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The kinetics of O2− ions were numerically studied in He and Ar with a small admixture of O2 in a strong electric field through a Monte Carlo simulation. Cross sections for elastic scattering between O2− ions and He and Ar atoms were determined by adjusting cross section values to obtain good agreement between calculated and measured ion mobilities in pure helium and argon. Ion transport coefficients, mean energy, and detachment rate were calculated for various mole fractions of O2 and values of the reduced electric field. It was shown that the O2− ion mobility, mean energy, and detachment rate in He:O2 mixtures are much higher than those in Ar:O2 mixtures and air. A great difference between the properties of O2− ions in He–O2 and Ar–O2 mixtures was explained on the basis of the difference in the polarizability and mass for He and Ar atoms. Calculated detachment rates can be used for simulating guided ionization waves in He and Ar flows ejected into ambient air. [ABSTRACT FROM AUTHOR]

In this paper, the high-accuracy ion mobility coefficient based on the Chapman–Enskog approximation to the solution of the Boltzmann equation for low pressure radio frequency plasma discharges is presented. We employ two-dimensional fluid simulations of the argon filled axisymmetric reactor, where the effect of new ion-kinetics-based fluid closure is compared to theoretical expressions and experimental data. The spatial profiles of plasma composition in the low pressure radio frequency capacitively coupled plasma are presented, which includes the metastable reactions in the simulation. Moreover, inelastic collision integrals terms, due to charge exchange inelastic collisions between ions and neutral species, have been also considered. A Monte Carlo simulation of kinetic ion energy distribution of impinging on the radio frequency powered electrode provides a measure of accuracy of the new transport model. From our simulation, the results that mirror the influence of ion mobility coefficient obtained by the Chapman–Enskog method on plasma physical quantities under different pressures, frequencies, and electrode gaps is in good agreement with experimental measurement results and theoretical expressions. [ABSTRACT FROM AUTHOR]

A self-consistent method for computation and testing of transport coefficients in low pressure radio frequency plasma discharges is detailed by comparing the experimental fitting formula and previous results gained from the Langevin expression. In this paper, for the first time, we apply the Chapman–Enskog method for calculating transport coefficients of ions to a weakly ionized plasma which is laid in a thermodynamic non-equilibrium and chemical non-equilibrium state. The ion diffusion coefficient is obtained by using the Chapman–Enskog method expanded to the second order approximation. The corresponding ion mobility coefficient is deduced from Einstein's relation. A two-dimensional axisymmetric fluid model of capacitively coupled plasmas is employed to determine the plasma composition. Moreover, the comparison of plasma physical characteristics using ion transport coefficients calculated by the Chapman–Enskog method with those derived from theoretical and experimental methods is also investigated. The results show that the ion transport coefficients obtained by the Chapman–Enskog method and their effect on plasma physical quantities are in good agreement with the experimental measurement and theoretical expression. Furthermore, the results indicate that the set of exponential and Morse potentials is more reasonable for the ion–neutral particles' interaction. [ABSTRACT FROM AUTHOR]

Strong impacts of drifts on the divertor plasma in–out asymmetry and detachment are demonstrated in DIII-D with an open divertor configuration. For forward toroidal field, BT, i.e., with the ion B × ∇B drift toward the divertor, the particle flux to the inner divertor, as represented by the Langmuir probe measured ion saturation current (Jsat), exhibits a double peak structure, with electron temperature, lower at the inner target. Reversing the BT direction reverses both the radial and poloidal E × B flows, leading to a broad particle flux profile in the outboard scrape-off layer (SOL) with a similar double-peak structure to that observed at the inner target with forward BT. The correlation of a double peak structure with divertor temperature profiles confirms physical coupling between the drift flow and sheath boundary condition and their strong impact on divertor profiles. In addition, under reversed BT conditions, increasing the density flattens the target temperature profile. However, Jsat remains high away from the strike point, rendering it difficult to achieve an "effective" detached plasma, i.e., with effective reduction in both peak heat flux and peak temperature (in the far SOL). In contrast, divertor detachment with a cold and flat temperature profile can be achieved at both target plates with the forward BT. [ABSTRACT FROM AUTHOR]

Collisionless particle dynamics in a helical magnetic mirror is investigated by methods of the Hamilton dynamics. In the limit of small Larmor radius, the Hamiltonian is fully integrable and particle trajectories can be divided into three types, differing in the direction of the drift velocity. The structure of the trajectories of resonant particles and the width of the resonance are studied. The influence of weak axi-symmetrical corrugation on the particle dynamics and efficiency of plasma flow suppression through a helical mirror are discussed. [ABSTRACT FROM AUTHOR]

Ion temperature anisotropy in an expanding magnetized plasma is investigated using laser induced fluorescence. Parallel and perpendicular ion velocity distribution functions (IVDFs) were measured simultaneously with high spatial resolution in the expanding plasma. Large ion temperature anisotropies ( T ⊥ i / T ∥ i ∼ 10) are observed in a conical region at the periphery of the expanding plasma plume. A simple 2D Boris stepper model that incorporates the measured electric field structure is able to reproduce the gross features of the measured perpendicular IVDFs. A Nyquist stability analysis of the measured IVDFs suggests that multiple instabilities with k ⊥ ρ i ∼ 1 and k | | ρ i ∼ 0.2 are likely to be excited in these plasmas. [ABSTRACT FROM AUTHOR]

Magnetic cusps have been used for more than 50 years to limit charged particle losses to the walls and confine the plasma in a large variety of plasma sources or ion sources. Quantification of the effective loss area has been the subject of many experimental as well as theoretical investigations in the 1970s–1990s. In spite of this effort, there is no fully reliable expression of the effective wall loss as a function of cusp magnetic field, electron temperature, ion mass, gas pressure, etc. We describe in this paper a first attempt at obtaining scaling laws for the effective loss width of magnetic cusps based on two-dimensional particle-in-cell Monte Carlo collision simulations. The results show that the calculated leak width follows a 1/B scaling in the collisionless low B limit, is approximately proportional to the hybrid gyroradius with an ion velocity equal to the Bohm velocity, and is proportional to the square root of gas pressure in the collisional limit. [ABSTRACT FROM AUTHOR]

Spatially and temporally resolved ion flow measurements are performed inside the plasma source of an inductive radio frequency plasma thruster. Using the resulting data, the pure effects of the inductive current drive on the ion flow are identified. The cross field ion acceleration and the establishment of the cross field electric field are found in the upstream region, where the azimuthal current is induced by the superimposition of a time-varying magnetic field. Analyzing the electron and ion dynamics with two-fluid equations, the magnetized electrons form the in-plane Hall electric field to satisfy the electron force balance, which results in the electrostatic acceleration of unmagnetized ions. The enhanced density gradient forms a stronger Boltzmann electric field along the magnetic field. It generates a supersonic ion group along the magnetic field line, which increases the field-aligned ion flow velocity and the momentum thrust. [ABSTRACT FROM AUTHOR]

Inertial electrostatic confinement (IEC) is a method of confining and heating a plasma at benchtop scales to sufficient energies for nuclear fusion to occur. Ion velocity and flow direction were measured in an IEC discharge using laser induced fluorescence (LIF) on argon ions. A cathode of two parallel rings, with a common axis of symmetry, resulted in predominant discharge beams, otherwise known as microchannels, along this axis. The device was operated in the abnormal glow discharge regime where both current and voltage increase monotonically, replicating a conventional high voltage IEC device. It was found that argon ions accelerated and flowed outward from the midpoint between the rings along the axis; we have labeled this ion motion as being divergent. The predominant direction of ion flow in the discharge is opposite to the conventional ion focus model, where the discharge at the cathode center is assumed to be the result of ion flow toward it from outside of the cathode. An ion sheath model is shown to produce a virtual anode at the axial midpoint between the rings. The model also shows that ions within the virtual anode are accelerated outward with a spatial velocity profile that replicates those measured using LIF. [ABSTRACT FROM AUTHOR]

This paper overviews experimental and numerical results on acceleration of narrow energy spread ion beams by an electrostatic collisionless shockwave driven by 1 μm (Omega EP) and 10 μm (UCLA Neptune Laboratory) lasers in near critical density CH and He plasmas, respectively. Shock waves in CH targets produced high-energy ∼50 MeV protons (ΔE/E of ≤30%) and 314 MeV C6+ ions (ΔE/E of ≤10%). Observation of acceleration of both protons and carbon ions to similar velocities is consistent with reflection of particles off the moving potential of a shock front. For shocks driven by CO2 laser in a gas jet, ∼30 MeV peak in He ion spectrum was detected. Particle-in-cell simulations indicate that regardless of the target further control over its density profile is needed for optimization of accelerated ion beams in part of energy spread, yield and maximum kinetic energy. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The genesis of the ion axial velocity distribution function (VDF) is analyzed for collisionless Hall thruster discharges. An analytical form for the VDF is obtained from the Vlasov equation, by applying the Tonks–Langmuir theory in the thruster channel, under the simplifying assumptions of monoenergetic creation of ions and steady state. The equivalent set of 1D unsteady anisotropic moment equations is derived from the Vlasov equation, and simple phenomenological closures are formulated, assuming a polynomial shape for the ion VDF. The analytical results and the anisotropic moment equations are compared to collisionless particle-in-cell simulations, employing either a zero heat flux (Euler-like equations) or the polynomial-VDF closure for the heat flux. The analytical ion VDF and its moments are then compared to experimental measurements. [ABSTRACT FROM AUTHOR]

The Enhanced Pedestal (EP) H-mode regime is an attractive wide-pedestal high-βp scenario for the National Spherical Torus Experiment Upgrade (NSTX-U) and next-step devices as it achieves enhanced energy confinement (H98y,2 > 1.5), large normalized pressure (βN > 5), and significant bootstrap fraction (fBS > 0.6) at Ip/BT = 2 MA/T. This regime is realized when the edge ion collisionality becomes sufficiently small that a positive feedback interaction occurs between a reduction in the ion neoclassical energy transport and an increase in the particle transport from pressure-driven edge instabilities. The EP H-mode was most often observed as a transition following a large edge-localized mode in conditions with low edge neutral recycling. It is hypothesized that the onset of pressure-driven instabilities prior to the full recovery of the neutral density leads to a temporary period with an elevated ion temperature gradient that triggers the transition to EP H-mode. Linear CGYRO and M3D-C1 calculations are compared to beam emission spectroscopy and magnetic spectroscopy in order to describe the evolution of the edge particle transport mechanisms during the ELM recovery and the saturated EP H-mode state. The observations are consistent with the hypothesis that the onset of pressure-driven edge instabilities, such as the kinetic ballooning mode and kink-peeling mode, can be responsible for the increased particle transport in EP H-mode. [ABSTRACT FROM AUTHOR]

The physical processes taking place at the separatrix and scrape-off layer regions are crucial for the operation of tokamaks as they govern the interaction of hot plasma with the vessel walls. Numerical modeling of the edge with state-of-the-art codes attempts to elucidate the complex interactions between neoclassical drifts, turbulence, poloidal, and parallel flows that control the physical set-up of the SOL region. Here, we present the post-processing analysis of simulation results from the gyrokinetic code XGC1, comparing and contrasting edge turbulence characteristics from a simulation of the DIII-D tokamak against a simulation of the Alcator C-Mod tokamak. We find that the equilibrium E × B flux across the separatrix has a similar poloidal pattern in both discharges, which can be explained by ∇ B -drifts and trapped ion excursions. However, collisionality is noted to play a major role in the way that it prevents local charge accumulations from having more global effects in the C-Mod case. In both cases, turbulent electron heat flux is observed to be higher than the ion one and is possibly related to the need of electrons to maintain quasineutrality through the only channel available to them for exiting the confinement. By Fourier analysis, we identify turbulent frequencies and growth rates of the dominant mode in both simulations. In the case of C-Mod, these numbers point to the presence of a drift wave. In the DIII-D case, further linear simulations with the Gene code reveal a trapped electron mode. Furthermore, using a blob detection and tracking tool, we present the amplitude and size distributions of the blobs from both simulations. The amplitude distributions are in qualitative agreement with experimental observations, while the size distributions are consistent with the fact that most of the blobs are not connecting to the divertor plates and suggest that they are generated by the shearing of the turbulent modes. [ABSTRACT FROM AUTHOR]

The present particle-in-cell simulation for grain–plasma interaction at cryogenic temperatures springs from recent experimental realization of ultracold dusty plasmas with atoms and ions at superfluid helium temperatures. In this work, we discuss the results of particle-in-cell simulations (taking into account ion-atom collisions) for dust particle charge, ion drag force, and interaction between grains in ultracold dusty plasmas. The single grain as well as two grain system is investigated, considering both streaming ions and equilibrium ions. The dependence of plasma mediated inter-grain interactions on the streaming velocity of ions and inter-particle separation is delineated in detail. Additionally, ion density distribution around grains is discussed. The interactions at cryogenic temperatures are distinct from those at room temperature by their differences in magnitude and interaction mechanism details. Most remarkable features are (i) the inter-grain attraction in equilibrium plasmas due to plasma absorption on the surface of grains, (ii) the ion drag force directed against ion streaming velocity in the case of a single dust particle, and (iii) non-linear response of the plasma polarization around grains with an increase in the streaming velocity of ions. [ABSTRACT FROM AUTHOR]

A molecular dynamics simulation of ion flow past dust grains is used to investigate the interaction between a pair of charged dust particles and streaming ions. The charging and dynamics of the grains are coupled and derived from the ion–dust interactions, allowing for detailed analysis of the ion wakefield structure and wakefield-mediated interaction as the dust particles change position. When a downstream grain oscillates vertically within the wake, it decharges by up to 30% as it approaches the upstream grain and then recharges as it recedes. There is an apparent hysteresis in charging depending on whether the grain is approaching or receding from a region of higher ion density. Maps of the ion-mediated dust–dust interaction force show that the radial extent of the wake region, which provides an attractive restoring force on the downstream particle, increases as the ion flow velocity decreases, though the restoring effect becomes weaker. As also shown in recent numerical results, there is no net attractive vertical force between the two grains. Instead, the reduced ion drag on the downstream particle allows it to "draft" in the wakefield of the upstream particle. [ABSTRACT FROM AUTHOR]

The kinematic lower bound for the single scattering of neutrons produced in deuterium-tritium (DT) fusion reactions produces a backscatter edge in the measured neutron spectrum. The energy spectrum of backscattered neutrons is dependent on the scattering ion velocity distribution. As the neutrons preferentially scatter in the densest regions of the capsule, the neutron backscatter edge presents a unique measurement of the hydrodynamic conditions in the dense DT fuel. It is shown that the spectral shape of the edge is determined by the scattering rate weighted fluid velocity and temperature of the dense DT fuel layer during neutron production. In order to fit the neutron spectrum, a model for the various backgrounds around the backscatter edge is developed and tested on synthetic data produced from hydrodynamic simulations of OMEGA implosions. It is determined that the analysis could be utilized on current inertial confinement fusion experiments in order to measure the dense fuel properties. [ABSTRACT FROM AUTHOR]

The characterization of an electric propulsion device cathode is performed in the so-called diode configuration with an external anode. The anode acts as a physical boundary for the cathode plasma discharge; therefore, it influences cathode operation and performances. In this study, four different anodes—namely, a disk, a plate, a long cylinder, and a short cylinder—have been used with a flat disk LaB6 emitter 5 A-class cathode to examine the anode geometry impact on cathode discharge properties. Current–voltage curves, discharge oscillations, electron parameters, and ion velocities have been measured for currents in the 2 A to 12 A range and xenon mass flow rates varied from 0.4 mg/s to 1 mg/s with a fixed cathode-to-anode distance. The set of results clearly supports the fact that the anode geometry strongly influences the cathode characteristics both at the macroscopic and the microscopic scale. [ABSTRACT FROM AUTHOR]

Geodesic modes are typically excited by a minor concentration of energetic ions, but unstable mode frequencies are substantially different from Geodesic Acoustic Modes (GAMs) and are named EGAM (Energetic particle GAM). The EGAM instability driven by Neutral Beam Injection (NBI) has been observed in DIII-D tokamak experiments. The problem of the geodesic mode instability is analytically studied using a full drift kinetic equation. To analyze the instability condition, an ionization NBI location is assumed to be on the high field side of tokamaks. A minority NBI ion distribution is modeled by an energetic ion tail in the untrapped-passing region that remains between a magnetic axis and the trapped NBI boundary. The EGAM instability condition is defined by the parallel NBI ion velocity v | | ≈ (1.2 − 1.5) ω R 0 q 0 that has to be above the effective EGAM phase velocity. In this case, the EGAM frequency is ≈ 50 % below the standard stable GAM frequency, which is reduced by a small concentration of energetic NBI ions. Qualitative comparison of the developed geodesic mode theory with NBI heating experiments in the midregion of the tokamak plasma is discussed. [ABSTRACT FROM AUTHOR]

In this paper, we present ideas that were part of the miniconference on the crossover between High Energy Density Plasmas (HEDP) and Ultracold Neutral Plasmas (UNPs) at the 60th Annual Meeting of the American Physical Society Division of Plasma Physics, November 2018. We give an overview of UNP experiments with an emphasis on measurements of the time-evolving ion density and velocity distributions, the electron-ion thermalization rate, and plasma self-assembly—all just inside the strongly coupled plasma regime. We also present theoretical and computational models that were developed to understand a subset of HEDP experiments. However, because HEDP experiments display similar degrees of strong coupling, many aspects of these models can be vetted using precision studies of UNPs. This comparison is important because some statistical assumptions used for ideal plasmas are of questionable validity in the strongly coupled plasma regime. We summarize two theoretical approaches that extend kinetic theories into the strong-coupling regime and show good agreement for momentum transfer and self-diffusion. As capabilities improve, both computationally and experimentally, UNP measurements may help guide the ongoing development of HEDP-appropriate plasma models. Future opportunities in viscosity, energy relaxation, and magnetized plasmas are discussed. [ABSTRACT FROM AUTHOR]

We present gyrokinetic simulations with the GENE code addressing the near-edge region of an L-mode plasma in the DIII-D tokamak. At radial position ρ = 0.80, simulations with the ion temperature gradient (ITG) increased by 40% above the nominal value give electron and ion heat fluxes that are in simultaneous agreement with the experiment. This gradient increase is consistent with the combined statistical and systematic uncertainty σ of the charge exchange recombination spectroscopy measurements at the 1.6σ level. Multiscale simulations are carried out with a realistic mass ratio and geometry for the first time in the near-edge. These multiscale simulations suggest that the highly unstable ion temperature gradient (ITG) modes of the flux-matched ion-scale simulations suppress electron-scale transport, such that ion-scale simulations are sufficient at this location. At radial position ρ = 0.90, nonlinear simulations show a hybrid state of ITG and trapped electron modes, which was not expected from linear simulations. The nonlinear simulations reproduce the total experimental heat flux with the inclusion of E × B shear effects and an increase in the electron temperature gradient by ∼23%. This gradient increase is compatible with the combined statistical and systematic uncertainty of the Thomson scattering data at the 1.3σ level. These results are consistent with previous findings that gyrokinetic simulations are able to reproduce the experimental heat fluxes by varying input parameters close to their experimental uncertainties, pushing the validation frontier closer to the edge region. [ABSTRACT FROM AUTHOR]

The gyrokinetic toroidal code has been upgraded for global simulations by coupling the core and scrape-off layer regions across the separatrix with field-aligned particle-grid interpolations. A fully kinetic particle pusher for high frequency waves (ion cyclotron frequency and beyond) and a guiding center pusher for low frequency waves have been implemented using cylindrical coordinates in a global toroidal geometry. The two integrators correctly capture the particle orbits and agree well with each other, conserving energy and canonical angular momentum. As a verification and application of this new capability, ion guiding center simulations have been carried out to study ion orbit losses at the edge of the DIII-D tokamak for single null magnetic separatrix discharges. The ion loss conditions are examined as a function of the pitch angle for cases without and with a radial electric field. The simulations show good agreement with past theoretical results and with the experimentally observed feature in which high energy ions flow out along the ion drift orbits and then hit the divertor plates. A measure of the ion direct orbit loss fraction shows that the loss fraction increases with the ion energy for DIII-D in the initial velocity space. Finally, as a further verification of the capability of the new code, self-consistent simulations of zonal flows in the core region of the DIII-D tokamak were carried out. All DIII-D simulations were performed in the absence of turbulence. [ABSTRACT FROM AUTHOR]

Using 2.5 dimensional kinetic particle-in-cell simulations, we simulate reconnection conditions appropriate for the magnetosheath and solar wind, i.e., plasma beta (ratio of gas pressure to magnetic pressure) greater than 1 and low magnetic shear (strong guide field). Changing the simulation domain size, we find that the ion response varies greatly. For reconnecting regions with scales comparable to the ion inertial length, the ions do not respond to the reconnection dynamics leading to "electron-only" reconnection with very large quasisteady reconnection rates. Note that in these simulations, the ion Larmor radius is comparable to the ion inertial length. The transition to a more traditional "ion-coupled" reconnection is gradual as the reconnection domain size increases, with the ions becoming frozen-in in the exhaust when the magnetic island width in the normal direction reaches many ion inertial lengths. During this transition, the quasisteady reconnection rate decreases until the ions are fully coupled, ultimately reaching an asymptotic value. The scaling of the ion outflow velocity with the exhaust width during this electron-only to ion-coupled transition is found to be consistent with a theoretical model of a newly reconnected field line. In order to have a fully frozen-in ion exhaust with ion flows comparable to the reconnection Alfvén speed, an exhaust width of at least several ion inertial lengths is needed. In turbulent systems with reconnection occurring between magnetic bubbles associated with fluctuations, using geometric arguments, we estimate that fully ion-coupled reconnection requires magnetic bubble length scales of at least several tens of ion inertial lengths. [ABSTRACT FROM AUTHOR]

The relaxation of an ion-electron plasma due to collisions, in a uniform background magnetic field, is studied. Based on the recently found ion velocity transfer by a binary collision, the diffusion coefficients are derived. The Fokker-Planck equation is treated in the framework of the diffusion approximation, from which the drag term is extracted. The diffusion and drag coefficients are written appropriately in parallel and perpendicular components, relative to the direction of the magnetic field. Then, the relaxation times of the ion parallel and perpendicular temperatures are calculated. Finally, the energy loss rate of a test ion moving in an electron plasma is obtained, and a specific case in the conditions of the ALPHA experiment is presented. [ABSTRACT FROM AUTHOR]

New features and peculiarities of anomalous self-sustained Hall E × B discharge have been discovered. The presence of a minimum density of ions n upon the magnetic field induction B is confirmed for a wide range of discharge parameters. Likewise, the operation mode for a maximum density at the optimal values of the radial and longitudinal components of magnetic induction was discovered. Both effects are accompanied by the changes in the position of the combustion zone E × B of the discharge in the anode-cathode interval and the transformation of the distribution function of the ion energy. The threshold nature of the processes in the plasma E × B of the discharge can be manifested on the curves n = f(B) as an abrupt 3 to 4 times decrease in density upon the increase in B by not more than 10%. An abrupt increase in the density of ions (up to 16 times), occurring with a slight increase in the density of neutrals (∼1.2 times), is recorded when there are jumps of the anode layer from the anode region to the cathode one and vice versa. A thin structure of ion energy spectra explained by the isomagnetic potential jumps, which generate ion density jumps in a narrow energy range, was found. In the search for the reasons for the restructuring of the discharge and generation of isomagnetic jumps, the possible effect of nonlocal gradient-drift and electron-cyclotron drift instabilities on electron mobility and ion acceleration is briefly discussed. [ABSTRACT FROM AUTHOR]

We use charge exchange recombination spectroscopy to make the first localized measurements of impurity ion flow velocity profiles in the reversed field pinch. Measurements in improved confinement plasmas reveal an intrinsic flow profile that is peaked on the axis and mostly parallel to the equilibrium magnetic field. The toroidal flow decreases in time at off-axis locations where tearing modes are resonant, giving rise to a highly sheared flow profile near the axis. The tearing mode phase velocity correlates strongly with toroidal flow near the resonant surface and weakly with flow in other locations, providing an opportunity to verify the commonly held assumption that the plasma and mode move together at the resonant surface. Mechanisms for the observed momentum loss during the improved confinement period are evaluated, and it is found that eddy currents in the conducting shell caused by the rotation of the dominant tearing mode dominate over other losses. [ABSTRACT FROM AUTHOR]

Formation of a plasma sheath is studied in collisional and magnetized low pressure plasma. The magnetic field has a tilted configuration. The model provides plasma profiles for a wide range of magnetic field strengths and tilting angles. It is shown that the sheath entry velocity in the magnetized case should be chosen for all three components of ion velocity rather than a single component (which is directed along the sheath). It is found in the collisionless case that the net magnitude of the ion velocity gains the characteristic ion speed (Bohm speed). This, in turn, is found to retain the monotonic variation of the ion density. Ionization is considered with constant ionizing frequency. Comments are made on the sheath thickness and the retention of the Debye sheath at a low angle, for low pressure magnetized plasma. [ABSTRACT FROM AUTHOR]

We perform fluid simulations to examine the effect of ion thermal velocity on the formation and dynamics of solitary waves in an unmagnetized two-component plasma consisting of ions and electrons. Based on the linear and nonlinear fluid theories, some of the previous studies have reported that the plasma with the electron temperature greater than the ion temperature (i.e., Te > Ti) supports ion acoustic solitary waves (IASWs), whereas the plasma with Te ≪ Ti supports electron acoustic waves (EASWs). In this paper, we have considered a wide range of ion temperatures (with fixed electron temperature) to examine the criteria of temperature and thermal velocities in the generation of EASWs and IASWs in plasmas. Our simulation shows that the plasma with Ti > Te possesses two wave modes depending on the ratio of its thermal velocities. When the ratio of electron to ion thermal velocities R = Vthe/Vthi > 1, the system supports the generation of IASWs, whereas for R < 1, it supports the generation of EASWs. The analysis of characteristics like the amplitude, width, and phase speed of these solitary waves implies that the EASWs have a negative potential, whereas the IASWs have the positive potential. The transition from IASWs to EASWs occurs when the phase speed of the solitary wave exceeds the limiting value of 3 V the . This simulation study presents the detailed investigation of the evolution of EASWs and IASWs generated in plasmas having Ti > Te, which will have implications in modeling such waves in space and laboratory plasmas. [ABSTRACT FROM AUTHOR]

Collisional kinetic modifications of ion distributions in interpenetrating flows are investigated by irradiating two opposing targets, either CD/CD or CD/CH, on the National Ignition Facility. In the CD/CD case, neutron time-of-flight diagnostics are successfully used to infer the ion temperature, 5–6 keV, and velocity, 500 km/s per flow, of the flows using a multi-fluid approximation of beam-beam nuclear fusion. These values are found to be in agreement with simulations and other diagnostics. However, for CD/CH, the multi-fluid assumption breaks down, as fusion is quasi-thermonuclear in this case and thus more dependent on the details of the ion velocity distribution. Using kinetic-ion, hydrodynamic-electron, and hybrid particle-in-cell modeling, this is found to be partially due to a skewed deviation from a Maxwellian in the ion velocity distribution function resulting from ion-ion collisions. This skew causes a downshift in the mean neutron velocity that partially resolves the observation in the CD/CH case. We note that the discrepancy is not completely resolved via collisional effects alone and may be a signature of collisionless electromagnetic interactions such as the Weibel-filamentation instability. [ABSTRACT FROM AUTHOR]

Argon and xenon ion velocity distribution functions are measured in Ar-He, Ar-Xe, and Xe-He expanding helicon plasmas to determine if ion beam velocity is enhanced by the presence of lighter ions. Contrary to observations in mixed gas sheath experiments, we find that adding a lighter ion does not increase the ion beam speed. The predominant effect is a reduction of ion beam velocity consistent with increased drag arising from increased gas pressure under all conditions: constant total gas pressure, equal plasma densities of different ions, and very different plasma densities of different ions. These results suggest that the physics responsible for the acceleration of multiple ion species in simple sheaths is not responsible for the ion acceleration observed in expanding helicon plasmas. [ABSTRACT FROM AUTHOR]

The integration of kinetic effects in fluid models is important for global simulations of the Earth's magnetosphere. In particular, it has been shown that ion kinetics play a crucial role in the dynamics of large reconnecting systems, and that higher-order fluid moment models can account for some of these effects. Here, we use a ten-moment model for electrons and ions, which includes the off diagonal elements of the pressure tensor that are important for magnetic reconnection. Kinetic effects are recovered by using a nonlocal heat flux closure, which approximates linear Landau damping in the fluid framework. The closure is tested using the island coalescence problem, which is sensitive to ion dynamics. We demonstrate that the nonlocal closure is able to self-consistently reproduce the structure of the ion diffusion region, pressure tensor, and ion velocity without the need for finetuning of relaxation coefficients present in earlier models. [ABSTRACT FROM AUTHOR]

The drift velocities of ions in a constant homogeneous electric field are calculated using Monte Carlo simulations for noble-gas and some metal vapors. The ion mobility is analyzed as a function of the field strength and gas temperature. A general approximate formula for the dependence of the drift velocity on the reduced field and gas temperature is derived. The results of calculations of kinetic characteristics of ions crossing the surface of the target are presented. The authors focus on the angular and energy distributions of ions and differences between the distributions of the average volume and the average flow on the surface. [ABSTRACT FROM AUTHOR]

In order to directly compare phase spaces obtained at different energies, particle positions and momenta have been rescaled with (2/ )1/4( x,0/ k p)1/2 and ( /2)1/4(k p x,0)1/2, respectively. Erratum: "Adiabatic matching of particle bunches in a plasma-based accelerator in the presence of ion motion" [Phys. Witness bunch density distribution at injection ( i m c 2=50 MeV, m c 2=0 GeV) (a) and after a m c 2=10 GeV energy gain with adiabatic matching (b). [Extracted from the article]

The self-consistent 1D kinetic Harris-like model of a collisionless current sheet is developed for the case of the current sheet experiencing the impact of an external uniform gravity field. The ambipolar Pannekoek-Rosseland electric field appears in the system as a result of the additional drift motion of ions and electrons. This produces separation of charges, which is responsible for corresponding changes of the current sheet form. The presence of gravitation leads to formation of asymmetric distributions of the magnetic field as well as the plasma and the current density changes. Our estimations show that gravity-forced disruptions of the current sheet profile may occur in the Mercurial magnetosphere and, most probable, in the Io plasma torus near the Jupiter. Also, the model can be applied to magnetospheres of exoplanets. [ABSTRACT FROM AUTHOR]

The first simulations employing a kinetic treatment of both fuel and shell ions to model inertial confinement fusion experiments are presented, including results showing the importance of kinetic physics processes in altering fusion burn. A pair of direct drive capsule implosions performed at the OMEGA facility with two different gas fills of deuterium, tritium, and helium-3 are analyzed. During implosion shock convergence, highly non-Maxwellian ion velocity distributions and separations in the density and temperature amongst the ion species are observed. Diffusion of fuel into the capsule shell is identified as a principal process that degrades fusion burn performance. [ABSTRACT FROM AUTHOR]

We present a model for electron transport across the magnetic field of a Hall thruster and integrate this model into 2-D hybrid particle-in-cell simulations. The model is based on a simple scaling of the turbulent electron energy dissipation rate and the assumption that this dissipation results in Ohmic heating. Implementing the model into 2-D hybrid simulations is straightforward and leverages the existing framework for solving the electron fluid equations. The model recovers the axial variation in the mobility seen in experiments, predicting the generation of a transport barrier which anchors the region of plasma acceleration. The predicted xenon neutral and ion velocities are found to be in good agreement with laser-induced fluorescence measurements. [ABSTRACT FROM AUTHOR]

Radial transport in low temperature annular plasmas is investigated theoretically in this paper. The electrons are assumed to be in quasi-equilibrium due to their high temperature and light inertial mass. The ions are not in equilibrium and their transport is analyzed in three different situations: a low electric field (LEF) model, an intermediate electric field (IEF) model, and a high electric field (HEF) model. The universal IEF model smoothly connects the LEF and HEF models at their respective electric field strength limits and gives more accurate results of the ion mobility coefficient and effective ion temperature over the entire electric field strength range. Annular modelling is applied to an argon plasma and numerical results of the density peak position, the annular boundary loss coefficient and the electron temperature are given as functions of the annular geometry ratio and Paschen number. [ABSTRACT FROM AUTHOR]

We performed kinetic Vlasov simulations of the plasma-wall transition for a low-pressure argon discharge without external magnetic fields, using the same plasma parameters as in the experiments of Claire et al. [Phys. Plasmas 13, 062103 (2006)]. Experimentally, it was found that the ion velocity distribution function is highly asymmetric in the presheath, but, surprisingly, becomes again close to Maxwellian inside the sheath. Here, we show that this "thermalization" can be explained by purely collisionless effects that are akin to the velocity bunching phenomenon observed in charged particles beams. Such collisionless thermalization is also observed in the presheath region close to the sheath entrance, although it is much weaker there and in practice probably swamped by collisional processes (standard or enhanced by instabilities). [ABSTRACT FROM AUTHOR]

This paper presents the experimental measurement and numerical analysis of the current-voltage characteristics of dc corona discharges in air between coaxial cylinders. The current-voltage characteristics for both positive and negative corona discharges were measured within a specially designed corona cage. Then the measured results were fitted by different empirical formulae and analyzed by the fluid model. The current-voltage characteristics between coaxial cylinders can be expressed as I = C(U - U0)m, where m is within the range 1.5-2.0, which is similar to the point-plane electrode system. The ionization region has no significant effect on the current-voltage characteristic under a low corona current, while it will affect the distribution for the negative corona under a high corona current. The surface onset fields and ion mobilities were emphatically discussed. [ABSTRACT FROM AUTHOR]

Dynamics of coupled electrostatic electron and ion nonlinear oscillations in a collisionless plasma is studied with reference to a kinetic description. Proceeding from the exact solution of Vlasov-Maxwell equations written as a function of linear functions in the electron and ion velocities, we arrive at the two coupled nonlinear equations which describe the evolution of the system. [ABSTRACT FROM AUTHOR]

Symmetric and asymmetric capacitively coupled radio-frequency plasmas in oxygen at 40 Pa, 300 V voltage amplitude and a discharge gap of 40 mm are investigated by means of one-dimensional numerical semi-kinetic fluid modeling on the basis of a simplified reaction scheme including the dominant positive and negative ions, background gas, and electrons. An improved treatment, by accounting for the dependence of ion mobilities on E/N, is compared to the standard approach, based on using zero-field mobility values only. The charged particle dynamics as a result of direct electron impact ionization of oxygen, secondary electron release from the electrodes, the spatial distribution of all involved particles as well as impact of geometry and model modification on ion energies is analyzed and compared to independent simulations and experiments. [ABSTRACT FROM AUTHOR]

In a recent paper [Phys. Plasmas 25, 061203 (2018)], the authors have presented the analysis of the electric ion drift velocity experienced by heavy ions created in a plasma submitted to a low magnetic field. Unfortunately, they have used the classical E × B drift formula that is valid only in slab geometry. The authors have not taken into account that the cylindrical geometry induces a slow electric drift of the ions around the axis of the column. Moreover, the low magnetization of the ions induces a Larmor radius that is larger than the diameter of the plasma column. The movement of the ions immediately after their creation is parallel to the local electric field, not perpendicular as indicated by the authors. Most often, the ions are neutralized before experiencing the electric drift calculated along the classical guiding center theory. This has not been taken into account carefully by the authors so that the theoretical analysis of the Laser Induced Fluorescence measurements presented in this paper is clearly invalid. [ABSTRACT FROM AUTHOR]

We thank Th. Pierre for his comments. The aim of our essentially experimental paper was to trigger new discussions about the comparison between m = 1 , m = 2 instabilities in linear magnetized plasma. We are glad that this is the case. [ABSTRACT FROM AUTHOR]