Surface impurities and “clean-up” techniques in ORMAK

The detrimental role played by impurities in thermonuclear devices is now well-known. Experimental studies to identify those impurities residing on the liner, or first wall, of the Oak Ridge Tokamak (ORMAK) have been carried out in the laboratory using Auger Electron (AES) and X-ray Photoelectron (XPS) spectroscopic techniques. Additionally, liner measurements have been made in situ using a small Soft X-ray Appearance Potential Spectrometer (SXAPS). Oxygen, iron and carbon were found to be the major surface impurities and, as confirmed by plasma diagnostics, also the major plasma impurities. Glow discharge cleaning of gold and stainless steel surfaces has been studied using various gases and gas mixtures. Oxygen discharges are very effective and hydrogen moderately effective in removing carbon and hydrocarbon deposits from both types of surfaces. Other parameters involved in the contamination-decontamination process, such as pressure and temperature, have been studied using techniques to controllably contaminate surfaces with hydrocarbons.     

Erosion research of CX-2002U carbon composites under low-temperature high-flux hydrogen plasma

The net erosion yield of CX-2002 U carbon fiber composites under high-flux low-temperature hydrogen plasma is investigated using a linear plasma device. It is found that the net erosion yield decreases rapidly first, and then tends to saturate with the increase of hydrogen–plasmaflux. When the temperature of the sample eroded by hydrogen plasma is above 300 °C, the hybridization of electrons outside the carbon atom would change. Then the carbon atoms combine with hydrogen atoms to form massive spherical nanoparticles of hydrocarbon compounds and deposit on the surface at the flux condition of 1.77?×?1022 m-2·s-1. Under the irradiation of hydrogen plasma loaded with negative bias, the surface morphology of the matrix carbon is changed dramatically. Moreover, the energy dependence of mass loss does not increase in proportion to the increase of hydrogen–plasma energy, but reaches a peak around 20 V negative bias voltage. Based on the analysis of different samples, it can be concluded that the enhancement of energy could make a contribution to chemical erosion and enlarge the size of pores existing on the surface.     

Investigations of the origins of deposited carbon species in gaps between divertor tiles using a kinetic code system

In order to investigate the origins of deposited carbon species in gaps, the simulations have been performed using a kinetic code system. At low plasma temperatures, the deposited carbon species mainly originate from top surfaces of the tile, while at high plasma temperatures the deposited carbon species are basically derived from side surfaces of the tile. A substantial variation of the deposition rate of carbon species originating from side surfaces is obtained due to physical sputtering and topography advantage. The deposited carbon species derived from top surfaces and side surfaces demonstrate different deposition characteristics for physical sputtering and chemical erosion: (i) for deposited carbon species from top surfaces, the deposition ratio for physical sputtering increases evidently and deposition rate virtually increases by one order of magnitude with increasing plasma temperature; and the deposition ratio for chemical erosion reduces correspondingly and deposition rate decreases gradually; (ii) for deposited carbon species from side surfaces, the deposited carbon species principally arise from physical sputtering; the deposition rates for chemical erosion are of the order of magnitude of 10¹⁵ m^?2 s^?1 for all studied plasma temperatures, and the deposition rates for physical sputtering can be two to three orders of magnitude greater than that for chemical erosion.     

Study of deuterium retention on lithiated tungsten exposed to high-flux deuterium plasma using laser-induced breakdown spectroscopy

Tungsten is under consideration for use as a plasma-facing material in the divertor region of ITER. Lithiation can significantly improve plasma performance in long-pulse tokamaks like EAST. The investigation of lithiated tungsten is important for understanding the lithium conditioning effects for EAST, where tungsten will be used as a plasma-facing material. In this paper, a few important issues of lithiated tungsten interacting with high-flux deuterium plasma have been studied, such as the effect of lithiation on deuterium retention, the profile of elemental distribution, and the chemical state of lithiated tungsten. Deuterium retention inside both pure and lithiated tungsten has been investigated for the first time in the linear plasma simulator Magnum-PSI by in-situ laser induced breakdown spectroscopy (LIBS). The results indicate that, after deuterium plasma exposure, deuterium retention could be saturated in the lithiation layer, and the lithium in the lithiated layer is chemically bound with deuterium. Moreover, the lithiation can inhibit the blistering on the tungsten surface. These results can be valuable for the application of LIBS as a diagnostic technique for plasma-facing components of tokamaks.     

ICPECVD类金刚石膜沉积过程的发射光谱诊断研究

利用发射光谱(Optical emission spectroscopy,OES)对感应耦合等离子体增强化学气相沉积(Inductivelycoupled plasma enhance chemical vapor d印osition,ICPECVD)类金刚石(Diamondlike carbon,DLC)膜过程中的各种基团进行分析,并对不同条件下薄膜沉积速率以及薄膜显微硬度进行测试.分析结果发现,感应耦合等离子体源激发甲烷等离子体中存在比较突出的碳氢离子成分,从而促进形成高硬度的DLC膜.而且射频功率、沉积气压等沉积参数的变化对DLC薄膜沉积过程的中性基团、离子基团以及原子氢等成分都有着明显影响,从而最终影响薄膜沉积过程及薄膜性质.     

Desorption of silver atoms from benzene-covered Ag(1 1 1) by energetic Ar bombardment

Experiments have been conducted to gain insight into the processes of desorption of neutral species from surfaces covered with organic molecules due to bombardment with keV particles. The system is comprised of benzene molecules adsorbed onto Ag(1 1 1) and bombarded with 8 keV Ar⁺ ions. Molecular dynamics (MD) simulations of the same system have been performed. Results show that the presence of the benzene alters the yield, the kinetic energy distributions, and the angular distributions of the silver atoms. These changes of the desorption characteristics are the result of collisions between the Ag atoms and the benzene molecules adsorbed to the surface. As more benzene is adsorbed to the surface, the changes to the Ag atom desorption characteristics become more pronounced. The simulations reproduce the modifications to the Ag atom energy and angle distributions.     

Ion impact effect on carbon coatings studied in a double plasma device

Hard carbon thin films were formed with a novel deposition technique based on double plasma operation: hydrocarbon ions are extracted from a driver plasma and injected onto a substrate in a target plasma. This technique allows us to avoid the ion-induced charge-up on the film surface. As a result, two pronounced effects of ion bombardment on the carbon film were found. Firstly, the harness of the films gradually increases with increasing ion energy E_i, and a Vickers hardness number of 4000 kg/mm² is obtained at E_i = 200 eV. Secondly, the hydrogen atom content in the carbon films is reduced to $\text{1}\text{10}$ by the ion impact.     

Incorporation of Nitrogen into Amorphous Carbon Films Produced by Surface-Wave Plasma Chemical Vapor Deposition

In order to study the influence of nitrogen incorporated into amorphous carbon films, nitrogenated amorphous carbon films have been deposited by using surface wave plasma chemical vapor deposition under various ratios of N2/CH4 gas flow. Optical emission spectroscopy has been used to monitor plasma features near the deposition zone. After deposition, the samples are checked by Raman spectroscopy and x-ray photo spectroscopy (XPS). Optical emission intensities of CH and N atom in the plasma are found to be enhanced with the increase in the N2/CH4 gas flow ratio, and then reach their maximums when the N2/CH4 gas flow ratio is 5%. A contrary variation is found in Raman spectra of deposited films. The intensity ratio of the D band to the G band (ID/IG) and the peak positions of the G and D bands all reach their minimums when the N2/CH4 gas flow ratio is 5%. These show that the structure of amorphous carbon films has been significantly modified by introduction of nitrogen。     

Modelling of carbon erosion and re-deposition for the EAST movable limiter

The movable limiter at the mid-plane of the Experimental Advanced Superconducting Tokamak(EAST)with carbon coatings on the surface was exposed to edge plasma to study the material erosion and re-deposition.After the experiments,the carbon erosion and re-deposition is modelled using the 3D Monte Carlo code ERO.The geometry of the movable limiter,3D configuration of the plasma parameters and electromagnetic fields under both limiter and divertor configurations have been implemented into the code.In the simulations,the main uncertain parameters such as carbon concentrationρ_c in the background plasma and cross-field transport coefficient D_⊥in the vicinity of surface according to the'funneling model',have been studied in comparison with experiments.The parameterρ_c mainly influences the net erosion and deposition profiles of the two sides of the movable limiter,while D_⊥mostly changes the profiles on the top surface.     

The chemical interaction of hydrogen discharges with non metals

The impurity ions in an r.f. discharge have been directly extracted and then both energy and mass analysed. A variety of surfaces have been exposed to the plasma including borosilicate glass, alumina and carbon. In each case a large variety of species were produced initially, due to desorption of impurities from the surface. After some hours exposure to the plasma many of these species decayed away and relatively simple ion mass spectra were observed. The principal species with oxide surfaces were OH⁺, OH⁺₂ and OH⁺₃ and the corresponding OD⁺, OD⁺₂, OD⁺₃ in a deuterium plasma. Two other major impurities were mass 28 and 29. These are attributed to CO⁺ and COH⁺. When the carbon sample was exposed to the plasma the main species observed were CH⁺₃, CH⁺₄ and the series C₂H⁺₂ to C₂H⁺₆.     

Recent Results Obtained with High Field, Internally Amplifying Semiconductor Radiation Detectors

Characterization of the gallium diffused junctions found useful as amplifying radiation detectors indicate a rather surprising window-junction depth relationship. The window, at only the self bias of the junction, has been measured to be a micron or so although the junction depth is ~50 microns. This is a result of the unusual diffusion process used - diffusion to 75 microns with subsequent removal of the heavily acceptor doped, first 25 micron region. Because of potential for low energy detection of these structures (which require thin windows) stress is placed upon window measurements. Measurements made of the response of these structures as photon detectors in the near infrared (0.7 - 1.1 micron) wavelength region are reported. Also initial results which indicate that the x-ray cutoff for these structures lies in the 44-60 Angstrom range.     

Initial damage processes for diamond film exposure to hydrogen plasma

Diamond is considered to be a possible alternative to other carbon based materials as a plasma facing material in nuclear fusion devices due to its high thermal conductivity and resistance to chemical erosion. In this work CVD diamond films were exposed to hydrogen plasma in the MAGnetized Plasma Interaction Experiment (MAGPIE): a linear plasma device at the Australian National University which simulates plasma conditions relevant to nuclear fusion. Various negative sample stage biases of magnitude less than 500 V were applied to control the energies of impinging ions. Characterisation results from SEM, Raman spectroscopy and ERDA are presented. No measureable quantity of hydrogen retention was observed, this is either due to no incorporation of hydrogen into the diamond structure or due to initial incorporation as a hydrocarbon followed by subsequent etching back into the plasma. A model is presented for the initial stages of diamond erosion in fusion relevant hydrogen plasma that involves chemical erosion of non-diamond material from the surface by hydrogen radicals and damage to the subsurface region from energetic hydrogen ions. These results show that the initial damage processes in this plasma regime are comparable to previous studies of the fundamental processes as reported for less extreme plasma such as in the development of diamond films.     

An object Kinetic Monte Carlo Simulation of the dynamics of helium and point defects in tungsten

In the near surface of plasma facing materials, high concentrations of hydrogen and helium isotopes can build up, which will interact with the point defects resulting from the bombardment of the surface as well as with the impurities of the materials. It is important to develop an understanding of the evolution of W microstructure in such conditions and to be able to model this evolution. The task is very complex, as many elements have to be included in the model which must be all parameterized correctly. Isochronal annealings experiments are simple experiments which can help in the making of more complicated models. In this work, an object Kinetic Monte Carlo technique parameterized on ab initio calculations as been used to model He desorption in W. The He atoms and the self interstitial atoms have been found to be very mobile but they can bind quite strongly with impurities such as carbon or molybdenum atoms. The evolution of the number of defects in the Kinetic Monte Carlo simulation was found to be in good agreement with the resistivity changes observed during an He desorption experiment of above threshold He implantation in a thin wire of tungsten.     

High-temperature thermodynamic properties of hypo- and hyper-stoichiometric uranium carbides

Measurements of carbon activity over univariant and bivariant UC_x compositions, including the difficult region near stoichiometry, have been made in the temperature range 2155–2455°K. A flowing-gas technique was used with H₂-CH₄ mixtures in order to fix the carbon potential in the gas stream. The shape of the isotherms showed a sharp decrease in carbon activity near the stoichiometric composition as the composition of the condensed phase was reduced toward the lower phase boundary in the hypostoichiometric region. With argon as the carrier gas, a congruent vaporization composition of C/U ≈ 1.05 was obtained. Values for the free energy and heats of formation, relative partial molar enthalpies and entropies of solution, enthalpy of vaporization and uranium activity are presented.     

Sputtering of hydrocarbons by ion-induced breaking of chemical bonds

While the physical sputtering of atoms caused by keV and MeV ions has been studied extensively both by molecular dynamics (MD) simulation and experiments, the mechanisms leading to atom and molecule erosion at energies 1–100 eV are not very well understood. We now describe how low-energy hydrogen ions can cause erosion of carbon atoms and hydrocarbon molecules by entering the region of a carbon–carbon chemical bond and hence breaking it, a process we call ‘swift chemical sputtering'. In the particular case of hydrogen bombardment of carbon-based materials, we further show that this can lead to erosion yields far exceeding those expected for a physical sputtering process.     

Surface bioactivity of plasma implanted silicon and amorphous carbon

1 Introduction tors.[10,11] In addition, it has been shown that the criti- cal concentration of ionic products dissolved from the Since the discovery of SiO2-based bioglass in bioactive glass composed of soluble silicon and cal-1969,[1] possible applications of silicon-containing cium ions can enhance osteogenesis th…     

Redeposition of amorphous hydrogenated carbon films during thermal decomposition

Thermally induced decomposition of hard and soft amorphous hydrocarbon films was investigated by thermal effusion spectroscopy. Released species were detected by a sensitive quadrupole mass spectrometer using two different experimental setups for thermal effusion. Species released in a molecular beam setup were detected in direct line of sight to the sample surface, while species released in a remote UHV oven had no direct line of sight to the mass spectrometer. Soft, hydrogen-rich carbon films exhibit a desorption maximum at T ≈ 740 K while hard films with a low hydrogen content have their maximum at T ≈ 870 K. Additionally, the spectrum of released species differs dramatically between hard and soft films. We found a significant redeposition of species released from soft films. From the redeposited fraction of material we estimated an average redeposition probability of about 50% for species released from soft films.     

Effect of methane content and the oscillating electric field between electrodes on atmospheric Ar/methane plasma jet and DLC coating deposition

In this work, the effects of the methane gas flow and the internal oscillating electric field between electrodes on radio-frequency(RF) atmospheric pressure argon/methane plasma jet and process of diamond-like carbon(DLC) film deposition have been investigated. Properties of RF atmospheric Ar/methane plasma jet such as active species density, length, electron temperature,appearance and ionization process of argon/methane plasma jet are changed due to the changing of methane flow content and electric field vector and its gradient. With increasing methane flow,the formation of C2 hydrocarbon and CH band content is decreased because injected electrical energy to a mixture of Ar/methane gases is insufficient to stabilize the ionization process of methane gas and the electrical-chemical reaction rate is decreased. With shortening the gas gap between two electrodes, electric field strength and its gradient are increased leading to more energy injection to the electron. Electrical-chemical reactions are strengthened leading to increasing the CH band content. These phenomena introduce the Ar/methane plasma jet in different modes causing to deposit the DLC film with different structures and properties. With using quartz glass and alumina ceramic as dielectric barriers tubes, RF atmospheric pressure Ar/methane plasma jet has been used to deposit DLC coating in different modes. Increasing methane content and shortening the gas gap leads to decreasing sp3 bonded content and the quality of the deposited film.     

On the Physics of Improved Confinement During Pulsed Poloidal Current Drive in MST Reversed-Field Pinch

Reduction of core-resonant magnetic fluctuations and improved confinement in the Madison Symmetric Torus reversed-field pinch (MST RFP) have been routinely achieved by applying the surface poloidal electric field. The created inductive poloidal electric field drives current in plasma which leads to the improved confinement. To study the effect we developed a relatively simple 1-D model in cylindrical geometry which assumes poloidal and axial symmetry during the drive. We use resistive magnetohydrodynamics model with realistic plasma parameters and assume that there is a vacuum gap between plasma boundary and conducting wall of the vessel. Evolution of plasma density is taken into account and plasma boundary moves self-consistently with momentum equation. We start from an initial unstable equilibrium and examine stability of plasma configuration at intermediate moments of time during the drive. For this we calculate the growth rates of unstable eigenmodes in the plasma. Our results show that the modifications to the plasma current profile during the drive are stabilizing. The initial stabilization is due to the direct modification of the current profile near the edge. It enhances later in time due to the flattening of λ profile in the core region as plasma and magnetic field compress inward during the drive.     

Nonequilibrium modeling study on plasma flow features in a low-power nitrogen/hydrogen arcjet thruster

Gasdynamic flow features in an electrothermal arcjet thruster with a mixture of 1:2 nitrogen/ hydrogen as the working gas have been studied by a two-temperature numerical simulation.Seven species and 17 kinetic processes are included in the chemical kinetic model used to represent dissociation,ionization,and the corresponding recombination reactions in this nitrogen/hydrogen mixture system.Based on the gas flow characteristics inside the arcjet nozzle,a new method is introduced to define the edge of the cold boundary layer,which is more convenient to analyze the evolution and development of plasma flow in an arcjet thruster.The results show that the arcjet thruster performance is determined largely by the exchange of energy and momentum between the low-density,high-temperature arc region and the high-density,coolttow region near the nozzle wall.A significant thermal nonequilibrium is found in the cold boundary layer in the expansion portion of the nozzle.The important chemical kinetic processes determining the distribution of hydrogen and nitrogen species in different flow regions are presented.It has been shown that the reaction rate of hydrogen species ionization impacted by electrons is much higher than that of nitrogen species ionization in the center of the constrictor of the arcjet thruster.This indicates that hydrogen species is very important in the conversion of applied electric energy into thermal energy in the constrictor region of the arcjet thruster.