Origin of impurities in hydrogen plasmas

Prehandling and discharge cleaning methods applied in toroidal magnetic systems with metallic walls have been surveyed and their efficiency in depressing the initial concentration of low Z contaminants (mainly oxygen, but also carbon) have been compared. Chemical reactions between atomic (H° and H$^{+}$ ) hydrogen species and chemically bound oxygen and lattice-bound carbon in the surface-near layers are found to play a dominant role in the impurity release. Discharge cleaning in Tokamaks tends to crack and ionize the resulting water and hydrocarbons, leading to impurity ions which recombine chemically with the walls at the end of the discharge cycle. An ultrahigh vacuum apparatus has been built, in which the cleaning efficiency of H° and H$^{+}$ is investigated. The flux densities of the atomic species have been varied between 10$^{14}$ and 5 x 10$^{16}$ atoms/cm$^{2}$ s, covering the range of the averaged values of present day confinementdevices and future reactors. The cleaning efficiency is found to be high. A chemical kinetics model describes the mechanisms involved. An H$_{2}$ blistering-like effect is observed and is attributed to the H + H$^{2}$ recombination underneath the surface. This process influences the recycling phenomena and probably also accounts for the early appearance of metallic impurities in tokamak experiments. Conclusions to be shown from our measurements concerning phenomena observed in confinement systems are presented

Influence of bulk and surface phenomena on the hydrogen permeation through metals

We discuss the permeation of hydrogen through metals and alloys such as iron, nicket, steels and Inconel wherein H dissolves endothermically from an H$_{2}$ gas. We assume first that trapping centers, surface contaminatian layers, the saturation of the H surface coverage and the irrplantation profile when energetic Ions drive the permeation can be neglected, that a quasi-equilibrium exists between the H atom concentration $\gamma$ in the adsorbed layer and c in the near surface layers and that the H solubility and diffusivity are homogeneous in the membrane. We evaluate thereafter separately the influence of these various effects and identify the parameter domains where appreciable corrections result. The permeation phenomenon is complex even when these simplifications are male : the Penetration rate is proportional to the flux of thermal molecules, atoms or energetic ions - depending upon the rase - which strike the surface; the diffusion in the metal is proportional to the gradient of c ; the release rate depends an c$^{2}$; the time-dependent diffusion equation includes a double spatial derivative of c. Permeation can only be fully described when computer codes such as PERI described elsewhere is used. Simple analytical relations are however obtained in several limiting cases. They are the object of this report. Same of them had already been derived by other authors but they were not shown to be part of a single, seif consistent permeation model. A comparison of predicted and experimental results shows that the simplified model describes surprisingly accurately the hydrogen exchange between gas and metal solutions. Two extreme regimes exist where either the surface reactions or the bulk diffusion titelt the flow. The first arises usually when c is small and the second when c is large. A broad, continuous domain separates them, where both surface and volume effects are important. A permeation number W is introduced ; it allows to identify the flow regime which corresponds to a given experimental situation. With molecular hydrogen upstream, the steady state flux is proportional to the driving pressure P when the permeation number W, i. e. P, is small and to $\sqrt{p}$ when W>>1. Its variation is evaluated analytically in the intermediate domain. An universal function is obtained which involves twa material constants - the permeability and the rate constant of the recombination steil - when the surfaces of the membrane are identical ("symmetrical membrane"). The transient flux variation which follows a change of the driving pressure is described by analytical equations in both extreme cases : the equation of Daynes and Barmer applies when tel whereas the build-up of the permeation flow and the outgassing of hydrogen from metal samples obey other simple equations when W << 1. The equations valid for asymmetric membranes (with differing up- and downstream surfaces) are derived. [...

Rechencode PERI : Permeation von Wasserstoff durch Metallmembranen, Freisetzung aus Metalloberflächen und Inventar in Metallwänden

Das Rechenprogramm PERI wurde als Hilfsmittel entwickelt, um experimentelle Beobachtungen bei der Permeation von Wasserstoff durch Metallmembranen und bei der Wasserstofffreisetzung aus Metallwänden (Langmuir-Effekt) zu interpretieren. Solche Experimente dienen zur Bestimmung der Materialkonstanten, die die Wasserstoffdiffusion sowie seine Löslichkeit und seinen Austritt aus dem Material regeln. Ihre Kenntnis ist beim Betreiben von Fusionsanlagen notwendig, damit das Wasserstoffrecycling, das Inventar in der Wand und die Permeation (z .B . von Tritium) nach außen berechnet werden können. Experimente haben gezeigt, daß die Permeationsflußdichte bei niedrigen Primärdrücken ($\precsim$ , 1 Torr) diesem direkt proportional werden kann und nicht - wie nach der Richardson-Gleichung erwartet - der Quadratwurzel aus dem Druck. Die Gültigkeit der Richardson-Gleichung wird aber häufig bei derBestimmung der Materialkonstanten vorausgesetzt. Eine ausführliche Referenzliste hierzu befindet sich in dem Review-Bericht /16/.Die Beobachtungen lassen sich erklären mit der Vorstellung, daß die Konzentration der Wasserstoffatome in Oberflächennähe sowohl auf der Ein- als auf der Austrittsseite zeitlich veränderliche Größen sind und die Rate der freigesetztn Wasserstoffmoleküle quadratisch zur Konzentration wächst. In diesem Modell /1,2,16/ wird daher die Diffusionsgleichung für den Transport im Metallgitter zugrunde gelegt zusammen mit quadratischen Randbedingungen, die die Freisetzung nach außen beinhalten. Zur Diffusionskonstanten D tritt eine weitere Ratenkonstante, k$_{r}$ , für die Wasserstofffreisetzung. Die Lösung des auftretenden Gleichungssystems ist in Grenzfällen möglich (z.B. im stationären Zustand). Zur Bestimmung von D und k$_{r}$ aus dem instationären Zustand benötigt man die vom Rechencode PERI für den allgemeinen Fall ermittelten Lösungen. Der Rechencode PERI - PERI steht für $\underline{PE}$rmeation, $\underline{R}$ecycling und Inventar - erlaubt, die einseitige Beaufschlagung einer Metallwand (z.B. Stahl) mit Wasserstoffatomen oder -molekülen zu simulieren. Es sind zunächst die Wasserstoffkonzentration innerhalb der Wand als Funktion von Ort und Zeit berechnet und daraus die permeierenden und recyclierenden Wasserstoffflüsse und das Wasserstoffinventar in Abhängigkeit von der Zeit. Darüber hinaus läßt sich der Einfluß eines Wärmestroms berücksichtigen, der die Wand auf der Eintrittsseite des Wasserstoffs erreicht. Die Haupteingabedaten sind neben der Wanddicke die Teilchen- und Wärmeflußdichten und die temperaturabhängigen Materialkonstanten. Das Rechenprogramm gliedert sich in zwei Teile: TEIL I, der die Berechnung nur für ein einzelnes Wasserstoffisotop (z.B. Deuterium) erlaubt, und TEIL II, der die Berechnungen für zwei Isotope (z.B. Deuterium und Tritium) gestattet. Im folgenden werden das dem Code zugrunde liegende Modell skizziert und die berechneten Gleichungen genannt. Die Abschnitte enthalten im einzelnen: 1. Programm TEIL I (Berechnungen mit 1 Isotope), 2. Programm TEIL II (Berechnungen mit 2 Isotopen), 3. die berechneten und ausgegebenen Größen, 4. die Unterprogramme und Funktionen, 5. Beispiele und Hinweise zur Benutzung, 6. Test des Rechenprogramms. Die Liste der FORTRAN IV Statements befindet sich im Amhang

The behaviour of rg-cleaning discharges in large vessels

The pressures at which RG-discharges can be ignited (P$_{I}$) and sustained (P$_{Q}$ )have been measured in an unbaked SS vessel of 33 m$^{2}$ surface area and 11 m$^{3}$ volume. Different rf-electrode arrangements for the application in fusion devices have been tested. P$_{Q}$ = 5 x 10$^{-4}$ mbar can be achieved with an rf-anode in the center of the vessel. Higher pressures are necessary to sustain the discharge from anodes in portholes. Ignition is easily achieved by gaspuffing in the very vicinity of the rf antenna. No bipolar arcs of considerable lifetime have been observed, whereas numerous unipolar arcs were triggered by surface contaminations. Their rate increases strongly with increasing current but decreases exponentially with time for fixed Plasma conditions. A careful discharge conditioning by RG discharges in nonbakable devices or a prehandling at elevated wall temperature seem to be necessary. If they can be used, special means against unipolar ares and their transition into a stable bipolar arc become unnecessary

Data from: Phylogeography, historical demography, and habitat suitability modelling of freshwater fishes inhabiting seasonally fluctuating Mediterranean river systems: a case study using the Iberian cyprinid Squalius valentinus

The Mediterranean freshwater fish fauna has evolved under constraints imposed by the seasonal weather/hydrological patterns that define the Mediterranean climate. These conditions have influenced the genetic and demographic structure of aquatic communities since their origins in the Mid-Pliocene. Freshwater species in Mediterranean-type climates will likely constitute genetically well-differentiated populations, to varying extents depending on basin size, as a consequence of fragmentation resulting from drought/flood cycles. We developed an integrative framework to study the spatial patterns in genetic diversity, demographic trends, habitat suitability modelling and landscape genetics, to evaluate the evolutionary response of Mediterranean-type freshwater fish to seasonal fluctuations in weather. To test this evolutionary response, the model species used was Squalius valentinus, an endemic cyprinid of the Spanish Levantine area, where seasonal weather fluctuations are extreme, although our findings may be extrapolated to other Mediterranean-type species. Our results underscore the significant role of the Mediterranean climate, along with Pleistocene glaciations, in diversification of S. valentinus. We found higher nuclear diversity in larger drainage basins, but higher mitochondrial diversity correlated to habitat suitability rather than basin size. We also found strong correlation between genetic structure and climatic factors associated with Mediterranean seasonality. Demographic and migration analyses suggested population expansion during glacial periods that also contributed to the current genetic structure of S. valentinus populations. The inferred models support the significant contribution of precipitation and temperature to S. valentinus habitat suitability and allow recognizing areas of habitat stability. We highlight the importance of stable habitat conditions, fostered by typical karstic springs found on the Mediterranean littoral coasts, for the preservation of freshwater species inhabiting seasonally fluctuating river systems