Alexandre Bès

Characterization of high density matrix microwave argon plasmas by laser absorption and electric probe diagnostics

Microwave plasma sources distributed on a planar matrix configuration can produce uniform bi-dimensional plasmas free from magnetic field in the 100 Pa pressure range. In argon, such uniform sheets of plasma have been obtained with ion densities in the range of 1012 to 1013 cm−3 with microwave power ranging from 0.4 to 2 kW. The electrical characterization of the plasma has been investigated using a cylindrical Langmuir probe. A first feature concerns the plasma potential that exhibits quite high values due to the large increase in the electron temperature towards the source plane where the microwave electric field is applied. Secondly, the decrease in the electron temperature observed when increasing the microwave power can be justified by the apparition of multi-step ionization mechanisms via metastable and higher excited states of Ar atom. The concentration and temperature of Ar(3P2) metastables have been measured by laser diode absorption spectroscopy. The results indicate that the presence of Ar(3P2) metastables is significant at 2 cm from the source plane with concentration and temperature values varying from 1010 to 1011 cm−3 and from 500 K to 1300 K, respectively, as functions of argon pressure and microwave power. An analytical model using a few simplifying assumptions provides a plasma picture in good agreement with the experimental results.

Production of H− Ions by Surface Mechanisms in Cs‐free Multi‐dipolar Microwave Plasma

This study demonstrates the feasibility of the recently proposed idea of enhancing recombinations of the hydrogen atoms from the plasma on a surface in order to produce highly vibrationally excited molecules that can be attached and dissociated by the cold electrons of the plasma, hence creating negative ions that could be used as a Cs‐free negative ion source. The negative ion density was obtained for a) two distinct materials, i.e. tantalum and stainless steel, and for b) two different degrees of molecular hydrogen dissociation, the higher degree of dissociation resulting from the cooling of the walls of the source. The relative negative ion density n‐/ne was measured by laser photodetachment and the electron density was obtained from Langmuir probe measurements. The pre‐sheath was studied by emissive and conventional Langmuir probes to evaluate the potential drop near the surface. Laser photodetachment measurements performed in the vicinity of the investigated material consisting of a disk inserted in ...

Control of particle flux and energy on substrate in an inverted cylindrical magnetron for plasma PVD

Inverted cylindrical magnetrons (ICMs) are often used in dc, pulsed dc or mid-frequency ac mode for coating complex objects with thin films deposited by plasma PVD. Since in such a configuration the substrate is inherently surrounded by the target and hence by the plasma, the energy flux of the impinging particles represents the main contribution to the substrate heating. This can readily constitute a limiting factor in the deposition process, especially when it is not possible to cool and bias the substrate. This work concerns a dc-driven ICM configuration subjected to several constraints: not only is the substrate surface area small by comparison to the cathode surface area, but its imposed potential is the ground one, thus itself constituting the anode surface of the considered setup. Several important substrate heating factors are highlighted and, in order to reduce the most prominent of them, a means to raise the plasma potential is proposed. This is achieved by positively polarizing two additional electrodes with respect to the ground. This additional surface generates a redistribution of the current and consequently regulates the electron flux on the substrate. The results are shown as a function of bias applied on the auxiliary electrodes and discussed in terms of the impact on the substrate heating.

Investigation of H− production by surface interaction of the plasma generated in “Camembert III” reactor via distributed electron cyclotron resonance at 2.45GHz (abstract)a)

When considering the state-of-the-art on H− ion sources, ions can be produced either by plasma-surface interaction and/or inside the plasma volume. For the production of negative ions by surface ionization, a low work function material is required. For this purpose, cesium has been used in many cases at LBNL, JAEA, KEK, and in other facilities [M. Bacal, Nucl. FusionNUFUAU0029-5515 46, 250 (2006)]. Despite an enhancement in the negative ion production (by a factor of 2.5 in JAEA source), the use of cesium could lead to many drawbacks in the plasma functioning of ITER, for example. An alternative material to cesium could lead to an important improvement for negative ion source.For this purpose, both theoretical and experimental studies must be undertaken. Surface mechanisms have to be taken into account both for creation and loss mechanisms: (i)By recycling the atomic hydrogen into highly vibrationally excited molecular hydrogen via recombinative desorption on specific surfaces (fresh tantalum on surface i...

Characterization of X-ray gas attenuator plasmas by optical emission and tunable laser absorption spectroscopies

X-ray gas attenuators are used in high-energy synchrotron beamlines as high-pass filters to reduce the incident power on downstream optical elements. The absorption of the X-ray beam ionizes and heats up the gas, creating plasma around the beam path and hence temperature and density gradients between the center and the walls of the attenuator vessel. The objective of this work is to demonstrate experimentally the generation of plasma by the X-ray beam and to investigate its spatial distribution by measuring some of its parameters, simultaneously with the X-ray power absorption. The gases used in this study were argon and krypton between 13 and 530 mbar. The distribution of the 2p excited states of both gases was measured using optical emission spectroscopy, and the density of argon metastable atoms in the 1s5 state was deduced using tunable laser absorption spectroscopy. The amount of power absorbed was measured using calorimetry and X-ray transmission. The results showed a plasma confined around the X-ray beam path, its size determined mainly by the spatial dimensions of the X-ray beam and not by the absorbed power or the gas pressure. In addition, the X-ray absorption showed a hot central region at a temperature varying between 400 and 1100 K, depending on the incident beam power and on the gas used. The results show that the plasma generated by the X-ray beam plays an essential role in the X-ray absorption. Therefore, plasma processes must be taken into account in the design and modeling of gas attenuators.

Experimental study of H atom recombination on different surfaces in relation to H- negative ion production

Volume production of H- negative ions is mostly attributed to the dissociative attachment (DA) of electrons to ro-vibrationally excited molecules.1,2 Apart from the main formation path for enriching the plasma with these molecules (i.e. radiative decay of singlet states excited by collisions with energetic electrons, i.e. EV excitation)3, an additional formation process refers to recombination of hydrogen atoms on the surface of materials which face the plasma.4,5 In this work, the importance of the later process is evaluated by considering various materials. Pyrex, Stainless Steel, Highly Oriented Pyrolytic Graphite - HOPG, and Yttrium, are sequentially tested in the ECR-driven H- negative ion source ROSAE III. This source is specially designed to promote as much as possible surface recombination only on the surface of the specimen under test, limiting at the same time the formation of ro-vibrational states via other processes (e.g. EV excitation). Optical emission spectroscopy does prove a high degree of dissociation in this source. Furthermore, electron and negative ion densities are measured by means of electrostatic probe and laser photodetachment6, respectively. The effectiveness of the above materials for the production of ro-vibrational states is thus evaluated indirectly, i.e. by comparing the values of the produced negative ion densities, assuming H- production through DA mainly. The results suggest that, under the present conditions, the formation of ro-vibrational states is apparently dominated by process other than surface recombination.

Detection of rovibrationally excited molecular hydrogen in the electronic ground state via synchrotron radiation

We describe an original setup named SCHEME (Source of exCited HydrogEn MolEcules) designed to study the recombinative desorption mechanisms of H atoms on a surface by means of high-brilliance monochromatic synchrotron radiation (SR). H atoms are produced on electrically heated filaments under vacuum (in the absence of any discharge) and subsequently recombine on a surface to produce excited hydrogen molecules, namely, H2( v″, J″). Once these levels are produced, they are probed in the vacuum chamber with 5–40 eV (248–31 nm) SR-photons. A preliminary test with Krypton has demonstrated the feasibility of SCHEME to obtain simultaneously VUV-synchrotron radiation induced fluorescence and VUV-absorption signals with room-temperature gas. In order to evaluate the hydrogen molecule dissociation induced by the heated filaments, the absolute density of H atoms has been measured by VUV-absorption. At 1800 K and 100 mTorr, we estimated a dissociation degree of 10−4, which corresponds to an absolute density of 1.8×10...