Charge Exchange Model Research Articles

SOLAR WIND CHARGE EXCHANGE EMISSION FROM THE HELIUM FOCUSING CONE: MODEL TO DATA COMPARISON

A model for heliospheric solar wind charge exchange (SWCX) X-ray emission is applied to a series of XMM-Newton observations of the interplanetary focusing cone of interstellar helium. The X-ray data are from three coupled observations of the South Ecliptic Pole (SEP; to observe the cone) and the Hubble Deep Field-North (HDF-N, to monitor global variations of the SWCX emission due to variations in the solar wind (SW)) from the period 2003 November 24 to December 15. There is good qualitative agreement between the model predictions and the data, after the SEP data are corrected using the HDF-N data, with the maximum SWCX flux observed at an ecliptic longitude of ~72°, consistent with the central longitude of the He cone. We observe a total excess of 2.1 ± 1.3 line unit (LU) in the O VII line and 2.0 ± 0.9 LU in the O VIII line. However, the SWCX emission model, which was adjusted for SW conditions appropriate for late 2003, predicts an excess from the He cone of only 0.5 LU and 0.2 LU, respectively, in the O VII and O VIII lines. We discuss the model to data comparison and provide possible explanations for the discrepancies. We also qualitatively re-examine our SWCX model predictions in the keV band with data from the ROSAT All-Sky Survey toward the North Ecliptic Pole and SEP, when the He cone was probably first detected in soft X-rays.

Enhanced field ionization of water adsorbed on a carbon monoxide-covered, platinum field emitter tip

The influence of carbon monoxide, adsorbed on a platinum field emitter tip, on field ionization of adsorbed water was examined. Ramped field desorption (RFD) measurements of water ionization were performed at 108K for water layer thicknesses up to 80ML on a clean or CO-saturated tip surface. In RFD the applied field is ramped linearly in time until water ionization is detected, giving the onset field of ionization. Water ionization yields hydrated hydroxide ions and protons; the hydroxide ions remain within the water layer on the tip, while the hydrated protons are emitted into vacuum. At a low water coverage of 1.5ML, the CO adlayer substantially reduced the onset field of ionization (that is, facilitated ionization) of water by 40%, from a value of 0.43V/Å for water on clean Pt to 0.26V/Å for water on CO-covered Pt. The extent of the reduction gradually diminished with thicker coverages of water and was absent at coverages of 20ML or greater. The characteristic decay length of the field enhancement was 4.7±1ML. The results were analyzed with the charge exchange model of ionization kinetics and changes in dipole moments of water adsorbed without and with CO. The analysis reveals that a change in water structure (dipole moment) caused by CO is an important contributor in field enhancement and that the dipole moment for hydrated hydroxide ion in an ice-like layer must be greater than that for bulk ice-like water. The significance of these results with respect to electrochemical oxidation of CO is discussed.

Analytical approximations for the sheath potential profile

The space charge sheath in front of a plane negative wall is frequently described with rather poor approximations since Poisson's equation cannot be solved analytically. Accounting for the correct boundary conditions at the sheath edge, convenient analytical approximations are constructed both in the fluid and in the kinetic approach. The fluid solution is universal and does not depend on the special plasma properties. In the kinetic analysis, in contrast, different plasma models must be distinguished because the space charge formation depends on the ion velocity distribution at the sheath edge. The differences are significant in particular at low sheath potentials. Explicit formulae are given for the Tonks–Langmuir model and for the charge exchange model. All approximations represent the sheath potential profiles with surprising accuracy.

Spectral Study of the Galactic Ridge X-Ray Emission with Suzaku

Abstract In order to carry out a precise spectral study of the Galactic Ridge X-ray Emission using Suzaku, we have observed a typical Galactic plane field at $(l,b) =$ (28D46, $-$0D20), which is already deeply observed with Chandra, and known to be devoid of bright X-ray point sources. Thanks to the low background and high spectral resolution of Suzaku, we were able to resolve three narrow iron K-emission lines from low-ionized (6.41keV), helium-like (6.67keV), and hydrogenic ions (7.00keV). The cosmic-ray ion charge-exchange model or the non-equilibrium ionization plasma model are unlikely to explain these line features, since they require either broad emission lines or lines at intermediate ionization states. Collisional ionization equilibrium plasma is the likely origin for the 6.67keV and 7.00keV lines; however, the origin of the 6.41keV line, which is due to fluorescence from cold material, has not been elucidated. We could also precisely measure the absolute X-ray surface brightness in the direction of the Galactic plane. Excluding point sources brighter than $\sim$2 $\times 10^{-13}$ergs$^{-1}$cm$^{-2}$ (2-10keV), the total surface brightness on the Galactic plane is $\sim$6.1 $\times 10^{-11}$ergs$^{-1}$cm$^{-2}$deg$^{-2}$ (2-10keV), including the contribution of the cosmic X-ray background, which is estimated to be $\sim$1.3$\times 10^{-11}$ergs$^{-1}$cm$^{-2}$deg$^{-2}$.

Effect of radiation treatment on the explosive conduction kinetics of heavy metal azides

A study is performed of the effect of preliminary irradiation on the explosive conduction kinetics of whisker crystals of silver and lead azides under laser initiation. The rate of explosive decomposition of lead azide is shown to be a nonmonotonic function of the radiation dose, exhibiting a rise at low doses (tens of thousands of radians) and a fall at high doses (hundreds of thousands of radians). The results are analyzed using the previously proposed model of charge exchange of reaction centers during initiation. It is concluded that this model can be used to describe the effect of radiation treatment on the early stages of explosive decomposition.

Four-body continuum-distorted-wave model for charge exchange between hydrogenlike projectiles and atoms

The four-body continuum-distorted-wave model is formulated and used to investigate single electron capture in fast collisions of hydrogenlike projectiles with one- and multi-electron targets. The presented method of an analytical calculation of scattering integrals yields the total cross sections in terms of four-dimensional numerical quadratures. As an illustration, total cross sections are computed for electron capture processes in ${\mathrm{He}}^{+}\text{\ensuremath{-}}\mathrm{H}$, ${\mathrm{He}}^{+}\text{\ensuremath{-}}\mathrm{He}$, and ${\mathrm{Li}}^{2+}\text{\ensuremath{-}}\mathrm{He}$ collisions at intermediate and high impact energies. The contribution from the interelectron interaction during the collision is evaluated. The prior and post total cross sections are found to be in good agreement with the available experimental data.

A study of the anomalous field evaporation of Sm(Co 0.68Fe 0.20Cu 0.10Zr 0.02) 7.5 alloy by 3D atom probe

3D atom probe analysis of the composition of a Sm(Co 0.68Fe 0.20Cu 0.10Zr 0.02) 7.5 alloy was conducted by varying the probing temperature from 10.6 to 65 K and pulse fraction from 10% to 20%. It was found that the preferential evaporation of Sm occurred at 65 K, due to the very low evaporation field of Sm, 15.2 V/nm calculated by using the charge exchange model. With decreasing the specimen temperature, preferential evaporation of Sm was alleviated. The optimum analysis conditions which give reasonably good measurement of the composition were: the specimen temperature of 20 K and a pulse fraction ⩾15%. The effects of the specimen temperature and pulse fraction on the measured composition of the alloy are discussed, based on the charge exchange model.

Electron transfer in collisions of dipole-bound anions with polar targets

Charge transfer in collisions between dipole-bound CH 3CN − negative ions and the polar target CH 3NO 2 is examined using a Penning ion trap. A long-lived CH 3 NO 2 - ion signal is observed. It is argued that this results from creation of dipole-bound CH 3 NO 2 - ions that rapidly evolve into longer-lived ions that have predominantly valence-bound character. The charge transfer cross section is sizable, ∼1.4 × 10 −12 cm 2, but is consistent with a simplified near-resonant charge exchange model.

Plasma-sheath transition in the kinetic Tonks-Langmuir model

The famous Tonks-Langmuir model of a plane symmetric discharge is supplemented by a kinetic theory of the plasma-sheath transition on the “intermediate scale.” The mathematical structure of the transition is identical to that found for the collision dominated charge exchange model [K.-U. Riemann, J. Phys. D: Appl. Phys. 36, 2811 (2003)]. It reveals the significance of the asymptotic sheath edge and clarifies the role of the kinetic Bohm criterion. From the matching conditions an approximation for the plasma eigenvalue problem is obtained that is correct up to terms of the order λD∕L. Matched asymptotic potential curves are constructed and compared with numerical solutions. They show an excellent agreement, exceeding by far the expected range of validity in λD∕L.

Comment on “Entropy lowering in ion-atom collisions”

The recent experimental result by Nguyen et al. [Phys. Rev. A 71, 062714 (2005)] on the ratio of cross sections for charge exchange processes ${\mathrm{Rb}}^{+}+\mathrm{Rb}(5s)\ensuremath{\rightarrow}\mathrm{Rb}(5p)+{\mathrm{Rb}}^{+}$ and ${\mathrm{Rb}}^{+}+\mathrm{Rb}(5p)\ensuremath{\rightarrow}\mathrm{Rb}(5s)+{\mathrm{Rb}}^{+}$ is quantitatively derived from simple considerations within the general framework of the quasimolecular theory. Contrary to the expectations, applicability of the Demkov model for charge exchange with small energy defect is not shattered.

Continuum distorted wave – Born initial state (CDW-BIS) model for single charge exchange

The differential and total cross sections for single electron capture in fast collisions of a proton with helium are examined within the four-body distorted-wave formalism. A special emphasis is given to a proper inclusion of dynamic electron-electron correlation effects. For this purpose, a new four-body quantum-mechanical model termed the Continuum Distorted Wave - Born Initial State (CDW-BIS) approximation is introduced. With regard to a previous work of Mancev [J.Phys.B 36, 93 (2003)], where the prior form of one-channel distorted wave transition amplitude was used for describing charge exchange, in the present model the dielectronic interaction V$_{12}$=1/r$_{12}$=1/|\vec r r$_1$− \vec r$_2$| explicitly appears in the perturbation potential of the post transition probability amplitude. The inclusion of the potential V$_{12}$ is essential for the description of the Thomas projectile - $_1$ - e$_2$ scattering. Specifically, we consider asymmetric p+He collisions and the total cross sections are carried out at a large energy range from 20 to 10000 keV. The differential cross section are calculated at impact energies 400 and 630~keV. The theoretical results are compared with the available experimental data and very good agreement is found.

Ionization of Al recoiled and sputtered from Al(100)

The absolute ionization probability of energetic $(>500\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ particles recoiled from Al(100) by 2 and $5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ ${\mathrm{Xe}}^{+}$ bombardment was measured with time-of-flight spectroscopy. These values were then used to calibrate the energy and angular distributions of low-energy $(10--600\phantom{\rule{0.3em}{0ex}}\mathrm{eV})$ sputtered ions collected with an electrostatic analyzer. The independent-particle model of nonadiabatic surface-atom charge exchange, which is typically used to analyze single scattering events, was applied to the ion fractions of the recoiled and sputtered atoms. The model describes all the experimental data from a few eV to the keV range if a different surface electronic temperature is used for recoiling and sputtering. This suggests that the ionization process depends on the instantaneous surface condition at the time of ion emission.

Kinetic analysis of the collisional plasma–sheath transition

The subdivision of the plasma boundary layer into a quasi-neutral presheath and a collision-free sheath is strictly valid only in the asymptotic limit λD/λ → 0, where λD is the electron Debye length and λ the ion mean free path. This trivial fact requires one to discuss the applicability and correct interpretation of the plasma–sheath concept for finite ratios λD/λ. In a former fluid analysis (Riemann K-U 1997 Phys. Plasmas 4 4158) based on the charge exchange model the plasma–sheath transition was investigated on an intermediate scale. The analysis showed that there is no need and no justification for a modification of the Bohm criterion for finite λD. The former results and conclusions are confirmed and improved by a new kinetic analysis. The kinetic analysis of the plasma–sheath transition can be generalized to other presheath mechanisms.

Unified model of hole trapping, 1/f noise, and thermally stimulated current in MOS devices

Capture cross-section data from the literature and recent density-functional theory (DFT) calculations strongly suggest that the 1/f noise of MOS devices is caused by the thermally activated capture and emission of carriers at O vacancy centers near the Si-SiO/sub 2/ interface. At least two kinds of defects can contribute to the noise of unirradiated and irradiated devices. The first is a "dimer" vacancy associated with the E/sub /spl delta//' center, which is shown by DFT to be a metastable electron trap when the Si-Si spacing is stretched beyond its equilibrium value, as likely can occur in strained SiO/sub 2/ near the Si-SiO/sub 2/ interface. The second is a neutral or positively charged E/spl gamma/' center, which has two configurations. One is puckered (one of the Si atoms relaxes through its local bonding plane) and forms a dipole after electron capture (E(/spl gamma//sup 4/)'). The other is shown via DFT to be a different kind of puckered configuration, including a fivefold coordinated Si (E(/spl gamma//sup 5/)'); this does not form a dipole. These results strongly suggest a common model of charge exchange with O vacancies can account for much of the oxide-trap charge and 1/f noise in MOS devices. Applying this model to thermally stimulated current (TSC) experiments suggests hole emission in TSC may well be due to the thermal relaxation of a puckered E/spl gamma/' into a dimer E/sub /spl delta//'. This can explain literature discrepancies between optical and thermal estimates of trapped-hole energies.

A new approach to Auger and quasi-resonant processes in ion–surface collisions

We present a theoretical model of charge exchange between an ion and a solid surface which includes resonant transfer and Auger-coupling processes. The Auger processes are treated through an equation of motion approach which results in an Auger self-energy identical to that obtained through the second-order Keldysh–Green functions formalism. The model which we report involves a Lorentzian-shaped level width for the resonant processes and a wide band form for the Auger processes. It permits an exact solution of the problem in terms of well-known functions. The roles played by the characteristic parameters of the problem, which are related to the atom velocity, energy level positions, interaction potential and band structure are illustrated in some simple cases.

Kinetic Simulation of an Ion Acoustic Wave with Charge Exchange Process in Direct Current Discharge Plasma

A charge exchange model is set and its simulation results are compared to those of previous friction model in the lower current branch of a discharge plasma. A remarkable delocalization of a high f...

Interdefect charge exchange in silicon particle detectors at cryogenic temperatures

Silicon particle detectors in the next generation of experiments at the CERN Large Hadron Collider will be exposed to a very challenging radiation environment. The principal obstacle to long-term operation arises from changes in detector doping concentration (N/sub eff/), which lead to an increase in the bias required to deplete the detector and hence achieve efficient charge collection. We have previously presented a model of interdefect charge exchange between closely spaced centers in the dense terminal clusters formed by hadron irradiation. This manifestly non-Shockley-Read-Hall (SRH) mechanism leads to a marked increase in carrier generation rate and negative space charge over the SRH prediction. There is currently much interest in the subject of cryogenic detector operation as a means of improving radiation hardness. Our motivation, however, is primarily to investigate our model further by testing its predictions over a range of temperatures. We present measurements of spectra from /sup 241/Am alpha particles and 1064-nm laser pulses as a function of bias between 120 and 290 K. Values of N/sub eff/ and substrate type are extracted from the spectra and compared with the model. The model is implemented in both a commercial finite-element device simulator (ISE-TCAD) and a purpose-built simulation of interdefect charge exchange. Deviations from the model are explored and comments made as to possible future directions for investigation of this difficult problem.

High-temperature field evaporation of rhenium

High-temperature field emission of Re, Pt, Ta, and W is studied by field-emission methods. Metal ions are found to evaporate mainly from the tops of thermal-field microprotrusions produced by high electric fields and temperatures on the emitter surface. For fi eld intensities of up to F=1–2 V/A and temperatures of 1500–2000 K, the ion currents i are recorded from the entire emitter surface. They range from several tenths of nanoamperes to several nanoamperes. The activation energies of field evaporation determined from the Arrhenius plots logi=f(1/T) are found to be appreciably lower than those calculated within the charge exchange model for known parameters of the process and the metals evaporated. Reasons for such a difference in the activation energies and mechanisms of ion evaporation at high F and T are discussed.

Pion double charge exchange on4He

The doubly differential cross sections for the $^4$He$(\pi^+,\pi^-) 4p$ reaction were calculated using both a two-nucleon sequential single charge exchange model and an intranuclear cascade code. Final state interactions between the two final protons which were the initial neutrons were included in both methods. At incident pion energies of 240 and 270 MeV the low-energy peak observed experimentally in the energy spectrum of the final pions can be understood only if the contribution of pion production is included. The calculated cross sections are compared with data.

Neutral beam heating in the START spherical tokamak

The tight aspect ratios (typically A≈1.4) and low magnetic field of spherical tokamak (ST) plasmas, when combined with densities approaching the Greenwald limit, provide a significant challenge for all currently available auxiliary heating and current drive schemes. NBI heating and current drive are difficult to interpret in sub-megampere machines, as in order to achieve suitable penetration into the plasma core, fast ions have to be highly suprathermal and, as a result of the low magnetic field, can be non-adiabatic (i.e. non-conserving of magnetic moment µ0). The physics of NBI heating in START is discussed. The neutral beam injector deployed on START was clearly successful, having been instrumental in producing a world record tokamak toroidal beta of ≈40%. A fast ion Monte Carlo code (LOCUST) is described that was developed to model non-adiabatic fast ion topologies together with a high level of charge exchange loss and cross-field transport (present in START due to an envelope of high density gas surrounding the plasma). Model predictions compare well with experimental data, collected using a scanning neutral particle analyser, bolometric instruments and equilibrium reconstruction using EFIT. In particular, beta calculations based upon reconstruction of the pressure profile (by combining measurements from Thomson scattering, charge exchange recombination spectroscopy and model predictions for the fast ion distribution function) agree well with beta values calculated using EFIT alone (the routine method for calculation of START beta). These results thus provide increased confidence in the ability of STs to sustain high beta high confinement H mode plasmas and in addition indicate that the injected fast ions in collisional START plasmas evolve mainly due to collisional and charge exchange processes, without driving any significant performance degrading fast particle MHD activity.