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Advances in Magnum-PSI probe diagnosis in support of plasma-surface interaction studies
No full textMagnum-PSI is a linear plasma generator equipped with a superconducting magnet, assuring fusion devices relevant conditions at plasma–surface interface. The plasma column was diagnosed using 64 probes embedded in the target. The cross-sectional distributions of plasma parameters (floating potential, ion saturation current and electron temperature) were measured for hydrogen and deuterium plasmas under various discharge conditions. The radial profile of the floating potential across the plasma column can be described by a reversed Mexican hat-like wavelet, having the most negative potential at the center of the plasma column. The negativity of the floating potential diminishes when the discharge current increases or the magnetic field decreases. The axial gradient of the floating potential is reduced by increasing the magnetic field. The ion saturation current is maximum at the center of the plasma column, increasing with both the discharge current and magnetic field. The ion flux to the target, estimated from Thomson scattering (TS) data, was confirmed by probe measurements. The electron temperature estimated from the ion branch of the probe current-voltage characteristic is few times larger than that obtained from TS. By increasing the gas pressure in the target chamber, the time-dependent ion saturation current measured by probes changes from a constant average current (when the plasma column is attached to the target) to a fluctuating average current with scattered peaks (in a partially detached regime) which vanishes completely in the fully detached regime. With respect to hydrogen, the plasma column is wider in deuterium and is characterized by less negative floating potential distributions
Aliovalent Calcium Doping of Yttrium Oxyhydride Thin Films and Implications for Photochromism
No full textTo develop an understanding of the photochromic effect in rare-earth metal oxyhydride thin films (REH3–2xOx, here RE = Y), we explore the aliovalent doping of the RE cation. We prepared Ca-doped yttrium oxyhydride thin films ((CazY1–z)HxOy) by reactive magnetron cosputtering with Ca doping concentrations between 0 and 36 at. %. All of the films are semiconductors with a constant optical band gap for Ca content below 15%, while the band gap expands for compositions above 15%. Ca doping affects the photochromic properties, resulting in (1) a lower photochromic contrast, likely due to a lower H– concentration, and (2) a faster bleaching speed, caused by a higher pre-exponential factor. Overall, these results point to the importance of the H– concentration for the formation of a “darkened” phase and the local rearrangement of these H– for the kinetics of the process.</p
Effects from the Target Plate Geometry on Fluctuations of Helium Plasma in the Linear Divertor Simulator Magnum-PSI
No full textWith the help of the linear divertor simulation device Magnum-PSI, a fluctuation investigation of the impact of the target plate geometry was conducted. We simultaneously quantify coherent low-frequency fluctuations with a newly built 70-GHz microwave reflectometry system, a reciprocating probe, a light emission detector system, and a fast-framing camera system. The strong low-frequency fluctuations were observed at both the electron density and the plasma radiations by moving the target plate along the magnetic field line. Furthermore, a strong peak in fluctuation intensity and the influence of the target plate tilt angle on the fluctuation intensity were noted
Extension of the flux fit method for estimating power deposition profiles
Full text linkThe flux fit method is used to self-consistently estimate the power deposition profile and heat transport profiles from temperature measurements originating from perturbative experiments with a modulated source. This Letter improves on this method by addressing the limitations and assumptions. The most crucial improvement is the additional freedom in the source deposition profile. Allowing for a variable central deposition location and height and including a skewness parameter produces deposition profiles more consistent with the measurement data, but still wider than equilibrium ray tracing in two different DIII-D discharges. Moreover, we show that the quality of the estimated deposition profile is key to the accuracy of diffusivity and convectivity estimates, but inversely, the estimated transport parameters hardly affect the quality of the power deposition estimate. Using this method, we show that the power deposition profile estimate is broadened with respect to ray-tracing by about 1.7–1.8 times in two DIII-D discharges.</p
Iron carbide formation on thin iron films grown on Cu(100): FCC iron stabilized by a stable surface carbide
No full textThin iron films evaporated onto Cu(1 0 0) were carburized using ethylene to produce iron carbide surfaces for use as model systems in experimental research. XPS and AES confirm that ethylene dissociation produces a pure iron carbide. A maximum of 0.5 ML carbon can be deposited for film thicknesses below 12 ML where Fe grows as γ-iron (FCC). For thick, BCC-Fe(1 1 0) films, post-treatment with ethylene leads to carbon coverages beyond 0.5 ML where some carbon diffuses into the bulk. The film remains α-iron (BCC) and a different surface carbide with a (4 × 3) unit cell is found. On the thin FCC-Fe(1 0 0) films, carbon reconstructs the surface into a p4g(2 × 2)-Fe2C layer which has a special stability and acts as a carbon trap that prevents carbon diffusion into the bulk. Fe2C is thermally stable up to 700 K above which Fe diffuses into the copper substrate while leaving graphitic carbon behind. Carbon segregates to the surface during evaporation of iron on top of an Fe2C-covered FCC-Fe film and causes the film to retain the FCC structure up to a thickness of at least 30 ML, far beyond 12 ML where BCC-Fe forms on Cu(1 0 0) in absence of surface carbon.</p
A ReaxFF molecular dynamics study of hydrogen diffusion in ruthenium - the role of grain boundaries
No full textRuthenium (Ru) thin films are used as protective caps for the multilayer mirrors in extreme ultraviolet lithography machines. When these mirrors are exposed to atomic hydrogen (H), it can permeate through Ru, leading to the formation of hydrogen-filled blisters on the mirrors. H has been shown to exhibit low solubility in bulk Ru, but the nature of H diffusion through Ru and its contribution to the mechanisms of blistering remain unknown. This work makes use of reactive molecular dynamics simulations to study the influence of imperfections in a Ru film on the behavior of H. For the Ru/H system, a ReaxFF force field which reproduces structures and energies obtained from quantum-mechanical calculations was parametrized. Molecular dynamics simulations have been performed with the newly developed force field to study the effect of tilt and twist grain boundaries on the overall diffusion behavior of H in Ru. Our simulations show that the tilt and twist grain boundaries provide energetically favorable sites for hydrogen atoms and act as sinks and highways for H. They therefore block H transport across their planes and favor diffusion along their planes. This results in the accumulation of hydrogen at the grain boundaries. The strong effect of the grain boundaries on hydrogen diffusion suggests tailoring the morphology of ruthenium thin films as a means to curb the rate of hydrogen permeation
A spectroscopic inference and SOLPS-ITER comparison of flux-resolved edge plasma parameters in detachment experiments on TCV
No full textThis work presents the use of a collisional-radiative model to infer plasma parameters from 2D emissivities of several deuterium Balmer lines. The emissivities were obtained by MANTIS, an absolutely calibrated, 10-camera imaging polychromator with <br/
Plasmas for in-situ resource utilization on Mars: fuels, life-support and agriculture
No full textThis work discusses the potential of combining non-thermal plasmas and conducting membranes for in situ resource utilization (ISRU) on Mars. By converting different molecules directly from the Martian atmosphere, plasmas can create the necessary feed-stock and base chemicals for processing fuels, breathing oxygen, building materials, and fertilizers. Different plasma sources operate according to different principles and are associated with distinct dominant physicochemical mechanisms. This diversity allows exploring different energy transfer pathways leading to CO2 dissociation, including direct electron-impact processes, plasma chemistry mediated by vibrationally and electronically excited states, and thermally driven dissociation. The coupling of plasmas with membranes is still a technology under development, but a synergistic effect between plasma decomposition and oxygen permeation across conducting membranes is anticipated. The emerging technology is versatile, scalable, and has the potential to deliver high rates of production of molecules per kilogram of instrumentation sent to space. Therefore, it will likely play a very relevant role in future ISRU strategies
Correction to \u27Impedance Spectra and Surface Coverages Simulated Directly from the Electrochemical Reaction Mechanism: A Nonlinear State Space Approach, 2019\u27
No full text2019 paper DOI: http://dx.doi.org/10.1021/acs.jpcc.9b01836<br/