Resonant Charge Exchange Collisions Research Articles

Azimuthal ion movement in HiPIMS plasmas—part I: velocity distribution function

Magnetron sputtering discharges feature complex magnetic field configurations to confine the electrons close to the cathode surface. This magnetic field configuration gives rise to a strong electron drift in azimuthal direction, with typical drift velocities on the order of 100 km s−1. In high power impulse magnetron sputtering plasmas, the ions have also been observed to follow the movement of electrons with velocities of a few km s−1, despite being not magnetized. In this work, we report on measurements of the azimuthal ion velocity using spatially resolved optical emission spectroscopy, allowing for a more direct measurement compared to experiments performed using mass spectrometry. The azimuthal ion velocities increase with target distance, peaking at about 1.55 km s−1 for argon ions and 1.25 km s−1 for titanium ions. Titanium neutrals are also found to follow the azimuthal ion movement which is explained with resonant charge exchange collisions. The experiments are then compared to a simple test-particle simulation of the titanium ion movement, yielding good agreement to the experiments when only considering the momentum transfer from electrons to ions via Coulomb collisions as the only source of acceleration in azimuthal direction. Based on these results, we propose this momentum transfer as the primary source for ion acceleration in azimuthal direction.

Isotope selective three-step photoionization of 176Yb

Atomic vapor laser isotope separation of 176 Y b has been investigated through the density matrix analysis of laser–atom interactions in a three-step photoionization process. For the first time, to our knowledge, the method has been applied for both odd and even isotopes of Yb. The effects of peak power densities of the lasers, bandwidth of the excitation lasers, angular divergence, and number density of the atoms on the degree of enrichment have been studied. Peak power densities of the first, second, and third excitation lasers of 20, 20, and 40 , 000 W / c m 2 , respectively, and full angle divergence of the atomic beam of 120° were found to be optimum for the isotope separation of 176 Y b . The effects of resonant charge exchange collisions and atomic number densities on the degree of enrichment and production rates have also been studied. It has also been observed that the atomic number densities in the laser–atom interaction region should be limited to 1 × 1 0 12 a t o m s / c m 3 to obtain ∼ 90 % degree of enrichment of the 176 Y b isotope. At higher number densities of 1 × 1 0 13 a t o m s / c m 3 , the degree of enrichment decreases to 43.9%. The optimum range of bandwidths of excitation lasers is found be 90–120 MHz. At optimum conditions, the computed production rate of 22 mg/h is in good agreement with the previously reported value of 20 mg/h.

Extreme plasma convection and frictional heating of the ionosphere: ISR observations

AbstractExtremely elevated ion temperatures observed with the Poker Flat and Resolute Bay incoherent scatter radars in the E and F regions of the polar ionosphere are discussed. Our observations include one‐dimensional, line‐of‐sight ion temperatures (Ti,1d) that at times can rise to up to ∼8000°K. While examining the accuracy of the derived temperatures is difficult due to the several potential sources of error and uncertainty, we find that at altitudes of ∼130–250 km the line‐of‐sight ion temperatures obtained at relatively small aspect angles (for instance, at 22.5° away from the magnetic field direction) are below the expected values for the average or three‐dimensional ion temperatures (Ti). We find that this difference matches well with the theoretical expectation from anisotropic ion velocity distributions that emerge from polarization elastic scattering collisions between NO+ and N2. These results are therefore the clearest detection of anisotropic ion velocity distributions with the Poker Flat Incoherent Scatter Radar and Resolute Bay Incoherent Scatter Radar facilities. Moreover, at higher altitudes (>∼300 km), where the resonant charge exchange collisions prevail, and for a very high ion temperature event (Ti∼8000°K), we observe that the incoherent scatter radar (ISR) spectra obtained at large aspect angles (e.g., 55°) look like those expected from non‐Maxwellian plasma. At similar altitudes, the measured one‐dimensional temperatures at an aspect angle of 22.5° are found to exceed the expected values from the frictional heating and resonant charge exchange collisions by about 1000°K. A portion of this offset is thought to reflect the role of Coulomb collisions.

Large numbers of cold positronium atoms created in laser-selected Rydberg states using resonant charge exchange

Lasers are used to control the production of highly excited positronium atoms (Ps*). The laser light excites Cs atoms to Rydberg states that have a large cross section for resonant charge-exchange collisions with cold trapped positrons. For each trial with 30 million trapped positrons, more than 700 000 of the created Ps* have trajectories near the axis of the apparatus, and are detected using Stark ionization. This number of Ps* is 500 times higher than realized in an earlier proof-of-principle demonstration (2004 Phys. Lett. B 597 257). A second charge exchange of these near-axis Ps* with trapped antiprotons could be used to produce cold antihydrogen, and this antihydrogen production is expected to be increased by a similar factor.

Physics of the Advanced Plasma Source: a review of recent experimental and modeling approaches

The Advanced Plasma Source (APS), a gridless hot cathode glow discharge capable of generating an ion beam with an energy of up to 150 eV and a flux of 1019s−1, is a standard industrial tool for the process of plasma ion-assisted deposition (PIAD). This manuscript details the results of recent experimental and modeling work aimed at a physical understanding of the APS. A three-zone model is proposed which consists of (i) the ionization zone (the source itself) where the plasma is very dense, hot, and has a high ionization rate, (ii) the acceleration zone (of ∼20 cm extension) where a strong outward-directed electric field accelerates the primary ions to a high kinetic energy, and (iii) a drift zone (the rest of the process chamber) where the emerging plasma beam is further modified by resonant charge exchange collisions that neutralize some of the energetic ions and generate, at the same time, a flux of slow ions.

Ion–Neutral Collision Modeling Using Classical Scattering With Spin-Orbit Free Interaction Potential

A particle-in-cell Monte Carlo collision model is developed to explore dominant collisional effects on high-velocity xenon ions incident to a quiescent xenon gas at low neutral pressures. The range of neutral pressure and collisionality examined are applicable for electric propulsion as well as plasma processing devices; therefore, the computational technique described herein can be applied to more complex simulations of those devices. Momentum and resonant charge-exchange collisions between ions and background neutrals are implemented using two different models, classical scattering with spin-orbit free potential and variable-hard-sphere model, depending on the incident particle energy. The primary and charge-exchange ions are tracked separately, and their trajectories within a well-defined “Test Cell” domain are determined. Predicted electrode currents as a function of the Test Cell pressure are compared with electrode currents measured in an ion gun experiment. The simulation results agree well with the experiment up to a Test Cell pressure corresponding to a mean free path of the Test Cell length and then start to deviate with increasing collisionality at higher pressures. This discrepancy at higher pressures is likely due to the increasing influence of secondary electrons emitted from electrodes due to the high-velocity impacts of heavy species (i.e., beam ions and fast neutrals created by charge-exchange interaction) at the electrode surfaces.

Ionization enhanced ion collection by a small floating grain in plasmas

It is demonstrated that the ionization events in the vicinity of a small floating grain can increase the ion flux to its surface. In this respect, the effect of electron impact ionization is fully analogous to that of the ion-neutral resonant charge exchange collisions. Both processes create slow ion which cannot overcome electrical attraction of the grain and eventually fall onto its surface. The relative importance of ionization and ion-neutral collisions is roughly given by the ratio of the corresponding frequencies. We have evaluated this ratio for neon and argon plasmas to demonstrate that ionization enhanced ion collection can indeed be an important factor affecting grain charging in realistic experimental conditions.

Cooling and stabilization by collisions in a mixed ion–atom system

In mixed systems of trapped ions and cold atoms, the ions and atoms can coexist at different temperatures. This is primarily due to their different trapping and cooling mechanisms. The key questions of how ions can cool collisionally with cold atoms and whether the combined system allows stable coexistence, need to be answered. Here we experimentally demonstrate that rubidium ions cool in contact with magneto-optically trapped rubidium atoms, contrary to the general experimental expectation of ion heating. The cooling process is explained theoretically and substantiated with numerical simulations, which include resonant charge exchange collisions. The mechanism of single collision swap cooling of ions with atoms is discussed. Finally, it is experimentally and numerically demonstrated that the combined ion-atom system is intrinsically stable, which is critical for future cold chemistry experiments with such systems.

Ion drift in a gas in an external electric field

A model of ion collisions with gas atoms is constructed that takes into account resonant ion-atom charge exchange, polarization interaction, and short-range (gas-kinetic) collisions. The cross sections for resonant charge exchange of the ions of noble gases, as well as rubidium, cesium, and mercury ions, with atoms are calculated based on the experimental data and the results of numerical simulations of ion collisions with parent gas atoms in a uniform electric field. The effect of the electric field, the temperature of the buffer gas atoms, and the percentage composition of the gas mixture on the ion velocity distribution function is investigated.

A non-Maxwellian kinetic approach for charging of dust particles in discharge plasmas

Nanoparticle charging in a capacitively coupled radio frequency discharge in argon is studied using a particle in cell Monte Carlo collisions method. The plasma parameters and dust potential were calculated self-consistently for different unmovable dust profiles. A new method for definition of the dust floating potential is proposed, based on the information about electron and ion energy distribution functions, obtained during the kinetic simulations. This approach provides an accurate balance of the electron and ion currents on the dust particle surface and allows us to precisely calculate the dust floating potential. A comparison of the obtained floating potentials with the results of the traditional orbital motion limit (OML) theory shows that in the presence of the ion resonant charge exchange collisions, even when the OML approximation is valid, its results are correct only in the region of a weak electric field, where the ion drift velocity is much smaller than the thermal one. With increasing ion drift velocity, the absolute value of the calculated dust potential becomes significantly smaller than the theory predicts. This is explained by a non-Maxwellian shape of the ion energy distribution function for the case of fast ion drift.

Paschen breakdown curve by one-dimensional PIC-MCC simulation

The voltage at which a gas confined in a gap ignites, i.e. the breakdown voltage, is a function of the total pressure (p) and the gap size (d). The understanding of gas discharge has been important in applications of weakly ionized plasmas and studying the properties of the plasma medium, such as nanotechnoogy, ion sources and thrusters [1-2]. We obtained the Paschen breakdown curve by considering the energy and angle dependent secondary electron emission coefficient γse for ions impinging on a MgO surface and compared it with the results of fluid simulation. For the dc breakdown, a parallel electrode assumed to be coated with MgO was considered. The gap distance between two parallel electrodes was fixed 100μm and the gas pressure varied between 100 to 1000Torr. For these simulations, Ne-Xe mixture gas was employed and an elastic, excitation and ionization collisions between electrons and neutrals as well as elastic and resonant charge exchange collisions between ions and neutrals were added to the original XPDP1 code [3]. Figure 1 shows the Paschen breakdown curves by one-dimensional PIC-MCC simulation and twodimensional fluid simulation at typical pd values used in PDP cells (1-10Torr cm). Four symbols correspond to the data from one-dimensional PIC simulation results with the energy and angle dependent γse. The solid lines were obtained using our two-dimensional fluid simulation. The constant γse used in fluid simulation are 0.2 and 0.05 for Ne and Xe ions, respectively. While the simulation results had some discrepancies, the trend of these curves is obtained. The breakdown voltage increases as the Xe content is increased and minimum voltages are shifted to higher pd values around 3Torr cm in PIC simulations [4]. The Paschen breakdown curves for Ne-Xe mixture gas have been investigated using one-

O+ ion temperature partition coefficients β∥ and β⊥: The Effect of O+–O+ Coulomb self‐collisions

We have computed altitude profiles for O+ ion temperature partition coefficients β∥ and β⊥ in the auroral ionosphere for different values of electric field with the use of a Monte Carlo simulation. The Monte Carlo model includes the effect of E × B drift, O+–O (resonant charge exchange and polarization interaction) collisions, and O+–O+ Coulomb self‐collisions. These effects were included self‐consistently in the computations. At low altitudes, the O+ ion concentration is small compared with the O concentration; thus the role of O+–O+ Coulomb self‐collisions in isotropizing the O+ ion velocity distribution is negligible and hence non‐Maxwellian features (β⊥ > β∥) are obtained due to the effect of O+–O collisions. However, as altitude increases, the O+ concentration increases and consequently, the role of O+–O+ Coulomb collisions becomes significant in transferring energy from the field‐perpendicular direction to the field‐parallel direction. This explains the increase of β∥ and the decrease of β⊥ with altitude. Also, we have investigated the variation of β∥ and β⊥ with the electric field and found that as electric field increases, O+ ion temperature increases, β∥ decreases, and β⊥ increases due to the interplay between E × B drift and O+–O collisions. In other words, the effect of O+–O+ Coulomb collisions becomes less important because these Coulomb collisions are in turn dependent on O+ ion temperature. Therefore the combined effects of E × B drift, O+–O collisions, and O+–O+ Coulomb collisions determine the altitude profiles of β∥ and β⊥. Finally, a comparison has been made between the Monte Carlo calculations obtained in this paper and observations of the O+ ion temperature partition coefficient β∥. The comparison showed a remarkably close agreement in the corresponding results for the altitude variation of β∥. This close agreement provides further evidence that the Monte Carlo model described in this paper is a powerful tool for studying O+ ion behavior in the auroral ionosphere, especially when O+–O+ Coulomb self‐collisions are included. As a result of the comparison, we were able to predict the real values of the convection electric field in the auroral ionosphere for three ion heating events.

First Laser-Controlled Antihydrogen Production

Lasers are used for the first time to control the production of antihydrogen (H ). Sequential, resonant charge exchange collisions are involved in a method that is very different than the only other method used so far-producing slow H during positron cooling of antiprotons in a nested Penning trap. Two attractive features are that the laser frequencies determine the H binding energy, and that the production of extremely cold H should be possible in principle-likely close to what is needed for confinement in a trap, as needed for precise laser spectroscopy.

Laser-controlled production of Rydberg positronium via charge exchange collisions

Lasers are used to choose the binding energies of highly excited positronium atoms (Ps*) created via charge exchange collisions. The lasers directly excite cesium (Cs) atoms to highly excited states (Cs*). These large Cs*atoms have a large cross section for resonant charge exchange collisions with trapped positrons. Highly excited Ps* is formed with a binding energy that is determined by the initial laser excitation. These Ps* should have a large cross section for resonant charge exchange collisions with trapped antiprotons—suggesting a possible new way to produce cold antihydrogen.

Screening effects on resonant charge-exchange collisions in strong coupled plasmas

Plasmas screening effects on the resonant charge-exchange processes by stripped ions from hydrogenic ions in strongly coupled plasmas are investigated using the ion-sphere model. The semiclassical straight-line trajectory analysis is applied to describe the dynamic behavior of the collision system. The plasma screening effects on the resonant charge-exchange probability in strongly coupled plasmas are found to be decreased with increasing ion sphere radius and collision energy. It is found that the maximum position of the resonant charge-exchange probability is receded from target nucleus with a decrease of the ion-sphere radius and however is almost unchanged with increasing collision energy.

Plasma monitoring of plasma-assisted nitriding of aluminium alloys

Abstract To improve the plasma-assisted nitriding process of aluminium alloys it is necessary to obtain knowledge about the underlying reaction mechanisms. A suitable diagnostic tool to clarify these mechanisms is plasma monitoring, which provides mass- and energy-resolved analysis of the ions hitting the substrate surface in a glow discharge. Application of this technique to plasma-assisted nitriding of pure aluminium and the 2024 aluminium alloy is demonstrated. Mass spectra and energy distributions of the major ions were recorded during sputtering in an argon atmosphere and nitriding in pure nitrogen. The energy distributions of the gas ions are mainly determined by resonant charge exchange collisions, whereas the ions of the sputtered metal atoms reach the cathode almost without interaction. The influence of the process parameters, temperature and working pressure, on the discharge characteristic were examined. Changing the substrate temperature did not significantly affect the ion energy distributions, whereas a reduction of the working pressure increased the collision probability of the ions due to a disproportionate elongation of the cathode fall.

Kinetic modeling of O+ upflows resulting from E × B convection heating in the high‐latitude F region ionosphere

We report here the results of modeling work aimed at understanding the development of ionospheric O+ field‐aligned upflows that develop in response to high‐latitude E × B drift induced frictional heating. The model used is a collisional semikinetic model which includes ion‐neutral resonant charge exchange and polarization collisions as well as Coulomb self‐collisions. It also includes the process of chemical removal of O+ as well as all of the macroscopic forces: ambipolar electric, gravity, magnetic mirror, and centripetal. Model results show the development of several types of non‐Maxwellian velocity distributions including toroids at low altitude, distributions with large heat flow in the perpendicular component at intermediate altitudes, and distributions with a separate upflowing population or upward superthermal tail at high altitudes. Whenever the convection electric field increases from a small value (< 25 mV/m) to a large value (100‐200 mV/m) in 6 min or less large upflows develop with parallel drift speeds which peak (below 1000 km) at values between 500 m/s and 2 km/s, parallel fluxes which peak between 6.0 × 108 and 3.2 × 109 cm−2 s−1, and parallel per particle heat flows which peak between 8.0 × 10−9 and 8.0 × 10−8 ergs cm/s. The higher values in these ranges occur for a cooler neutral atmosphere, with a larger convection electric field that is turned on quickly. The model produces field‐aligned O+ flow speeds that are larger than those produced by a 20‐moment generalized transport model but smaller then those produced by an isotropic hydrodynamic model for comparable values of the convection electric field and convection turn on times. The model results compare favorably with some topside satellite and radar data.

Derivation of the ion temperature partition coefficient β∥ from the study of ion frictional heating events

This paper reports the results of a study evaluating the ion temperature partition coefficient β∥ over a range of F region altitudes. The data have been selected from three EISCAT CP‐0 experiments, each of which displayed clear evidence of ion frictional heating. The data were averaged at a time resolution of 30 s. These observations, made in an observing direction parallel to the Earth's magnetic field, have the advantage that the line‐of‐sight ion thermal velocity distribution can be closely approximated to a Maxwellian, while still giving a line‐of‐sight ion temperature which can be interpreted on the basis of a well‐known energy balance equation. The technique for determining β∥ depends upon fitting the variation of the field‐parallel ion temperature to simplified forms of this balance equation. The method is an extension of the curve‐fitting approach previously used by Glatthor and Hernández (1990) to deduce both ion temperature partition coefficients at a single F region altitude. The results of this procedure are compared to the theoretical predictions for β∥ obtained from an equation originally due to St.‐Maurice and Hanson (1982). By introducing measured and modeled parameters, it is found that values of β∥ close to those expected for resonant charge exchange collisions are predicted around the F region peak. At greater heights, however, the increased influence of Coulomb collisions is predicted to give rise to an increase in β∥ with altitude, corresponding to ion thermal velocity distributions which tend toward isotropy. This height dependence, which has been theoretically predicted in some other recent studies, would be an important factor in calculations of the energy balance in the upper F region. While the experimentally derived values of β∥ are close to the RCE predictions near the F region peak, the expected increase in β∥ with altitude is seen in only one of the three selected events. In the remaining cases, the increases are notably smaller. Possible reasons for this discrepancy are discussed. Changes in ion and neutral composition, together with effects such as heat conduction and thermal diffusion can act to bias the curve‐fitting technique. The degree of coupling between the ionized and neutral atmospheres is also an important factor, and there is considerable uncertainty in some of the terms used in the theoretical part of the study. Nonetheless, the results suggest that Coulomb collisions play a potentially important role in determining the energy balance of the upper F region. A clear need exists for further studies in this area to establish more fully the contribution of Coulomb processes.

Survey of electrons in the Uranian magnetosphere: Voyager 2 observations

We present results of an analysis of the Voyager 2 plasma science experiment (PLS) electron measurements made during the Uranus encounter. The energy coverage is 10 eV ≤ E ≤ 5950 eV. The electron distribution functions are composed of a thermal (cold) and a non‐Maxwellian suprathermal (hot) component. Within the inner magnetosphere the electron density ne ranged between 0.02 cm−3 and 1.0 cm−3, while the electron temperatures Te ranged between ∼10 eV near the terminator and ∼100 eV within the nightside hemisphere. Thermal (suprathermal) electron temperatures Tc (Th) ranged from ∼7 to 10 eV (20–200 eV) near the terminator and from ∼10 to 30 eV (500 eV to 2 keV) within the nightside hemisphere. The observations revealed no significant electron fluxes above the detection limit at all observed energies in the dayside hemisphere, but electron fluxes orders of magnitude larger extended over the full PLS energy range in the nightside hemisphere. The large day‐night asymmetry together with the spin axis alignment with the solar direction and the large tilt ∼60° of the planetary magnetic dipole indicate that solar wind driven time dependent magnetospheric convection will be an important transport mechanism within the Uranian magnetosphere. The energy‐time evolution of the plasma boundary for ions and electrons near the terminator (L ∼ 6.7) at 1648 spacecraft event time (SCET) on January 24, 1986, is remarkably similar to that observed during substorms by spacecraft within the earth's magnetosphere at geosynchronous orbit in the dusk‐midnight quadrant. We show that Miranda cannot be the cause of the plasma sheet inner edge and that present “steady state” shielding models cannot account for the thinness of the “inner edge” (< 500 km) because of expected periodic merging (enhanced convection) every ∼17 hours on the dayside magnetopause. Resonant charge exchange collisions between protons and Uranus' hydrogen cloud are used to estimate an upper limit for the hydrogen corona density NH ∼200 cm−3 at L = 5. Using the local time asymmetry in the E ∼ 6 keV electron fluxes and the observation of E > 20 keV electron fluxes at all local times sampled by the spacecraft (Krimigis et al., 1986), we estimate that the steady state convection time τconv of the plasma is between 1 and 3 days. Finally, the local time asymmetry in the radio emissions from Uranus is consistent with the day‐night asymmetry in the keV electron fluxes observed by PLS which may provide the free energy for the radio emission.

Temperature dependence of Cs-Cs+charge-exchange cross sections

A charge-exchange optical-pumping experiment to study the temperature dependence of the Cs-Cs+ charge-exchange cross section sigma has been performed. Cs+ ions are polarised by resonant charge-exchange collisions with optically pumped Cs atoms. The Cs+ ion resonance linewidths are measured over the temperature range 20-26 degrees C. They are used to study the charge-exchange cross section as a function of temperature. The cross section sigma (Cs+ to Cs) varies inversely as the square root of the temperature. The charge-exchange cross section sigma (Cs+ to Cs) has also been re-measured and found to be (6.6+or-0.5)*10-14 cm2, in good agreement with theoretical calculations.