Charge Exchange Cross Sections Research Articles

Ion energy distribution in extremely high electric field discharges

Singly charged ion energy distribution functions (IEDFs) have been investigated by the particle-in-cell/Monte Carlo method in practical discharges at extremely high reduced electric field E/n, i.e. the ratio of electric field and gas density, up to 700 kTd in helium. It is found in high E/n regime that IEDFs deviate from Maxwellian form due to complex processes: (a) ions undergo a transition to runaway regime, (b) the copious energetic fast neutrals generated by charge exchange produce slow ions via ionization collisions, which load in the low-energy part of IEDF, as well as commonly assumed energy-dependent charge exchange cross section. For E/n > 100 kTd, the ratio of the volumetric ionization rate and charge exchange rate exceeds 10% due to heavy particles impact ionization. The ion mean energy close to the cathode is approximately reduced by in comparison with the value obtained by the reduced model with only the charge exchange collision considered. Charge production attributed to fast-neutral impact ionization not only causes the decrease of ion mean energy, but also enhances ion non-equilibrium.

A gas cell apparatus for measuring charge exchange cross sections with multicharged ions.

A gas cell apparatus to measure charge exchange cross sections for charge state- and energy-resolved ion beams with neutrals is described. The design features a short well-defined interaction region required for beams of multicharged ions with high cross sections. Our method includes measuring the beam transmission at four different neutral pressures and extracting the cross section from the slope of a beam loss vs pressure plot. The design and procedure were tested for Ar+ interacting with neutral Ar gas over the incident ion energy range of 1.0-5.0 keV. The charge exchange cross sections agree well with previous complementary measurement techniques.

The role of charge-exchange cross-section for pickup protons and neutrals in the inner heliosheath

The process of deceleration of the solar wind downstream of the termination shock is studied on the basis of a one-dimensional multi-component model. It is assumed that the solar wind consists of thermal protons, electrons and interstellar pickup protons. The protons interact with interstellar hydrogen atoms by charge-exchange. Two cases are considered. In the first one, the charge-exchange cross-section for thermal protons and hydrogen atoms is the same as for pickup protons and atoms. Under this condition, there is a strong dependence of the solar wind velocity on the downstream temperature of pickup protons. When the proton temperature is close to 10 keV, the change in the velocity with the distance from the termination shock is similar to that measured on the Voyager 1 spacecraft: linear velocity decrease is accompanied by an extended transition region with near-zero velocity. However, with a more careful approach to the choice of the charge-exchange cross-section, the situation changes dramatically. The strong dependence of the solar wind speed on the pickup proton temperature disappears and the transition region in the heliosheath disappears as well, at least at reasonable distances from the TS.

Particle Simulations of the Effects of Atomic Oxygen on Ion Thruster Plumes

Ion thruster plumes interacting with ambient atomic oxygen are simulated using direct simulation Monte Carlo/particle-in-cell code with automatic mesh refinement/octree grids. Because of the disparity among the species concentrations and velocities, particle weighting factors with multiple species weighting time steps have been used. To calculate the elastic cross section for collisions, the variable hard sphere collision cross section was compared with the higher-fidelity Lennard–Jones model and found to agree well at the relevant collision energies. Because momentum and charge exchange cross sections for and O are not available in the literature, analytic approaches were used to infer these values. The simulations provide a number of interesting results about the role of and collisions. The effect of incoming freestream atomic oxygen direction on the ion thruster plume at 185 and 300 km altitudes was studied by changing the plume angle with respect to the freestream atomic oxygen, corresponding to satellite orientation change in orbit. It was observed that the freestream atomic oxygen affects the backflux of Xe at the lower altitude. This effect lessens as the altitude increases and is not observable at 300 km, and it was observed that Xe fluxes scale in a linear fashion with the density of O. The spatial distribution of the xenon ion fluxes was not influenced by collisions with freestream atomic oxygen.

Charge transfer in low-energy collisions of H with He+ and H+ with He in excited states

The charge transfer process in collisions of excited (n = 2, 3) hydrogen atoms with He+ and in collisions of excited helium atoms with H+ is studied theoretically. A combination of a fully quantum-mechanical method and a semi-classical approach is employed to calculate the charge-exchange cross sections at collision energies from 0.1 eV u−1 up to 1 keV u−1. These methods are based on accurate ab initio potential energy curves and non-adiabatic couplings for the molecular ion HeH+. Charge transfer can occur either in singlet or in triplet states, and the differences between the singlet and triplet spin manifolds are discussed. The dependence of the cross section on the quantum numbers n and l of the initial state is demonstrated. The isotope effect on the charge transfer cross sections, arising at low collision energy when H is substituted by D or T, is investigated. Rate coefficients are calculated for all isotopes up to 106 K. Finally, the impact of the present calculations on models of laboratory plasmas is discussed.

Incoherent Scatter Spectra Based On Monte Carlo Simulations of Ion Velocity Distributions Under Strong Ion Frictional Heating

AbstractUnder strong electric field conditions often found at high latitudes, the ion velocity distribution of the weakly ionized F region plasma can differ enough from a Maxwellian shape to substantially change incoherent scatter (IS) spectra and thus the analysis of those spectra. With the goal to provide a quantitative and reliable description of the IS spectra, this study directly uses for the first time an advanced Monte Carlo calculation of the ion velocity distribution to derive IS spectra for a range of electric fields and aspect angles. For most cases the spectra associated with NO+ maintains a shape that closely resembles that of a spectrum derived from a Maxwellian distribution with the same line‐of‐sight ion temperature as the equivalent Monte Carlo simulated distribution. This study also fully characterizes the spectral shape as well as the ion temperature and its anisotropy for two different models of the resonant charge exchange between O+ and O. It confirms that the distortions from the Maxwellian shape can be substantial for this particular interaction. The distortions are also such that along the magnetic field direction, the extracted apparent electron temperature is always greater than the real temperature. This work also includes a determination of the stability of the plasma against magnetic field‐aligned electrostatic instabilities. It is found that the NO+ distribution is always stable, whereas the O+ distribution may or may not be stable, depending on the model chosen for the resonant charge exchange cross section in collisions with the background atomic oxygen gas.

Measurement of Absolute Single and Double Charge Exchange Cross Sections for Si(7–10)+ at 0.88–2.50 KeV/u Impacting He and H2

Abstract First measurements are reported for total, absolute, single, and double charge-exchange (CE) cross sections for Si(7–10)+ ions in the solar wind (SW), colliding with the prevalent astrophysical species H2 and He found in comets, the interplanetary medium, and planetary atmospheres. Measurements are in the energy range 0.88–2.50 keV/u, corresponding to the energies of the “fast” and “slow” ion components of the SW. Limited agreement is found with single-exchange results calculated in the classical over-the-barrier model. These CE data are useful for understanding, in laboratory and astrophysical plasmas, photon emissions in the X-ray to visible energy range arising from resulting ion-stabilization steps.

Ion energy distribution function in the wall layer at a negative wall potential with respect to the plasma

On the basis of the kinetic approach, the self-consistent problem of the gas discharge ion distribution function in the sheath near a surface at a negative potential with respect to the plasma is solved. For the first time, the solution takes into account the dependence of the ion charge exchange cross section from the atom on the ion energy, as well as the real ion distribution function in the unperturbed plasma. It is shown that the dependence of the charge exchange cross section on the ion energy significantly affects the shape of the ion distribution function. It is found that the mean energy of the ions near the wall depends on the electron mean energy in the unperturbed plasma. It was also found that, at the same electron mean energy, the form of the distribution function has practically no effect on the ion distribution function in the wall sheath. The calculations are in good agreement with the known mass spectrometric measurements of the ion distribution function. The obtained results give an opportunity to develop a self-consistent solution of the wall layer structure problem including the quasi-neutral presheath.

Line Ratios for Solar Wind Charge Exchange with Comets

Abstract Charge exchange (CX) has emerged in X-ray emission modeling as a significant process that must be considered in many astrophysical environments—particularly comets. Comets host an interaction between solar wind ions and cometary neutrals to promote solar wind charge exchange (SWCX). X-ray observatories provide astronomers and astrophysicists with data for many X-ray emitting comets that are impossible to accurately model without reliable CX data. Here, we utilize a streamlined set of computer programs that incorporate the multi-channel Landau–Zener theory and a cascade model for X-ray emission to generate cross sections and X-ray line ratios for a variety of bare and non-bare ion single electron capture (SEC) collisions. Namely, we consider collisions between the solar wind constituent bare and H-like ions of C, N, O, Ne, Na, Mg, Al, and Si and the cometary neutrals H2O, CO, CO2, OH, and O. To exemplify the application of this data, we model the X-ray emission of Comet C/2000 WM1 (linear) using the CX package in SPEX and find excellent agreement with observations made with the XMM-Newton RGS detector. Our analyses show that the X-ray intensity is dominated by SWCX with H, while H2O plays a secondary role. This is the first time, to our knowledge, that CX cross sections have been implemented into a X-ray spectral fitting package to determine the H to H2O ratio in cometary atmospheres. The CX data sets are incorporated into the modeling packages SPEX and Kronos.

Antinucleon-nucleon interaction at next-to-next-to-next-to-leading order in chiral effective field theory

Results for the antinucleon-nucleon left(overline{N};Nright) interaction obtained at next-to-next-to-next-to-leading order in chiral effective field theory (EFT) are reported. A new local regularization scheme is used for the pion-exchange contributions that has been recently suggested and applied in a pertinent study of the N N force within chiral EFT. Furthermore, an alternative strategy for estimating the uncertainty is utilized that no longer depends on a variation of the cutoffs. The low-energy constants associated with the arising contact terms are fixed by a fit to the phase shifts and inelasticities provided by a phase-shift analysis of overline{p}p scattering data. An excellent description of the overline{N};N amplitudes is achieved at the highest order considered. Moreover, because of the quantitative reproduction of partial waves up to J = 3, there is also a nice agreement on the level of overline{p}p observables. Specifically, total and integrated elastic and charge-exchange cross sections agree well with the results from the partial-wave analysis up to laboratory energies of 300 MeV, while differential cross sections and analyzing powers are described quantitatively up to 200-250 MeV. The low-energy structure of the overline{N};N amplitudes is also considered and compared to data from antiprotonic hydrogen.

Semi-empirical scaling for ion–atom single charge exchange cross sections in the intermediate velocity regime

We present a semi-empirical scaling law for non-resonant ion–atom single charge exchange cross sections for collisions with velocities from and ions with positive charge . Non-resonant cross sections tend to have a velocity peak at collision velocities with exponential decay around this peak. We construct a scaling formula for the location of this peak then choose a functional form for the cross section curve and scale it. The velocity at which the cross section peaks, vm, is proportional to the energy defect of the collision, , which we predict with the decay approximation. The value of the cross section maximum is proportional to the charge state q, inversely proportional to the target ionization energy IT, and inversely proportional to vm. For the shape of the cross section curve, we use a function that decays exponentially asymptotically at high and low velocities. We scale this function with parameters , where the are the target and projectile atomic numbers. For the more than 100 cross section curves that we use to find the scaling rules, the scaling law predicts cross sections within a little over a factor of 2 on average.

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.

Formation of the charge distribution of fast multicharged ions passing through a carbon target

The equilibrium and nonequilibrium charge distributions of ions with nuclear charges from 10 to 18 are analyzed by evaluating ion charge-exchange cross sections with the density effect taken into account. It is shown that the increase in the cross section for electron loss and the decrease in the cross section for electron capture by ions in a solid target (carbon) compared with gases is maximum in the region of 0.1–0.2MeV/nucleon. It is also shown that, in the case of a solid target, the energy by ion cross section for electron loss by an ion becomes maximum decreases, which is explained by a decrease in the binding energy of the active electron in the ion because of the presence of excited states. This leads to an increase in the average equilibrium ion charge in solid targets compared with the value in gases. The results of calculations agree with the experimental data in the entire considered energy region. The proposed method for calculating the ion charge-exchange cross sections in solid targets also makes it possible to qualitatively describe the dependence of the average ion charge on the target thickness and calculate the thickness (required to determine the equilibrium charge distribution) for different energies and charges of the ion nucleus.

Measurement ofσABSandσCXofπ+on carbon by the Dual Use Experiment at TRIUMF (DUET)

The DUET Collaboration reports on the measurements of the absorption ($\sigma_{\mathrm{ABS}}$) and charge exchange ($\sigma_{\mathrm{CX}}$) cross sections of positively charged pions on carbon nuclei for the momentum range 201.6 MeV$/c$ to 295.1 MeV$/c$. The uncertainties on the absorption and charge exchange cross sections are $\sim$9.5\% and $\sim$18\%, respectively. The results are in good agreement with previous experiments. A covariance matrix correlating the 5 $\sigma_{\mathrm{ABS}}$ and 5 $\sigma_{\mathrm{CX}}$ measured data points is also reported

Ion velocity distribution function in intrinsic gas at cryogenic gas temperatures

The influence of elastic scattering on the ion distribution function in the plasma of an intrinsic gas in weak fields has been considered. An analytical expression valid for cryogenic temperatures of atoms has been obtained. The reduced He+–He, Ar+–Ar mobilities as functions of the temperature of atoms in a range of 4–1000 K have been calculated in the approximation of the zero field taking into account elastic collisions; the calculated dependences well agree with the available experimental data. It has been demonstrated that elastic collisions play an important role in the formation of the ion distribution function at low temperatures. The results of measurement of the ion mobility in the limit of the zero field at low temperatures can be used to obtain data on the ratio of elastic scattering and resonance charge exchange cross sections.

Precision measurements of cross-sections for inelastic processes in collisions of alkali metal ions with atoms of rare gases

A multifaceted experimental study of collisions of Na[Formula: see text] and K[Formula: see text] ions in the energy range of 0.5–10 keV with He and Ar atoms is presented. Absolute cross-sections for charge-exchange, ionization, stripping and excitation processes were measured using a refined version of the transfer electric field method, angle- and energy-dependent collection of product ions, energy loss and optical spectroscopy methods. The experimental data and the schematic correlation diagrams are employed to analyze and determine the mechanisms for these processes.

Comment on “Ion distribution function in a plasma with uniform electric field” [Phys. Plasmas 19, 113703 (2012)

The comparison between experimental data of ion distribution function at the parent gas plasma obtained by the authors and results of calculations presented by Lampe et al. are considered. It is shown that the experimental and calculated angular distributions of ions in the case at least of argon differ considerably. The analysis of Lampe et al. assumptions showed that the main reasons of these discrepancies were the assumptions of ion distribution function independence on field orientation and independence of charge exchange cross-section on the relative velocity of ion and atom.

Integral cross section measurements and product recoil velocity distributions of Xe(2+) + N2 hyperthermal charge-transfer collisions.

Charge exchange from doubly charged rare gas cations to simple diatomics proceeds with a large cross section and results in populations of many vibrational and electronic product states. The charge exchange between Xe(2+) and N2, in particular, is known to create N2 (+) in both the A and B electronic states. In this work, we present integral charge exchange cross section measurements of the Xe(2+) + N2 reaction as well as axial recoil velocity distributions of the Xe(+) and N2 (+) product ions for collision energies between 0.3 and 100 eV in the center-of-mass (COM) frame. Total charge-exchange cross sections decrease from 70 Å(2) to about 40 Å(2) with increasing collision energy through this range. Analysis of the axial velocity distributions indicates that a Xe(2+) - N2 complex exists at low collision energies but is absent by 17.6 eV COM. Analysis of the axial velocity distributions reveals evidence for complexes with lifetimes comparable to the rotational period at low collision energies. The velocity distributions are consistent with quasi-resonant single charge transfer at high collision energies.

Observation of large enhancements of charge exchange cross sections with neutron-rich carbon isotopes

Production cross sections of nitrogen isotopes from high-energy (∼950 MeV per nucleon) carbon isotopes on hydrogen and carbon targets have been measured for the first time for a wide range of isotopes (A = 12 to 19). The fragment separator FRS at GSI was used to deliver C-isotope beams. The cross sections of the production of N-isotopes were determined by charge measurements of forward-going fragments. The cross sections show a rapid increase with the number of neutrons in the projectile. Since the production of nitrogen is mostly due to charge-exchange (Cex) reactions below the proton separation energies, the present data suggests a concentration of Gamow-Teller and/or Fermi transition strength at low excitation energies for neutron-rich carbon isotopes. It was also observed that the Cex cross sections were enhanced much more strongly for neutron-rich isotopes in the C-target data.

Equilibrium thickness of carbon target interacting with nitrogen and neon ion beams

The method for calculation of the target thickness which is required for the formation of equilibrium charge distribution of ions is proposed. The description of nonequilibrium processes is based on empirical estimations of charge-exchange cross sections, taking the density effect for solids into account. The variation of the average charge and the width of the nonequilibrium charge distribution as a function of the target thickness is analyzed. The results of calculations for nitrogen and neon ions in carbon are compared with experimental data.