Charge Exchange Events Research Articles

Initial study on thermal stability of cold spray tantalum coating irradiated with deuterium for fusion applications

Removal of neutral hydrogen atoms in the plasma edge reduces the number of charge exchange events and thus, the net energy losses in the plasma, significantly improving performance of fusion devices. Effective control of the residual pressure of hydrogen isotopes (HIs) in the plasma edge may be achieved by utilizing a hydrogen absorbing first wall interface capable of withstanding the harsh fusion environment. In this study, we have investigated tantalum (Ta) coating deposited by cold spray technology on 316L stainless steel substrate as a potential plasma-facing material surface. High fluence low energy deuterium plasma irradiation experiments and subsequent thermal annealing cycles associated with thermal desorption spectrometry (TDS) demonstrated superior structural stability of the Ta coating. TDS experiments revealed the outgassing of deuterium (as measure of its retention) for cold spray Ta coatings to be three times higher than bulk Ta and two orders of magnitude greater than bulk polycrystalline W. X-ray photoelectron spectroscopy revealed evolution of oxidation states upon deuterium irradiation and a partial recovery of the metallic signature of Ta after the thermal treatment at 1100 K.

Modeling of geocoronal solar wind charge exchange events detected with Suzaku

Abstract A model of geocoronal solar wind charge exchange (SWCX) emission was built and compared to five Suzaku detections of bright geocoronal SWCX events. An exospheric neutral hydrogen distribution model, charge exchange cross-sections, solar wind ion data taken with the ACE and WIND satellites, and magnetic field models of the Earth’s magnetosphere are all combined in order to predict time-variable geocoronal SWCX emission depending on line-of-sight directions of the Suzaku satellite. The modeled average intensities of O vii emission lines were consistent with the observed ones within a factor of three in four out of the five cases except for an event in which a line-of-sight direction was toward the night side of the high-latitude magnetosheath and a major geomagnetic storm was observed. Those of O viii emission lines were underestimated by a factor of three or more in all five cases. On the other hand, the modeled O vii and O viii light curves reproduced the observed ones after being scaled by ratios between the observed and modeled average intensities. In particular, short-term variations due to line-of-sight directions traversing cusp regions during an orbital motion of the Suzaku satellite were reproduced. These results are discussed in the context of model uncertainties.

Details of the neutral energy distribution and ionization source using spectrally resolved Balmer-alpha measurements on DIII-D.

Spectrally resolved passive Balmer-α (D-α, H-α) measurements from the DIII-D 16 channel edge main-ion charge exchange recombination system confirm the presence of higher energy neutrals ("thermal" neutrals) in addition to the cold neutrals that recycle off the walls in the edge region of DIII-D plasmas. Charge exchange between thermal ions and edge neutrals transfers energy and momentum between the populations giving rise to thermal neutrals with energies approximating the ions in the pedestal region. Multiple charge exchange events in succession allow an electron to effectively take a random walk, transferring from ion to ion, providing a pathway of increasing energy and velocity, permitting a neutral to get deeper into the plasma before a final ionization event that contributes to the ion and electron particle fueling. Spectrally resolved measurements provide information about the density and velocity distribution of these neutrals, which has been historically valuable for validating Monte Carlo neutral models, which include the multi stage charge exchange dynamics. Here, a multi-channel set of such measurements is used to specifically isolate the details of the thermal neutrals that are responsible for fueling inside the pedestal top. Being able to separate the thermal from the cold emission overcomes several challenges associated with optical filter-based neutral density measurements. The neutral dynamics, deeper fueling by the thermal neutrals, and spectral measurement are modeled with the FIDASIM Monte Carlo collisional radiative code, which also produces synthetic spectra with a shape that is in close agreement with the measurements. By scaling the number of neutrals in the simulation to match the intensity of the thermal emission, we show it is possible to obtain local neutral densities and ionization source rates.

Feasibility Studies of Charge Exchange Measurements in pp Collisions at the LHC

(1) Pions produced in the development of extended atmospheric cosmic ray air showers subsequently decay to muons. The measured yield of those muons is generally underestimated by current phenomenological models and event generators optimized for cosmic ray physics. The importance of those disagreements motivates the feasibility studies for testing these models at the Large Hadron Collider (LHC) energies, at the highest center-of-mass energies achievable in a laboratory. The interaction of a nucleus and a virtual pion created in a charge exchange reaction at the LHC is a similar process to those contributing to the development of air showers in case of cosmic rays. The crucial problem of such an analysis is the selection of charge exchange events with the highest possible efficiency and high purity from proton–proton collisions at the LHC. (2) For this we consider distributions of various measurable quantities given by event generators commonly used in cosmic ray physics. (3) We examine the expected distributions of energy deposited in different calorimeters of an LHC experiment. We consider the geometrical acceptance and energy resolution of the detectors at the Compact Muon Solenoid (CMS) experiment, as an example. We determine a working point cut from the various options for event selection, and compare signal and background predictions using different models for a representative simple observable, such as average transverse momentum or charge particle yield. (4) A set of event selection cuts along these considerations is proposed, with the aim of achieving optimal efficiency and purity.

Auroral hydrogen emissions: a historic survey

Abstract. Auroral spectroscopy provided the first tool for remotely sensing the compositions and dynamics of the high-latitude ionosphere. In 1885, Balmer discovered that the visible hydrogen spectrum consists of a series of discrete lines whose wavelengths follow a simple mathematical pattern, which ranks among the first steps toward developing this tool. On 18 October 1939 Lars Vegard discovered the Hα (656.3 nm) and Hβ (486.1 nm) spectral lines of Balmer series emissions, emanating from a diffuse structure, located equatorward of the auroral zone. Intense, first positive bands of N2+ nearly covered the Hα emissions. With more advanced instrumentation after World War II, auroral spectroscopists Vegard, Gartlein and Meinel investigated other characteristics of the auroral hydrogen emissions. The first three lines of the Balmer series, including Hγ at 410 nm, were identified in ground-based measurements prior to the space age. Based on satellite observations, the Balmer lines Hδ and Hε at 410.13 and 396.97 nm, respectively, as well as extreme ultraviolet (EUV) Lyman α (121.6 nm) hydrogen emissions, were also detected. Doppler blue shifts in hydrogen emissions, established in the 1940s, indicated that emitting particles had energies well into the kiloelectron volt range, corresponding to velocities >1000 km s−1. Systematic spatial separations between the locations of electron- and proton-generated aurorae were also established. These observations in turn, suggested that protons, ultimately of solar origin, precipitate into the topside ionosphere, where they undergo charge-exchange events with atmospheric neutrals. Newly generated hydrogen atoms were left in excited states and emitted the observed Balmer radiation. Sounding rocket data showed that most of the hydrogen radiation came from altitudes between 105 and 120 km. Space-age data from satellite-borne sensors made two significant contributions: (1) energetic particle detectors demonstrated the existence of regions in the magnetosphere, conjugate to nightside proton aurora, where conditions for breaking the first adiabatic invariants of kiloelectron volt protons prevail, allowing them to precipitate through filled loss cones. (2) EUV imagers showed that dayside hydrogen emissions appear in response to changes in solar wind dynamic pressure or the polarity of the north–south component of the interplanetary magnetic field.

Synthetic diagnostic for assessing spatial averaging of charge exchange recombination spectroscopy measurements.

A synthetic charge exchange recombination spectroscopy diagnostic based on the FIDASIM modeling suite has been created for the DIII-D tokamak. This synthetic diagnostic assumes that the ions have Maxwellian distribution functions on each flux surface and models emission from charge exchange events between the beam neutrals and a fully ionized impurity. This work was motivated by the observation of non-Gaussian spectra that may be caused by spatial averaging, atomic physics, or non-Maxwellian distribution functions. Measurements of non-Gaussian spectra commonly observed in the high confinement mode pedestal and in plasmas with large core gradients are compared to the synthetic diagnostic. Spatial averaging alone cannot account for the observations in these two cases, opening up the possibility of there being other causes such as non-Maxwellian distribution functions. The synthetic diagnostic has also been used to resolve a long-standing issue: it is shown that the lower temperatures measured by using vertical view chords relative to tangential view chords are due to increased spatial averaging for vertical views due to the DIII-D neutral beams being approximately twice as tall as they are wide.

Charge-exchange source terms in magnetohydrodynamic plasmas

In the modeling of space plasma environments, source terms are often used to couple separate species of particles and/or fluids. There have been many techniques developed over the years to make such coupling more tractable while maintaining maximum physical fidelity. In our current application we use the formalism of the Boltzmann collision integral to compute source terms due to charge-exchange events in the heliosphere. The charge-exchange cross sections often encountered in heliospheric interactions can be fit to laboratory data, but in most cases cannot be directly integrated over analytically. Therefore, researchers often employ various levels of approximation, either semi-analytic or numerical. We explore several assumptions to the charge-exchange source term integrals, namely using Maxwellian velocity spaces for like-mass species and either hard-sphere, power-law, or exact forms of the cross section.

Effects of solar wind speed on the secondary energetic neutral source of the Interstellar Boundary Explorer ribbon

The Interstellar Boundary EXplorer (IBEX) ribbon is an intense energetic neutral atom (ENA) emission feature encircling the sky, spanning energies ≤0.5–6 keV. The ribbon may be produced by the “secondary ENA” mechanism, where ENAs emitted from a source plasma population inside the heliosphere propagate outside the heliopause, undergo two charge-exchange events, and become secondary ENAs that may be directed back toward Earth and detected by IBEX. In this scenario, the source plasma population is governed by the interaction of the solar wind (SW) with the interstellar medium and is thus sensitive to the global SW properties. Moreover, this scenario predicts that the distance to the source of secondary ENAs depends on the ENA energy and SW speed, which in turn may affect the shape of the ribbon. In this paper, we use a computational model of the heliosphere with simplified SW boundary conditions to analyze the influence of ENA energy and SW speed, independent of time and latitude, on the global spatial and geometric properties of the ribbon. We find a strong dependence of the simulated ribbon energy spectrum and spatial symmetry on SW speed and ENA energy, and only a slight dependence on ribbon geometry. Our results suggest a significant number of primary ENAs from the inner heliosheath may contribute to the pickup ion source population outside the heliopause, depending on the ENA energy and SW speed. The lack of variation in the simulated ribbon center as a function of ENA energy and SW speed, in contrast to the observations, implies that the asymmetry of the SW plays an important role in determining the position of the ribbon. Comparisons to the IBEX data also signify the ribbon’s dependence on the properties of the local interstellar medium, particularly the interstellar magnetic field.

STRUCTURE OF THE INTERSTELLAR BOUNDARY EXPLORER RIBBON FROM SECONDARY CHARGE-EXCHANGE AT THE SOLAR–INTERSTELLAR INTERFACE

In 2009, the Interstellar Boundary Explorer discovered a bright of energetic neutral atom (ENA) flux in the energy range ≤0.4–6 keV, encircling a large portion of the sky. This observation was not previously predicted by any models or theories, and since its discovery, it has been the subject of numerous studies of its origin and properties. One of the most studied mechanisms for its creation is the process. Here, solar wind ions, neutralized by charge-exchange with interstellar atoms, propagate outside the heliopause; experience two charge-exchange events in the dense outer heliosheath; and then propagate back inside the heliosphere, preferentially in the direction perpendicular to the local interstellar magnetic field. This process has been extensively analyzed using state-of-the-art modeling and simulation techniques, but it has been difficult to visualize. In this Letter, we show the three-dimensional structure of the source of the ribbon, providing a physical picture of the spatial and energy scales over which the secondary ENA process occurs. These results help us understand how the ribbon is generated and further supports a secondary ENA process as the leading ribbon source mechanism.

Observation of a narrow baryon resonance with positive strangeness formed inK+Xe collisions

The charge-exchange reaction ${K}^{+}\mathrm{Xe}\ensuremath{\rightarrow}{K}^{0}p{\mathrm{Xe}}^{\ensuremath{'}}$ is investigated using the data of the DIANA experiment. The distribution of the $p{K}^{0}$ effective mass shows a prominent enhancement near 1538 MeV formed by nearly 80 events above the background, whose width is consistent with being entirely due to the experimental resolution. Under the selections based on a simulation of ${K}^{+}$Xe collisions, the statistical significance of the signal reaches 5.5$\ensuremath{\sigma}$. We interpret this observation as strong evidence for formation of a pentaquark baryon with positive strangeness, ${\ensuremath{\Theta}}^{+}(uudd\overline{s})$, in the charge-exchange reaction ${K}^{+}n\ensuremath{\rightarrow}{K}^{0}p$ on a bound neutron. The mass of the ${\ensuremath{\Theta}}^{+}$ baryon is measured as $m({\ensuremath{\Theta}}^{+})=1538\ifmmode\pm\else\textpm\fi{}2$ MeV. Using the ratio between the numbers of resonant and nonresonant charge-exchange events in the peak region, the intrinsic width of this baryon resonance is determined as $\ensuremath{\Gamma}({\ensuremath{\Theta}}^{+})=0.34\ifmmode\pm\else\textpm\fi{}0.10$ MeV.

Purity of Ion Beams: Analysis and Simulation of Mass Spectra and Mass Interferences in Ion Implantation

This paper shows that charge exchange events and dissociation reactions of ions may impact the purity of the ion beam in ion implantation, leading to contamination of the implanted target. Physical relations are derived that explain why unwanted ions are transported in the ion beam despite of a magnetic mass separation. Based on those relations, the simulation tool ENCOTION (ENergetic COntamination simulaTION) has been developed. ENCOTION is a very powerful tool for the simulation of transport mechanisms of ions through a magnet analyzer and for the simulation of mass spectra, as will be demonstrated in this paper.

Proposal for making a beam of antihydrogen by two charge exchange events

We have performed calculations of two successive charge transfers in a geometry that could generate a beam of antihydrogen atoms () or reliably produce cold without having to reach extremely cold plasma temperatures. The basic idea is similar to that proposed by Hessels et al (1998 Phys. Rev. A 57 1668) except that the order of the charge transfers is reversed. A beam of highly-excited (Rydberg) Cs atoms passes through an antiproton () plasma where a charge transfer can take place; the result is an exotic Rydberg atom (Cs+ ion and a bound ) which has approximately the original velocity of the Cs atom since the Cs+ mass is much greater than that of the . This exotic Rydberg atom travels into a positron plasma where a second charge exchange gives antihydrogen (). The velocity distribution of the resulting is directly related to the original velocity of the Rydberg Cs atom. The binding energy of the is roughly that of the original Cs Rydberg atom; thus, the starting state of the can be controlled by choosing the initial state of the Cs atom. Because the Cs+ binding energy can be controlled in the charge transfer, this first step, by itself, could be of interest to the exotic atom community.

Using channeling properties for studying the impact-parameter dependence of electron capture by 20-MeV/u uranium ions in a silicon crystal

The impact parameter dependence of electron capture by 20 MeV/u U$^{91+}$ ions has been studied by means of channeling in a 11 $\mu$m thick silicon crystal. Such ions are far from their equilibrium charge state in matter, and channeling offers a unique opportunity to study electron capture in conditions going from the extreme case of a single capture event (for the best channeled ions) to the case of multiple charge exchange events leading to the charge state equilibrium (for unchanneled ions). For each incident ion, the charge state at emergence, energy loss, electron emission and X-ray yields are measured. The correlations between these quantities are studied. The data are reproduced by simulations based on the ion flux distribution. We show that the Mechanical Electron Capture (MEC) dominates at impact parameters smaller than 0.5 Angstrom, whereas Radiative Electron Capture (REC) is the only process occurring beyond. Specific features associated to highly charged heavy ions at intermediate velocities are discussed, in particular ionization following capture into highly excited states, and local electron density enhancement due to the electron gas polarization. The measured impact parameter dependence of capture probabilities is compared to CDW-EIS (continuum distorted waves – eikonal initial state) calculations, extrapolated to n>5 final states.

Further evidence for formation of a narrow baryon resonance with positive strangeness in K + collisions with Xe nuclei

We have continued our investigation of the charge-exchange reaction K^+ Xe --> K^0 p Xe' in the bubble chamber DIANA. In agreement with our previous results based on part of the present statistics, formation of a narrow p K^0 resonance with mass of 1537+-2 MeV/c^2 is observed in the elementary transition K^+ n --> K^0 p on a neutron bound in the Xenon nucleus. Visible width of the peak is consistent with being entirely due to instrumental resolution and allows to place an upper limit on its intrinsic width: \Gamma < 9 MeV/c^2. A more precise estimate of the resonance intrinsic width, \Gamma = 0.36+-0.11 MeV/c^2, is obtained from the ratio between the numbers of resonant and non-resonant charge-exchange events. The signal is observed in a restricted interval of incident K^+ momentum, that is consistent with smearing of a narrow p K^0 resonance by Fermi motion of the target neutron. Statistical significance of the signal is some 7.3, 5.3, and 4.3 standard deviations for the estimators S/sqrt{B}, S/sqrt{S+B}, and S/sqrt{S+2B}, respectively. This observation confirms and reinforces our earlier results, and offers strong evidence for formation of a pentaquark baryon with positive strangeness in the charge-exchange reaction K^+ n --> K^0 p on a bound neutron.

Additional peaks in mass spectra due to charge exchange events and dissociation of molecular ions during extraction

It is well-known that charge exchange events and dissociation of molecular ions after extraction and prior to mass analysis result in changes to a so-called apparent mass of the ion and increase the possibility of energetic contamination in ion implantation. It was found that if charge exchange events and dissociation of molecular ions already take place during extraction, additional peaks occur in a mass spectrum. They were observed for all of the used source feed materials, such as AsH3, BF3, and argon. In this paper, the origin of these peaks is discussed and a model that calculates the apparent mass as a function of the extraction voltage is presented.

Capture from random flux to channeling for protons transmitted through a Si foil

Flux redistribution and formation of channeling peaks have been observed in the geometry of initially wide-angle incidence for protons transmitted through monocrystalline Si membranes. The channeling peaks are generated by the crystalline medium as a result of multiple charge-exchange events due to the impact-parameter dependence of the electron capture and loss cross-sections. This explains successfully the detection of channeling peaks only at a low enough residual proton energy: Epf⩽0.6 MeV, after the transmission.

Secondary electron yield induced by slowly moving heavy ions

The secondary electron yield induced by slowly moving ( v = 0.6 v 0, v 0 - Bohr velocity) heavy ions was researched on a thin carbon foil in forward and backward direction in respect to the ion beam. Only for the backwards electron emission an oscillatory behaviour of the yield was observed in dependence of the atomic number Z 1 of the projectiles while in forward direction the electron yield increases nearly monotonically with Z 1. For the forward electron emission the yield is somewhat higher. The excess of electrons between forward and backward electron emission seems to be proportional to the mean charge of the emerging ions which indicates a surface effect for the forward electron emission. The Z 1-oscillation in the backwards electron emission corresponds to oscillations as was found by energy loss measurements on an argon gas target. These oscillations correlate with the number of charge exchange events in gases. From the common oscillatory behaviour it was concluded that charge exchanges during single collision events are everywhere dominant for the energy loss of slowly moving heavy ions in gases as well in solids.

Experimental and Theoretical Investigations on the Interrelation of Charge Exchange Processes and Energy Loss of Particles at Metal Surfaces

ABSTRACTWe present experiments and a theoretical model for the energy loss distribution of specularly reflected particles after slow (2-5 keV) He ions have been impinging on a Ni(110) surface under grazing incidence. The energy spectra of the backscattered particles are asymmetric with the low-energy tail falling off more slowly than the high-energy tail does.This asymmetry is accounted for by considering charge exchange events during the interaction with the surface. The main neutralization channel for this system is the Auger-neutralization from the conduction band into the He-1s ground state. The transition rate for this process was calculated from first principles. Assuming that the ‘friction coefficient’ for the system particle-surface depends on the charge state of the particle, we calculate the energy lost by a particle during the interaction as a sum of different contributions belonging to different charge states.

Neutralization of Fast Molecular IonsH2+andN2+at Surfaces

Energy spectra of the products of surface scattering of ${\mathrm{H}}_{2}^{+}$ and ${\mathrm{N}}_{2}^{+}$ in the primary-beam energy range from 200 to 600 eV are presented. Large neutral-molecular yields are observed for incident ${\mathrm{N}}_{2}^{+}$, i.e., beams of neutral molecules can be produced with optical properties similar to that of the incident beam. The results provide a new experimental method to study resonant and Auger-type processes in ion-surface charge-exchange events.

Experimental study of π− double charge exchange with 7Li

Abstract We have observed 150 double charge exchange events, π − + 7 Li → π + + anything , at an incident π − lab kinetic energy of 102 MeV. The π + were recorded in an emulsion stack at 30° to the incident π − beam. No significant peak due to the hypothetical reaction π − + 7 Li → π + + 7 H(g.s.) was observed in the part of the π + energy spectrum corresponding to a 7 H binding energy between −5 MeV and +25 MeV. Our new upper limit for the corresponding differential cross section is 1.0 × 10 −31 cm 2 sr (90% C.L.). The π + spectrum was recorded down to low energies (20 MeV E π + π + , the proton and the six neutrons. The differential double charge exchange cross section integrated over all π + energies was determined as ( dσ dΩ ) tot = (4.2 ± 1.7) × 10 −30 cm 2 sr .