Ion-neutral Collisions Research Articles

Integrated accretion disc angular momentum removal and astrophysical jet acceleration mechanism

Ions and neutrals in the weakly ionized plasma of an accretion disc are tightly bound because of the high ion–neutral collision frequency. A cluster of a statistically large number of ions and neutrals behaves as a fluid element having the charge of the ions and the mass of the neutrals. This fluid element is effectively a metaparticle having such an extremely small charge-to-mass ratio that its cyclotron frequency can be of the order of the Kepler angular frequency. In this case, metaparticles with a critical charge-to-mass ratio can have zero canonical angular momentum. Zero canonical angular momentum metaparticles experience no centrifugal force and spiral inwards towards the central body. Accumulation of these inward spiralling metaparticles near the central body produces radially and axially outward electric fields. The axially outward electric field drives an out-of-plane poloidal electric current along arched poloidal flux surfaces in the highly ionized volume outside the disc. This out-of-plane current and its associated magnetic field produce forces that drive bidirectional astrophysical jets flowing normal to and away from the disc. The poloidal electric current circuit removes angular momentum from the accreting mass and deposits this removed angular momentum at near infinite radius in the disc plane. The disc region is an electric power source (E⋅J 0).

Models, assumptions, and experimental tests of flows near boundaries in magnetized plasmas

We present the first measurements of ion flows in three dimensions (3Ds) using laser-induced fluorescence in the plasma boundary region. Measurements are performed upstream from a grounded stainless steel limiter plate at various angles (ψ=16° to 80°) to the background magnetic field in two argon helicon experiments (MARIA at the University of Wisconsin-Madison and HELIX at West Virginia University). The Chodura magnetic presheath model for collisionless plasmas [R. Chodura, Phys. Fluids 25, 1628 (1982)] is shown to be inaccurate for systems with sufficient ion-neutral collisions and ionization such as tokamak scrape off layers. A 3D ion fluid model that accounts for ionization and charge-exchange collisions is found to accurately describe the measured ion flows in regions where the ion flux tubes do not intersect the boundary. Ion acceleration in the E→×B→ direction is observed within a few ion Larmor radii of the grounded plate for ψ=80°. We argue that fully 3D ion and neutral acceleration in the plasma boundary are uniquely caused by the long-range presheath electric fields, and that models that omit presheath effects under-predict observed wall erosion in tokamak divertors and Hall thruster channel walls.

ALFVÉN WAVE HEATING OF THE SOLAR CHROMOSPHERE: 1.5D MODELS

ABSTRACT Physical processes that may lead to solar chromospheric heating are analyzed using high-resolution 1.5D non-ideal MHD modeling. We demonstrate that it is possible to heat the chromospheric plasma by direct resistive dissipation of high-frequency Alfvén waves through Pedersen resistivity. However, this is unlikely to be sufficient to balance radiative and conductive losses unless unrealistic field strengths or photospheric velocities are used. The precise heating profile is determined by the input driving spectrum, since in 1.5D there is no possibility of Alfvén wave turbulence. The inclusion of the Hall term does not affect the heating rates. If plasma compressibility is taken into account, shocks are produced through the ponderomotive coupling of Alfvén waves to slow modes and shock heating dominates the resistive dissipation. In 1.5D shock coalescence amplifies the effects of shocks, and for compressible simulations with realistic driver spectra, the heating rate exceeds that required to match radiative and conductive losses. Thus, while the heating rates for these 1.5D simulations are an overestimate, they do show that ponderomotive coupling of Alfvén waves to sound waves is more important in chromospheric heating than Pedersen dissipation through ion–neutral collisions.

Influence of ion-neutral collision parameters on dynamic structure of magnetized sheath during plasma immersion ion implantation

AbstractA cold magnetized plasma sheath is considered to examine the gas pressure effect on the sheath dynamics. A fluid model is used to describe the plasma sheath dynamic. The governing fluid equations in the plasma are solved from plasma center to the target using the finite difference method and some convenient initial and boundary conditions at the plasma center and target. It is found that, the ion-neutral collision has significant effect on the dynamic characteristics of the high-voltage sheath in the plasma immersion ion implantation (PIII). It means that, the temporal profile of the ion dose on the target and sheath width are decreased by increasing the gas pressure. Also, the gas pressure substantially diminishes the temporal psychograph of ion incident angle on the target.

Ion-Acoustic Vortices in Two-Electron-Temperature Magnetoplasma with Cairn’s Distributed Electrons and in the Presence of Ion Shear Flow

Linear and nonlinear characteristics of electrostatic waves in a multicomponent magnetoplasma comprising of Boltzmann distributed electrons, Cairn’s distributed hot electrons, and cold dynamic ions are studied. It is found that the effect of superthermal electrons, ion-neutral collisions, and ion shear flow modifies the propagation of ion-acoustic and drift waves. The growth rate of the ion shear flow instability varies with the addition of Cairn’s distributed hot electrons. It is also investigated that the behavior of different type of vortices changes with the inclusion of superthermal hot electrons. The relevance of this investigation in space plasmas such as in auroral region and geomagnetic tail is also pointed out.

Effect of collision parameter on magnetized electronegative plasma sheath structure

The structure of an electronegative plasma sheath in an oblique magnetic field is investigated. Moreover, the collisions between positive ions and neutral particles are taken into account. It is assumed that the system consists of hot electrons, hot negative ions, and cold positive ions. Also the negative ions and the electrons are assumed to be described by the Boltzmann distributions of their own temperatures, and the accelerated positive ions are treated by the continuity and momentum balance equations through the sheath region. In addition, it is assumed that the collision cross section has a power law dependence on the positive velocity. After theoretical derivation, an exact expression of sheath criterion is obtained. The numerical simulation results include the density distributions of the positive ions for different invariable ion Mach numbers satisfying Bohm criterion, and the comparison of net space charge distribution between variable and invariable ion Mach numbers. Furthermore, three kinds of charged particle densities, the net space charges, and the spatial electric potentials in the sheath are studied numerically for different collision parameters under the condition of the fixed ion Mach number. The results show that the ion Mach number has not only the lower limit but also the upper limit. The ion Mach number affects the sheath structure by influencing the distribution of the positive ion density, and different conclusions can be obtained because ion Mach number is adopted as variable or invariable value when discussing the effects of the other variables which can result in a variety of the ion Mach numbers on the sheath formation. The reason is that the actual sheath structure modification brought on by the variation of a parameter can be divided into two parts. One is the sheath formation change caused directly by the variation of the parameter, and the other is the sheath formation change caused by the Bohm criterion modification which the variation of the parameter results in. Therefore, an identical ion Mach number should be adopted when studying the direct effects of a parameter variety on plasma sheath structure. In addition, it is concluded that the collisions between positive ions and neutral particles make positive ion density curve higher and electron density curve lower than the case without collisions. Negative ion density does not change significantly no matter whether there exists collision. Besides, there is a peak in the profile of the net space charge while in the presence of ion-neutral collision, and the net space charge peak moves toward the sheath edge. The spatial potential increases and the sheath thickness decreases on account of the presence of the collisions between ions and neutral particles.

Direct measurements of classical and enhanced gradient-aligned cross-field ion flows in a helicon plasma source using laser-induced fluorescence

Direct laser induced fluorescence measurements are shown of cross-field ion flows normal to an absorbing boundary that is aligned parallel to the axial magnetic field in a helicon plasma. We show Langmuir and emissive probe measurements of local density and plasma potential in the same region, as well as floating probe spectra near the boundary. With these measurements, we investigate the influence of ion-neutral collisionality on radial ion transport by varying the ratio of the ion gyro-radius, ρi, to the ion-neutral collision length, λ, over the range 0.34 ≤ ρiλ−1 ≤ 1.60. Classical drift-diffusion transport along density and potential gradients is sufficient to describe flow profiles for most cases. For two parameter regimes (ρiλ−1 = 0.65 and 0.44), low-frequency electrostatic fluctuations (f < 10 kHz) and enhanced cross-field bulk ion flow to the boundary are observed.

Spatiotemporal study of gas heating mechanisms in a radio-frequency electrothermal plasma micro-thruster

A spatiotemporal study of neutral gas temperature during the first 100 s of operation for a radio-frequency electrothermal plasma micro-thruster operating on nitrogen at 60 W and 1.5 Torr is performed to identify the heating mechanisms involved. Neutral gas temperature is estimated from rovibrational band fitting of the nitrogen second positive system. A set of baffles are used to restrict the optical image and separate the heating mechanisms occurring in the central bulk discharge region and near the thruster walls. For each spatial region there are three distinct gas heating mechanisms being fast heating from ion-neutral collisions with timescales of tens of milliseconds, intermediate heating with timescales of 10 s from ion bombardment on the inner thruster tube surface creating wall heating, and slow heating with timescales of 100 s from gradual warming of the entire thruster housing. The results are discussed in relation to optimising the thermal properties of future thruster designs.

Formation of sporadic-E (Es) layers under the influence of AGWs evolving in a horizontal shear flow

The formation of mid-latitude sporadic-E (Es) layer under the influence of atmospheric gravity waves (AGWs), evolving in the background horizontal wind with a horizontal linear shear (horizontal shear flow), is studied. The AGWs, which are in-situ excited in the horizontal shear flow, interact with metallic ions through ion-neutral collisions and Lorentz forcing, influence the horizontal and vertical motion of ions, and lead to their convergence into thin horizontal layers. In order to investigate the formation of sporadic-E, 2-D numerical simulations are performed and temporal evolution of multilayered sporadic-E is demonstrated.It is found that ion/electron density of Es layer depends on the value of the horizontal shear of a magnetic north-directed background wind. The increase of the shear parameter in the cyclonic type background wind leads to the increase of horizontal convergence of charged particles. It is shown that the plasma density of Es layers also depends on the horizontal and vertical amplitudes of AGWs' velocity perturbations, which increase in the horizontal shear flow and cause a formation of multilayered sporadic-E.The Es layers mainly move downwards, but at certain spatial location, where temporal changes in wave phase caused by background shear flow and AGW are small, these layers are almost horizontal. The vertical spatial location of the horizontal Es layers is determined by the vertical wavelength of atmospheric gravity waves.

Results of the Ion–Neutral Collision Frequency Measurements in the Lower Thermosphere by the Method of Resonance Scattering of the Radio Waves by Artificial Periodic Irregularities

UDC 550.388.2+551.510.536 We present for the first time the experimental data on the frequency of ion–neutral collisions at the altitudes of the ionospheric E layer, which were obtained by the method of resonance scattering of the radio waves by artificial periodic irregularities of the ionospheric plasma. The measurements were carried out in different time periods from 1990 to 2012. Time and altitude dependences of the collision frequency are given. The dependence of the collision frequency on the solar and geomagnetic activity has been analyzed. Collisions of electrons and ions with atoms and molecules play an important role in physics of the ionosphere and atmosphere. In particular, they determine the friction force and should be taken into account in the equations of motion of a medium when its viscosity increases. The frequency of ion–neutral collisions affects the energy and momentum transfer processes [1–4]. When propagation of internal gravity waves above 100 km is considered, it is needed to allow for dissipation due to increasing viscosity of the medium [4–7], which is directly related to collisions. The frequency νin of collisions between ions and neutral molecules should also be taken into account when the low-frequency processes associated with the plasma conductivity are analyzed (see, e. g., [8]). However, the experimental data on νin are fairly scarce. Meanwhile, considerable variations in the ion–neutral collision frequency can be observed in the upper ionosphere. These variations are due most frequently to the neutral atmosphere density variations caused by the transmission of internal gravity and tidal waves. Variations from day to day can be due to propagation of planetary waves. At the altitudes of the upper mesosphere and lower thermosphere, the neutral atmosphere density variations according to the lidar data can exceed 20%, while the temperature variations can reach 70 K [9, 10]. Observations suggest that during propagation of gravity waves the collision frequency can be changed by a factor of 1.5–2 for half an hour [9, 11]. The collision frequency variations can also be a consequence of variations in the composition of both the ion component and the neutral atoms and molecules. In particular, in the ionospheric E region, variations in the ion–neutral collision frequency can be caused by the penetration of metal meteor particles and ions that contribute to the formation of both the sporadic layers of the electron density and the layer with increased content of Na, K, Fe, etc. atoms [12–15]. In this paper, we present the results of determining the ion–molecule collision frequency obtained by the method of resonant scattering of radio waves by artificial periodic irregularities of the plasma at the altitudes of the ionospheric E region.

Three-dimensional two-fluid Braginskii simulations of the large plasma device

The Large Plasma Device (LAPD) is modeled using the 3D Global Braginskii Solver code. Comparisons to experimental measurements are made in the low-bias regime in which there is an intrinsic E × B rotation of the plasma. In the simulations, this rotation is caused primarily by sheath effects and may be a likely mechanism for the intrinsic rotation seen in LAPD. Simulations show strong qualitative agreement with the data, particularly the radial dependence of the density fluctuations, cross-correlation lengths, radial flux dependence outside of the cathode edge, and camera imagery. Kelvin Helmholtz (KH) turbulence at relatively large scales is the dominant driver of cross-field transport in these simulations with smaller-scale drift waves and sheath modes playing a secondary role. Plasma holes and blobs arising from KH vortices in the simulations are consistent with the scale sizes and overall appearance of those in LAPD camera images. The addition of ion-neutral collisions in the simulations at previously theorized values reduces the radial particle flux by about a factor of two, from values that are somewhat larger than the experimentally measured flux to values that are somewhat lower than the measurements. This reduction is due to a modest stabilizing contribution of the collisions on the KH-modes driving the turbulent transport.

THE LINE WIDTH DIFFERENCE OF NEUTRALS AND IONS INDUCED BY MHD TURBULENCE

We address the problem of the difference of line widths of neutrals and ions observed from molecular clouds and explore whether this difference can arise from the effects of magnetohydrodynamic (MHD) turbulence acting on partially ionized gas. Among the three fundamental modes of MHD turbulence, we find fast modes do not contribute to linewidth differences, whereas slow modes can have an effect on different line widths for certain parameters. We focus on Alfv\'{e}nic component because they contain most of the turbulent energy, and consider the damping of this component taking into account both neutral-ion collisions and neutral viscosity. We consider different regimes of turbulence corresponding to different media magnetizations and turbulent drivings. In the case of super-Alfv\'{e}nic turbulence, when the damping scale of Alfv\'{e}nic turbulence is below $l_A$, where $l_A$ is the injection scale of anisotropic GS95-type turbulence, the linewidth difference does not depend on the magnetic field strength. While for other turbulent regimes, the dependence is present. For instance, the difference between the squares of the neutral and ion velocity dispersions in strong sub-Alfv\'{e}nic turbulence allows evaluation of magnetic field. We discuss earlier findings on the neutral-ion linewidth differences in the literature and compare the expressions for magnetic field we obtain with those published earlier.

Ion thermal and dispersion effects in Farley-Buneman instabilities

Farley-Buneman modes are an example of the collisional instability, which is thought to be the dominant mechanism for the irregularities in low ionosphere region. Despite high collisionality due to electron-neutral and ion-neutral collisions, the kinetic effects associated with finite temperature are important for determination of the mode frequencies and growth rate. This is especially important for ion component that is largely unmagnetized due to low ion cyclotron frequency. The ion thermal effects are strongly pronounced for shorter wavelengths and are crucial for the growth rate cut-off at high wavenumbers. We develop an extended fluid model for ion dynamics to incorporate the effects of ion thermal motion. The model is based on the extended MHD model that includes the evolution equations for higher order moments such as ion viscosity and ion heat flux. We also develop the generalized Chapman-Enskog closure model that provides exact linear closures based on the linearized kinetic equation. The results of these models are compared and tested against the linear kinetic model. The dispersion of Farley-Buneman modes and growth rate behavior are investigated in the short wavelength region.

How Closely Related Are Conformations of Protein Ions Sampled by IM-MS to Native Solution Structures?

Here, we critically evaluate the effects of changes in the ion internal energy (E(int)) on ion-neutral collision cross sections (CCS) of ions of two structurally diverse proteins, specifically the [M + 6H](6+) ion of ubiquitin (ubq(6+)), the [M + 5H](5+) ion of the intrinsically disordered protein (IDP) apo-metallothionein-2A (MT), and its partially- and fully-metalated isoform, the [CdiMT](5+) ion. The ion-neutral CCS for ions formed by "native-state" ESI show a strong dependence on E(int). Collisional activation is used to increase E(int) prior to the ions entering and within the traveling wave (TW) ion mobility analyzer. Comparisons of experimental CCSs with those generated by molecular dynamics (MD) simulation for solution-phase ions and solvent-free ions as a function of temperature provide new insights about conformational preferences and retention of solution conformations. The E(int)-dependent CCSs, which reveal increased conformational diversity of the ion population, are discussed in terms of folding/unfolding of solvent-free ions. For example, ubiquitin ions that have low internal energies retain native-like conformations, whereas ions that are heated by collisional activation possess higher internal energies and yield a broader range of CCS owing to increased conformational diversity due to losses of secondary and tertiary structures. In contrast, the CCS profile for the IDP apoMT is consistent with kinetic trapping of an ion population composed of a wide range of conformers, and as the E(int) is increased, these structurally labile conformers unfold to an elongated conformation.

Weakly dissipative solitons in dense relativistic-degenerate plasma

We investigate the features of weakly nonlinear waves in a collisional dense plasma consisting of ultra-relativistic degenerate electrons and non-relativistic degenerate ions. In weak dissipation limit, the dynamics of low frequency nonlinear ion (solitary) wave is described by solving a damped Korteweg-deVries equation. The analytical and numerical analysis shows the existence of weakly dissipative solitons evolving with time. The characteristics of soliton evolution with plasma number density and slow ion-neutral collision rate are discussed with some detail. The relevance of the study with degenerate plasmas in ultra-dense astrophysical objects, particularly white dwarf stars is also pointed out.

Backward mapping solutions of the Boltzmann equation in cylindrically symmetric, uniformly charged auroral ionosphere

By applying a backward mapping technique, we solve the Boltzmann equation to investigate the effects of ion-neutral collisions on the ion velocity distribution and related transport properties in cylindrically symmetric, uniformly charged auroral ionosphere. Such a charge geometry introduces a radial electric field which increases linearly with distance from the axis of symmetry. In order to obtain complete analytical solutions for gaining physical insights into more complicated problems, we have substituted a relaxation collision model for the Boltzmann collision integral in the Boltzmann equation. Our calculations show that collisions drive the velocity distribution to a “horseshoe” shape after a few collision times. This feature extends to all radial positions as long as the electric field keeps increasing linearly versus radius. If the electric field is introduced suddenly, there is a transition from the collision-free pulsating Maxwellian distributions obtained in previous work (Ma and St.-Maurice, J. Geophys. Res., 113:A05312, 2008) to the “horseshoe” shapes on a time scale of within the few collision times. We also show how the transport properties evolve in a similar fashion, from oscillating to a non-oscillating features over the same time interval.

Prospects for observing the magnetorotational instability in the plasma Couette experiment

Many astrophysical disks, such as protoplanetary disks, are in a regime where non-ideal, plasma-specific magnetohydrodynamic (MHD) effects can significantly influence the behaviour of the magnetorotational instability (MRI). The possibility of studying these effects in the plasma Couette experiment (PCX) is discussed. An incompressible, dissipative global stability analysis is developed to include plasma-specific two-fluid effects and neutral collisions, which are inherently absent in analyses of Taylor–Couette flows (TCFs) in liquid metal experiments. It is shown that with boundary driven flows, a ion-neutral collision drag body force significantly affects the azimuthal velocity profile, thus limiting the flows to regime where the MRI is not present. Electrically driven flow (EDF) is proposed as an alternative body force flow drive in which the MRI can destabilize at more easily achievable plasma parameters. Scenarios for reaching MRI relevant parameter space and necessary hardware upgrades are described.

Potential around a dust grain in collisional plasma

The ion neutral collision can lead to interesting phenomena in dust charging, totally different from the expectations based on the traditional orbit motion limited theory. The potential around a dust grain is investigated for the collisional plasma considering the presence of ion neutral collisions. Fluid equations are solved for the one dimensional radial coordinate. It is observed that with the gradual increase in ion neutral collision, the potential structure around the dust grain changes its shape and is different from the usual Debye-Hückel potential. The shift however starts from a certain value of ion neutral collision and the electron-ion density varies accordingly. The potential variation is interesting and reconfirms the fact that there exists a region of attraction for negative charges. The collision modeling is done for the full range of plasma, i.e., considering the bulk and the sheath jointly. The potential variation with collision is also shown explicitly and the variation is found to cope up with the earlier observations.

Particle in cell simulation of a radiofrequency plasma jet expanding in vacuum

The effect of a pressure gradient (∼133 Pa–0.133 Pa) on electron and ion energy distributions in a radiofrequency (rf at 13.56 MHz) argon plasma jet is studied using a 1D-3v Particle In Cell (PIC) simulation. The PIC domain is three times that of the 0.018 m long plasma cavity and the total simulation time is 1 ms. Ion heating and acceleration up to a drift velocity about 2000 m s−1 are measured along the jet's main expansion axis. Elastic and charge exchange ion-neutral collisions histograms computed at equilibrium during 0.74 ms show that charge exchange collisions act as the main neutral heating mechanism.

Measurement of the force exerted on the surface of an object immersed in a plasma

An experiment is described in which the force exerted by a low-pressure rf plasma on one side of a test surface is measured. The test surface is part of a cubic object immersed in the plasma and is mounted on a pendulum whose small deviations are recorded by a digital camera. The force balance is calibrated by means of an electrostatic method. The measured forces on the surface (4.8 cm2) are in the micronewton range and increase with the plasma density, while an increased collisionality reduces the force. The measured forces are discussed on the basis of a simple model taking into account the momentum fluxes across the sheath edge. It is concluded that ion-neutral collisions in the presheath enhance the force caused by electron pressure and ion flux.