C. Pardanaud

Raman spectroscopy investigation of the H content of heated hard amorphous carbon layers

We revisit here how Raman spectroscopy can be used to estimate the H content in hard hydrogenated amorphous carbon layers. The H content was varied from 2 at.% to 30 at.%, using heat treatments of a a-C:H, from room temperature to 1300 K and was determined independently using ion beam analysis. We examine the correlation of various Raman parameters and the consistency of their thermal evolution with thermo-desorption results. We identify a weak band at 860 cm-1 attributed to H bonded to C(sp2). We show that the HD/HG parameter (Height ratio between the D and G bands) is quasi-linear in the full range of H content and can thus be used to estimate the H content. Conversely, we show that the m/HG parameter (ratio between the photoluminescence background, m, and the height of the G band), often used to estimate the H content, should be used with care, first because it is sensitive to various photoluminescence quenching processes and second because it is not sensitive to H bonded to C(sp2).

Enhanced negative ion yields on diamond surfaces at elevated temperatures

Boron-doped polycrystalline diamond (BDD) and highly oriented pyrolytic graphite (HOPG) surfaces were exposed to low pressure hydrogen plasma. The relative yields of surface-produced H− ions were measured by an energy analyser quadrupole mass spectrometer. The highest H− yield was obtained at 400 °C for a BDD surface and at room temperature for an HOPG surface. At low ion bombardment energy, the maximum yield on a BDD surface is about 5 times higher than that on an HOPG surface, which has been the best carbon material so far for surface production of H− ions in caesium-free plasma. Raman measurements revealed surface modifications after plasma exposure.

Deuterium Inventory in Tore Supra: status of post-mortem analyses

A dedicated study on fuel retention has been launched in Tore Supra which included a D wall-loading campaign and the dismantling of the main limiter (Deuterium Inventory in Tore Supra, DITS project). This paper presents new results of post-mortem analyses performed on selected limiter tiles. Scanning electron microscopy shows thick layers with poloidally oriented tip-shaped structures in deposition zones. In the erosion zone deposits inside the open porosities of the bulk material of the tiles were found. Raman microscopy indicates that hard deuterated amorphous carbon layers are deposited on the limiter tiles and on their gap sides. Secondary ion mass spectrometry D depth profiles show an increase of the D content near the bottom of the gap, in agreement with previous results. The campaign markers (13C and 11B) are found in only 2 out of 7 measurements in the deposited layers. Finally, the agreement of the SIMS data with previous nuclear reaction analysis (NRA) measurements is discussed, especially discrepancies possibly due to the non-uniformity of the samples.

Negative-ion production on carbon materials in hydrogen plasma: influence of the carbon hybridization state and the hydrogen content on H− yield

Highly oriented polycrystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond, ultra-nanocrystalline diamond and diamond-like carbon surfaces are exposed to low-pressure hydrogen plasma in a 13.56MHz plasma reactor. Relative yields of surface-produced H− ions due to bombardment of positive ions from the plasma are measured by an energy analyser cum quadrupole mass spectrometer. Irrespective of plasma conditions (0.2 and 2 Pa), HOPG surfaces show the highest yield at room temperature (RT), while at high temperature (HT), the highest yield (∼3-5 times compared to HOPG surface at RT) is observed on BDD surfaces. The shapes of ion distribution functions are compared at RT and HT to demonstrate the mechanism of ion generation at the surface. Raman spectroscopy analyses of the plasma-exposed samples reveal surface modifications influencing H− production yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving the surface ionization efficiency of carbon materials under the chosen plasma conditions.

Raman micro-spectroscopy as a tool to measure the absorption coefficient and the erosion rate of hydrogenated amorphous carbon films heat-treated under hydrogen bombardment

We present a fast and simple way to determine the erosion rate and absorption coefficient of hydrogenated amorphous carbon films exposed to a hydrogen atomic source based on ex-situ Raman micro-spectroscopy. Results are compared to ellipsometry measurement. The method is applied to films eroded at different temperatures. A maximum of the erosion rate is found at ~ 450 °C in agreement with previous results. This technique is suitable for future quantitative studies on the erosion of thin carbonaceous films, especially of interest for plasma wall interactions occurring in thermonuclear fusion devices.

Modelling of the micrometric erosion pattern observed on the Tore Supra limiter tiles

Recently, the surface of carbon fibre composite tiles of the toroidal pump limiter of Tore Supra has been analysed by scanning electron and atomic force microscopies. In regions where fibres are perpendicular to the surface, a specific erosion pattern has been observed. It is constituted of a striation oriented with an angle oblique with respect to the magnetic field. The characteristic wavelength of this structure is micrometric, and similar to the fibre size. Modelling has been undertaken to reproduce this micrometric pattern. It is shown to originate from the carbon composite structure, for which it has been found by measurement using laboratory plasma that the erosion rate of the fibres is different from that of the surrounding matrix. Modelling emphasizes the effect of the impinging flux angle distributions of deuterium ions and carbon impurities that are preliminarily determined from computation of the magnetic sheath. In the case of deuterium the sheath is shown to have little effect on the particle trajectories for the simulation parameters considered here, although when impurities are included the sheath deflection is significant. Furthermore this study shows how the fibre organization in the composite influences the striation direction and points out the importance of the angular dependence of the sputtering yield.

Organic multishell isostructural host-guest crystals: fullerenes C-60 inside a nitroxide open framework

The dinitroxide biradical crystallizes forming hexagonal open frameworks with one-dimensional corrugated channels filled with crystallization solvent. The large pockets constitutive of the channels allowed the inclusion of C(60) in the paramagnetic network. The rapidity and high fidelity of crystal growth were used to prepare isostructural multilayer host-guest crystals successively stained with C(60).

Structural analysis of eroded carbon fiber composite tiles of Tore Supra: insights on ion transport and erosion parameters

As part of the deuterium inventory in the Tore Supra project, a detailed structural analysis of the eroded tiles (carbon fiber composite tiles) of the main plasma-facing component of Tore Supra, the toroidal pump limiter, has been performed. It combines scanning electron microscopy, atomic force microscopy and Raman microscopy. A tile cross-section has been scanned, showing the eroded profile of the tile and allowing the net erosion rate to be determined. Ripples are observed on the surface of the eroded tiles and this pattern gives a clear indication of the ion flux direction. The height profile along the valleys indicates that the fibers are eroded faster than the matrix, the difference in the erosion rates probably originating from differences in microstructure.

In-situ characterisation of the dynamics of a growing dust particle cloud in a direct-current argon glow discharge

The growth and the dynamics of a tungsten nanoparticle cloud were investigated in a direct-current low pressure argon glow discharge. Real-time analyses of the dust particle size and number concentration were performed in-situ by light extinction spectrometry, while spatial dynamics of the cloud was investigated with the laser light-sheet scattering method. Additional off-line electron microscopy and Raman spectroscopy measurements were also performed for comparison purpose. This experimental work reveals the existence of an agglomeration phase followed by the appearance of a new dust particle generation. While growing, the dust cloud is pushed towards the anode and the discharge edge. Afterwards, a new dust particle generation can grow in the space freed by the first generation of nanoparticles. The continuous growth, below the light extinction spectrometry scanning positions, explains the apparent dissimilarities observed between the in-line optical and the off-line electron microscopy analyses.

Long-term H-release of hard and intermediate between hard and soft amorphous carbon evidenced by in situ Raman microscopy under isothermal heating

We study the kinetics of the H release from plasma-deposited hydrogenated amorphous carbon films under isothermal heating at 450, 500 and 600 °C for long times up to several days using in situ Raman microscopy. Four Raman parameters are analyzed. They allow the identification of different processes such as the carbon network reorganization and the H release from sp 3 or sp 2 carbon atoms and the corresponding timescales. Carbon reorganization with aromatization and loss of sp 3 hybridization occurs first in 100 minutes at 500 °C. The final organization is similar at all investigated temperatures. Full H release from sp 3 carbon occurs on a longer timescale of about 10 hours while H release from sp 2 carbon atoms is only partial, even after several days. All these processes occur more rapidly with higher initial H content, in agreement with what is known about the stability of these types of films. A quantitative analysis of these kinetics studies gives valuable information about the microscopic processes at the origin of the H release through the determination of activation energies.