Ion-neutral Collisions Research Articles

Tearing Modes in Partially Ionized Astrophysical Plasma

Abstract In many astrophysical environments the plasma is only partially ionized, and therefore the interaction of charged and neutral particles may alter both the triggering of reconnection and its subsequent dynamical evolution. We derive the tearing mode maximum growth rate for partially ionized plasmas in the cases of weak and strong coupling between the plasma and the neutrals. In addition, critical scalings for current sheet aspect ratios are presented in terms of Lundquist number and ion–neutral collision frequencies for which the tearing mode becomes fast, or ideal. In the decoupled regime the standard tearing mode is recovered with a small correction that depends on the ion–neutral collision frequency; in the intermediate regime collisions with neutrals are shown to stabilize current sheets, resulting in larger critical aspect ratios for ideal tearing to occur. In the coupled regime, the growth rate depends on the density ratio between ions and neutrals through the collision frequency between these two species.

Numerical Analysis of Artificial Electron Heating Effects on Polar Mesospheric Winter Echoes

In this paper, an analytical model is used to analyze the modulated polar mesospheric winter echoes (PMWE). The winter parameters were introduced to simulate the effects of different parameters during the artificial electron heating of PMWE. The important role of the charged dust particle in the creation of PMWE is confirmed again. It is found that during the heating of PMWE, the increases of the dust size, dust charge, electron temperature, initial electron density, and ion-neutral collision frequency cause the increase of the electron density irregularity, and hence the PMWE strength. However, with increasing the dust density, the electron density irregularity and the PMWE strength decrease.

Influence of varying magnetic field on nonlinear wave excitations in collisional quantum plasmas

Abstract The nonlinear wave excitations arising from the spatially varying magnetic field in the quantum plasma environment are investigated in the frame work of quantum hydrodynamic model. In the weakly nonlinear, dispersive and dissipative limit it is shown that the varying magnetic field and collision-induced excitations can be described by a modified form of Korteweg-de Vries–Burgers’ type model equation. It is found that the dissipation terms (Burgers’ and collisional term) arise due to spatially varying magnetic field and the ion-neutral collisions. The numerical solutions of this equation predict that the localized soliton solutions decay algebraically due to the combined effect of varying magnetic field and collision by radiating oscillatory pulses behind the propagating soliton.

Electronegative magnetized plasma sheath properties in the presence of non-Maxwellian electrons with a homogeneous ion source

The three-fluid model was employed to study electronegative magnetized plasma sheath properties in the presence of non-Maxwellian electrons. The Bohm condition for the electronegative magnetized plasma was extended in the presence of a homogeneous ionization source term and an ion-neutral collision force. The effect of the following physical parameters: the degree of non-Maxwellian electrons, ion-neutral collisions, obliqueness of the magnetic field and source frequency on the plasma sheath parameters are numerically analyzed and presented. It was found that the magnitude of the potential increased in all cases and hence the velocity of the ions increased towards the wall. The plasma sheath parameters were not significantly varied about 2λ De from the presheath—sheath interface, but after that, the sheath parameters abruptly changed towards the wall. The effect of ion-neutral collisions and the obliqueness of the magnetic field on the potential distribution, net charge density and drift velocity of ions are discussed. Furthermore, a comparison of plasma sheath characteristics with and without the ion source term is systematically presented.

Inevitable consequences of ion–neutral damping of intermediate MHD waves in Sun-like stars

ABSTRACT In the context of the solar atmosphere, we re-examine the role of neutral and ionized species in dissipating the ordered energy of intermediate-mode MHD waves into heat. We solve conservation equations for the hydrodynamics and for hydrogen and helium ionization stages, along closed tubes of magnetic field. First, we examine the evolution of coronal plasma under conditions where coronal heating has abruptly ceased. We find that cool (<105K) structures are formed lasting for several hours. MHD waves of modest amplitude can heat the plasma through ion–neutral collisions with sufficient energy rates to support the plasma against gravity. Then we examine a calculation starting from a cooler atmosphere. The calculation shows that warm (>104) K long (> several Mm) tubes of plasma arise by the same mechanism. We speculate on the relevance of these solutions to observe properties of the Sun and similar stars whose atmospheres are permeated with emerging magnetic fields and stirred by convection. Perhaps this elementary process might help to explain the presence of ‘cool loops’ in the solar transition region and the production of broad components of transition region lines. The production of ionized hydrogen from such a simple and perhaps inevitable mechanism may be an important step towards finding the more complex mechanisms needed to generate coronae with temperatures in excess of 106K, independent of a star’s metallicity.

Ion Mobility Shift of Isotopologues in a High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) at Elevated Effective Temperatures.

Ion mobility spectrometers (IMS) separate ions mainly by ion-neutral collision cross section and to a lesser extent by ion mass and effective temperature. When investigating isotopologues, the difference in collision cross section can be assumed negligible. Since the mobility shift of isotopologues is thus mainly caused by their difference in mass and effective temperature, the investigation of isotopologues can provide important insights into the theory of ion mobility. However, in classical IMS operated at ambient pressure, cluster formation with neutral molecules occurs, which significantly influences the mobility shift of isotopologues and thus makes a sound investigation of the effect of ion mass and effective temperature on the ion mobility difficult. In this work, the relative ion mobility of several organic compounds and their 13C-labeled isotopologues is studied in a High Kinetic Energy Ion Mobility Spectrometer (HiKE-IMS) at high reduced electric fields up to 120 Td, which allows the investigation of nonclustered ion species and thus enables a sound investigation of the mobility shift of isotopologues. The results show that the measured relative ion mobilities of isotopologues having the same effective temperature and, thus, their ion mass dominating the relative ion mobility agree well with theoretical relative ion mobilities predicted by the theory of ion mobility.

Influence of the Ion Mass in the Radial to Orbital Transition in Weakly Collisional Low-Pressure Plasmas Using Cylindrical Langmuir Probes

This paper presents an experimentally observed transition from the validity of the radial theories to the validity of the orbital theories that model the ion current collected by a cylindrical Langmuir probe immersed in low-pressure, low-temperature helium plasma when it is negatively biased with respect to the plasma potential, as a function of the positive ion-neutral collision mean free path to the Debye length ratio Λ=λ+/λD. The study has been also conducted on argon and neon plasmas, which allows a comparison based on the mass of the ions, although no transition has been observed for these gases. As the radial or orbital behavior of the ions is essential to establish the validity of the different sheath theories, a theoretical analysis of such a transition not only as a function of the parameters Λ and β=T+/Te, T+ and Te being the positive ion and electron temperature, respectively, but also as a function of the ion mass is provided. This study allows us to recognize the importance of the mass of the ion as the parameter that explains the transition in helium plasmas. Motivated by these theoretical arguments, a novel set of measurements has been performed to study the relationship between the Λ and β parameters in the transition that demonstrate that the effect of the ion mean free path cannot be completely ignored and also that its influence on the ion current collected by the probe is less important than the effect of the ion temperature.

Magnetoacoustic Waves in Partially Ionized Two-Fluid Heat-Releasing Plasma

A dispersion relation for magnetoacoustic waves in partially ionized heat-releasing plasma in an external magnetic field is derived in a compact form. The increments of slow and fast modified magnetoacoustic waves are calculated in the case of isentropic instability; their strong dependence on the degree of ionization and frequency of ion—neutral collisions is showed.

Numerical simulations of the lower solar atmosphere heating by two-fluid nonlinear Alfvén waves

Context. We present new insight into the long-standing problem of plasma heating in the lower solar atmosphere in terms of collisional dissipation caused by two-fluid Alfvén waves. Aims. Using numerical simulations, we study Alfvén wave propagation and dissipation in a magnetic flux tube and their heating effect. Methods. We set up 2.5-dimensional numerical simulations with a semi-empirical model of a stratified solar atmosphere and a force-free magnetic field mimicking a magnetic flux tube. We consider a partially ionized plasma consisting of ion + electron and neutral fluids, which are coupled by ion-neutral collisions. Results. We find that Alfvén waves, which are directly generated by a monochromatic driver at the bottom of the photosphere, experience strong damping. Low-amplitude waves do not thermalize sufficient wave energy to heat the solar atmospheric plasma. However, Alfvén waves with amplitudes greater than 0.1 km s−1 drive through ponderomotive force magneto-acoustic waves in higher atmospheric layers. These waves are damped by ion-neutral collisions, and the thermal energy released in this process leads to heating of the upper photosphere and the chromosphere. Conclusions. We infer that, as a result of ion-neutral collisions, the energy carried initially by Alfvén waves is thermalized in the upper photosphere and the chromosphere, and the corresponding heating rate is large enough to compensate radiative and thermal-conduction energy losses therein.

Three-dimensional cross-field flows at the plasma-material interface in an oblique magnetic field

This article describes experimental evidence that the magnetic presheath is a fully three-dimensional structure modified by ion–neutral collisions. Velocity distributions of both ions and neutrals, obtained via laser-induced fluorescence, show that cross field ion drifts do not result from entrainment of ions in a flowing neutral background. Ion flows parallel to E×B arise and accelerate to as much as 0.2cs within several ion gyroradii of the boundary surface, where cs is the sound speed. Within measurement resolution, the onset of the E×B aligned flow occurs at the same distance to the surface that ions begin to deflect from travel along magnetic field lines. Collisional fluid and particle-in-cell simulations of the boundary region are compared to the experimental measurements. We find that, in contrast to the classical collisionless Chodura model, collisional effects between the ions and the non-flowing neutral population are essential to quantitatively predict the observed ion drift velocities. No momentum coupling between ions and neutrals, separable from noise and other effects, is observed in either signal. We discuss several explanations and implications of this observation.

Evaluation of Trapped Ion Mobility Spectrometry Source Conditions Using Benzylammonium Thermometer Ions.

A key aspect of reduced pressure ion mobility spectrometry (IMS) experiments is to identify experimental conditions that minimize the role of collisional energy transfer that allows for assessing effective ion-neutral collision cross sections of metabolites, peptides, and proteins in "native-like" or compact states. Across two separate experimental campaigns using a prototype trapped ion mobility spectrometer (TIMS) coupled to a time-of-flight mass spectrometer, we present independent findings that support the results recently published by Morsa et al. using a different set of thermometer ions (Morsa et al. Anal. Chem. 2020, 92 (6), 4573-4582). First, using five para-substituted benzylammonium ions, we conducted survival yield experiments to assess ion internal energy across different experimental settings. Results from the present set of experiments illustrate that greater ion heating occurs at lower pressures and higher voltage settings applied to the TIMS. At the "softest" settings where the benzylammonium thermometer ions have an effective average energy of 1.73 eV, we observe the majority of bradykinin in the compact state. Under more extreme operating conditions where the energy of the benzylammonium ions varies from 1.83 to 1.86 eV, the bradykinin transitions from the compact to the elongated state. In addition to independently confirming the findings of Morsa et al., we also report the mobilities for the benzylammonium parent and fragment ions using the tandem drift-tube-TIMS calibration procedure described by Naylor et al. ( J. Am. Soc. Mass Spectrom. 2019, 30 (10), 2152-2162).

3D Numerical Simulations of Solar Quiet Chromosphere Wave Heating

Abstract Despite numerous observational and theoretical attempts, the heating problem of the solar chromosphere still remains unsolved. We develop a novel 3D two-fluid model that accounts for dynamics of charged species and neutrals, and use it to perform the numerical simulations of granulation driven jets and associated waves in a quiet region of the solar chromosphere. The energy carried by the waves is dissipated through ion–neutral collisions, which are sufficient to balance radiative energy losses and to sustain the quasi-stationary atmosphere whose ion and neutral number densities, ionization fraction, and temperature profiles are relatively close to the observationally based semi-empirical model. Additional verification of our results is provided by a good fit of the numerically predicted waveperiod variations with height to the recent observational data. These observational validations of the numerical results demonstrate that the wave heating problem of a quiet region of the chromosphere may be solved.

Ion current collection by double flush-mounted probe in intermediate-pressure plasmas

A double flush-mounted probe method is proposed to analyze the characteristics of intermediate-pressure plasmas (10–100 Pa) by a sheath criterion. The analytical model mainly focuses on the effect of the ion drift velocity in the sheath edge and then provides a correction factor for the measured ion-collection current. The reduction of the ion current is attributed to ion–neutral collisions leading to the modified Bohm velocity. To validate this method, the ion density is calculated and compared with that measured by two types of cylindrical probes, and the results agree well with the data measured by other probes. On the basis of this work, the double flush-mounted probe can be adopted to diagnose plasma parameters accurately, without a reference electrode, in collisional and certain complex plasma environments, such as the reentry plasma sheath.

Enabling resolution of isomeric peptides using tri-state ion gating and Fourier-transform ion mobility spectrometry

Using Fourier-Transform ion gate modulation technique, we compare the ability of the tri-state ion shutter (3S-IS) to the two-state ion shutter (2S-IS) in separating three pairs of isomeric peptide including 1.Gly-Arg-Gly-Asp-Ser (GRGDS) / Ser-Asp-Gly-Arg-Gly (SDGRG); 2.Sar-Arg-Gly-Asp-Ser-Pro (SRGDSP) / Gly-Arg-Gly-Asp-Thr-Pro (GRGTP); 3.Kemptide / (Val6, Ala7)-Kemptide using electrospray ionization and ion mobility spectrometry. Mobility separation was evaluated for peptide individually and as simple mixtures. Baseline resolution of both singly and doubly charged ions of the isomeric pentapeptide mixture of GRGDS / SDGRG was attainable with the described IMS system using the 3S-IS configuration, illustrating the capacity of the present instrument to resolve isomeric compounds with differences in ion neutral collision cross section (CCS) of less than 1% for the singly charged ions. However, with the 2S-IS, both singly and doubly charged ions of the same peptide mixture were unresolved in the mobility domain. To our knowledge, this is the first-time baseline separation has been reported for the singly charged ions of the isomeric reversed sequence pentapeptide mixture using Fourier transformed drift tube IMS with nitrogen as the drift gas. For all the peptide mixtures, the ion counts for the ion mixture recorded with the 3S-IS were substantially higher (> 50%) in comparison to the 2S-IS. The resolving power of the instrument ranged between 82 to 128 for the target analyte ions analyzed in a mixture using the 3S-IS. Whereas, the resolving power of the 2S-IS ranged between 60 and 100 for the target analytes. Overall, a 20% increase in resolving power was obtained with the 3S-IS in comparison to the 2S-IS. Separation of the different isomeric peptide ion mixture depicted in this present study clearly shows the unique size-to-charge separation ability of IMS that complements the mass-to-charge ratio measurement capacity of mass spectrometry.

Day-to-day variability of the bottomside ionosphere

This study is focused on investigating how well the day-to-day variability of HF propagation in the bottomside ionosphere can be simulated using a state-of-the-art ionosphere model coupled to a whole atmosphere model driven by the lower atmosphere. The SAMI3 ionosphere model has been coupled to the WACCM-X whole atmosphere model, which was nudged with NAVGEM-HA to incorporate the effects of lower atmospheric weather. The resulting ionospheric specifications were used as input for an HF propagation model (MoJo) in order to compare to observations from the NOAA ionosphere network. The results indicate that the amount of variability at the F-region peak can be successfully reproduced by the model, but that the simulations do not show enough variability in the bottomside of the F-region, particularly below 200 km altitude. Further comparisons with data and a simple modeling study are used to demonstrate that the most likely source of this missing variability are low-altitude ion transport terms, which are not currently included in the standard SAMI3 model. These transport terms are required to simulate the motion perpendicular to the magnetic field in regions where the motion becomes strongly dominated by ion-neutral collisions.

Wave heating of the solar atmosphere without shocks

Context. We investigate the wave heating problem of a solar quiet region and present its plausible solution without involving shock formation. Aims. We aim to use numerical simulations to study wave propagation and dissipation in the partially ionized solar atmosphere, whose model includes both neutrals and ions. Methods. We used a 2.5D two-fluid model of the solar atmosphere to study the wave generation and propagation. The source of these waves is the solar convection located beneath the photosphere. Results. The energy carried by the waves is dissipated through ion-neutral collisions, which replace shocks used in some previous studies as the main source of local heating in quiet regions. Conclusions. We show that the resulting wave dissipation is sufficient to balance radiative and thermal energy losses, and to sustain a quasi-stationary atmosphere whose averaged temperature profile agrees well with the observationally based semi-empirical model of Avrett & Loeser (2008, ApJS, 175, 229).

Ultracold ions wake in dusty plasmas

Motivated by the recent experimental realization of ultracold dusty plasma (2019 Sci. Rep. 9 3261), we present the results of particle-in-cell simulation with Monte-Carlo-collisions for wake behind a dust particle due to focusing of ions at superfluid helium temperature (∼2 K). Dynamical screening (wakefield) defines structural and dynamical properties of charged dust particles in plasmas such as phase transition, crystal formation, vibration modes (waves) etc. Here, we delineate in detail the dependence of wake strength on the streaming velocity of ions and on the ion-neutral charge exchange collision frequency (neutrals density) in the ultracold dusty plasma. Lowering the temperature to ultracold level leads to a wake pattern behind a dust particle that completely differs from the wake at normal conditions. For wide range of parameters, most remarkable features of the wakefield are (i) the formation of wake pattern with two maxima split in transverse to ion flow direction in the downstream area, (ii) pronounced inverse V shape of the wakefield closely resembling the wake in quark-gluon plasma and dense quantum plasma (warm dense matter), and (iii) the inter-dust attraction region in transverse direction. The latter shows that molecule-like interaction between dust particles is realized in ultracold dusty plasmas. These observations show a fundamental difference of ultracold dusty plasma physics from well studied complex plasmas at normal conditions.

Excitations of low-frequency solitary waves in collisional magnetized plasmas with two kappa-distributed electron populations

The electrostatic excitations of low-frequency solitary waves are considered in a collisional magnetized plasma consisting of two-electron populations with kappa distribution. A nonlinear damped Korteweg–de Vries (dK–dV) equation is derived by using the reductive perturbation method. In the present plasma model, both positive and negative potential solitons can be supported. The numerical results reveal that ion-neutral collision frequency modifies features of the electrostatic solitary waves significantly. The effects of wave propagation angle, spectral index of cold (hot) electrons, temperature ratio of cold to hot electron, ion temperature and the strength of the external magnetic field on electrostatic solitary wave structures are also discussed here.

Breaking Down Structural Diversity for Comprehensive Prediction of Ion-Neutral Collision Cross Sections.

Identification of unknowns is a bottleneck for large-scale untargeted analyses like metabolomics or drug metabolite identification. Ion mobility-mass spectrometry (IM-MS) provides rapid two-dimensional separation of ions based on their mobility through a neutral buffer gas. The mobility of an ion is related to its collision cross section (CCS) with the buffer gas, a physical property that is determined by the size and shape of the ion. This structural dependency makes CCS a promising characteristic for compound identification, but this utility is limited by the availability of high-quality reference CCS values. CCS prediction using machine learning (ML) has recently shown promise in the field, but accurate and broadly applicable models are still lacking. Here we present a novel ML approach that employs a comprehensive collection of CCS values covering a wide range of chemical space. Using this diverse database, we identified the structural characteristics, represented by molecular quantum numbers (MQNs), that contribute to variance in CCS and assessed the performance of a variety of ML algorithms in predicting CCS. We found that by breaking down the chemical structural diversity using unsupervised clustering based on the MQNs, specific and accurate prediction models for each cluster can be trained, which showed superior performance than a single model trained with all data. Using this approach, we have robustly trained and characterized a CCS prediction model with high accuracy on diverse chemical structures. An all-in-one web interface (https://CCSbase.net) was built for querying the CCS database and accessing the predictive model to support unknown compound identifications.

Sculpted ultracold neutral plasmas.

Ultracold neutral plasma (UNP) experiments allow for careful control of plasma properties across Coulomb coupling regimes. Here, we examine how UNPs can be used to study heterogeneous, nonequilibrium phenomena, including nonlinear waves, transport, hydrodynamics, kinetics, stopping power, and instabilities. Through a series of molecular dynamics simulations, we have explored UNPs formed with spatially modulated ionizing radiation. We have developed a computational model for such sculpted UNPs that includes an ionic screened Coulomb interaction with a spatiotemporal screening length, and Langevin-based spatial ion-electron and ion-neutral collisions. We have also developed a hydrodynamics model and have extracted its field quantities (density, flow velocity, and temperature) from the molecular dynamics simulation data, allowing us to investigate kinetics by examining moment ratios and phase-space dynamics; we find that it is possible to create UNPs that vary from nearly perfect fluids (Euler limit) to highly kinetic plasmas. We have examined plasmas in three geometries: a solid rod, a hollow rod, and a gapped slab; we have studied basic properties of these plasmas, including the spatial Coulomb coupling parameter. By varying the initial conditions, we find that we can design experimental plasmas that would allow the exploration of a wide range of phenomena, including shock and blast waves, stopping power, two-stream instabilities, and much more. Using an evaporative cooling geometry, our results suggest that much larger Coulomb couplings can be achieved, possibly in excess of 10.