Plasma Recombination Research Articles

N-changing population of Rydberg states by low-energy electron-Rydberg collisions.

Inelastic n-changing collisions play an important role in the evolution of Rydberg atoms into ultracold plasmas. However, for the initially intermediate n (n ∼ 40) Rydberg states, these collisions can hardly be observed due to the low electron temperature in ultracold plasmas. In this work, we designed an experimental scheme to facilitate collisions between free electrons at 1.5eV and intermediate n Rydberg atoms. Using the field ionization technique, we measured the state distributions resulting from the evolution of initially cold rubidium atoms in the 45P3/2 Rydberg state. The experimentally obtained probability of inelastic collisions excitation agrees well with the Monte Carlo simulation results. In addition, our experimental results indicate that the n-changing population induced by hot electrons is significant for lower nP Rydberg states. Our work plays a significant role in calculating the rates of electron-ion three-body recombination in ultracold plasmas.

Tearing-mediated Reconnection in Magnetohydrodynamic Poorly Ionized Plasmas. I. Onset and Linear Evolution

In high-Lundquist-number plasmas, reconnection proceeds via the onset of tearing, followed by a nonlinear phase during which plasmoids continuously form, merge, and are ejected from the current sheet (CS). This process is understood in fully ionized, magnetohydrodynamic plasmas. However, many plasma environments, such as star-forming molecular clouds and the solar chromosphere, are poorly ionized. We use theory and computation to study tearing-mediated reconnection in such poorly ionized systems. In this paper, we focus on the onset and linear evolution of this process. In poorly ionized plasmas, magnetic nulls on scales below v A,n0/ν ni0, with v A,n0 the neutral Alfvén speed and ν ni0 the neutral–ion collision frequency, will self-sharpen via ambipolar diffusion. This sharpening occurs at an increasing rate, inhibiting the onset of reconnection. Once the CS becomes thin enough, however, ions decouple from neutrals and the thinning of the CS slows, allowing the onset of tearing in a time of order νni0−1 . We find that the wavelength and growth rate of the mode that first disrupts the forming sheet can be predicted from a poorly ionized tearing dispersion relation; as the plasma recombination rate increases and ionization fraction decreases, the growth rate becomes an increasing multiple of ν ni0 and the wavelength becomes a decreasing fraction of v A,n0/ν ni0.

Lasing emission from ZnO hierarchical spherical microcavity

We investigated lasing characteristics of ZnO hierarchical micro-spherical particles with diameters of 1–5 μm . The lasing emission consists of a small number of discrete laser peaks unlike conventional random lasing from ZnO nanopowder-assembly. Theoretical calculations based on a many-body theory reveal that the optical gain is achieved at the observed lowest lasing threshold value, 4 mJ⋅cm−2⋅pulse−1 , which corresponds to the excited carrier density ∼2.4×1025 m−3 . Because the carrier density is much higher than the Mott density, the gain origin for lasing is electron-hole plasma recombination. The lasing frequency mode shift ( ∼1.2 meV ) is due to the refractive index change induced by exciting high carrier density up to 6.1×1025 m−3 . By changing hierarchical structures via controlling growth condition and performing the annealing treatment and performing the calculation of scattering efficiency of ZnO particles, we found that the hierarchy of the micro-spherical particle plays a crucial role of the lasing feedback: the strong light scattering at the interface between outer nanoparticles with the sizes of 100–200 nm and smaller ones with the sizes of 10–60 nm consisting in the micro-spherical particle results in a light confinement. Furthermore, it has been confirmed that each discrete lasing mode shows strong gain competition with each other probably due to spatial overlap between the modes. These results suggest that both random scattering and microcavity modes contribute to the lasing oscillation.

Py3BR: A software for computing atomic three-body recombination rates.

The three-body recombination reaction, or ternary association, is a termolecular reaction leading to a molecule after a three-body encounter that plays a vital role in many relevant scenarios in chemical physics. Here, we introduce the Python 3-Body Recombination program, which is dedicated to the computation of atomic three-body recombination rate coefficients. The software is based on a classical trajectory approach in hyperspherical coordinates after mapping the three-body problem as a single particle in a higher-dimensional space. This theoretical approach is fully general and applicable to any ion-atom-atom or atom-atom-atom three-body process. The predictive power of the methodology has been tested in several different experimental scenarios, reaching a good description of every system. The code structure is presented alongside examples and tests to ensure the software's capacity. In addition, the performance of the software after parallelization is shown.

Kinetic mechanisms and species densities in N2/(0–5%)H2 afterglows

N atoms absolute concentrations were obtained in the afterglows of N2/(x = 0–5%)H2 mixtures by optical emission spectroscopy (OES) coupled with NO titration in the 2–6 Torr pressure range and at distances to the discharge varying between 20 and 50 cm. Band intensity measurements were then used to obtain absolute concentrations of N2(A) and N2(X, v > 13) metastable molecules in the same conditions. From these data, absolute NH and H-atoms densities were deduced from a set of kinetic mechanisms. Independently, absolute N and H atoms densities were also measured using the TALIF technique. The comparison between TALIF and OES results suggests that the NH species can be generated outside the discharge (i.e. in the afterglow) by the 3-body recombination process N+H+N2→NH+N2, with a rate coefficient of 1.0 10−33 cm6 s−1, the NH(A) state being itself produced by the reaction N2(X,v>13)+NH→NH(A)+N2, with a rate equal to 5 10−11 cm3 s−1. The agreement between the TALIF and OES measurements is further improved considering the role of N(2D) metastable atoms in the H and NH kinetics.

Transition from MAR to EIR of deuterium plasma detachment in NAGDIS-II

Volumetric plasma recombination in a detached deuterium (D) plasma was experimentally studied in the linear plasma device NAGDIS-II. The radial distributions of the electron temperature (T e ) and the electron density (n e ) were measured by upstream and downstream laser Thomson scattering (LTS) systems and D-atom excited states of the Balmer series (n = 3–7) were measured by optical emission spectroscopy (OES). The results indicated that the electron static pressure, particle flux and heat flux present a two-step reduction with an increasing neutral pressure, suggesting that molecule activated recombination (MAR) and electron–ion recombination (EIR) were activated, respectively. The difference in the distributions of the atomic state population densities was observed to be a feature of the transition from the MAR-dominant phase to the EIR-dominant phase. The transition between the two phases had a distinct boundary at T e ∼0.7 eV, which was considered a transition point between the phases. The ratios of the atomic state population densities showed different features in the two phases, and are expected to be useful in characterizing the phase and degree of plasma detachment.

Molecular data needs for advanced collisional-radiative modelling for hydrogen plasmas

Population models for molecular hydrogen (H2) are applicable in various fields of plasma physics and particularly in fusion research: they are necessary for the evaluation of plasma diagnostics (e.g. optical emission spectroscopy) or can be used to evaluate effective reaction rates for molecular processes (e.g. molecular-assisted recombination in divertor plasmas). The accuracy and completeness of population models for molecular hydrogen is strongly linked to the accuracy and availability of molecular reaction data. While there are recently huge improvements in the field of electron impact cross sections, the shortfalls regarding input data availability persist in the area of collisions between heavy particles and H2. An overview of the status of population models for H and H2 based on the Yacora solver is given. The data needs for collisional-radiative modelling are demonstrated by means of three examples comprising different detail levels, namely a purely electronic collisional-radiative model for the singlet system of H2, a corona model for the Fulcher-α band and a vibrationally resolved collisional-radiative model for the electronic ground state X1 of H2.Graphical abstractElectronic energy level diagram of the hydrogen molecule delimiting the population models discussed in this work

Energy and particle balance during plasma detachment in a long-leg divertor configuration

Comprehensive studies of energy and particle balances in the transition to plasma detachment in an alternative divertor configuration with long outer legs are shown. Numerical simulations are performed with the 2D code suite SOLPS 4.3, using a disconnected double null grid with narrow, tightly baffled long poloidal leg divertors at the outer lower target and outer upper target. A particle count scan is performed using the ‘closed gas box’ model, where the tunable parameter in the simulations is the total number of deuterium particles in the simulation space and all other parameters are held fixed, including a constant input power and trace neon impurity radiation, to assess the physics of the transition to detachment in the system as the particle count increases. Three main aspects of the physics of divertor detachment are addressed: the criteria for the local onset of divertor detachment in each of the divertors, the distribution of heat flux and other plasma parameters between the four divertors as each divertor transitions to detachment, and the role of perpendicular transport in the transition to the detached regime. A synergistic mechanism by which the cross-field transport is reduced by factors associated with the onset of plasma recombination effects is identified. These results are compared to the existing understanding of the physics of the transition to plasma detachment in standard divertors.

Dielectronic recombination rate coefficients of carbon-like Kr30+

Dielectronic recombination (DR) rate coefficients for carbon-like Kr30+ have been measured over the collision energy of 0–60 eV using the heavy-ion storage ring CSRe at the Institute of Modern Physics in Lanzhou, China. The present DR spectrum covers the resonances associated with the , and core excitations. The corresponding DR resonance energies and strengths have been calculated by using the flexible atomic code (FAC) to understand the measured results. An overall agreement has been obtained between the experiment and theory, except for the data at the collision energies below 7 eV and in 35–38 eV, where the electronic correlation effect is strong. In particular, the resonances from the trielectronic recombination due to have been identified with the help of the FAC calculation. Temperature-dependent plasma recombination rate coefficients were derived from the measured DR rate coefficients for the temperature range 103–107 K and compared with our FAC calculations as well as the previous AUTOSTRUCTURE calculations by Zatsarinny et al (2004 Astron. Astrophys. 417 1173–81). The FAC and AUTOSTRUCTURE calculations are in good agreement with the presently derived plasma rate coefficients. The present work provides the benchmark data for astrophysical and laboratory plasma modeling.

Absolute dielectronic recombination rate coefficients of highly charged ions at the storage ring CSRm and CSRe

Dielectronic recombination (DR) is one of the dominant electron–ion recombination mechanisms for most highly charged ions (HCIs) in cosmic plasmas, and thus, it determines the charge state distribution and ionization balance therein. To reliably interpret spectra from cosmic sources and model the astrophysical plasmas, precise DR rate coefficients are required to build up an accurate understanding of the ionization balance of the sources. The main cooler storage ring (CSRm) and the experimental cooler storage ring (CSRe) at the Heavy-Ion Research Facility in Lanzhou (HIRFL) are both equipped with electron cooling devices, which provide an excellent experimental platform for electron-ion collision studies for HCIs. Here, the status of the DR experiments at the HIRFL-CSR is outlined, and the DR measurements with Na-like Kr25+ ions at the CSRm and CSRe are taken as examples. In addition, the plasma recombination rate coefficients for Ar12+, 14+, Ca14+, 16+, 17+, Ni19+, and Kr25+ ions obtained at the HIRFL-CSR are provided. All the data presented in this paper are openly available at https://doi.org/10.57760/sciencedb.j00113.00092.

Two-step photoionization of barium atoms through excited levels in optical dense approximation

The kinetics of two-step photoionization of barium atoms through excited levels was studied by numerical simulation method in the optically dense approximation. The first laser at wavelength nm ( nm) pumps transition (). The second laser with nm ( nm) ionizes atoms from the level (). The third probe laser with wavelength nm causes resonant fluorescence of barium ions. It is shown that the metastable from the triplet in barium is strongly populated through levels. Therefore, the applying of the ionizing laser tuned to a continuum from this metastable can yield a significant increase in the ionization efficiency. The laser-induced fluorescence method makes it possible to estimate the photoionization yield of barium ions with an error of up to 8%. The loading rate of the barium ion beam into the ion trap, obtained numerically, is consistent with the experimental data [Opt. Exp. 19(17), 16438 (2011)]. The deceleration of ultracold plasma recombination due to the heating of electrons in superelastic collisions for the characteristic times of a two-stage process is predicted.

Three-body recombination of ultracold barium plasma created by two-step photoionization of atoms through an excited 6s6p P11 level

The kinetics of ionization and recombination of an ultracold barium plasma created in a two-step process, taking into account the transfer of resonant radiation in 3D cylindrical geometry, is studied by numerical simulation. At the first step, a pump laser excites the upper level of the resonant transition 6s2 S10↔6s6p P11 (λ1=553.5 nm). At the second step, the laser with quantum energy exceeding the ionization potential from the level 6s6p P11 (λ2=417.79 nm) ionizes the atoms. A scheme is proposed for increasing the efficiency of electron yield: at the second ionization step, the laser radiation with frequency corresponding to the continuum from the metastable D32 is used. The electron temperature from the initial value 0.1 K during the action of the pump and ionizing lasers increases by more than 200 times due to superelastic processes. As a result, the time of three-body recombination of plasma increases significantly. The results of numerical simulation indirectly confirm the fact of Killian et al. [Phys. Rev. Lett. 83(23), 4776 (1999)] that the deceleration of recombination of ultracold xenon plasma can be explained by the heating of electrons in superelastic quenching collisions.

1D model of tokamak scrape-off layer and divertor plasmas: Dimensionless parameters and the results of numerical simulations

Solutions of a 1D model of the scrape-off layer and a divertor plasma in a tokamak are analyzed on the basis of dimensionless parameters governing the physics of both plasma and neutral transport and atomic physics processes. Numerical simulations based on fluid plasma and Monte Carlo neutral modeling confirm earlier results obtained with simplified analytic considerations and numerical simulations performed with fluid neutral description and demonstrate bifurcation of plasma parameters. These simulations also confirm that the major physical reasons for the rollover of the plasma flux to the target, which is the key feature of the phenomenon of divertor plasma detachment, are the impurity radiation loss and plasma recombination. It is suggested that simulations of the edge plasma based on dimensionless parameters provide a powerful tool for checking the consistency of the physics incorporated in existing complex 2D codes used for edge plasma modeling.

Divertor plasma detachment: roles of plasma momentum, energy, and particle balances

Contrary to statements from some recent papers, the bifurcation of the scrape-off layer and divertor plasma parameters, as described within a 1D model, does exist. The analysis of both numerical simulations and experimental data shows that the real physical reasons that can cause a strong reduction of the plasma particle flux to the target are impurity radiation loss and volumetric plasma recombination. Fitting the parameter describing the ‘plasma momentum removal’, based on the results of 2D numerical simulations, then using this fit in subsequent analytic expressions for , and finally interpreting the evolution of as the result of elastic plasma-neutral interactions break the causality principle and lead to a wrong physical picture of divertor plasma detachment.

Two-fluid numerical model of chromospheric heating and plasma outflows in a quiet-Sun

\textbf{Purpose:} This paper addresses long-standing solar physics problems, namely, the heating of the solar chromosphere and the origin of the solar wind. Our aim is to reveal the related mechanisms behind chromospheric heating and plasma outflows in a quiet-Sun. \textbf{Methods:} The approach is based on a two-fluid numerical model that accounts for thermal non-equilibrium (ionization/recombination), non-adiabatic, and non-ideal dynamics of protons+electrons and hydrogen atoms. The model is applied to numerically simulate the propagation and dissipation of granulation-generated waves in the chromosphere and plasma flows inside a quiet region. \textbf{Results:} The obtained results demonstrate that collisions between protons+electrons and hydrogen atoms supplemented by plasma viscosity, magnetic resistivity, and recombination lead to thermal energy release, which compensates radiative and thermal losses in the chromosphere, and sustains the atmosphere with vertical profiles of averaged temperature and periods of generated waves that are consistent with recent observational data. \textbf{Conclusion:} Our model conjectures a most robust and global physical picture of granulation-generated wave motions, plasma flows, and subsequent heating, which form and dynamically couple the various layers of the solar atmosphere.

New insights on divertor parallel flows,E × B drifts, and fluctuations from in situ, two-dimensional probe measurement in the Tokamak à Configuration Variable

In situ, two-dimensional (2D) Langmuir probe measurements across a large part of the TCV outer divertor are reported in L-mode discharges with and without divertor baffles. This provides detailed insights into time averaged profiles, particle fluxes, and fluctuation behavior in different divertor regimes. The presence of the baffles is shown to substantially increase the divertor neutral pressure for a given upstream density and to facilitate the access to detachment, an effect that increases with plasma current. The detailed, 2D probe measurements allow for a divertor particle balance, including ion flux contributions from parallel flows and E × B drifts. The poloidal flux contribution from the latter is often comparable or even larger than the former, and the divertor parallel flow direction reverses in some conditions, pointing away from the target. In most conditions, the integrated particle flux at the outer target can be predominantly ascribed to ionization along the outer divertor leg, consistent with a closed-box approximation of the divertor. The exception is a strongly detached divertor, achieved here only with baffles, where the total poloidal ion flux even decreases towards the outer target, indicative of significant plasma recombination. The most striking observation from relative density fluctuation measurements along the outer divertor leg is the transition from poloidally uniform fluctuation levels in attached conditions to fluctuations strongly peaking near the X-point when approaching detachment.

A laser-induced breakdown spectroscopy method to assess the stochasticity of plasma-flame transition in sprays

An experimental approach is presented to evaluate the impact of plasma composition arising from pulse-to-pulse energy and mixture fluctuations on the non-resonant laser-induced ignition of sprays. This allows for spark events to be conditioned on the successful or failed establishment of a flame kernel, a phase dominated by plasma decomposition and recombination reactions and the on-set of combustion reactions, that is, independent of the subsequent flame growth phase controlled by propagation phenomena only, such as fuel availability and turbulent strain. For that, laser-induced breakdown spectroscopy of the spark-generated plasma is carried out, followed by OH high-speed imaging of the kernel. Exploratory experiments in spatially uniform and polydisperse kerosene droplet distributions in a jet suggest that the hydrogen concentration in the plasma deriving from the fuel dissociated by the spark is closely related to the generated OH radicals levels and, in turn, with the success of establishing a flame kernel. This suggests that the ignition process is heavily controlled by mixture fluctuations at the spark, inherent of spray flows. The instantaneous mixture at the spark is estimated with a stochastic model, with the probability density function of the equivalence ratio exhibiting values higher than twice the mean value, while the highest probability occurs at lean conditions between the gaseous equivalence ratio and the overall equivalence ratio. The findings corroborate insights on the early-phase ignition obtained from direct numerical simulations, and the framework paves the way for the development of smart online engine-health tools to assess relight capability in future aeroengines.

Combining nucleotide variations and structure variations for improving astaxanthin biosynthesis

BackgroundMutational technology has been used to achieve genome-wide variations in laboratory and industrial microorganisms. Genetic polymorphisms of natural genome evolution include nucleotide variations and structural variations, which inspired us to suggest that both types of genotypic variations are potentially useful in improving the performance of chassis cells for industrial applications. However, highly efficient approaches that simultaneously generate structural and nucleotide variations are still lacking.ResultsThe aim of this study was to develop a method of increasing biosynthesis of astaxanthin in yeast by Combining Nucleotide variations And Structure variations (CNAS), which were generated by combinations of Atmospheric and room temperature plasma (ARTP) and Synthetic Chromosome Recombination and Modification by LoxP-Mediated Evolution (SCRaMbLE) system. CNAS was applied to increase the biosynthesis of astaxanthin in yeast and resulted in improvements of 2.2- and 7.0-fold in the yield of astaxanthin. Furthermore, this method was shown to be able to generate structures (deletion, duplication, and inversion) as well as nucleotide variations (SNPs and InDels) simultaneously. Additionally, genetic analysis of the genotypic variations of an astaxanthin improved strain revealed that the deletion of YJR116W and the C2481G mutation of YOL084W enhanced yield of astaxanthin, suggesting a genotype-to-phenotype relationship.ConclusionsThis study demonstrated that the CNAS strategy could generate both structure variations and nucleotide variations, allowing the enhancement of astaxanthin yield by different genotypes in yeast. Overall, this study provided a valuable tool for generating genomic variation diversity that has desirable phenotypes as well as for knowing the relationship between genotypes and phenotypes in evolutionary processes.

SPACE: 3D parallel solvers for Vlasov-Maxwell and Vlasov-Poisson equations for relativistic plasmas with atomic transformations

A parallel, relativistic, three-dimensional particle-in-cell code SPACE has been developed for the simulation of electromagnetic fields, relativistic particle beams, and plasmas. In addition to the standard second-order Particle-in-Cell (PIC) algorithm, SPACE includes efficient novel algorithms to resolve atomic physics processes such as multi-level ionization of plasma atoms, recombination, and electron attachment to dopants in dense neutral gases. SPACE also contains a highly adaptive particle-based method, called Adaptive Particle-in-Cloud (AP-Cloud), for solving the Vlasov-Poisson problems. It eliminates the traditional Cartesian mesh of PIC and replaces it with an adaptive octree data structure. The code's algorithms, structure, capabilities, parallelization strategy, and performance have been discussed. Typical examples of SPACE applications to accelerator science and engineering problems are described.

Ultrafast Nanoscopy of High-Density Exciton Phases in WSe2.

The density-driven transition of an exciton gas into an electron-hole plasma remains a compelling question in condensed matter physics. In two-dimensional transition metal dichalcogenides, strongly bound excitons can undergo this phase change after transient injection of electron-hole pairs. Unfortunately, unavoidable nanoscale inhomogeneity in these materials has impeded quantitative investigation into this elusive transition. Here, we demonstrate how ultrafast polarization nanoscopy can capture the Mott transition through the density-dependent recombination dynamics of electron-hole pairs within a WSe2 homobilayer. For increasing carrier density, an initial monomolecular recombination of optically dark excitons transitions continuously into a bimolecular recombination of an unbound electron-hole plasma above 7 × 1012 cm-2. We resolve how the Mott transition modulates over nanometer length scales, directly evidencing the strong inhomogeneity in stacked monolayers. Our results demonstrate how ultrafast polarization nanoscopy could unveil the interplay of strong electronic correlations and interlayer coupling within a diverse range of stacked and twisted two-dimensional materials.