Optical observations on critical ionization velocity experiments in space

Although the existence of the critical ionization velocity (CIV) is known from laboratory experiments, no agreement has been reached as to whether CIV exists in the natural space environment. In this paper we move towards more realistic models of CIV and present the first fully three‐dimensional, electromagnetic particle‐in‐cell Monte‐Carlo collision (PIC‐MCC) simulations of typical space‐based CIV experiments. In our model, the released neutral gas is taken to be a spherical cloud traveling across a magnetized ambient plasma. Simulations are performed for neutral clouds with various sizes and densities. The effects of the cloud parameters on ionization yield, wave energy growth, electron heating, momentum coupling, and the three‐dimensional structure of the newly ionized plasma are discussed. The simulations suggest that the quantitative characteristics of momentum transfers among the ion beam, neutral cloud, and plasma waves is the key indicator of whether CIV can occur in space. The missing factors in space‐based CIV experiments may be the conditions necessary for a continuous enhancement of the beam ion momentum. For a typical shaped charge release experiment, favorable CIV conditions may exist only in a very narrow, intermediate spatial region some distance from the release point due to the effects of the cloud density and size. When CIV does occur, the newly ionized plasma from the cloud forms a very complex structure due to the combined forces from the geomagnetic field, the motion induced emf, and the polarization. Hence the detection of CIV also critically depends on the sensor location.

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

The energy dependence of the total cross section for symmetric charge exchange in helium is measured in the range from 400-2000 eV. Beam-attenuation techniques are used, and all measurements are made on the forward-scattered ions and the high-energy neutrals resulting from the charge exchange. Cross-section values are given at 100-eV intervals, and the results show a cross section decreasing as the energy increases, with a value at 1000 eV of 1.04 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$. The absolute cross sections are accurate to \ifmmode\pm\else\textpm\fi{} 12%. Relative cross-section values accurate to \ifmmode\pm\else\textpm\fi{} 6% are reported.

theoretical rate equations in order to determine the average charge exchange rates. These rates were compared to charge exchange rates of an ion with a known charge exchange cross section (Ar{sup 11+}) measured in a similar manner in order to determine the average charge exchange cross sections for the highly charged ions. The energy of interaction between the highly charged ions and hydrogen was estimated to be 4 eV in the center of mass frame. The mean charge exchange cross sections were 9 {times} 10{sup {minus}14} cm{sup 2} for Xe{sup 43+} to Xe{sup 46+} and 2 {times} 10{sup {minus}13} cm{sup 2} for Th{sup 73+} to Th{sup 79+}. Double capture was approximately 20--25% of the total for both xenon and thorium. A fit indicated that the cross sections were approximately proportional to q. This is consistent with a linear dependence of cross section on q within the measurement uncertainties.

The authors calculate the differential cross sections for charge exchange at high energies by means of the continuum-distorted-wave (CDW) approximation. The calculations show the importance of contributions from second-order terms in the electron-nucleus interaction for both the magnitude of the cross section and its variation with angle. The internuclear repulsion is included exactly within a first-order expansion in the ratio of the electron to proton mass. Comparisons are made with recent experimental data on H+-He and H+-Ar collisions.

The interpenetration of laser produced plasmas can be detected by the occurrence of charge exchange reactions. Such reactions only occur if highly charged ions collide with ions of a significantly lower charge in suitable energy levels. This leads to selective population of a specific level of the highly charged ion resulting in an enhancement of particular lines in the region of the interaction. In case of interpenetrating carbon plasmas these reactions lead to enhanced line emission at 18.2 nm (H α line of C Vl).We present spatially and temporally resolved investigations of the interaction region of colliding plasmas;we show results of calculations of the cross sections for charge exchange obtained with a semi-classical model and compare measured spectra with calculated ones obtained from a collisional radiative model.

In the heliosphere, especially in the inner heliosheath, mass-, momentum-,\nand energy loading induced by the ionization of neutral interstellar species\nplays an important, but for some species, especially Helium, an underestimated\nrole. We discuss the implementation of charge exchange and electron impact\nprocesses for interstellar neutral Hydrogen and Helium and their implications\nfor further modeling. Especially, we emphasize the importance of electron\nimpact and a more sophisticated numerical treatment of the charge exchange\nreactions. Moreover, we discuss the non-resonant charge exchange effects. The\nrate coefficients are discussed and the influence of the cross-sections in the\n(M)HD equations for different reactions are revised as well as their\nrepresentation in the collision integrals. Electron impact is in some regions\nof the heliosphere, particularly in the heliotail, more effective than charge\nexchange, and the ionization of neutral interstellar Helium contributes about\n40% to the mass- and momentum loading in the inner heliosheath. The charge\nexchange cross-sections need to be modeled with higher accuracy, especially in\nview of the latest developments in their description. The ionization of Helium\nand electron impact ionization of Hydrogen needs to be taken into account for\nthe modeling of the heliosheath and, in general, astrosheaths. Moreover, the\ncharge exchange cross-sections need to be handled in a more sophisticated way,\neither by developing better analytic approximations or by solving the collision\nintegrals numerically.\n

Highly charged xenon (35+ to 46+) and thorium (72+ to 79+) ions were produced in an Electron Beam Ion Trap (EBIT). The ions were extracted from EBIT in a short pulse. Ions of one charge state were selected using an electromagnet. The ions were recaptured at low energy in a cryogenic Penning trap (RETRAP). As the ions captured electrons from molecular hydrogen, populations of the various charge states were obtained by measuring the image currents induced by the ions on the electrodes of the trap. Data on the number of ions in each charge state vs. time were compared to theoretical rate equations in order to determine the average charge exchange rates. These rates were compared to charge exchange rates of an ion with a known charge exchange cross section (Ar{sup 11+}) measured in a similar manner in order to determine the average charge exchange cross sections for the highly charged ions. The energy of interaction between the highly charged ions and hydrogen was estimated to be 4 eV in the center of mass frame. The mean charge exchange cross sections were 9 {times} 10{sup {minus}14} cm{sup 2} for Xe{sup 43+} to Xe{sup 46+} and 2 {times} 10{sup {minus}13} cm{sup 2} for Th{sup 73+} to Th{sup 79+}. Double capture was approximately 20--25% of the total for both xenon and thorium. A fit indicated that the cross sections were approximately proportional to q. This is consistent with a linear dependence of cross section on q within the measurement uncertainties.

Plasma production in a gas release from a body moving in low Earth orbit may occur via Alfven's critical ionization velocity (CIV) process. The effects of the collisional processes of charge exchange and elastic collisions of neutrals with ions and electrons are examined by numerical simulations of a plasma with a beam of fast neutral molecules. The rate of electron heating via plasma interactions increases as the fraction of ions in the fast beam increases. When the initial beam fraction is 0.005, increasing the neutral number density nN (for nN < 1012 cm−3) increases the number of excitations and ionizations by electrons. This increased electron heating results from ambient charge exchange collisions increasing the fraction of beam ions. When the initial beam fraction is 0.5, the number of ionizations and excitations is shown to go through a maximum as nN increases. The initial increase is the result of the shorter collision times for the fast electrons, and the later decrease is the result of elastic collisions that disrupt the electron heating. In the higher density regime, the lower energy process of excitation is favored over ionization. Resonant charge exchange is an important process in maintaining the energy of the fast ion beam, and it lowers the threshold velocity for the CIV process.

The proportion of N4+ photofragmenting to yield N2+ in X 2Σg+v≳0 states has been measured by observing the yield of the fragments’ charge transfer reaction with Ar. Such a determination is possible because N2+ in the v=0 level has a cross section for charge exchange with Ar that is 2 orders of magnitude smaller than in higher vibrational levels. The fraction of N2+(v≳0) fragments increases from 0.30 at 620 nm to 0.37 at 266 nm. Calculations, which treat the N2+ and N2 fragments as quantum harmonic oscillators and the dissociation coordinate classically, predict an N2+ fragment vibrational content that is of the same order as that observed. The substantial population of vibrationally quiescent N2+ fragments, along with previous kinetic energy release studies of N4+ photodissociation, that show a limited fraction of the available energy appears as translational motion [M. F. Jarrold, A. J. Illies, and M. T. Bowers, J. Chem. Phys. 81, 214 (1984)], suggest that the fate of much of the photon’s energy is either in product rotational motion or in the neutral N2 vibration. Mechanisms for the dissociation are discussed.

For pt.I see ibid., vol.24, no.18, p.4049-60 (1991). The authors report a treatment of slow C3++H collisions for impact energies E=0.04-9 keV amu-1, using an expansion in terms of 22 molecular states modified with a common translation factor. Calculated partial and total cross sections for charge exchange are in good agreement with measured ones. Cross sections and values for the alignment parameter for C3+ excitation are predicted. A detailed study of the collision dynamics is presented.

The physics of multicharged ions has considerably developed during the last years1. Aside from purely academic and fundamental interest, the initial motivation arose from fusion research. Indeed from simple theoretical arguments it could be predicted that cross sections for charge exchange between highly charged impurities present in tokomak devices, and hydrogen atom would be very large (10-15–10-14 cm2). Knowledge of the plasma properties would therefore require measurement of the corresponding cross sections as well as a better understanding of the collision process. A large number of experiments were therefore undertaken to determine the total (all processes included) electron capture cross section for a variety of multicharged ions. In contrast, very little work was devoted to the identification of the states which are effectively populated by the collision process. A similar effort was undertaken in theory, for the collision energy range of interest for nuclear fusion (say few keV/Nucleon). At these energies the electron velocity v is much larger than the collision velocity V so that the electron motion adapts itself adiabatically (or quasi-adiabatically) to the nuclear motion as in a molecule.

X-ray emission from a comet was observed for the first time in 1996. One of the mechanisms believed to be contributing to this surprisingly strong emission is the interaction of highly charged solar wind ions with cometary gases. Reported herein are total absolute charge-exchange and normalized line-emission (X-ray) cross sections for collisions of high-charge state (+3 to +10) C, N, O, and Ne ions with the cometary species H2O and CO2. It is found that in several cases the double charge-exchange cross sections can be large, and in the case of C3+ they are equal to those for single charge exchange. Present results are compared to cross section values used in recent comet models. The importance of applying accurate cross sections, including double charge exchange, to obtain absolute line-emission intensities is emphasized.

Nuclear continuum charge exchange of hadrons at [formula omitted

Heavy-ion charge exchange in the eikonal approximation