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Vertically oriented single wall nanotubes (SWNTs) and few walled nanotubes (FWNTs) have been grown by electronic cyclotron resonance plasma enhanced chemical vapor deposition (PECVD) on silica flat substrates. The impact of the plasma parameters on SWNT and FWNT growth has been investigated using two different etching gas mixtures, namely, C2H2/NH3 and C2H2/H2 with various ratios and applied bias voltages. Kinetic studies are also proposed in order to describe the FWNT growth mechanism by plasma techniques. A key role played by the reactive gas (NH3 and H2) is observed in the PECVD process, contrary to multiwalled nanotube growth. It is demonstrated that the balance between FWNT growth versus FWNT etching can be widely modulated by varying the gas mixture and bias voltage. It is shown that the use of hydrogen for hydrocarbon gas dilution restricts the destruction of SWNT and FWNT by the plasma species (ions and radicals). [ABSTRACT FROM AUTHOR]

Ta 3 N 5 has ideal properties for the efficient water splitting under sunlight illumination, but its formation is only triggered by the incorporation of oxygen in its lattice. As a result, material properties may deviate from those of an ideal Ta 3 N 5 crystal, which can promote or impede the water splitting. This paper is to quantify variations in key properties relevant for the photoelectrolysis as a function of oxygen incorporation and to determine an optimum oxygen concentration. Thin films were prepared by direct current magnetron sputtering in an Ar/N 2 /O 2 gas mixture at two different N 2 and varying O 2 concentrations. The films were characterized by grazing-incidence x-ray diffraction (GI-XRD), total transmission and reflectance spectroscopy (TTRS) from which the band gap is deduced, photocurrent measurements on which the calculation of the efficiency-mobility-lifetime ( ημτ ) product is based and photoelectrolysis experiments. Finally, the stability of films was determined by x-ray photoelectron spectroscopy (XPS) measured before and after the photoelectrolysis. We show that material properties degrade with oxygen incorporation during the film growth. At the highest oxygen concentrations investigated, the material becomes amorphous and shows strong surface oxidation and a possible reaction with the electrolyte during the photoelectrolysis. Samples deposited at an oxygen concentration below 1%, exhibit the Ta 3 N 5 phase, a band gap below 2.0 eV, a high mobility-lifetime product, photoelectrolytic activity and a good relative stability against surface oxidation during the photoelectrolysis. [ABSTRACT FROM AUTHOR]

A pseudo-3D modeling approach, based on a particle-in-cell (PIC)-Monte Carlo collisions algorithm, has been developed for the study of large- and short-scale organization of the plasma in a planar magnetron. This extension of conventional PIC modeling permits the observation of spontaneous organization of the magnetron plasma, under the influence of crossed electric and magnetic fields, into the well-known, large-scale regions of enhanced ionization and density known as spokes. The nature of complex three-dimensional electron trajectories around such structures, and non-uniform ionization within them, is revealed. This modeling provides direct numerical evidence for the existence of high-amplitude internal spoke electric fields, proposed in earlier works. A 3D phenomenological model, consistent with numerical results, is proposed. Electron density fluctuations in the megahertz range, with characteristics similar to the electron cyclotron drift instability experimentally identified in a recent Letter, are also found. Published by AIP Publishing. [ABSTRACT FROM AUTHOR]

Electron transport across magnetic barriers is crucial in all magnetized plasmas. It governs not only the plasma parameters in the volume, but also the fluxes of charged particles towards the electrodes and walls. It is particularly important in high-power impulse magnetron sputtering (HiPIMS) reactors, influencing the quality of the deposited thin films, since this type of discharge is characterized by an increased ionization fraction of the sputtered material. Transport coefficients of electron clouds released both from the cathode and from several locations in the discharge volume are calculated for a HiPIMS discharge with pre-ionization operated in argon at 0.67 Pa and for very short pulses (few µs) using the a posteriori Monte Carlo simulation technique. For this type of discharge electron transport is characterized by strong temporal and spatial dependence. Both drift velocity and diffusion coefficient depend on the releasing position of the electron cloud. They exhibit minimum values at the centre of the race-track for the secondary electrons released from the cathode. The diffusion coefficient of the same electrons increases from 2 to 4 times when the cathode voltage is doubled, in the first 1.5 µs of the pulse. These parameters are discussed with respect to empirical Bohm diffusion. [ABSTRACT FROM AUTHOR]

The fluid description is widely used for the multi-dimensional modeling of low temperature plasmas with complex chemistries due to their relative low computational cost. It relies, however, on a series of simplifying assumptions and some truncation of the moment equations for describing the non-equilibrium between the electrons, positive ions, negative ions, and the neutrals. In this paper, the classical assumption of isothermal negative ions is revisited for electronegative plasmas and, more particularly, for the fluid modeling of the transition between the plasma and its sheath. To do so, and in contrast to previous studies, the energy balance equation for the negative ions is also computed, and it allows us to derive the polytropic coefficient γ of the negative ions in addition to one of the positive ions. Strong variations in the sheath and presheath of the negative ions temperature and their polytropic coefficient are observed. The polytropic coefficient is shown to be a strongly varying function of space having for consequence that the negative ions are isothermal only in a very narrow extension of the presheath. For the case considered in this paper, both positive and negative ion flows are nearly adiabatic at the sheath-edge and become adiabatic inside the sheath. This paper shows that classical fluid modeling assumptions need to be verified for each system under consideration, most particularly while modeling the transition from plasma to a wall. [ABSTRACT FROM AUTHOR]

Abstract: This paper deals with different methods for the manufacturing of near-field optical probes with nanometric aperture. After the wet chemical etching of the fiber, two metallization processes are compared: the well-known thermal evaporation versus the novel arrangement of plasma sputtering. Further, it is reported an original controlled nano-indentation in the smooth softness surface to produce the apex aperture of the tapered fiber. These apertures present thin protrusions, but they show good optical and topographic resolutions. Besides, the probe sensitivity is discussed with respect to the multi- and single-mode of the primary optical fiber for imaging sub-wavelength dimension objects in the collection mode. [Copyright &y& Elsevier]

Abstract: This paper reports on carbon nanotubes (CNT) arrays grown on patterned catalysts by plasma-enhanced chemical vapor deposition (PECVD). CNT growth has been performed by low pressure high-density plasma process at 700°C in acetylene/ammonia mixture. Prior to CNT growth, a standard lift-off process has been used for catalyst patterning. The chemical impact of the lift-off sequence on the catalyst activity is discussed with respect to the amount of Ni pre-deposited. In particular, it discusses the impact of this wet treatment on CNT growth when the catalyst amount is lower than a threshold value. [Copyright &y& Elsevier]

Abstract: This paper is dedicated to carbon nanotubes grown by electron cyclotron resonance plasma-enhanced chemical vapour deposition. It has been shown that carbon nanotubes (CNTs) can be grown at temperature as low as 550 °C by this technique. X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy were used to analyse nitrogen-doped CNTs. Independently of the synthesis temperature, about 4 at.% nitrogen are incorporated in the CNTs. A good correlation was found between XPS and NEXAFS results. Four components (397.3, 398.5, 400.8 and 402.5 eV) compose N 1s XPS spectra related to different chemical environments of nitrogen. This interpretation can be used as a simple check method of the presence of CNTs on as-deposited PECVD samples. [Copyright &y& Elsevier]

This letter reports low-pressure, room-temperature growth of carbon nanofibers containing nitrogen by plasma chemical vapor deposition arrangement. By alternating pure acetylene plasma and afterglow pure nitrogen high dense plasma, a fine control of the fibers growth kinetic is obtained. This layer-by-layer deposition technique takes advantage of nitrogen chemical etching effects during the growth of nitrogen-doped carbon nanofibers. [ABSTRACT FROM AUTHOR]

In negative ion sources for the ITER Neutral Beam Injection system, the co-extraction of electrons is one of the main limiting factors. The current of co-extracted electrons can be decreased by applying a positive bias voltage to the Plasma Grid (PG) with respect to its source walls. Simulations using three-dimensional Particle-in-Cell Monte Carlo Collision (3D-PIC MCC) model are a powerful tool for studying the extraction region of such ion sources. However, the inclusion of both PG and source walls in the simulation domain is difficult due to numerical constraints. This study uses the 3D-PIC MCC code ONIX to explore the effects of particle injection models on plasma characteristics, using a flux injection model to regulate particle influx for a flat transition in potential from the bulk plasma to the simulation domain. Biasing of the PG above floating potential is possible using the flux injection scheme and results in a notable reduction in co-extracted electrons, corroborating with established experimental observations. [ABSTRACT FROM AUTHOR]

The extraction of negatively charged particles from a negative ion source is one of the crucial issues in the development of the neutral beam injector system for future experimental reactor ITER. Full 3D electrostatic particle-in-cell Monte Carlo collision code-ONIX [S. Mochalskyy et al., Nucl. Fusion 50, 105011 (2010)]-is used to simulate the hydrogen plasma behaviour and the extracted particle features in the vicinity of the plasma grid, both sides of the aperture. It is found that the contribution to the extracted negative ion current of ions born in the volume is small compared with that of ions created at the plasma grid walls. The parametric study with respect to the rate of negative ions released from the walls shows an optimum rate. Beyond this optimum, a double layer builds-up by the negative ion charge density close to the grid aperture surface reducing thus extraction probability, and therefore the extracted current. The effect of the extraction potential and magnetic field magnitudes on the extraction is also discussed. Results are in good agreement with available experimental data. [ABSTRACT FROM AUTHOR]

Pulsed systems are extensively used to produce active species such as atoms, radicals, excited states, etc. The tunable diode laser absorption spectroscopy (TD-LAS) is successfully used to quantify the density of absorbing species, but especially for stationary or slow changing systems. The time resolved-direct absorption profile (TR-DAP) measurement method by TD-LAS, with time resolution of μs is proposed here as an extension of the regular use of diode laser absorption spectroscopy. The spectral narrowness of laser diodes, especially in the blue range (∼0.01 pm), combined with the nanosecond fast trigger of the magnetron pulsed plasma and long trace recording on the oscilloscope (period of second scale) permit the detection of the sputtered titanium metal evolution in the afterglow (∼ms). TR-DAP method can follow the time-dependence of the temperature (Doppler profile) and the density (deduced from the absorbance) of any medium and heavy species in a pulsed system. [ABSTRACT FROM AUTHOR]

The recent developments in nanosecond pulsed power supplies facilitate the emission of high density electron bursts but their safe operation demands avoiding breakdowns. Using the theoretical and numerical modeling of the electron emission phenomena from a tip (micro-protrusion), the breakdown threshold (pre-breakdown) is analyzed considering it as the highest value of the voltage preserving the system out of the thermo-emission instability regime. However, the space charge that builds up in front of the tip limits the performance of these electron sources by decreasing the local electric field and consequently the thermo-field emission as well as the temperature of the emissive surface. Hence, it is found that the system can safely hold higher voltages (without breakdown) in the presence of dense space charge. In direct current, for a titanium elliptic tip, the highest operation voltage increases by about 15%, whereas for a tungsten hyperbolic tip, it increases by 70%. Remarkably, the emitted current close to the pre-breakdown voltage stays unchanged with or without taking into account the space charge. Surprisingly, when very short pulses (3 ns) are applied to a tungsten hyperbolic tip, the pre-breakdown voltage additionally increases by 30%, and the Coulomb screening, very effective in front of the tip apex, enlarges the electron emission area by 60%, releasing about 1.3 times more electrons compared to vacuum emission (without the space charge). Moreover, the ring effect, experimentally discovered by Dyke and Trolan [Phys. Rev. 89, 799 (1953)] on the radial electron density distribution, can be microscopically observed and understood with your model. [ABSTRACT FROM AUTHOR]

In the neutralizer of the ITER Neutral Beam Injector, a 1 MeV-D- beam passes through an structure filled with D2 gas, where negative ions are mainly converted to fast D0 atoms. Once that the beam is neutralized no further optical correction is possible, i.e., transport from the neutralizer to the confinement chamber is ballistic. Because of this, the transport through the neutralizer determines ultimately the geometrical properties of the neutral beams. The ionization of the buffer gas (D2) filling the neutralizer induced by the D beam creates a rarefied and low temperature plasma (ionization degree ≈10-3, electron temperature ≈20 eV). This plasma can screen the electrostatic well of the D beam and, consequently, affect the properties of the extracted beam and the energy transport to the neutralizer walls. On the other hand, the plasma will eventually escape from the neutralizer and move back in the accelerator chain, toward the accelerating grids and the source. We present particle-in-cell simulations of the beam propagation and plasma formation through the neutralizer. Particle-particle and particle-wall collisions are treated using a Monte Carlo approach. Simulations show that the secondary plasma effectively screens the beam space charge preventing beam radial expansion due to Coulomb repulsion between beam ions. First results suggest that the current of plasma ions (D2+) into the accelerator would be of the order of I(D-)/100, with I(D-) the negative ion current. [ABSTRACT FROM AUTHOR]

Abstract: High Power Impulse Magnetron Sputtering (HiPIMS) with pre-ionization was successfully used for metal deposition at very low-pressures by feeding one cathode of a conventional dual magnetron deposition system, the other one being radio frequency (RF) supplied. By adjusting the pulse length and RF power, this hybrid RF/HiPIMS co-deposition process improved the deposition rate of niobium in a-NbSi films as well as the film homogeneity on 4in. Si substrates. Moreover, the films obtained by hybrid RF/HiPIMS showed good response for both superconducting critical temperature transition (Tc) and normal resistivity (R), which compared favorably to films obtained by Electron Beam Physical Vapor Deposition (EB PVD) and standard DC/RF MS-PVD co-sputtering deposition currently used in the micro-fabrication of cryogenic detector. [Copyright &y& Elsevier]

Population densities of excited states of argon atoms in a high power impulse magnetron sputtering (HiPIMS) discharge are examined using a global discharge model and a collisional-radiative model. Here, the ionization region model (IRM) and the Orsay Boltzmann equation for electrons coupled with ionization and excited states kinetics (OBELIX) model are combined to obtain the population densities of the excited levels of the argon atom in a HiPIMS discharge. The IRM is a global plasma chemistry model based on particle and energy conservation of HiPIMS discharges. OBELIX is a collisional-radiative model where the electron energy distribution is calculated self-consistently from an isotropic Boltzmann equation. The collisional model constitutes 65 individual and effective excited levels of the argon atom. We demonstrate that the reduced population density of high-lying excited argon states scales with (p *) − 6 , where p * is the effective quantum number, indicating the presence of a multistep ladder-like excitation scheme, also called an excitation saturation. The reason for this is the dominance of electron impact processes in the population and de-population of high-lying argon states in combination with a negligible electron–ion recombination. [ABSTRACT FROM AUTHOR]

The magnetic field is a key feature that distinguishes magnetron sputtering from simple diode sputtering. It effectively increases the residence time of electrons close to the cathode surface and by that increases the energy efficiency of the discharge. This becomes apparent in high power impulse magnetron sputtering (HiPIMS) discharges, as small changes in the magnetic field can result in large variations in the discharge characteristics, notably the peak discharge current and/or the discharge voltage during a pulse. Here, we analyze the influence of the magnetic field on the electron density and temperature, how the discharge voltage is split between the cathode sheath and the ionization region, and the electron heating mechanism in a HiPIMS discharge. We relate the results to the energy efficiency of the discharge and discuss them in terms of the probability of target species ionization. The energy efficiency of the discharge is related to the fraction of pulse power absorbed by the electrons. Ohmic heating of electrons in the ionization region leads to higher energy efficiency than electron energization in the sheath. We find that the electron density and ionization probability of the sputtered species depend largely on the discharge current. The results suggest ways to adjust electron density and electron temperature using the discharge current and the magnetic field, respectively, and how they influence the ionization probability. [ABSTRACT FROM AUTHOR]

The oxygen dynamics in a reactive Ar/O2 high power impulse magnetron sputtering discharge has been studied using a new reactive ionization region model. The aim has been to identify the dominating physical and chemical reactions in the plasma and on the surfaces of the reactor affecting the oxygen plasma chemistry. We explore the temporal evolution of the density of the ground state oxygen molecule O2(Xµ ∑-g, the singlet metastable oxygen molecules O2 (aµΔg) and O2(bµ ∑-g, the oxygen atom in the ground state O(³P), the metastable oxygen atom O(¹D), the positive ions O+2 and O+, and the negative ion O-. We furthermore investigate the reaction rates for the gain and loss of these species. The density of atomic oxygen increases significantly as we move from the metal mode to the transition mode, and finally into the compound (poisoned) mode. The main gain rate responsible for the increase is sputtering of atomic oxygen from the oxidized target. Both in the poisoned mode and in the transition mode, sputtering makes up more than 80% of the total gain rate for atomic oxygen. We also investigate the possibility of depositing stoichiometric TiO2 in the transition mode. [ABSTRACT FROM AUTHOR]

Two separate scientific communities are shown to have studied one common phenomenon, azimuthally rotating dense plasma structures, also called spokes, in pulsed-power E × B discharges, starting from quite different approaches. The first body of work is motivated by fundamental plasma science and concerns a phenomenon called the critical ionization velocity, CIV, while the other body of work is motivated by the applied plasma science of high power impulse magnetron sputtering (HiPIMS). Here we make use of this situation by applying experimental observations, and theoretical analysis, from the CIV literature to HiPIMS discharges. For a practical example, we take data from observed spokes in HiPIMS discharges and focus on their role in charged particle transport, and in electron energization. We also touch upon the closely related questions of how they channel the cross-B discharge current, how they maintain their internal potential structure and how they influence the energy spectrum of the ions? New particle-in-cell Monte Carlo collisional simulations that shed light on the azimuthal drift and expansion of the spokes are also presented. [ABSTRACT FROM AUTHOR]

Abstract: The crystalline structure profile of the compound layer obtained by pulsed plasma nitriding in steel gears is reported. The nitrogen depth profile obtained by Radio Frequency Glow Discharge Optical Emission Spectroscopy is correlated with both the nano-hardness and the crystalline ε-Fe3N/γ′-Fe4N phases identified in the nitrided layer by X-ray diffraction. These results show the importance to control the nitriding parameters to avoid abrupt hardness changes along the case that can jeopardize the gear performance. [Copyright &y& Elsevier]

This article reports a new approach for probe manufacturing, which is the key component in scanning near-field optical microscope (SNOM). The wet-etching process, to create the tip at the apex of a tapered fiber, has been optimized. Typical tip features are short tapers, large cone angles (30°), and very small diameters (<50 nm). Next process steps are performed in an original arrangement of plasma device, based on a modified hollow cathode discharge. It is used for both, to remove the dust particles or the etching residues from the tip surface and to coat the tapered region with a metallic ultrathin and compact film. To complete the probe’s fabrication, the tips are opened by dry electrolytic erosion. These probes have been successfully tested for SNOM applications. [ABSTRACT FROM AUTHOR]

A correction to the article "Extracted Current Saturation in Negative Ion Sources," by S. Mochalskyy, A. F. Lifschitz and T. Minea that was published in 2012 is presented.

A simple and reproducible method for nanoaperture opening of metal film covered tapered optical fibers is reported as the last step of the manufacturing process for near-field scanning optical microscopy (NSOM) probes. It uses a microspark between the biased tip and another plane electrode working as a coronalike discharge in point-to-plane configuration. The resultant high electric field is enhanced at the extremity of the metallized tip self-focusing the discharge ions. The opening process is performed in situ in a specially designed plasma reactor which also allows, for the optical fiber tips, their surface cleaning and silver coating. Nanoaperture diameter can be controlled between 30 and 100 nm. These probes are able to generate subwavelength resolved images and are appropriate for high resolution NSOM. [ABSTRACT FROM AUTHOR]

Ta 3 N 5 belongs to the group of transition metal nitrides with the cation in a high oxidation state. These are typically challenging to synthesize owing to the low reactivity of nitrogen. This applies similarly to Ta 3 N 5 that crystallizes only in the presence of oxygen during synthesis. Typical preparation methods are ammonolysis of oxidized Ta or magnetron sputtering of a Ta target in an atmosphere of Ar, N 2 and O 2. However, the material typically obtained by either synthesis method is of varying degrees of crystallinity and the key parameter affecting the crystallinity remains elusive. In this study, we examine the role of oxygen for the crystallinity of Ta 3 N 5 samples by studying. Thin film samples prepared by magnetron sputtering reveal that oxygen is indeed the central driver for Ta 3 N 5 crystallinity. While little oxygen in the films yields the metallic δ-TaN phase, excess oxygen results in low crystallinity Ta 3 N 5 or Ta-O-N films. Ta 3 N 5 samples with a high degree of crystallinity are obtained by limiting the oxygen supply to the sample during the deposition. A comparison with other studies suggests a fundamental oxygen incorporation limit above which the crystallinity of Ta 3 N 5 is compromised. The most crystalline sample from this study contains 4.4 at.% of oxygen. It is grown onto a Si(100) substrate, covered with a 30 nm-thick metallic diffusion layer. For this sample, we observe Ta 3 N 5 grains between 80 and 120 nm in size. • The degree of crystallinity of Ta 3 N 5 thin films is determined by its oxygen content. • A fundamental limit of oxygen incorporation in Ta 3 N 5 crystalline films is suggested. • Ta interlayer between Si substrate and Ta 3 N 5 promotes crystallization. • Large Ta 3 N 5 grain sizes between 80 and 120 nm are obtained. [ABSTRACT FROM AUTHOR]

Abstract DLC based coatings are now widely used for many applications. Me-doped DLC (Me = Metal) are studied intensively in order to optimize properties such as low compressive stress, low friction coefficient and higher tribological performance. These Me-DLC coatings are often deposited by means of a PVD/PECVD hybrid process, but few studies attempt to describe the phenomena involved in these complex processes. In this work, DLC is obtained in the Ar + H 2 + C 2 H 2 mixture by RF-PECVD and the metal, which is in-situ, is added by magnetron sputtering of pure metal targets. Plasma is investigated by means of Optical Emission Spectroscopy and the main phenomena between the target and the substrate are described. The results show that atomic aluminium emission line intensity decreases continuously, moving away from the Al target whereas Ti and Nb lines are weak near electrodes and reach a maximum in the centre of the target-substrates gap. This difference is due to the low mass of Al as well as its rapid thermalisation. Addition of a metallic element (Al, Ti or Nb) in DLC seems to lead to a disorder within the carbon matrix. Carbides precipitate in Ti-DLC and Nb-DLC for high metal contents. The addition of Ti or Nb in the DLC matrix leads to open morphologies associated with high surface roughness. Like the non-doped DLC, Al-DLC coatings have a compact morphology and low surface roughness. Highlights • Magnetron sputtering and PECVD (Plasma Enhanced Chemical Vapor Deposition) are combined to deposit Me-DLC films (Me = metal). • Al emission line intensity decreases continuously from the Al target to the substrates because of its fast thermalization • Ti or Nb density is low near the electrodes' sheaths and is maximum in the center of the target-substrates gap • Additional RF power on substrates favours ionization and, as a consequence, the metal sputtering. • -Low metal contents in DLC lead to a disorder within the carbon matrix • -Above a metal content threshold, when the metal content further increases, order is favoured • -Carbides precipitate in Ti-DLC and Nb-DLC for high Ti or Nb contents. • -Addition of Ti or Nb in the DLC matrix leads to open morphologies and high surface roughness. • -Al-DLC coatings keep a compact morphology and low surface roughness. [ABSTRACT FROM AUTHOR]

Ta 3 N 5 is a promising candidate for a variety of applications, most notably as a photoactive material for solar water splitting. It is typically synthesized in a two-step process in which oxidized tantalum is annealed in NH 3 . Magnetron sputtering is an alternative synthesis method that is little explored to date, as first tries resulted in a small degree of crystallinity of the samples. In this paper, we report on the addition of an axial magnetic field to the conventional magnetron configuration which guides ionic species from the negative glow onto the growing film of Ta 3 N 5 . This ion-assisted growth is shown to result in a high degree of crystallinity, i.e. amorphous content, which is typical for conventionally sputtered films, is largely suppressed. The surface of such prepared films is nanostructured by a dense population of grains. [ABSTRACT FROM AUTHOR]

We use an ionization region model to explore the ionization processes in the high power impulse magnetron sputtering (HiPIMS) discharge in argon with a titanium target. In conventional dc magnetron sputtering (dcMS), stepwise ionization can be an important route for ionization of the argon gas. However, in the HiPIMS discharge stepwise ionization is found to be negligible during the breakdown phase of the HiPIMS pulse and becomes significant (but never dominating) only later in the pulse. For the sputtered species, Penning ionization can be a significant ionization mechanism in the dcMS discharges, while in the HiPIMS discharge Penning ionization is always negligible as compared to electron impact ionization. The main reasons for these differences are a higher plasma density in the HiPIMS discharge, and a higher electron temperature. Furthermore, we explore the ionization fraction and the ionized flux fraction of the sputtered vapor and compare with recent experimental work. [ABSTRACT FROM AUTHOR]

Non-ohmic heating will be used in the experimental nuclear fusion reactor ITER to reach thermonuclear temperatures. Two heating mechanism will be implemented, i.e. microwaves resonant with ion and electron cyclotron frequencies and energetic neutral beam injection, which contributes also to the current drive. Each one of the two neutral beam injector planned for ITER will deliver 16 MW of 1 MeV D0 beam. In the injector, negative ions D− coming from a 40 A negative ion source are electrostatically accelerated to 1 MeV, and stripped of their extra electron by collision with a target gas in a structure known as the neutralizer. Residual charged particles are deflected after the neutralizer in an electrostatic ion dump (E-RID). The ionization of the deuterium buffer gas filling the neutralizer induced by the D− beam creates a rarefied plasma which is expected to efficiently screens the Coulomb repulsion of the beam. Moreover, this plasma can eventually escape from the neutralizer and move back in the accelerator, towards the accelerating grids and the negative ion source. The transport of the beam through the neutralizer and the RID and the related plasma properties were studied using a 3D electrostatic particle-in-cell code called OBI-3 (Orsay Beam Injector 3 dimensional). Particle–particle and particle–wall collisions are treated using the Monte Carlo collision approach. Simulations show that the secondary plasma effectively screens the beam space charge preventing beam transverse expansion. Plasma ions created in the neutralizer form an upstream current with a magnitude of ∼0.5% of the negative ion current. Gas breakdown leading to arc formation in the RID was not observed. Finally, results for the propagation of non-ideal beams coming from simulations of the extraction and consecutive acceleration taken from Revel et al 2013 Nucl. Fusion53 073027 are presented. [ABSTRACT FROM AUTHOR]

The future experimental fusion reactor International Thermonuclear Experimental Reactor will include two neutral beam injectors, which will contribute to heat the plasma to thermonuclear temperatures and to sustain the magnetic equilibrium by driving the toroidal current. Each injector will consist of a 40 A negative ion source, followed by a 1 MeV electrostatic accelerator and a gas neutralizer. The modelling of the transport of beam particles through the accelerator is a key ingredient in the design of the accelerator optics. It has been recently shown (Mochalskyy et al 2012 J. Appl. Phys. 111 113303) that the space distribution of the negative ion beam at the extraction plane of the ion source drastically differs from the ones used in previous studies of the accelerator optics. A numerical study of the transport of beam particles through the accelerator using these realistic geometries for the input beam is presented here. The simulations are performed with a new 3D particle-in-cell Monte Carlo collision code called ONAC (Orsay Negative ion Accelerator). The code is able to deal with accurate 3D magnetic field configurations and electrode geometries. Simulations show that the initial beam distribution drastically affects the beam transport and the power load on the grids. A significant fraction of beam particles collides with the accelerating grids, with a corresponding power loss of ~20% of the total power carried by the beam. The inclusion of beam loading effects (intrinsically taken into account in PIC codes) and of a fully 3D model is shown to be necessary to provide a correct description of the transport. [ABSTRACT FROM AUTHOR]

The influence of nano-structure and composition of the substrate on the properties of carbon nanotubes (CNTs) is presented. The samples are obtained following a sequential in situ deposition routine. First, TiNxOy films are grown on a crystalline silicon substrate. Immediately, dispersed nickel catalyst particles are deposited on the film. The non-stoichiometric TiNxOy films and Ni particles are grown by ion beam sputtering of Ti and Ni targets, respectively. Soon after that, the CNTs are grown by feeding acetylene gas into the chamber and maintaining the substrate at 973 K. In situ x-ray photoelectron spectroscopy allows compositional and structural analysis in all the stages of the sample growth process. The CNTs are further studied by scanning and transmission electron microscopy techniques, showing different population densities, sizes and diameters as a function of the oxygen content in the TiNxOy films. The results show that oxygen influences the surface diffusion mobility of the precursor carbon atoms involved in the growth of nanotubes suggesting the inhibition of catalyst particle coarsening. It is concluded that, in addition to acting as a diffusion barrier between the catalyst particles and the silicon support, the TiNxOy films modify the growth kinetics of the CNTs. [ABSTRACT FROM AUTHOR]

Abstract: To achieve thermonuclear temperatures necessary to produce fusion reactions in the ITER Tokamak, additional heating systems are required. One of the main method to heat the plasma ions in ITER will be the injection of energetic neutrals (NBI). In the neutral beam injector, negative ions (D −) are electrostatically accelerated to 1MeV, and then stripped of their extra electron via collisions with a target gas, in a structure known as neutralizer. In the current ITER specification, the target gas is deuterium. It has been recently proposed to use lithium vapor instead of deuterium as target gas in the neutralizer. This would allow to reduce the gas load in the NBI vessel and to improve the neutralization efficiency. A Particle-in-Cell Monte Carlo code has been developed to study the transport of the beams and the plasma formation in the neutralizer. A comparison between Li and D 2 based neutralizers made with this code is presented here, as well as a parametric study on the geometry of the Li based neutralizer. Results demonstrate the feasibility of a Li based neutralizer, and its advantages with respect to the deuterium based one. [Copyright &y& Elsevier]

Time-resolved tunable diode-laser absorption spectroscopy measurements were performed on the argon metastable (Arm) level 3s23p5(2P° 3/2)4s excited at 801.478 nm, in the dense plasma region in front of the magnetron target in a high power impulse magnetron sputtering (HiPIMS) discharge. From the Doppler profile the evolution of the temperature and density was derived during the pulse as well as during the plasma decay, i.e. in the afterglow. It is shown that the Arm density sharply increases at the beginning of the discharge pulse, followed by a severe Arm depletion along with increasing gas temperature around the peak of the HiPIMS discharge current. The strong dynamics of these parameters involve many elementary processes such as electron-impact excitation, electron-impact de-excitation and ionization of Arm, gas rarefaction, electron temperature increase at the end of the pulse and gas diffusion. These phenomena are discussed with respect to several parameters: distance from the target, peak discharge current during the pulse, pulse length, and gas pressure. [ABSTRACT FROM AUTHOR]

The lower deposition rate for high power impulse magnetron sputtering (HiPIMS) compared with direct current magnetron sputtering for the same average power is often reported as a drawback. The often invoked reason is back-attraction of ionized sputtered material to the target due to a substantial negative potential profile, sometimes called an extended presheath, from the location of ionization toward the cathode. Recent studies in HiPIMS devices, using floating-emitting and swept-Langmuir probes, show that such extended potential profiles do exist, and that the electric fields Ez directed toward the target can be strong enough to seriously reduce ion transport to the substrate. However, they also show that the potential drops involved can vary by up to an order of magnitude from case to case. There is a clear need to understand the underlying mechanisms and identify the key discharge variables that can be used for minimizing the back-attraction. We here present a combined theoretical and experimental analysis of the problem of electric fields Ez in the ionization region part of HiPIMS discharges, and their effect on the transport of ionized sputtered material. In particular, we have investigated the possibility of a 'sweet spot' in parameter space in which the back-attraction of ionized sputtered material is low. It is concluded that a sweet spot might possibly exist for some carefully optimized discharges, but probably in a rather narrow window of parameters. As a measure of how far a discharge is from such a window, a Townsend product ΠTownsend is proposed. A parametric analysis of ΠTownsend shows that the search for a sweet spot is complicated by the fact that contradictory demands appear for several of the externally controllable parameters such as high/low working gas pressure, short/long pulse length, high/low pulse power and high/low magnetic field strength. [ABSTRACT FROM AUTHOR]

Abstract: This paper reviews the on-going design, R&D and procurement activities, mostly conducted within the ITER framework, on-going in Europe under the co-ordination of Fusion for Energy (F4E), in co-operation with the European Fusion Associations and aimed at the establishment of the ITER Heating Neutral Beam (HNB) system. [Copyright &y& Elsevier]

This work presents the changes induced at the nanometer scale by the nitrogen incorporation into amorphous carbon nitride a-CNx films deposited by radio-frequency (r.f.) magnetron sputtering. High N content films (20 to 33 at.%) have been analysed by electron energy loss spectroscopy (EELS) and by near edge X-ray absorption fine structure (NEXAFS). EELS spectra at C K and N K edges roughly indicate the increase of the corresponding π* features, associated to N sp2 and C sp2 bonds, vs. the N content. Fine structures of the π* region of the N K edge spectra show at least three contributions, with different evolution depending on the N content. The discrepancies between the results recorded by these two techniques are only outward and they can be explained by the influence of the plasma parameters (pressure lying from 0.5 to 13 Pa and various Ar/N2 mixtures) on the deposition process. All the results are in good agreement with the XPS analysis of the N 1s data. [Copyright &y& Elsevier]

Decreasing the co-extracted electron current while simultaneously keeping negative ion (NI) current sufficiently high is a crucial issue on the development plasma source system for ITER Neutral Beam Injector. To support finding the best extraction conditions the 3D Particle-in-Cell Monte Carlo Collision electrostatic code ONIX (Orsay Negative Ion eXtraction) has been developed. Close collaboration with experiments and other numerical models allows performing realistic simulations with relevant input parameters: plasma properties, geometry of the extraction aperture, full 3D magnetic field map, etc. For the first time ONIX has been benchmarked with commercial positive ions tracing code KOBRA3D. A very good agreement in terms of the meniscus position and depth has been found. Simulation of NI extraction with different e/NI ratio in bulk plasma shows high relevance of the direct negative ion extraction from the surface produced NI in order to obtain extracted NI current as in the experimental results from BATMAN testbed. [ABSTRACT FROM AUTHOR]

• Vibrational and rotational temperatures of methane are obtained by fitting the methane Raman spectrum. • Technique is validated using available data up to 860 K, but believed to be accurate at least up to 1500 K. • Vibrational-rotational non-equilibrium spectra are generated using pulsed microwave plasmas. This work describes the theoretical basis and implementation of the measurement of vibrational (T vib) and rotational (T rot) temperatures in CH 4 by fitting spontaneous Raman scattering spectra in the Pentad region. This method could be applied for thermal equilibrium temperature measurements applications, e.g. in combustion, or vibrational-rotational non-equilibrium applications, such as in plasma chemistry. The method of calculating these temperatures is validated against known temperature thermal equilibrium spectra up to 860 K from published data, giving an estimated relative error of 10%. This demonstrates that both the calculated stick spectrum and the algorithm to determine T vib and T rot for CH 4 is robust to 860 K, but we expect it is valid to 1500 K. Additionally, a number of non-equilibrium spectra generated with a pulsed microwave plasma are fitted to find T vib and T rot , further demonstrating the applicability of this method in fitting non-equilibrium spectra. [ABSTRACT FROM AUTHOR]