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The interaction between neutrals outside tokamak plasmas and fast ions that are generated by neutral beam injection and pass outside the plasma is studied. It is shown that at beam energies up to ~100 keV fast-ion losses are significant, up to 20% in the studied case, because of charge exchange reactions between the neutrals and the fast ions. About half of the fast ions that neutralize outside the plasma get lost to the wall while the other half re-enters the plasma and ionizes again, often on better confined orbits, which is visible as an enhanced beam power deposition in the plasma core (). As a result, the plasma performance as measured from beam-plasma fusion reactions is hardly changed despite the significant fast-ion losses toward the plasma edge. [ABSTRACT FROM AUTHOR]

Gas giant exoplanets orbiting at close distances to the parent star are subjected to large radiation and stellar wind fluxes. In this paper, hydrodynamic simulations of the planetary upper atmosphere and its interaction with the stellar wind are carried out to understand the possible flow regimes and how they affect the Lyα transmission spectrum. Following Tremblin and Chiang, charge exchange reactions are included to explore the role of energetic atoms as compared to thermal particles. In order to understand the role of the tail as compared to the leading edge of the planetary gas, the simulations were carried out under axisymmetry, and photoionization and stellar wind electron impact ionization reactions were included to limit the extent of the neutrals away from the planet. By varying the planetary gas temperature, two regimes are found. At high temperature, a supersonic planetary wind is found, which is turned around by the stellar wind and forms a tail behind the planet. At lower temperatures, the planetary wind is shut off when the stellar wind penetrates inside where the sonic point would have been. In this regime mass is lost by viscous interaction at the boundary between planetary and stellar wind gases. Absorption by cold hydrogen atoms is large near the planetary surface, and decreases away from the planet as expected. The hot hydrogen absorption is in an annulus and typically dominated by the tail, at large impact parameter, rather than by the thin leading edge of the mixing layer near the substellar point. [ABSTRACT FROM AUTHOR]

Capacitively coupled plasma (CCP) tools utilized for plasma etching of dielectric features utilize large amounts of power for processing. As a result, neutral gas heats up significantly during processing. The resulting gas density variations across the reactor can affect reaction rates, radical densities, plasma characteristics and uniformity within the reactor. In this paper, results from a two-dimensional computational investigation of an Ar/CF4 CCP discharge incorporating an energy equation solution for all ions and neutrals are discussed. The dominant neutral gas heating process is identified to be elastic collisions with ions while conduction is found to be the major mechanism of heat transport. Some species such as F and CF3 demonstrate higher temperatures than the feedstock gases owing to additional heating via charge-exchange reactions and/or Franck–Condon heating. Typical process parameters such as pressure, frequency of excitation, power and gas composition are varied to investigate their impact on gas temperature. At higher excitation frequency and/or pressure, increased elastic collisions with ions lead to greater heat generation. The heat generated per molecule of the radicals, however, decreases with increase in pressure leading to a decrease in gas temperature. The increase in neutral collision frequencies with pressure also results in the decrease in temperature difference between species in the plasma. As CF4 fraction increases, both the elastic collision cross-section and Franck–Condon heating sources increase, leading to higher gas temperatures. [ABSTRACT FROM AUTHOR]

The ion temperature is not frequently measured in the boundary of magnetic fusion devices. Comparisons among different ion temperature techniques and simulations are even rarer. Here we present a comparison of ion temperature measurements in the boundary of the Alcator C-Mod tokamak from three different diagnostics: charge exchange recombination spectroscopy (CXRS), an ion sensitive probe (ISP), and a retarding field analyzer (RFA). Comparison between CXRS and the ISP along with close examination of the ISP measurements reveals that the ISP is space charge limited. It is thus unable to measure ion temperature in the high density (>1019 m-3) boundary plasma of C-Mod with its present geometry. Comparison of ion temperatures measured by CXRS and the RFA shows fair agreement. Ion and electron parallel heat flow is analyzed with a simple 1D fluid code. The code takes divertor measurements as input and results are compared to the measured ratios of upstream ion to electron temperature, as inferred respectively by CXRS and a Langmuir probe. The analysis reveals the limits of the fluid model at high Knudsen number. The upstream temperature ratio is under predicted by a factor of 2. Heat flux limiters (kinetic corrections) to the fluid model are necessary to match experimental data. The values required are found to be close to those reported in kinetic simulations. The 1D code is benchmarked against the 2D plasma fluid code UEDGE with good agreement. [ABSTRACT FROM AUTHOR]

The installation of a new poloidal charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) has enabled the determination of the radial electric field, Er, using the radial force balance of impurity ions. Er has been derived from charge exchange (CX) spectra measured on different impurity species, such as He2+, B5+, C6+ and Ne10+. The resulting Er profiles are found to be identical within the uncertainties regardless of the impurity species used, thus, demonstrating the validity of the diagnostic technique. The Er profile has been compared to the main ion pressure gradient term, which is found to be the dominant contribution at the plasma edge, thus, supporting that the Er well is created by the main ion species. The Er profile has been measured in different confinement regimes including L-, I- and H-mode. The depth of the Er well and the magnitude of the Er shear are correlated with the ion pressure at the pedestal top. The temporal evolution of the measured CX profiles and the resulting Er have been studied during an edge-localized mode (ELM) cycle. At the ELM crash, the Er minimum is less deep resulting in a reduction of the E × B shear. Within 2ms after the ELM crash, the edge kinetic profiles have nearly recovered and the Er well is observed to recover simultaneously. In high density type-I ELM mitigated H-mode plasmas, obtained via externally applied magnetic perturbations (MPs) with toroidal mode number n = 2, no clear effect on Er due to the MPs has been observed. [ABSTRACT FROM AUTHOR]

Interstellar neutral hydrogen (ISN H) gas penetrates freely the heliopause. Inside the inner heliosheath, the charge-exchange interaction of this gas with the shocked solar wind and pickup ions creates energetic neutral atoms (ENAs). ISN H is strongly depleted inside the termination shock but a fraction reaches the Earth’s orbit. In these regions of the heliosphere, ISN H is the source population for interstellar pickup ions and for the heliospheric backscatter glow. The globally distributed flux (GDF) of ENAs created in the inner heliosheath has been sampled directly by the Interstellar Boundary Explorer. Based on these measurements, we calculate the density of the GDF ENA population at the Earth’s orbit. We find that this number density is between 10−4 and 10−3 cm−3, i.e., comparable in magnitude to the number density of ISN H in the downwind portion of the Earth’s orbit. Half of this atom population has energies less than ∼80 eV. This GDF population of neutral hydrogen is likely to provide a significant contribution to the intensity of heliospheric glow in the downwind hemisphere, may be the source of the inner source of hydrogen pickup ions, and may be responsible for the excess of production of pickup ions found in the analysis of magnetic wave events induced by the proton pickup process in the downwind region at 1 au from the Sun. [ABSTRACT FROM AUTHOR]

Hot atoms with chaotically directed velocities are generated by charge-exchange with plasma ions of neutrals recycling from the vessel wall of a fusion reactor. Some of them flee into openings in the vessel made for ducts guiding to diagnostic installations; in particular, to first mirrors for optical observations. On the one hand, hot atoms, hitting the mirror directly, can erode its surface. On the other hand, impurity species, released from the walls of the vessel and of the diagnostic duct, migrate to the mirror and can be deposited there. Both the erosion of and impurity deposition on the mirror decline its reflection properties. Models elaborated to describe processes above are outlined, including a 2D kinetic description for neutral species in the vicinity of the duct opening, an assessment for the erosion of the duct walls and mirror surface by hot atoms, estimates for influxes of the wall material into the duct and a consideration of the migration of impurity atoms along the duct. Calculations are done for the conditions predicted for a fusion reactor like DEMO. The rates for erosion of and impurity deposition on first mirrors of Mo are assessed versus input parameters such as the duct radius, the distance from the opening to the mirror, the density ng of the working gas in the duct, the probabilities for impurity sticking to the duct wall and mirror surface. It is demonstrated that, by ng exceeding a level of m−3, the mirror sputtering can be reduced to the target level of 1 nm per full power year. Moreover, for long enough ducts the erosion rate of impurities deposited onto the mirror exceeds their deposition one and no formation of impurity precipitations on the mirror surface has to be expected. [ABSTRACT FROM AUTHOR]

The reported observations of an unidentified X-ray line feature at ∼3.5 keV have driven a lively discussion about its possible dark matter origin. Motivated by this, we have measured the K-shell X-ray spectra of highly ionized bare sulfur ions following charge exchange with gaseous molecules in an electron beam ion trap, as a source of or a contributor to this X-ray line. We produced S16+ and S15+ ions and let them capture electrons in collision with those molecules with the electron beam turned off while recording X-ray spectra. We observed a charge-exchange-induced X-ray feature at the Lyman series limit (3.47 ± 0.06 keV). The inferred X-ray energy is in full agreement with the reported astrophysical observations and supports the novel scenario proposed by Gu et al. [ABSTRACT FROM AUTHOR]

Radial profiles of the density ratio of helium to hydrogen ions are measured using charge exchange spectroscopy with a two-wavelength spectrometer in the large helical device. Helium transport at the last closed flux surface (LCFS) and stochastic magnetic field layer outside the LCFS as well as in the core plasma is studied for a wide range of helium fractions, i.e. from hydrogen-dominated plasmas up to helium-dominated plasmas. The helium density profile becomes more peaked and inward convection velocity increases in the hydrogen-dominant plasma, while it becomes flat or hollow and the convection velocity is in the outward direction in the helium-dominant plasmas. The density gradient of helium at the LCFS is twice that of hydrogen and becomes steeper as the hydrogen becomes more dominant. [ABSTRACT FROM AUTHOR]

The effective ionic charge, , is determined through the integration of soft x-ray tomography and charge exchange recombination spectroscopy impurity density measurements in the Madison Symmetric Torus. is found is be 2.3  ±  0.1 in the core of high temperature, high current, improved confinement discharges, with a slightly hollow profile peaking near mid-radius. A Bayesian probability framework, developed as part of an on-going effort in Integrated Data Analysis, was used to incorporate these two measurements. This framework provides a method to address different systematic and statistical uncertainties associated with each diagnostic and to test hypothetical contributions to against the existing data set. The combined analysis provides much higher confidence in the result than previous single-diagnostic attempts to characterize using near-infrared bremsstrahlung or x-ray spectroscopy. [ABSTRACT FROM AUTHOR]

The velocity-space observation regions and sensitivities in fast-ion Dα (FIDA) spectroscopy measurements are often described by so-called weight functions. Here we derive expressions for FIDA weight functions accounting for the Doppler shift, Stark splitting, and the charge-exchange reaction and electron transition probabilities. Our approach yields an efficient way to calculate correctly scaled FIDA weight functions and implies simple analytic expressions for their boundaries that separate the triangular observable regions in (v‖, v⊥)-space from the unobservable regions. These boundaries are determined by the Doppler shift and Stark splitting and could until now only be found by numeric simulation. [ABSTRACT FROM AUTHOR]