Negative ions in low-pressure plasma sources are created either in the plasma volume by dissociative attachment or, at the plasma surface interaction due to surface ionization of backscattered or sputtered particles. Negative-ions formed on surfaces are accelerated towards the plasma by the sheath. They can influence the plasma kinetics through collisions with plasma species, or are self-extracted from the plasma thanks to the energy acquired in the sheath. Self-extraction of negative-ions can affect processes like sputtering, where the negative-ions formed on the cathode bombard the layer being deposited. In applications such as negative-ion sources for accelerator or fusion devices, it is taken advantage of negative-ion surface production. A low work-function material (usually caesium-covered metals) is in contact with the plasma and greatly enhances negative-ion production because of the low energy required to extract an electron from the surface. However, caesium free negative-ion sources would be greatly valuable for fusion applications because of the strong maintenance constraints induced by caesium injection.

This paper describes a theoretical analysis that can be used to study every periodic roughness pattern effects when Reynolds equation is valid. Numerical results using this new “averaged” equation point out the field of application of the method, particularly with respect to the roughness wavelength.

A new method involving mass spectrometry and modelling is described in this work, which may highlight the production mechanisms of negative ions on surface in low pressure plasmas. Positive hydrogen ions from plasma impact a sample which is biased negatively with respect to the plasma potential. Negative ions (NI) are produced on the surface through the ionization of sputtered and backscattered particles and detected according to their energy and mass by a mass spectrometer placed in front of the sample. The shape of the measured negative-ion energy distribution function (NIEDF) strongly differs from the NIEDF of the ions emitted by the sample because of the limited acceptance angle of the mass spectrometer. The reconstruction method proposed here allows to compute the distribution function in energy and angle (NIEADF) of the negative-ions emitted by the sample based on the NIEDF measurements at different tilt angles of the sample. The reconstruction algorithm does not depend on the NI surface production mechanism, so it can be applied to any type of surface and/or NI. The NIEADFs for HOPG (Highly Oriented Pyrolitic Graphite) and Gadolinium (low work-function metal) are presented and compared with the SRIM modelling. HOPG and Gd show comparable integrated NI yields, however the key differences in mechanisms of NI production can be identified. While for Gd the major process is backscattering of ions with the peak of NIEDF at 36 eV, in case of HOPG the sputtering contribution due to adsorbed H on the surface is also important and the NIEDF peak is found at 5 eV.

The characterization of a helicon plasma source has been done in argon plasmas using a rf compensated Langmuir probe. It is now well known that helicon discharges exhibit three modes of operation: a capacitive mode (known as the E mode), which is a low-density mode (around 109 cm−3), an inductive mode (known as the H mode) with higher densities (1010 cm−3), and a helicon-wave sustained mode (known as the W mode) with very high densities (1011–1012 cm−3). By increasing the injected power, the transitions between these modes can be observed and correspond to sudden increases of the plasma density and of the glow brightness. In this article the performances of the helicon source in terms of density and plasma potential are presented and discussed. Transitions between the different coupling modes are observed and explained thanks to two phenomenological models and a good agreement is found between the experimental values and the model results. It has been shown that the main parameter, which determines the coupling mode, is the plasma density, which depends on the rf power and the gas pressure. So the transition between E and H modes is observed when the density reaches a critical value: for this value the skin depth δ is equal to the radius of the Pyrex tube. A similar result has been shown for the H–W transition: a minimum density, the value of which depends on the magnetic field amplitude, is necessary to observe the first longitudinal helicon-wave mode. This value is also directly linked with the geometry of the reactor and of the antenna.

This work focuses on the production of negative-ions on graphite and diamond surfaces bombarded by positive ions in a low pressure (2 Pa) low power (20 W) capacitively coupled deuterium plasma. A sample is placed opposite a mass spectrometer and negatively biased so that surface produced negative ions can be self-extracted from the plasma and measured by the mass spectrometer. The ratio between negative-ion counts at mass spectrometer and positive ion current at sample surface defines a relative negative-ion yield. Changes in negative-ion production yields versus positive ion energy in the range 10-60 eV are analysed. While the negative-ion production yield is decreasing for diamond surfaces when increasing the positive ion impact energy, it is strongly increasing for graphite. This increase is attributed to the onset of the sputtering mechanisms between 20 and 40 eV which creates negative ions at rather low energy that are efficiently collected by the mass spectrometer. The same mechanism occurs for diamond but is mitigated by a strong decrease of the ionization probability due to defect creation and loss of diamond electronic properties.

Manuscript AbstractNumerical modelling is now used routinely to make predictions about the behaviour of environmental systems. Model calibration remains a critical step in the modelling process and different approaches have been taken to develop guidelines to support engineers and scientists in this task. This article reviews currently available guidelines for a river hydraulics modeller by dividing them into three types: on the calibration process, on hydraulic parameters, and on the use of hydraulic simulation codes. The article then presents an integration of selected guidelines within a knowledge-based calibration support system. A prototype called CaRMA-1 (Calibration of River Model Assistant) has been developed for supporting the calibration of models based on a specific 1D code. Two case studies illustrate the ability of the prototype to face operational situations in river hydraulics engineering, for which both data quality and quantity are not sufficient for an optimal calibration. Using CaRMA-1 allows the modeller to achieve the calibration task in accordance with good calibration practice implemented in the knowledge base. Relevant reasoning rules can easily be added to the knowledge base to extend the prototype range of applications. This study thus provides a framework for building operational support tools from various types of existing engineering guidelines.

2003) What kind of water models are needed for the implementation of the European water framework directive? ABSTRACTThe European Water Framework directive (WFD) promotes an integrated management approach, defines the river basin as the relevant management unit, and sets the objective of good ecological status for all waters in Europe before 2015. Based on research activities underway in France, this paper presents five examples of issues related to water modelling for the implementation of the WFD. The new concept of "good ecological status" calls for a necessary shift from a classical biogeochemical modelling to an ecological modelling, and for new kind of models which can describe the biological response of aquatic ecosystems to physical disturbance. The integrated management approach demands new kind of models, based on a more global approach, adapted to the scale of work and management to be done; this is illustrated with the rainfall-runoff models. In the field of hydraulics, the complexity is addressed through the creation of models, consisting of several modules, which are optimised as a function of the application desired. Finally, the need of tools in the process of allocating water resources among several actors in a river basin, as foreseen in the WFD, may be addressed by the use of multi-agent systems. All these examples show clearly that the hydrosystem complexity, the study of which requiring the use of physical, ecological, social and economic sciences, cannot be solved easily with a unique type of water modelling. Research is still needed to address this complexity, but also to provide in the same time tools and results for water managers.

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