Highly oriented poly crystalline graphite (HOPG), boron-doped diamond (BDD), nanocrystalline diamond (NCD), ultra-nano-crystalline diamond (uNCD), fullerenes C60 and C70 and Diamond Like Carbon (DLC) surfaces are exposed to low pressure hydrogen plasma in a 13.56 MHz plasma reactor. Relative yields of surface produced Hions due to bombardment of positive ions from the plasma are measured by an energy analyzer 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 (~ 5 times compared to HOPG surface at room temperature) is observed on BDD surfaces. The shapes of ion distribution functions (IDFs) 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 Hproduction yields, while further analyses strongly suggest that the hydrogen content of the material and the sp3/sp2 ratio are the key parameters in driving surface ionization efficiency of carbon materials under the chosen plasma conditions.

This work is devoted to the study of atomic oxygen recombination on a glass surface, mainly in connection with atomic sources development. In this paper we present a non-stationary model for atomic oxygen recombination on a fused silica surface. Kinetics equations for oxygen atoms, taking into account heterogeneous reactions between gaseous atoms and the surface (Eley-Rideal mechanisms), as well as homogeneous processes involving surface migration of adsorbed species (Langmuir-Hinshelwood mechanisms), are solved. Surface reaction coefficients are calculated, and the choice of numerical values for surface parameters is discussed. The solution to the equations is compared to our previous experiments concerning the influence of the surface state on atomic recombination. An estimation is made of surface reaction coefficient values.

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.

The formation of NO molecules during a single plasma pulse in a low-pressure dc discharge is measured using time resolved tunable diode laser absorption spectroscopy in the infrared region. The pulse duration ranges from 280 µs to 16 ms and the pulse current ranges from 20 to 80 mA. The gas pressure is 133 Pa. Experimental results show that NO density is about proportional to the product of the pulse current times the pulse duration. NO formation mechanisms are discussed. We show that reaction of oxygen atoms with vibrationally excited nitrogen molecules (N 2 (X, v > 12) + O) does not impact the NO concentration. Numerical computation of a simplified kinetics taking into account excited metastable state N 2 (A) for the NO formation shows good agreement.

This paper deals with a study of H − /D − negative ion surface production on diamond in low pressure H 2 /D 2 plasmas. A sample placed in the plasma is negatively biased with respect to plasma potential. Upon positive ion impacts on the sample, some negative ions are formed and detected according to their mass and energy by a mass spectrometer placed in front of the sample. The experimental methods developed to study negative ion surface production and obtain negative ion energy and angle distribution functions are first presented. Different diamond materials ranging from nanocrystalline to single crystal layers, either doped with boron or intrinsic, are then investigated and compared with graphite. The negative ion yields obtained are presented as a function of different experimental parameters such as the exposure time, the sample bias which determines the positive ion impact energy and the sample surface temperature. It is concluded from these experiments that the electronic properties of diamond materials, among them the negative electron affinity, seem to be favourable for negative-ion surface production. However, the negative ion yield decreases with the plasma induced defect density. fusion power-plant prototype producing electrical energy, targeting ∼1 GW of electrical power coupled to the grid [23,24]. In the ITER and DEMO devices, the heating of the plasma will mainly be produced by neutral beam injection (NBI). NBIs systems are key components in achieving high fusion energetic-performances. The ITER NBIs are required to inject 1 MeV beams of neutral deuterium atoms (D) into the tokamak, providing plasma heating and current drive. At such high velocities, much larger than classical electron orbit velocities of hydrogen atoms, the probability of electron capture from D + ions is too low, so that production of D relies on electron detachment from high-intensity D − beams. D − negative-ions are produced in a low-pressure plasma source and subsequently extracted and accelerated.The ITER negative ion source, currently under development at IPP Garching [7,25] in Germany, operates with a high-density, low-pressure inductively coupled plasma. Extracted D − current density of 200 A m −2 , over a large surface of 1.2 m 2 , with 5%-10% uniformity and low co-extracted electron-current (below one electron per negative ion), during long operation period (3600 s) is targeted. To reach such a high D − negative-ion current, the only up-to-date scientific solution is the use of caesium. Deuterium negative-ions are created at the extraction region by backscattering of positive ions or neutrals on the plasma grid. Deposition of caesium on the grid lowers the material work function and allows for high electron-capture efficiency by incident particles and thus, high negative ion yields. Studies conducted at IPP Garching show that the ITER negative-ion source can reach the required high current densities. However, drawbacks to the use of caesium have been identified. First, the caesium is continuously injected in the source a...

We measure H − negative ions by means of a mass spectrometer in a helicon plasma reactor. The H 2 plasma operates at a low injected RF power (50-300 W), in a capacitive regime, under low pressure conditions (between 0.4 and 1 Pa). A highly oriented pyrolytic graphite (HOPG) graphite sample centred in the expanding chamber and facing the mass spectrometer nozzle placed 40 mm away is negatively biased. Negative ions formed on the graphite surface upon positive ion bombardment are detected according to their energy by the mass spectrometer. We obtain the H − ion distribution function (IDF) showing two main features: first, a high energy tail attributed to negative ions created via two-electron capture following H + 2 and H + 3 impact on the HOPG sample and, second, a main peak which can be attributed to negative ions created on the surface by the sputtering of adsorbed hydrogen and/or two-electron capture. Finally, we show that negative ion production is proportional to the positive ion flux and strongly depends on the positive ion energy.

The time afterglow of a pulsed discharge is used to investigate the neutral-particle kinetics in N2-O2 low-pressure mixtures. The pressure is in the range 0.5-2 Torr at 300 K and the mixture composition in the range 0-20% of oxygen. Time-resolved emission spectroscopy on N2(B), N2(C), NO(A) and NO(B) is employed to monitor energy transfers involving the metastable state N2(A). The influence of N(4S) and O(3P) atoms on the kinetics is accurately treated using absolute concentration measurements by time-resolved absorption spectroscopy in the VUV range. It is shown that by an appropriate choice of the discharge repetition rate, the vibrational excitation of N2(X) can be neglected. A chemical model, containing few unknown parameters, is developed in order to fit the experimental fluorescences. The NO(X) kinetics are investigated and its absolute concentration is deduced. Furthermore, it is shown that the N2(A) density is probably higher for pulsed discharges than for stationary low-pressure DC discharges.

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.