Charge Exchange Collisions Research Articles

Physics of the high specific impulse alternative low power hybrid ion engine (alphie): Direct thrust measurements and plasma plume kinetics

The Alternative Low Power Ion Engine (alphie) is a high specific impulse plasma thruster different from conventional gridded ion engines (GIEs). It uses only one external cathode and ions and electrons flow through the open spaces of its two grids, whereas only ions are transported through the GIE ion optics. Ionizing electrons from the cathode move inward to the alphie ionization chamber and ions, which are neutralized by electrons from the same cathode, exit along the opposite direction. These currents together with the voltages applied to the grids produce a self-consistent electric field that accelerates the charges. The one-dimensional ion velocity distribution and the electron energy spectra in the collisionless alphie plasma plume are studied along its axial axis of symmetry. The thruster produces a mesothermal plasma flow with a non-monotone plasma potential profile along the axial direction. The ion populations observed are of those accelerated by the self-consistent electric field and a low velocity group that results from the charge exchange collisions in the thruster. Both populations remain essentially unaltered in the plasma flow. Conversely, the two electron groups observed merge along the axial direction of the plume following the changes in the plasma potential. The temperatures of ion populations are high by the neutral gas heating inside the thruster by high-energy ionizing electrons. The direct measurement of thrusts of 0.8–3.5 mN for argon gives 13 900–20 000 s specific impulses. These high values might be explained by the additional contribution to the thrust by the remaining non-ionized hot neutral gas effusion through the apertures of grids.

Charge-Exchange Byproduct Cold Protons in the Earth’s Magnetosphere

Owing to the spatial overlap of the ion plasma sheet (ring current) with the Earth’s neutral-hydrogen geocorona, there is a significant rate of occurrence of charge-exchange collisions in the dipolar portion of the Earth’s magnetosphere. During a charge-exchange collision between an energetic proton and a low-energy hydrogen atom, a low-energy proton is produced. These “byproduct” cold protons are trapped in the Earth’s magnetic field where they advect via E×B drift. In this report, the number density and behavior of this cold-proton population are assessed. Estimates of the rate of production of byproduct cold protons from charge exchange are in the vicinity of 1.14 cm−3 per day at geosynchronous orbit or about 5 tons per day for the entire dipolar magnetosphere. The production rate of cold protons owing to electron-impact ionization of the geocorona by the electron plasma sheet at geosynchronous orbit is about 12% of the charge-exchange production rate, but the production rate by solar photoionization of the neutral geocorona is comparable or larger than the charge-exchange production rate. The byproduct-ion production rates are smaller than observed early time refilling rates for the outer plasmasphere. Numerical simulations of the production and transport of cold charge-exchange byproduct protons find that they have very low densities on the nightside of geosynchronous orbit, and they can have densities of 0.2–0.3 cm−3 at geosynchronous orbit on the dayside. These dayside byproduct-proton densities might play a role in shortening the early phase of plasmaspheric refilling.

Statistical study of “trunk-like” heavy ion structures in the inner magnetosphere

We present a statistical study of “trunk-like” structures observed in He⁺ and O⁺ in the inner magnetosphere. The main characteristic of these structures is that the energy of the peak flux decreases earthward. Using observations from the Helium Oxygen Proton Electron (HOPE) instrument onboard Van Allen Probe A, we identify the trunks observed from November 2012 to June 2019 and obtain the universal time, <italic>L</italic> shell, magnetic local time (MLT), and energy information of each trunk’s root and tip. We then investigate the behavior of trunks in terms of their frequency of occurrence, temporal evolution, spatial and energy distribution, and trunk dependence on different geomagnetic indices. We find that (1) the trunks are always located at <italic>L</italic> = 1.5−4.0 and have a preferential location mainly concentrated at MLT = 18−24, (2) the sector within MLT = 14−16 is a forbidden zone without trunk roots, and (3) the energy of He⁺ trunks is the largest near dusk and gradually decreases in the counterclockwise direction, whereas the energy of O⁺ trunks is relatively evenly distributed with MLT and <italic>L</italic>. The differences between He⁺ and O⁺ trunks are probably due to the different charge exchange and Coulomb collision lifetime. The dependence on different geomagnetic indices indicates that the trunk structures occur more frequently during relatively quiet periods.

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.

The far-field plasma characteristics of LHT40 low-power Hall thruster for commercial aerospace applications

To achieve a better insight into the far-field plasma spatial distribution and evolution characteristics of the 300 W class low-power Hall thruster (LHT) for commercial aerospace applications, a dedicated and integrated plasma diagnostic system composed of seventeen Faraday cups (FC) and two triple Langmuir probes (TLP) is established to investigate the time-averaged in situ spatial distribution characteristics of far-field ions and electrons. The ion current density (ICD), plasma potential, plasma density, and electron temperature at 1000 mm downstream of 300 W class LHT for commercial aerospace applications in the azimuthal angle range of −90° to 90° were investigated under the conditions of different anode mass flow rates and discharge voltages. The results demonstrated that ICD, beam divergence angle, and mass utilization efficiency increased with increasing anode mass rate. The double-wings phenomenon was observed in the spatial distribution of ICD at large angles from the thruster axis, which is attributed to charge exchange collisions at increasing vacuum backpressure. The plasma electron temperature, electron density, and plasma potential parameters derived from the TLP decreased rapidly in the angle range from 0° to 30° and did not exhibit significant variations above 30°, which was also in good agreement with the results of the measured divergence angle of the FC. The discrepancy of average ion speed was calculated. The maximum error is better than 31.5% which checks the consistency between the TLP’s results and that of FC to some extent.

Investigation of dual-frequency effect on the ion energy and flux in Torr-regime plasma by a two-dimensional GPU-PIC simulation

Dual frequency (DF) capacitively coupled plasma (CCP) equipment has been widely used in semiconductor processing and requires particle-in-cell (PIC) simulations to investigate the nonlinear particle dynamics. It is believed that DF driving controls the electron density and ion energy separately, but a strong coupling exists between the two frequencies in reality. The two-dimensional (2D) PIC simulation of DF CCPs under deposition process conditions has not yet been reported due to the heavy computational load that can be resolved with a graphics processing unit (GPU) computation. This study quantitatively investigates the Torr-regime DF CCP using a 2D GPU-PIC simulation. The increase of the low-frequency (LF) power enhances ion transport, which results in the rise of plasma potential, the increase of sheath thickness, and the reduction of the electron density. The ions exhibit the Maxwellian distribution due to the frequent charge-exchange collisions. The ion flux keeps uniformity while electron density becomes abruptly nonuniform with the increase of the LF power, which is suitable to maintain the quality of deposited films.

Generation of Thrust Force in High Density and Low Temperature Plasma by Neutral Entrainment

The main objective of this article is to discuss the concept of a device that uses dense low temperature atmospheric plasma to generate thrust to enable vertical take-off and landing. Plasma consisting of atmospheric gases and potassium admixture have been enclosed in a toroidal chamber with the help of a magnetic field ( $B$ ) and a radial electric field ( $E$ ) placed perpendicular to it. The electric field induces the parallel current ( $j_{II} $ ) and drives the $E\times B$ drift in azimuthal direction, which in partially ionized and weakly magnetized plasma induces the Hall current ( $j_{\wedge } $ ). The Hall current induces the $B_{p} $ paramagnetic field with an intensity up to 5.5 T, and the parallel current induces the azimuthal field ${B}''_{\Phi } $ with an intensity up to 4 T. Ohmic heating controlled by these two currents raises the plasma temperature from 0.44 eV (injection temperature) to 1.38 eV (which is the upper value resulting from the emergence of electrothermal instability). Neutrals injected into the toroidal chamber are heated by charge exchange collisions (CEXs), then they are released through a cylindrical nozzle generating the thrust force ( $F_{T} $ ) of $\sim 2.91\cdot 10^{5} \textrm {N}$ at a temperature of ~ 1 eV. Power consumption at this thrust level attains $\sim 1.32\cdot 10^{9} \textrm {W}$ and can be covered by remote microwave or laser power sources.

Comparison among translational temperatures of , , and in inductively coupled plasmas

We compared the translational temperatures of , , and in low-pressure inductively coupled plasmas. We employed laser absorption spectroscopy for measuring the Doppler broadening widths of the transition lines, and they told us the translational temperatures of the three electronic excited states. From the comparison between the temperatures of and , we confirmed that the metastable 3 S 1 state can work as the probe for the temperature measurement of the ground state helium atoms. We observed higher temperatures for than in helium–argon mixture plasmas with total pressures less than 80 mTorr. The higher temperature is considered to be due to the selective heating of argon by the charge exchange collision with Ar+.

Transient modelling and simulation of the forced arc extinguish period of the circuit breakers

Plasma transport during continuous extinction processes from high-current arcs to fully extinction is an important issue in the study of vacuum arcs. In this research a 3D hybrid plasma model has been developed to study the extinguish period of the DC vacuum arc. The model treats the electrons as a massless fluid and ions as macro particles. The ion-neutral collision processes including both charge exchange collisions and momentum exchange collisions are considered with Monte-Carlo method. By this approach, the vacuum arc model consists of plasma jets ejected from multiple cathode spots is established and the behaviours of ions under the collisions with neutral atoms are simulated. It is shown that a large number of low-speed ions are generated in the interelectrode region due to the chare exchange collisions between ions and neutrals. And as the arc current drops to zero, the proportion of low-speed ions to the total ions at each moment becomes higher and higher.

Generation of Electron and Fast Atom Beams by a Grid Immersed in Plasma

We present a new method of product processing with beams of accelerated electrons and fast neutral atoms, which are generated by an immersed in plasma grid under a high negative voltage of 5 kV. The electrons appear due to secondary emission from the grid surface provoked by its bombardment with ions accelerated from the plasma. At the gas pressure not exceeding 0.1 Pa the ions with energy of 5 keV reach the grid without collisions in the space charge sheaths near its surface and their current in the grid circuit is by 2-3 times lower than the electron current. At higher pressures accelerated ions due to charge exchange collisions in the sheaths turn into fast neutral atoms leaving the sheaths and forming the beams. With the pressure increasing, the electron beam current diminishes and the current of fast atom beam grows.

Neutral flow interaction with a magnetic dipole plasma. I. Theory and scaling

The interaction between a high-speed neutral gas flow and dipole magnetized plasma is investigated theoretically to examine how mass, momentum, and energy are transferred to the plasma. Single-particle trajectory analysis reveals the existence of an ion-trapping region within which ions are born into closed orbits around the magnet, thus adding mass and energy to the plasma. Ion deceleration through trapping or deflection is analyzed to quantify momentum transfer to the magnetic field. The drag force and transfer rate of mass and energy from the flow to the plasma are found to scale with two dimensionless parameters: (1) a characteristic ion Larmor radius normalized by the magnet radius and (2) a characteristic neutral reaction rate normalized by the neutral gas rate of transit. The energy transfer rate is maximized at a specific reaction rate, above which increased reactivity rapidly decreases energy capture as the interaction moves away from the ion-trapping region. As the electron energy confinement time increases, there is a transition from a mode in which seed plasma is required to sustain the interaction to a high-density mode that is sustained primarily by mass and energy from the neutral flow. Two distinct flow-sustained regimes are identified that depend on the ratio of the effective ionization energy to kinetic energy of the neutral gas particles. One of the two regimes corresponds to the well-known critical ionization velocity phenomenon. The other, in which charge exchange collisions are the dominant energy transfer mechanism, has not been previously identified as a separate physical regime.

Demonstration of Muon-Beam Transverse Phase-Space Compression.

We demonstrate efficient transverse compression of a 12.5 MeV/c muon beam stopped in a helium gas target featuring a vertical density gradient and crossed electric and magnetic fields. The muon stop distribution extending vertically over 14mm was reduced to a 0.25mm size (rms) within 3.5 μs. The simulation including cross sections for low-energy μ^{+}-He elastic and charge exchange (μ^{+}↔ muonium) collisions describes the measurements well. By combining the transverse compression stage with a previously demonstrated longitudinal compression stage, we can improve the phase space density of a μ^{+} beam by a factor of 10^{10} with 10^{-3} efficiency.

Expansion of alternatively extracted ion–ion beam in a low pressure collisional medium

The aim of this paper is to investigate, through particle-in-cell with Monte Carlo collisions (PIC-MCC) simulations, the effects of collisions on the expansion of an ion–ion beam formed by the alternate extraction of oppositely charged ions. This beam is extracted from an ion–ion plasma, formed in the downstream of a radiofrequency magnetically filtered iodine electronegative plasma, by the mean of two extraction grids. In this grid system, the screen grid is biased with a square voltage waveform, in the low MHz range, while the acceleration grid is grounded. The collisionless interaction between the extracted ion packets involves and enhances electrostatic waves propagating at beam velocity. Our results show that in addition to these electrostatic waves, the presence of significant fraction of low energy ions, produced by charge exchange, beside extracted fast ions gives rise to two-stream instability. Furthermore, space and time dependent electric field might accelerate the slow ions, produced by charge exchange collision. This acceleration, involving Landau damping and its inverse mechanism, is mainly due to the interaction between ions and potential patterns inherent to two-stream instability.

Single charge exchange in collisions of energetic nuclei with biomolecules of interest to ion therapy

This study is on the theory of single electron capture by fast nuclei from a variety of molecular targets of biological significance with high relevance to ion therapy of deep-seated tumors. The adopted theoretical framework is that of the first principles of quantum physics. As such, no free, adjustable parameters are used. This is in sharp contrast to the associated existing cross section input data to Monte Carlo simulations that all abound with empirical/phenomenological formulae. The present theory has the well-established track of its predictive power. This means that the computed cross sections can confidently be used in the cases for which no experimental data exist. These cross sections are from the full continuum distorted wave method (CDW). We first compute atomic cross sections in the independent electron model and then generate the corresponding molecular cross sections. The latter follow from the former within the independent atom model accompanied by the Bragg additivity rule. The investigated atomic targets are from the backbone of DNA and/or RNA molecules. These are atomic hydrogen, carbon, nitrogen and oxygen (H, C, N, O). Neon is also added to this sequence of targets as an isoelectronic atomic counterpart of water vapor, methane and ammonia molecules. The studied molecular targets are H2O (water vapor), CO (carbon-monoxide), CO2 (carbon-dioxide), CH4 (methane), C2H4 (ethylene), C2H6 (ethane), C4H10 (butane) as well as the DNA/RNA nucleobases C4H4N2O2 (uracil), C5H5N5 (adenine), C5H5N5O (guanine), C5H6N2O2 (thymine) and C4H5N3O (cytosine). The obtained total cross sections for any electronic target shell are compared with the available experimental data and overall favorable agreement is recorded at intermediate and high impact energies, which is the validity domain of the CDW method.

Three-body boundary-corrected continuum-intermediate-state method for single charge exchange with the general transition amplitude (1s→nlm) applied to the p–H(1s), α–H(1s), and p–He(1s2) collisions with n≤4

We derive a general quantum-mechanical transition amplitude in the prior version of the three-body boundary-corrected continuum-intermediate-state (BCIS-3B) method for single charge exchange in collisions of heavy nuclei with atomic hydrogenlike targets. The obtained expression is valid for all the transitions from the initial ground to any final hydrogenlike bound states $(1s\ensuremath{\rightarrow}nlm),$ where ${n,l,m}$ is the usual triple of the quantum numbers (principal, angular, magnetic). The outcome is semianalytical: a judicious combination of an analytical calculation (as far as feasible) and the subsequent numerical computations (quadrature for integrals that could not be reduced to closed formulas). The final results for the $T$-matrix elements are a compact double integration computed by numerical quadratures over real variables in finite intervals. The ensuing total cross section is a single numerical quadrature of the squared absolute value of this transition amplitude integrated over the magnitude of the transverse momentum transfer vector. This can provide detailed information on the cross sections for each individual subshell ${n,l,m}$ of the newly formed hydrogenlike atom or ion comprised of a projectile nucleus and the captured electron. Using these general expressions, we computed a large number of state-selective and state-summed cross sections (both differential and total) for electron capture in the $p\text{\ensuremath{-}}\mathrm{H}$, $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{H}$, and $p\text{\ensuremath{-}}\mathrm{He}$ collisions at intermediate and high impact energies. The energy dependence of the cross sections for the individual shells and their subshells with the fixed quantum numbers $n$, ${n,l}$, and ${n,l,m}$ are reported via detailed tables and figures. Also, differential cross sections (state selective and summed) are computed for the $p\text{\ensuremath{-}}\mathrm{H}$ collisions at 60, 125, and 5000 keV. All the present results from the BCIS-3B method are found to be in very good agreement with the existing experimental data on differential and total cross sections, both state selective (whenever available) and state summed. The provided cross-section database can find important applications in several investigative fields ranging from plasma physics, astrophysics, and heavy-ion transport physics through fusion research and technology to hadron therapy.

A particle model of ion thruster plume Mo source based on grid erosion

Ion thruster's plume can bring serious contamination to satellite, especially the molybdenum (Mo) charge-exchange (CEX) ions in plume. Therefore, it is critical to describe the emission and movement of Mo atoms appropriately for the prediction of plume pollution. In this paper, a 3D Mo source model is constructed based on grid erosion. In this model, the emission of Mo atom is given by the erosion rate of grid. Then by using a 3D plume particle model and the 3D Mo source model, the Mo CEX ions of LIPS-300 ion thruster is calculated. The influences of the non-uniformity of upstream plasma, the CEX collision, and the Mo atom initial distribution on the Mo CEX ion distribution are considered. The results show that the above aspects have a significant effect on the distribution of Mo CEX ions. The simulation results of Mo CEX ions obtained by this paper's model are also compared with those based on the commonly used empirical hypothesis of Mo atoms. It is shown that the results based on empirical hypothesis are similar to those under the uniform initial distribution of Mo atoms, and the range of Mo CEX ion distribution is increased.

Ion Emission From a Positively Biased Hollow Cathode Plasma

Hollow cathode plasma contactors have an extensive history as spacecraft neutralizers, but questions remain on whether the technology can neutralize significant positive charging in tenuous space plasmas. Simulations and a representative semianalytical model (which we term the ion emission model) predict effective neutralization in these scenarios as ion current is emitted from the surface of the quasi-neutral contactor plasma according to the space-charge limit. This article focuses on experimental plasma measurements of a hollow cathode biased to several positive voltages with respect to a surrounding vacuum chamber in order to validate the ion emission model. Particular attention is paid to the emitted ions (those reaching the chamber wall) and to the ion-rich sheath region where the ion emission process takes place. Retarding potential analyzer (RPA) measurements were performed to understand ion flow velocity and direction as these parameters relate directly to the current predicted by the ion emission model. Planar probe measurements were performed at the chamber wall to compare local emission currents to model predictions. Evidence of collisions within the plasma (particularly charge exchange collisions) and simple models predicting ion energetics are presented. These experiments suggest that ion emission from the contactor plasma acts according to the ion emission model and adds to the physical understanding of spacecraft neutralization using hollow cathode plasma contactors as it may occur in tenuous space plasmas.

Multi-component MHD model for hydrogen-helium extended envelope of hot Jupiter

AbstractWe describe a numerical model of hot Jupiter extended envelope that interacts with stellar wind. Our model is based on approximation of multi-component magnetic hydrodynamic. The processes of ionization, recombination, dissosiation and chemical reactions in hydrogen-helium envelope are taken into account. In particular, the ionization of neutral hydrogen atoms takes place due to processes of photo-ionization, charge-exchange and thermal collisions. Further, this model is supposed to be used for research on biomarkers’ dynamics in extended envelopes of hot Jupiters.

Far-Field Plume Characterization of a 100-W Class Hall Thruster

The 100 W-class ISCT100-v2 Hall Thruster (HT) has been characterized in terms of far-field plume properties. By means of a Faraday Cup and a Retarding Potential Analyzer, both the ion current density and the ion energy distribution function have been measured over a 180 ∘ circular arc for different operating points. Measurements are compared to far-field plume characterizations performed with higher power Hall thrusters. The ion current density profiles remain unchanged whatever the HT input power, although an asymptotic limit is observed in the core of the plume at high discharge voltages and anode mass flow rates. In like manner, the ion energy distribution functions reveal that most of the beam energy is concentrated in the core of the plume [ − 40 ∘ ; 40 ∘ ] . Moreover, the fraction of low energy ion populations increases at large angles, owing to charge exchange and elastic collisions. Distinct plume regions are identified; they remain similar to the one described for high-power HTs. An efficiency analysis is also performed in terms of current utilization, mass utilization, and voltage utilization. The anode efficiency appears to be essentially affected by a low voltage utilization, the latter originating from the large surface-to-volume ratio inherent to low-power HTs. Experimental results also show that the background pressure clearly affects the plume structure and content.

How blobs affect plasma dynamics in tokamaks

The inelastic interactions between blobs and neutral particles can impact shoulder formation in a tokamak plasma through ionization and charge-exchange collisions.