity needed in the water circuit design.

Quantum Modeling of Water and Oxygen Adsorption on Beryllium Surface

This article reports on calculations carried out using first-principles density functional theory (DFT) on beryllium surface hydration. The water adsorption and dissociation reactions are investigated, with a first step considering only water molecules on the pure metal surface. The adsorbed water monolayer structure and the activation energies for dissociation are determined. Then the reactivity of coadsorbed H2O and O2 molecules is investigated, and it is shown that water is a very efficient catalyst for oxygen dissociation. The resulting surface structure is strongly perturbed during these processes, and we conclude that this can be considered as the first step of beryllium corrosion.

Numerical simulation on temperature and stress fields in beryllium during cutting process

The temperature and stress fields in beryllium during high speed cutting process were studied by employing a thermo-mechanically coupled finite element method (FEM). The results show that the temperatures in beryllium increase only a little during the cutting process. Both of the residual stresses for along and normal to the cutting direction are tensile stresses in the surface of beryllium after cutting. The cutting force and thrust force are about 280 and −250 kN/m at the steady stage, respectively. The main effects of coolant on the cutting process are to decrease the friction coefficient and heat between the tool and the workpiece, so to reduce the temperature, but almost no effects are made for stress. This study is helpful to enhance the understanding for stress formation and optimize the process parameters of beryllium.

Quantum Modeling of Beryllium Surface Oxidation and Hydrogen Adsorption

This article reports on calculations carried out using the first-principles density functional theory (DFT) on the oxidation process of the beryllium surface and the consequences on hydrogen atom retention. The oxygen molecule adsorption/dissociation reaction is investigated above the special symmetry points (above a surface atom on top and in bridge position, and in the hollow sites of the hexagonal lattice) of the beryllium basal surface. The corresponding energy profiles are built for both the triplet ground state and the singlet first excited state. The structure of the oxidized layer is proposed. A comparison is then given between hydrogen adsorption on partly oxidized beryllium and hydrogen adsorption on beryllium oxide.

Simulation of transient heat loads on high heat flux materials and components

In order to simulate transient events on beryllium, thermal shock experiments have been carried out in the electron beam facility JUDITH.Different grades of Chinese and Russian beryllium have been loaded in comparison to the ITER reference grade S65C. The pulse length was 5ms, and energy densities covered the range from 1.2 to 5MJ/m2. All tests have been performed at room temperature.In a second series of tests, the influence ELM-like conditions has been investigated. Multiple shot experiments with up to 10,000cycles on beryllium S65C were carried out at energy densities below the ones for the onset of crack formation in single shot experiments. Most loading parameters were similar to the single shot tests, but in addition the experiments were carried out on hot beryllium surfaces of 250°C.Post mortem examinations of the samples were carried out by optical microscopy, SEM, metallography and other diagnostic methods.

Wall conditioning on ITER

Like all tokamaks, ITER will require wall conditioning systems and strategies for successful operation from the point of view of plasma-facing surface preparation. Unlike today’s devices however, ITER will have to manage large quantities of tritium fuel, imposing on wall conditioning a major responsibility for tritium inventory control. It will also feature the largest plasma-facing beryllium surface ever used in a tokamak and its high duty cycle and long pulse are expected to lead to the rapid formation of deposited layers in which tritium can accumulate. This paper summarises the currently planned ITER wall conditioning systems and describes the strategy for their use throughout exploitation of the device.

Quantum modeling of hydrogen retention on partially oxidized beryllium

This article reports on calculations carried out using the First Principles density functional theory (DFT) on the oxidization process of the beryllium surface and the consequences on hydrogen atom retention. Detailed analysis of the electron density of states (DOS) provides indications on the oxidized surface reactivity, particularly towards hydrogen (deuterium) adsorption. A comparison is then given between hydrogen adsorption on partly oxidized beryllium and on beryllium oxide.

Collective electronic excitations in a potassium‐covered Be surface

AbstractWe investigate within a two‐component jellium model the collective electronic excitations in a one‐monolayer (1ML) potassium covered beryllium surface. The calculations of the surface dynamical response properties have been performed with one‐particle energies and wave functions derived from the Kohn‐Sham density‐functional theory. The dispersion relation for the surface plasmon modes as well as the real and imaginary parts of the induced dynamical charge density oscillations are presented. Comparison of the collective modes in the 1ML K/Be system with that of a clean Be surface is made. (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Condon-domain phase diagram and hysteresis size for beryllium

The Condon domain phase diagram for beryllium is determined in magnetic fields up to 10 T and at temperatures down to 1.3 K using a standard ac pick-up coil method to measure the de Haas-van Alphen (dHvA) effect. The detection of the transition point from the homogeneous state to the Condon domain state (CDS) is based on the extremely non-linear response to the modulation field resulting from a small irreversibility in the dHvA magnetization. The experimental results are compared with theoretical predictions calculated from the Fermi surface (FS) of beryllium. The width h_m of the hysteresis loop in the CDS is measured in a wide temperature and field region. A model for the hysteresis size is proposed and numerically calculated for the whole phase diagram.

Photoelectron driven acoustic surface plasmons inp(2×2)K/Be(0001):Ab initiocalculations

We report on low-energy dynamical surface response properties of the $p(2\ifmmode\times\else\texttimes\fi{}2)\text{K}/\text{Be}(0001)$ system calculated within a first-principles approach. It is shown that a partly occupied adsorbate-induced quantum-well band dramatically affects the dynamical properties of the beryllium surface. We demonstrate clear evidence that the observed anomalous features in photoemission spectra of this and a similar system can be explained by acoustic surface plasmon excitations.

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.

Correlations between Survival Probabilities and Ionization Energies of Slow Ions Colliding with Room-Temperature and Heated Surfaces of Carbon, Tungsten, and Beryllium

Survival probabilities, S(a) (%), of hydrocarbon ions C1, C2, and C3 and several nonhydrocarbon ions (Ar(+), N(2)(+), CO(2)(+)) on room-temperature (hydrocarbon-covered) and heated (600 degrees C) surfaces of carbon (HOPG), tungsten, and beryllium were experimentally determined using the ion-surface scattering method for several incident energies from a few electronvolts up to about 50 eV and for the incident angle of 30 degrees (with respect to the surface). A simple correlation between S(a) and the ionization energy (IE) of the incident ions was found in the semilogarithmic plot of S(a) versus IE. The plots of the data at 31 eV were linear for all studied surfaces and could be fitted by an empirical equation log S(a) = a - b(IE). The values of the parameters a and b were determined for all investigated room-temperature and heated surfaces and can be used to estimate unknown survival probabilities of ions on these surfaces from their ionization energies.

Testing of beryllium marker coatings in PISCES-B for the JET ITER-like wall

Beryllium has been chosen as the first wall material for ITER. In order to understand the issues of material migration and tritium retention associated with the use of beryllium, a largely beryllium first wall will be installed in JET. As part of the JET ITER-like wall, beryllium tiles with marker coatings are proposed as a diagnostic tool for studying the erosion and deposition of beryllium around the vessel. The nominal structure for these coatings is a ∼10 μm beryllium surface layer separated from the beryllium tile by a 2–3 μm metallic inter-layer. Two types of coatings are tested here; one with a nickel inter-layer and one with a copper/beryllium mixed inter-layer. The coating samples were deposited by DC magnetron sputtering at General Atomics and were exposed to deuterium plasma in PISCES-B. The results of this testing show that the beryllium/nickel marker coating would be suitable for installation in JET.

‘Mixed-material evolution analysis of the ITER divertor’

Evolution of the ITER outer divertor target surface is analyzed for the case of wall-sputtered beryllium transported to an initially tungsten divertor. Coupled codes for the convective edge plasma, impurity transport, and mixed-surface sputtering, give the wall-source beryllium flux to the divertor, the sputtering erosion/redeposition response, and the resulting beryllium surface content/growth. The analysis shows zero net Be growth over most (∼80%) of the divertor, due to high re-sputtering and reflection—with an equilibrium Be/W surface quickly formed (∼10 s)—but with a region near the strike point having substantial (∼1 nm/s) growth.

A tritium diagnostic and trap for JUDITH—First results in disruption simulation experiments with neutron irradiated beryllium during cyclic electron beam testing

In the present study, the effect of disruptions on beryllium has been studied. Disruptions are simulated in the electron beam facility JUDITH by high energetic pulses of up to 250 MJ/m 2. Under these loads, the beryllium surface may roughen combined with the forming of cracks. During the experiments, a special problem arises from the fact that during the neutron irradiation beryllium transmutes to tritium. This tritium is bound in the beryllium matrix, but during the heating of the samples, the tritium may be set free and through the vacuum pump it may be released to the environment. In order to avoid and to quantify this release of tritium, a special tritium trap has been constructed. In this tritium trap the gas is pumped by means of a metal bellows pump through a catalyst tube filled with copper oxide. At a temperature of 300 °C, the tritium is oxidized to HTO. This HTO is lead through gas washing bottles filled with water. Here approximately 98% of the released tritium is caught. The temperatures in the process are controlled by thermocouples, and the tritium content is controlled by a tritium gas monitor and in addition with a liquid scintillation counter (LSC).