Finite Ion Temperature Research Articles

Coherence of turbulent structures with varying drive in stellarator edge plasmas

This work investigates the parallel coherence of plasma filaments through numerical simulations using the hot-ion two-fluid hermes-2 model within the BOUT++ framework. Realistic field lines in the scrape-off layer (SOL) of magnetic fusion devices, especially in stellarator configurations possess a highly varying curvature along the magnetic field line. A varying curvature creates a parallel velocity gradient which might tear the filament apart. The main parameters controlling this process are the collisionality and the electron plasma beta. Simulations of realistic curvature variations along field lines in a circular ASDEX Upgrade-like tokamak and Wendelstein 7-X stellarator (W7-X) show the parallel displacement between different filament sections to correlate with the curvature. The rapidly varying W7-X curvature and the low average curvature drive reduce the propagation of the filament to only a few hundred meters per second. The effect of a finite ion temperature on filament propagation in a W7-X field line geometry is found to be a higher diamagnetic current resulting in stronger charge separation. This work supports simulations and experimental findings that filaments in W7-X are comparably slow due to the large major radius of the device. They do not perform ballistic motion and hence do not drive significant turbulence spreading in the SOL.

Study of magnetized multi-component plasma sheath containing charged dust particles in presence of oblique magnetic field: a fluid approach

The dynamics of low-temperature magnetized multi-component dusty plasma sheath structures have been investigated with finite ion temperature in presence of an oblique magnetic field using the one-dimensional multi-fluid model. The parametric changes inside the sheath are estimated in presence of charged dust species having nano-meter (nm) sizes. In presence of charged dust inside the sheath, the ions are found to get accumulated near the sheath edge, hence the ion density is decreased towards the wall. Further, with the increase in magnetic field strength, the peaking of ion densities near the sheath edge has been found to be intensified. The magnetic field orientation has also played a crucial role in the bunching of the ions near the sheath edge. An increase in the magnetic field obliqueness has also contributed to intensifying the ion bunching. It has also been observed that the sheath potential is substantially changed. In addition, we also investigated and presented the influence of dust species presence on the electron density inside the sheath. A qualitative explanation of the phenomenon that occurs due to the presence of dust species is presented.

Ion kinetic effects and instabilities in the plasma flow in the magnetic mirror

Kinetic effects in plasma flow due to a finite ion temperature and ion reflections in a converging–diverging magnetic nozzle are investigated with collisionless quasineutral hybrid simulations with kinetic ions and isothermal Boltzmann electrons. It is shown that in the cold ions limit, the velocity profile of the particles agrees well with the analytical theory, predicting the formation of the global accelerating potential due to the magnetic mirror with the maximum of the magnetic field and resulting in the transonic ion velocity profile. The global transonic ion velocity profile is also obtained for warm ions with isotropic and anisotropic distributions. Partial ion reflections are observed due to a combined effect of the magnetic mirror and time-dependent fluctuations of the potential as a result of the wave breaking and instabilities in the regions when the fluid solutions become multi-valued. Despite partial reflections, the flow of the passing ions still follows the global accelerating profile defined by the magnetic field profile. In simulations with reflecting boundary condition imitating the plasma source and allowing the transitions between trapped and passing ions, the global nature of the transonic accelerating solution is revealed as a constrain on the plasma exhaust velocity that ultimately defines plasma density in the source region.

On the electron sheath theory and its applications in plasma–surface interactions

In this work, an improved understanding of electron sheath theory is provided using both fluid and kinetic approaches while elaborating on their implications for plasma–surface interactions. A fluid model is proposed considering the electron presheath structure, avoiding the singularity in electron sheath Child–Langmuir law which overestimates the sheath potential. Subsequently, a kinetic model of electron sheath is established, showing considerably different sheath profiles in respect to the fluid model due to non-Maxwellian electron velocity distribution function and finite ion temperature. The kinetic model is then further generalized and involves a more realistic truncated ion velocity distribution function. It is demonstrated that such a distribution function yields a super-thermal electron sheath whose entering velocity at the sheath edge is greater than the Bohm criterion prediction. Furthermore, an attempt is made to describe the electron presheath–sheath coupling within the kinetic framework, showing a necessary compromise between a realistic sheath entrance and the inclusion of kinetic effects. Finally, the secondary electron emissions induced by sheath-accelerated plasma electrons in an electron sheath are analysed and the influence of backscattering is discussed.

Ion temperature effects on plasma flow in the magnetic mirror configuration

Effects of finite ion temperature on the plasma flow in the converging–diverging magnetic field, the magnetic mirror, or equivalently, magnetic nozzle configuration are studied using a quasineutral paraxial two-fluid MHD model with isothermal electrons and warm magnetized ions. The ion acceleration was studied with an emphasis on the role of the singularity at the sonic point transition. It is shown that the regularity of the sonic point defines a global solution describing plasma acceleration from subsonic to supersonic velocity. Stationary accelerating solutions were obtained and compared with the time dependent dynamics, confirming that the solutions of the time-dependent equations converge to the stationary solutions and, therefore, are stable. The effects of the ion pressure anisotropy were analyzed using the Chew–Golberger–Low model and its generalization. It is shown that the mirror force (manifested by the perpendicular ion pressure) enhances plasma acceleration. The role of ionization and charge exchange on plasma flow acceleration have been investigated.

Ion-acoustic shocklets in F-region of ionosphere with non-Maxwellian electrons

The evolution and propagation characteristics of ion-acoustic (IA) shocklets are examined in an unmagnetized collisionless plasma, comprised of warm ions and non-Maxwellian (Kappa/Cairns) electrons. Solving the fluid equations and using the frameworks of diagonalized-matrix/changing variable techniques, a nonlinear set of two characteristic wave equations is obtained, which admits dispersionless/dissipationless evolution of IA shocklets. Numerical analyses have also confirmed the excitation/deformation of IA shocklets and the wave overtaking/breaking effects at time τ>0 for nonlinear coupling of potential field. It is found that superthermal (Kappa distributed) electrons can enhance the overtaking/breaking of IA perturbations in comparison with nonthermal Cairns distributed electrons that lead to less steepened potential field to stabilize IA excitations against the nonlinear steepening. Finite ion temperature has significant impact on the wave coupling to lead the profiles of IA shocklets ahead. This study is important to understand the physical mechanism for excitation and deformation of IA shocklets relevant to the F-region of Ionosphere.

Raytracing Study of Source Regions of Whistler Mode Wave Power Distribution Relative to the Plasmapause

AbstractA comprehensive numerical raytracing study of whistler mode wave power with the inclusion of finite background electron and ion temperature is performed in order to investigate wave power distribution in relation to the plasmapause. Both Landau damping and linear growth of whistler mode waves are taken into account using a bi‐Maxwellian hot electron distribution as well as an isotropic hot electron distribution. Isotropic and bi‐Maxwellian distributions yield similar results of statistical spatial wave power for frequencies below 500 Hz. The effect of finite background temperature of ∼1 eV for electrons and ions are secondary in terms of the spatial distribution of whistler mode waves relative to the plasmapause. Three primary equatorial source locations at L=2, Lpp and L=5, corresponding to within the plasmasphere, at the plasmapause and outside the plasmapause, are investigated for MLT values of 00, 06, 12, and 18. At each location, waves are launched with a range of initial wave normal angles (−70° to 20°). The simulated wave power distributions are compared with observations from the EMFISIS instrument on Van Allen Probe A. Correspondence between the simulated distribution and the observations requires a weighting of the source regions. Results suggest that the majority of whistler mode power in the plasmasphere is sourced from within the plasmasphere itself and near the plasmapause. Only at noon (MLT 12) is wave power sourced primarily from at and outside the plasmapause.

Plasma-sheath transition in multi-fluid models with inertial terms under low pressure conditions: comparison with the classical and kinetic theory

We present high-order accuracy simulations of the plasma-sheath transition with an ion–electron multi-fluid model that considers the electron inertial terms and the ion pressure gradient. By means of a third-order accuracy time-dependent finite volume scheme, we solve the isothermal multi-fluid equations with realistic ion-to-electron mass ratios. We propose a numerical procedure and boundary conditions that retrieve a steady-state solution of a planar 1D floating sheath. The classical solution to this problem neglects the ion temperature since the model equations present a singularity for a finite ion temperature. The multi-fluid simulations provide a rigorous solution to the plasma-sheath transition without singularities. We compare our solution to the classical theory, finding perfect agreement when the ion-to-electron temperature tends to zero. We discuss the effect of the ion temperature, the electron inertia, and the elastic collisions with neutrals in low-pressure plasmas. Finally, relying on a kinetic approach, we derive analytical expressions for the electron macroscopic quantities inside a steady sheath that assumes a truncated Maxwellian electron velocity distribution function (EVDF) with a wall that collects the electron random flux. The derived analytical expressions are beyond the classical isothermal solution with Boltzmann electrons. The isothermal multi-fluid solution captures properly the first two moments of the EVDF inside the sheath. Our analytical expressions show the need for solving for higher-order moments to fully explain the electron physics inside the sheath, as opposite to the classical isothermal assumption. This work demonstrates that the multi-fluid simulations are able to capture the plasma-sheath transition under weakly collisional conditions with a solution that is consistent with the classical and the kinetic theory.

Nonlinear dynamics of laser-generated ion-plasma gratings: A unified description.

Laser-generated plasma gratings are dynamic optical elements for the manipulation of coherent light at high intensities, beyond the damage threshold of solid-state-based materials. Their formation, evolution, and final collapse require a detailed understanding. In this paper, we present a model to explain the nonlinear dynamics of high-amplitude plasma gratings in the spatially periodic ponderomotive potential generated by two identical counterpropagating lasers. Both fluid and kinetic aspects of the grating dynamics are analyzed. It is shown that the adiabatic electron compression plays a crucial role as the electron pressure may reflect the ions from the grating and induce the grating to break in an X-type manner. A single parameter is found to determine the behavior of the grating and distinguish three fundamentally different regimes for the ion dynamics: completely reflecting, partially reflecting or passing, and crossing. Criteria for saturation and lifetime of the grating as well as the effect of finite ion temperature are presented.

Characteristics of lower-hybrid surface waves

We have derived the dispersion relation for the lower-hybrid surface waves propagating perpendicularly to the magnetic field in the semi-bounded warm plasma. The mirror reflection boundary condition is adopted to obtain the surface mode of lower-hybrid plasma waves. The effects of magnetic-field strength and the finite ion temperature on the propagation of the lower-hybrid surface wave are investigated. The increase of ion temperature significantly increases the wave frequency, but an interesting hump of the group velocity appears in the region where the wavelength of the lower-hybrid surface wave is much larger than the electron Debye length. For cold ions, the surface wave is resonant near the lower-hybrid frequency as the wave number goes to infinity.

A detailed study on the structures of steady-state collisionless kinetic sheath near a dielectric wall with secondary electron emission. I. Classic sheath and its structure transition

It is well known that plasma sheath presents a classic sheath structure when the dielectric-wall total electron emission coefficient Γ ≤ Γc (<1). However, the structural transition of a classic sheath near a dielectric wall when Γ→Γc is controversial about transiting to a space-charge limited (SCL) sheath or inverse sheath. In this study, the classic sheath between a Maxwellian low-temperature plasma source and a dielectric surface that emits secondary electrons is carefully investigated using a 1D3V, steady-state, kinetic sheath model within a broad range of plasma electron temperatures Te. Using the Monte Carlo method to simulate secondary electron emission (SEE) events that are based on the self-consistent primary electron velocity distribution function at the wall and a detailed SEE model, it is found that the total emitted electron velocity distribution function (EEVDF) perpendicular to the dielectric wall approximately satisfies a three-temperature Maxwellian distribution. Due to the relatively high average energy of this total EEVDF, for cases of Te with cold plasma ion assumption: (1) the critical SCL sheath does not exist; (2) Γc reaches unit; (3) the sheath disappears when Te = Tec (i.e., Γ = 1); and (4) as Te increases, the classic sheath will transit to an inverse sheath structure. Further comparative calculations predict that the magnitude of emitted electrons' average energy may lead to different experimental results between thermionic emitting surfaces which have a “cold” half-Maxwellian EEVDF and SEE surfaces. However, when the finite plasma ion temperature is considered, at the transition point, the cold plasma ion assumption is expected to be invalid, and thus a fully kinetic sheath model should be built to reveal the potential new transition regime.

Removal of singularity in radial Langmuir probe models for non-zero ion temperature

We solve a radial theoretical model that describes the ion sheath around a cylindrical Langmuir probe with finite non-zero ion temperature in which singularity in an a priori unknown point prevents direct integration. The singularity appears naturally in fluid models when the velocity of the ions reaches the local ion speed of sound. The solutions are smooth and continuous and are valid from the plasma to the probe with no need for asymptotic matching. The solutions that we present are valid for any value of the positive ion to electron temperature ratio and for any constant polytropic coefficient. The model is numerically solved to obtain the electric potential and the ion population density profiles for any given positive ion current collected by the probe. The ion-current to probe-voltage characteristic curves and the Sonin plot are calculated in order to use the results of the model in plasma diagnosis. The proposed methodology is adaptable to other geometries and in the presence of other presheath mechanisms.

Magnetospheric whistler mode ray tracing in a warm background plasma with finite electron and ion temperature

AbstractWhistler mode waves play a major role in the energy dynamics of the Earth's magnetosphere. Numerical ray tracing has been used for many years to determine the propagation trajectories of whistler mode waves from various sources, both natural and anthropogenic. Previous work has been under the ideal cold plasma assumption even though temperatures of the background ions and electrons are in the range of several eV. We perform numerical ray tracing with the inclusion of finite electron and ion temperatures to more accurately model the plasma environment of the Earth's magnetosphere. Finite temperature effects are found to play a significant role in the whistler mode refractive index surface only when the wave frequency is near the lower hybrid resonance frequency and the wave normal angle is oblique. In such cases, the primary effect on whistler mode propagation is to lower the refractive index magnitude and increase the group velocity with slight modifications to the ray trajectories. Landau damping is shown to increase slightly with the inclusion of finite temperature.

E×B mean flows in finite ion temperature plasmas

The impact of ion pressure dynamics on E × B mean flows is investigated. Using a simplified, two-dimensional, drift ordered fluid model in the thin-layer approximation, three stresses in addition to the Reynolds stress are shown to modify the E × B mean flow. These additional terms in the stress tensor all require ion pressure fluctuations. Quasi-linear analysis shows that these additional stresses are as important as the Reynolds stress and hence must be taken into account in analysis of transport barriers in which sheared E × B mean flows are key ingredients.

Focusing characteristics of a 4 \u03c0parabolic mirror light-matter interface

BackgroundFocusing with a 4 π parabolic mirror allows for concentrating light from nearly the complete solid angle, whereas focusing with a single microscope objective limits the angle cone used for focusing to half solid angle at maximum. Increasing the solid angle by using deep parabolic mirrors comes at the cost of adding more complexity to the mirror’s fabrication process and might introduce errors that reduce the focusing quality.MethodsTo determine these errors, we experimentally examine the focusing properties of a 4 π parabolic mirror that was produced by single-point diamond turning. The properties are characterized with a single 174Yb + ion as a mobile point scatterer. The ion is trapped in a vacuum environment with a movable high optical access Paul trap.ResultsWe demonstrate an effective focal spot size of 209 nm in lateral and 551 nm in axial direction. Such tight focusing allows us to build an efficient light-matter interface.ConclusionOur findings agree with numerical simulations incorporating a finite ion temperature and interferometrically measured wavefront aberrations induced by the parabolic mirror. We point at further technological improvements and discuss the general scope of applications of a 4 π parabolic mirror.

Numerical simulations of blobs with ion dynamics

The transport of particles and energy into the scrape-off layer (SOL) region at the outboard midplane of medium-sized tokamaks, operating in low confinement mode, is investigated by applying the first-principle HESEL (hot edge-sol-electrostatic) model. HESEL is a four-field drift-fluid model including finite electron and ion temperature effects, drift wave dynamics on closed field lines, and sheath dynamics on open field lines. Particles and energy are mainly transported by intermittent blobs. Therefore, blobs have a significant influence on the corresponding profiles. The formation of a ‘shoulder’ in the SOL density profile can be obtained by increasing the collisionality or connection length, thus decreasing the efficiency of the SOL’s ability to remove plasma. As the ion pressure has a larger perpendicular but smaller parallel dissipation rate compared to the electron pressure, ion energy is transported far into the SOL. This implies that the ion temperature in the SOL exceeds the electron temperature by a factor of 2–4 and significantly broadens the power deposition profile.

Dust Charge Polarity Effect on Dust-Ion Acoustic Modulational Instability in a Nonthermal Dusty Plasma With Adiabatic Ions

The effects of dust charge polarity, ion temperature, and nonthermal electrons on dust-ion acoustic modulational instability and on the region (i.e., modulated wave number) for the formation of bright and dark envelope soliton structures are investigated. The dust particles are considered to be stationary and have positive and negative charges, while ions are taken dynamic and warm, and hot nonthermal electrons are assumed to be inertialess and kappa distributed in a dusty plasma. The Krylov–Bogoliubov–Mitropolsky method is used to derive the nonlinear Schrodinger equation (NLSE) for nonlinear amplitude modulation of dust-ion acoustic wave (DIAW) in plasmas. The dispersive and nonlinear coefficients of NLSE are obtained for DIAW, which depends on ion temperature, spectral indices of the nonthermal distributed electrons, dust density, and charge polarity of stationary dust particles. The modulationally stable and unstable regions of DIAW and its growth rate are investigated numerically. It is found that finite ion temperature, dust density, dust charge polarity, and spectral index of kappa for nonthermal electrons play a significant role to unstable the modulated DIAW modulated wave. The existence of positively and negatively charged dust particles and their observation in different regions of space plasma and in laboratory experiments is also pointed out.

Electrostatic fluctuations in a magnetized two-component plasma with q-nonextensive electrons and thermal ions

The effect of ion temperature on nonlinear low frequency electrostatic potential structures is studied in a two component magnetized plasma with q-nonextensive hot electrons. The standard Sagdeev pseudo-potential approach is used to investigate the existence of ion acoustic solitons and double layers and their dependence on finite ion temperature. Only solitons are found to exist, with the amplitude decreasing as the ion temperature increases. For parameters typical of the auroral plasma, our model yields a soliton electric field amplitude of 22 mV/m, which is within the range measured by satellites, and as such can be helpful in understanding the electrostatic fluctuations phenomena in space environments.

The GBS code for tokamak scrape-off layer simulations

We describe a new version of GBS, a 3D global, flux-driven plasma turbulence code to simulate the turbulent dynamics in the tokamak scrape-off layer (SOL), superseding the code presented by Ricci et al. (2012) [14]. The present work is driven by the objective of studying SOL turbulent dynamics in medium size tokamaks and beyond with a high-fidelity physics model. We emphasize an intertwining framework of improved physics models and the computational improvements that allow them. The model extensions include neutral atom physics, finite ion temperature, the addition of a closed field line region, and a non-Boussinesq treatment of the polarization drift. GBS has been completely refactored with the introduction of a 3-D Cartesian communicator and a scalable parallel multigrid solver. We report dramatically enhanced parallel scalability, with the possibility of treating electromagnetic fluctuations very efficiently. The method of manufactured solutions as a verification process has been carried out for this new code version, demonstrating the correct implementation of the physical model.

Ion temperature and gas pressure effects on the magnetized sheath dynamics during plasma immersion ion implantation

Here, a collisional magnetized plasma with finite ion temperature is considered to examine the effects of the ion temperature and gas pressure on the plasma-sheath dynamics. We use the two-fluid model of plasma-sheath where the nonlinear equations of a dynamic sheath are solved using a full implicit scheme of finite difference method along with some convenient initial and boundary conditions at the plasma center and target. It is found that the ion temperature only has a significant effect on the characteristics of low voltage sheath, while the gas pressure (collision rate) seriously affects the dynamic characteristics of the low and high voltage plasma-sheath. One can see, increasing the ion temperature in low voltage plasma-sheath causes to increase the temporal curve of the ion dose and the ion impact energy on the target, reduces the temporal curve of the sheath width, and has no any effect on the temporal curve of the ion incident angle on the target. However, rising the gas pressure in low and high voltage plasma-sheath reduces all of these temporal curves.