Charge Exchange Processes Research Articles

Implementation of SOLPS-ITER code with new Grad–Zhdanov module for D–T mixture

A new Grad–Zhdanov module is implemented into the SOLPS-ITER code for calculation of the parallel kinetic coefficients. A complete, multi-ion generalization is performed, relaxing the heavy impurity assumption. A JET-like D+T+Ne test simulation is conducted to demonstrate the ability to model a 50/50 deuterium (D) and tritium (T) mixture in SOLPS-ITER. More than 30% T build-up with respect to D is observed in different parts of the simulation domain, in particular at the high field side. A T predominance over D is also observed near both the inner and outer targets. It is a result of the different effective diffusion, dominated by charge-exchange processes, for the D and T neutral species. A simple 1D model is proposed to describe this phenomenon. The contribution to the differing D and T distribution in different regions from the Grad–Zhdanov thermal force is also studied. Due to the thermal force and the D/T poloidal flow from the low field side to the high field side, the prevalence of the T species over D is found at the high field side at the X-point level. The latter leads to an inner–outer divertor asymmetry of . However, the effect is relatively small due to the close D and T masses. It is further shown that the infinite ion mass difference limit, which is used for the derivation of the Zhdanov-Yushmanov analytical expressions and applied for the thermal force coefficient calculation previously used in the SOLPS-ITER code, overestimates significantly. Thus, the old SOLPS-ITER model should not be applied for D–T simulations. Finally, possible experimental studies of the D and T spatial separation due to the new effects revealed by this modelling are discussed.

Interatomic Coulombic decay in small helium clusters.

Interatomic Coulombic decay (ICD) is an ultrafast non-radiative electronic decay process wherein an excited atom transfers its excess energy to a neighboring species leading to the ionization of the latter. In helium clusters, ICD can take place, for example, after simultaneous ionization and excitation of one helium atom within the cluster. After ICD, two helium ions are created and the system undergoes a Coulomb explosion. In this work, we investigate theoretically ICD in small helium clusters containing between two and seven atoms and compare our findings to two sets of coincidence measurements on clusters of different mean sizes. We provide a prediction on the lifetime of the excited dimer and show that ICD is faster for larger clusters. This is due to (i) the increased number of neighboring atoms (and therefore the number of decay channels) and (ii) the substantial decrease of the interatomic distances. In order to provide more details on the decay dynamics, we report on the kinetic-energy distributions of the helium ions. These distributions clearly show that the ions may undergo charge exchange with the neutral atoms within the cluster, such process is known as frustrated Coulomb explosion. The probability for these charge-exchange processes increases with the size of the clusters and is reflected in our calculated and measured kinetic-energy distributions. These distributions are therefore characteristics of the size distribution of small helium clusters.

Collision integrals and viscosity coefficients of argon–carbon thermal plasmas: Comparison using different interaction potentials

The calculated values of collision integrals of the majority of binary interactions that can be involved in argon/carbon (Ar/C) plasmas are presented in this work. The studied plasmas are considered to be in local thermodynamic equilibrium, and calculations are performed from 1000 to 30 000 K. Computations have been carried out first for standard potentials and second for the Improved Lennard-Jones (ILJ) potential. In this paper, a clear and well detailed report is given for each methodology adopted to calculate the omega integrals, together with the main data needed to perform these computations. The obtained values of collision integrals have been compared with some data reported in the literature, and then, they are used to estimate the viscosity of two plasma systems, namely, pure argon and a mixture of argon and carbon (Ar/C). The calculation of viscosity coefficients is made on the basis of the Chapman–Enskog method and developed to the first approximation. The important contribution of the charge exchange process and its influence on the accuracy of the diffusion-type collision integrals of neutral–parent ion systems are emphasized. Although some discrepancies are observed, comparisons of our results with those of previously published studies show an overall satisfactory agreement in most of the cases. Our investigation of the data uncertainty further confirms the suggestion that the ILJ approach is an excellent candidate to provide collision integrals with acceptable accuracy when reliable experimental data or accurate theoretical calculations are unavailable. For that, all the necessary collision integrals needed to calculate reliable transport properties of the Ar/C plasma mixtures are reported in this work.

Benchmark n ℓ-resolved Cross Sections of Single and Double Charge Exchange Processes in 1.67–20 keV u−1 C4+ Collisions with He

State-resolved charge exchange (CX) cross sections are of the utmost importance for modeling related photon emissions existing in a broad range of astrophysical environments. With the cold-target recoil-ion momentum spectroscopy, we determined with high accuracy the state-resolved single and double CX cross sections at the quantum orbital angular momentum level for solar wind ion C4+ collisions with He in an energy range of 1.67–20 keV u−1, which allow one to benchmark the CX calculations in great detail, and to test the applicability of the analytical n- and ℓ-distribution models widely adopted by the astrophysical community. We found that the present measurements are well reproduced by the most recent state-of-the-art atomic–orbital close-coupling calculations. However, the CX models failed to give a consistent description on the measured ℓ distributions. The present work reveals that the velocity and collision partner species dependence effects as well as electronic correlations for multielectron processes should be included in an improved model. Alternatively, in future modeling to interpret high-resolution astrophysical observations the more elaborate quantum-mechanical calculations may be resorted to with confidence.

The radiative processes involving ion-atom alkali systems: The case of hydrogen and potassium

The importance of the symmetric as well as non-symmetric ion-atom absorption processes in broad spectral region within a model of solar photosphere and also within the corresponding sunspot models, has been previously emphasized. This motivated us to study the photo-association, charge-exchange and photo-dissociation absorption processes in the cases of non-symmetric and symmetric systems involving hydrogen and potassium atoms, ions, molecules and molecular-ions. We have calculated the data for molecules and molecular state characterizations needed for current computation. We determined the partial and average cross-sections and corresponding spectral absorption rate coefficients, as the functions of wavelengths, and temperatures (100 nm ⩽λ⩽1000 nm, 400 K ⩽T⩽10 000 K). Our calculations are performed for conditions that exist in different layers of weakly ionized stellar atmospheres. The obtained data are potentially of high impact and may have broad further applications, e.g., for modelling of laboratory plasmas and various astrophysical objects including geocosmical ones.

Investigation on charged particles in inductively coupled Ar/O2 plasmas: The role of Ar proportion

In this paper, a 2D fluid model is built to reveal the inductively coupled Ar/O2 plasma behavior at 300 W, 30 mTorr, in the gas mixture of 95% Ar −5% O2 and 10% Ar −90% O2. The reliability of the model is first verified by comparing the calculated results with the experimental data, and the consistent results are obtained. Then, the spatial distributions of the charged species densities are investigated. As Ar fraction decreases, the maximum densities of charged particles shift toward the coil significantly, and the O2+ becomes the dominant positive ion at higher O2 fractions. The main reaction mechanisms are also discussed. It is concluded that the electrons are generated by the ionization of background gases, and the Ar+ ions are primarily formed by the ionization of Ar. However, the charge exchange processes account for the most production of O2+ and O+ ions at a high Ar fraction. The loss at the walls is the most important process to the consumption of positive ions. For the O− ions, they are first generated by the dissociative attachment of O2 at ground state, followed by the metastable state, and they are destroyed either by the ion–ion recombination with Ar+ ions or the detachment with O atoms as the Ar proportion varies. Finally, the effect of the ion–ion recombination reaction rate coefficients is discussed. The results indicate that rate coefficients of the recombination reactions have a significant influence on the positive ion densities when the Ar fractions are high.

Post collision analyzer to study charge-exchange processes in ion-molecule collisions.

We have designed an electrostatic charge state analyzer for ion beams having energies in the range of 5-20 keV/q. It is primarily built to investigate the different ionization processes involved in the slow (v < 1 a.u.) impact of highly charged ions on molecules. The analyzer is a cylindrical deflector analyzer (CDA) based on a pair of concentric cylindrical sectors of radii 110.2 and 95mm, subtending an angle of 127° at its center. Additionally, an Einzel lens and a quadrupole deflector are deployed to focus and steer the ion beam. The compact design of the analyzer permits easy integration with an ion momentum spectrometer used for studying the fragmentation of the target molecules. The characterization of the CDA including its calibration and its transmission function is carried out using an ion beam delivered from an electron beam ion source. To check the performance of the setup, we have carried out experiments comprising the impact of Ar16+ projectiles on CO2 target molecules at an energy of 18 keV/q. With the help of the CDA, different charge exchange ionization processes, such as single capture, double capture, and triple capture of electrons by the projectile ion have been separated efficiently. The possibility of a modification in the geometry of CDA is discussed to further improve its performance.

Search for Nucleon Charge-Exchange Processes in the Fragmentation of Carbon Ions at an Energy of 300 MeV per Nucleon

Search for Nucleon Charge-Exchange Processes in the Fragmentation of Carbon Ions at an Energy of 300 MeV per Nucleon

Pion Production in νμ Charged Current Interactions on 40Ar in Deep Underground Neutrino Experiment

Understanding the pion generation and the consequences of final-state interactions (FSI) are critical for the data processing in all neutrino experiments. The energy utilized in modern neutrino researches of the resonance (RES) generation processes contributes significantly to the pion production. If a pion is absorbed in the nuclear matter after its production, the event may become unrecognizable from a quasielastic (QE) scattering process and act as a background. For oscillation experiments, estimating this background is critical, and it necessitates solid theoretical models for both pion generation at the primary vertex and after FSI. The number of pions created after FSI differs greatly from the number produced at the primary vertex due to FSI. Because neutrino detectors can only detect final-state particles, FSI obscures the proper information about particles created at the primary vertex. A detailed study of FSI is required to overcome this problem, which theoretical models incorporated in Monte Carlo (MC) neutrino event generators can provide. They should give theoretical results concerning the neutrino interactions for various researches, acting as a connection among both theoretical models and experimental data. In this paper, we provide simulated events for the pion creation in νμ charge current (CC) interactions on a 40Ar target in the Deep Underground Neutrino Experiment (DUNE) setup for two distinct MC generators: GENIE and NuWro. In comparison to GENIE (v-3.00.06), NuWro (v-19.02.2) is more opaque (less responsive) to the charge exchange and absorption processes; pions are more likely to be absorbed than produced during the intranuclear transport.

Exploring neutrinos from proton decays catalyzed by GUT monopoles in the Sun

We explore the neutrino signals from proton decays catalyzed by GUT monopoles in the Sun. Three typical proton decay modes, p ⟶ e + + (ρ 0, η, ω… ) ⟶ π +, p ⟶ μ + K 0 and p ⟶ ν̅eπ+ , have been analyzed for the Super-Kamiokande experiment. The monopole-induced neutrinos arise from interactions and subsequent decays of the proton decay products. To obtain the neutrino energy spectra, we use the Geant4 software to simulate propagations of daughter particles in the highly-dense solar center. It is found that K 0 can produce a large amount of 236 MeV monoenergetic νμ neutrinos through the charge exchange process K 0 + p ⟶ K + + n and the subsequent decay K + ⟶ μ + ν μ at rest. Based on this interesting feature, p ⟶ μ + K 0 can give the best discovery potential among three decay modes for most of the parameter space. In addition, we present the Super-Kamiokande sensitivities to the monopole flux for three proton decay modes.

Coexistence of Martian Proton Aurorae and Proton Cyclotron Waves during the Enhancement of Solar Wind Activity

Due to the lack of a global magnetic field, the charge exchange process between the solar wind protons and Martian hydrogen corona is of critical importance to Mars’ atmosphere. The energetic neutral atoms and pickup H+ produced during this process can further excite proton aurorae and proton cyclotron waves (PCWs) in the near-Mars space, for which the observational evidence however remains very limited. Here we report a particular event to show that the PCWs and proton aurorae were simultaneously recorded by the Mars Atmosphere and Volatile EvolutioN spacecraft during 10 consecutive orbits. As the byproducts of the charge exchange process, these two phenomena are found to be highly correlated in both space and time, with the correlation coefficient >0.7 between the auroral emissions and PCW amplitudes. Moreover, the wave and ultraviolet measurements show clearly that both the PCWs and proton aurora events tend to occur more frequently and intensively within the stream interaction region, as being strongly modulated by the solar wind activity. Our results indicate that the solar wind can directly transport energy and particles into the near-Mars environment, leading to the simultaneous enhancements of plasma wave activity and proton precipitation, which therefore helps comprehend the significant role of the solar wind activity and charge exchange process in driving the energy budget to the Martian upper atmosphere.

Divertor leg control of a quasi‐symmetry stellarator with external coils and its consequences for transport

AbstractA control method of the divertor configuration using external saddle loop coils is investigated for a quasi‐symmetry stellarator device. Multiple coils are used to modify the divertor configuration while keeping the rotational transform distribution in the core region the same. The external coils successfully shrink the core plasma region and extend the divertor legs. From the transport analysis with the EMC3‐EIRENE code, it is confirmed that the dependence of the electron temperature of the core plasma on the position of the plasma‐facing walls is weakened by the extension of the divertor legs. The increase of the divertor particle flux and the increase of energy loss due to radiation and charge exchange processes are also confirmed.

Single- and two-photon absorption laser-induced fluorescence spectroscopy in rare gases for gridded ion thruster diagnostics

Methods based on laser-induced fluorescence spectroscopy are widely used for spatially resolved non-intrusive diagnostics of atomic or molecular densities and velocity distributions in plasma applications. With regard to electric space propulsion, one focus is on the investigation of rare gases such as xenon or krypton, which are currently the favored propellants in gridded ion- and Hall-effect thrusters. For gridded ion engines, diagnostics of neutral atoms is of interest since charge-exchange processes between neutrals and ions are the main driver of accelerator grid erosion, which limits the lifetime of a gridded ion thruster. Extending the capabilities of the advanced electric propulsion diagnostics platform which has been developed by the IOM and partners, single- and two-photon absorption laser-induced fluorescence diagnostics have been set-up recently at our institute. Both experimental set-ups, and as a series of first applications, measurements of krypton neutrals in the plume of the radiofrequency ion thruster RIT-10 (ArianeGroup GmbH), and xenon neutrals within the discharge chamber of a gridded radiofrequency ion source developed at IOM, are presented.

A self-consistent multi-component model of plasma turbulence and kinetic neutral dynamics for the simulation of the tokamak boundary

A self-consistent model is presented for the simulation of a multi-component plasma in the tokamak boundary. A deuterium plasma is considered, with the plasma species that include electrons, deuterium atomic ions and deuterium molecular ions, while the deuterium atoms and molecules constitute the neutral species. The plasma and neutral models are coupled via a number of collisional interactions, which include dissociation, ionization, charge-exchange and recombination processes. The derivation of the three-fluid drift-reduced Braginskii equations used to describe the turbulent plasma dynamics is presented, including its boundary conditions. The kinetic advection equations for the neutral species are also derived, and their numerical implementation discussed. The first results of multi-component plasma simulations carried out by using the global Braginskii solver (GBS) code are then presented and analyzed, being compared with results obtained with the single-component plasma model.

Physics studies of ADITYA & ADITYA-U tokamak plasmas using spectroscopic diagnostics

Several spectroscopic diagnostics encompassing the spectral emission range from the x-ray to near infrared (NIR) have been developed, installed and operated for diagnosing and physics studies in the ADITYA and ADITYA-U tokamaks. The recycling and impurity influxes and plasma Z eff after lithium (Li) coating have been studied using a PMT (photomultiplier tube)-filter based system by capturing H α , O1+, C2+, and visible continuum emissions. Significant reduction in the Z eff values has been observed in the discharge with the Li coated walls. The measured radial profile of H α emission using a filter-PMT array, has been modelled using a neutral transport code. The results show substantial contributions from the molecular hydrogen and molecular hydrogen ion dissociation (∼56%) and charge-exchange (∼30%) processes in the measured H α emission. Furthermore, a high-resolution, 1 m spectrometer with charge coupled device detector capable of multi-track measurements, has been used to study impurity transport, neutral and ion temperature and intrinsic plasma rotation. By modelling the measured radial profile of O4+ spectral line emission using an impurity transport code, substantial contribution of edge fluctuations on the oxygen transport has been observed. The toroidal ( ∼ 20 km s−1 in core) and poloidal ( ∼ 4.5 km s−1 at edge) rotation velocities are measured using C5+ (529 nm) and C2+ (464.7 nm) passive line emissions respectively. The measurement of radial profile of toroidal plasma rotation revealed a reversal of rotation direction depending on the electron density content of the ADITYA-U plasmas. The neutral temperature measurements showed a poloidal asymmetry indicating a presence of asymmetrical source of neutral heating. Moreover, the modelling of measured Fe14+ and Fe15+ vacuum ultraviolet spectral lines has revealed the neo-classical nature of iron transport in ADITYA core. Fast visible camera images captured the formation of filament structures triggered by interchange instabilities during plasma disruptions.

2p excitation in target atoms in the interaction of slow ions with Al surfaces

This work reports on measurements of energy distribution of electrons emitted by Aluminum surfaces under the impact of singly charged noble gases and sodium ions, incident with energies up to 10 keV, focused on collisional excitation of target atoms. Analysis of the spectra give insight into several collisional excitation and charge exchange processes that occur during the cascade of atomic collisions, which are important in the understanding of charge fractions, electron emission and energy loss phenomena.

Photoelectrochemical activity of ZnO:Ag/rGO photo-anodes synthesized by two-steps sol-gel method

This work investigated the influence of silver plasmon and reduced graphene oxide (rGO) on the photoelectrochemical performance (PEC) of ZnO thin films synthesized by the sol-gel method. The physicochemical properties of the obtained photo-anodes were systematically studied by using several characterization techniques. The x-ray diffraction analysis showed that all samples presented hexagonal wurtzite structure with a polycrystalline nature. Raman and energy dispersive x-ray (EDX) studies confirmed the existence of both Ag and rGO in ZnO:Ag/rGO thin films. The estimated grain size obtained from scanning electron microscopy (SEM) analysis decreased with Ag doping, then increased to a maximum value after rGO addition. The UV-vis transmission spectra of the as-prepared ZnO:Ag and ZnO:Ag/rGO thin films have shown a reduction in the visible range with a redshift at the absorption edges. The bandgaps were estimated to be around 3.17 eV, 2.7 eV, and 2.52 eV for ZnO, ZnO:Ag, and ZnO:Ag/rGO, respectively. Moreover, the electrical measurements revealed that the charge exchange processes were enhanced at the ZnO:Ag/rGO/electrolyte interface, accompanied by an increase in the (PEC) performance compared to ZnO and ZnO:Ag photo-anodes. Consequently, the photocurrent density of ZnO:Ag/rGO (0.2 mA⋅cm−2) was around 4 and 2.22 times higher than photo-anodes based on undoped ZnO (0.05 mA⋅cm−2) and ZnO:Ag (0.09 mA⋅cm−2), respectively. Finally, from the flat band potential and donor density, deduced from the Mott–Schottky, it was clear that all the samples were n-type semiconductors with the highest carrier density for the ZnO:Ag/rGO photo-anode.

Characterization of a Multicusp Ion Source With Two-Grid Extraction System for Studying Extraction and Transport of Ion Beam

A low-energy multipole line cusp ion source with two-grid extraction system has been developed and characterized for studying the role of the charge exchange processes on ion beam extraction, transport, and neutralization. The ion source is operated for extraction of Ar <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> ion beam of current 50–240 mA at energies 850–1650 eV. The characterization of the ion beam is carried out at various axial positions along a 200–815-mm beam path with a background pressure of 4– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$20\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> Torr. An 11-channel Faraday cup array is used to measure the radial profiles of ion current density and angular beam divergence at these positions. The beam divergences are found to be in the range of 6°–15° for beam perveance of 0.50– <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.75\times10$ </tex-math></inline-formula> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−9</sup> AV <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3/2</sup> . The attenuation of ion beam current along the beam path is observed due to the charge exchange neutralization of the energetic ions in the presence of high background pressure. Since ion current density along the beam path can be reduced by both the charge exchange processes and beam divergence, the accounting of beam particles, i.e., energetic ions and neutrals, at larger distances is difficult by the measurements of ion current density using the Faraday cup alone. The flux of energetic particles (energetic ions and fast neutrals) is therefore obtained by a thrust measurements on a beam target fixed at a distance of 800 mm from the ion source. The neutralization of the ion beam obtained by the emission of electrons from a hot filament neutralizer is also investigated in the presence of the charge exchange processes. Space-charge neutralization is provided initially by the secondary electrons emitted from the vessel walls or by the charge exchange processes even without the neutralizer operating at the desired temperature. However, the current neutralization is achieved only by the electrons emitted from the hot filament neutralizer operating in a space-charge-limited mode.

On the Breakup Reaction in Three-Particle Coulomb Systems with Application to the Description of Dissociative Recombination and Charge-Exchange Processes in the Antiproton Physics

On the Breakup Reaction in Three-Particle Coulomb Systems with Application to the Description of Dissociative Recombination and Charge-Exchange Processes in the Antiproton Physics

Diffusive-like redistribution in state-changing collisions between Rydberg atoms and ground state atoms

Exploring the dynamics of inelastic and reactive collisions on the quantum level is a fundamental goal in quantum chemistry. Such collisions are of particular importance in connection with Rydberg atoms in dense environments since they may considerably influence both the lifetime and the quantum state of the scattered Rydberg atoms. Here, we report on the study of state-changing collisions between Rydberg atoms and ground state atoms. We employ high-resolution momentum spectroscopy to identify the final states. In contrast to previous studies, we find that the outcome of such collisions is not limited to a single hydrogenic manifold. We observe a redistribution of population over a wide range of final states. We also find that even the decay to states with the same angular momentum quantum number as the initial state, but different principal quantum number is possible. We model the underlying physical process in the framework of a short-lived Rydberg quasi-molecular complex, where a charge exchange process gives rise to an oscillating electric field that causes transitions within the Rydberg manifold. The distribution of final states shows a diffusive-like behavior.