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International audience; This paper provides perspectives on recent progress in understanding the physics of devices in which the external magnetic field is applied perpendicular to the discharge current. This configuration generates a strong electric field that acts to accelerate ions. The many applications of this set up include generation of thrust for spacecraft propulsion and separation of species in plasma mass separation devices. These “E × B” plasmas are subject to plasma–wall interaction effects and to various micro- and macroinstabilities. In many devices we also observe the emergence of anomalous transport. This perspective presents the current understanding of the physics of these phenomena and state-of-the-art computational results, identifies critical questions, and suggests directions for future research.

We propose a new method of estimating the mean diameter and dispersion of clusters formed in a cooling gas, right after the nucleation stage. Using a moment model developed by Friedlander [S.K. Friedlander, Ann. N.Y. Acad. Sci. 354 (1983)], we derive an analytic relationship for both cluster diameter and diameter dispersion as a function of two of the characteristic times of the system - the cooling time and primary constituents collision time. These formulas can be used to predict diameter and dispersion variation with process parameters such as the initial monomer pressure or cooling rate. It is also possible to use them as an input to the coagulation stage, without the need to compute complex cluster generation during the nucleation burst. We compared our results with a nodal code and got excellent agreement., Comment: 10 pages, 11 figures

In order to study the properties of short carbon arcs, a self-consistent model was implemented into a CFD code ANSYS-CFX. The model treats the transport of heat and electric current in the plasma and electrodes in a coupled manner and accounts for gas convection in the chamber. Multiple surface processes at the electrodes are modeled, including the formation of space-charge limited sheaths, ablation and deposition of carbon, and emission and absorption of radiation and electrons. The simulations show that the arc is constricted near the cathode and anode front surfaces, leading to the formation of electrode spots. The cathode spot is a well-known phenomenon, and mechanisms of its formation were reported elsewhere. However, the anode spot formation mechanism discovered in this work was not previously reported. We conclude that the spot formation is not related to plasma instability, as commonly believed in the case of constricted discharge columns, but rather occurs due to the highly nonlinear nature of heat balance in the anode. We additionally demonstrate this property with a reduced anode heat transfer model. We also show that the spot size increases with the arc current. This anode spot behavior was also confirmed in our experiments. Due to the anode spot formation, a large gradient of carbon gas density occurs near the anode, which drives a portion of the ablated carbon back to the anode at its periphery. This can consequently reduce the total ablation rate. Simulation results also show that the arc can reach the local chemical equilibrium state in the column region, while the local thermal equilibrium state is not typically achieved for experimental conditions. It shows that it is important to account for different electron and gas temperatures in the modeling of short carbon arcs.

We study, by numerical and analytical means, the evolution of a uniform one-dimensional collisionless plasma initiated between plane absorbing walls. The ensuing flow is described by rarefaction waves that propagate symmetrically inward from the boundaries, interact, and eventually vanish after crossing through, leading up to the asymptotic phase., Comment: Submitted to journal

Ionization gas sensors are ubiquitous tools that can monitor desired gases or detect abnormalities in real time to protect the environment of living organisms or to maintain clean and/or safe environment in industries. The sensors’ working principle is based on the fingerprinting of the breakdown voltage of one or more target gases using nanostructured materials. Fundamentally, nanomaterial-based ionization-gas sensors operate within a large framework of gas breakdown physics; signifying that an overall understanding of the gas breakdown mechanism is a crucial factor in the technological development of ionization gas sensors. Moreover, many studies have revealed that physical properties of nanomaterials play decisive roles in the gas breakdown physics and the performance of plasma-based gas sensors. Based on this insight, this review provides a comprehensive description of the foundation of both the gas breakdown physics and the nanomaterial-based ionization-gas-sensor technology, as well as introduces research trends on nanomaterial-based ionization gas sensors. The gas breakdown is reviewed, including the classical Townsend discharge theory and modified Paschen curves; and nanomaterial-based-electrodes proposed to improve the performance of ionization gas sensors are introduced. The secondary electron emission at the electrode surface is the key plasma–surface process that affects the performance of ionization gas sensors. Finally, we present our perspectives on possible future directions.

The Neutralized Drift Compression Experiment-II (NDCX-II) is an induction linac that generates intense pulses of 1.2 MeV helium ions for heating matter to extreme conditions. Here, we present recent results on optimizing beam transport. The NDCX-II beamline includes a 1-m-long drift section downstream of the last transport solenoid, which is filled with charge-neutralizing plasma that enables rapid longitudinal compression of an intense ion beam against space-charge forces. The transport section on NDCX-II consists of 28 solenoids. Finding optimal field settings for a group of solenoids requires knowledge of the envelope parameters of the beam. Imaging the beam on the scintillator gives the radius of the beam, but the envelope angle is not measured directly. We demonstrate how the parameters of the beam envelope (radius, envelop angle, and emittance) can be reconstructed from a series of images taken by varying the B-field strengths of a solenoid upstream of the scintillator. We use this technique to evaluate emittance at several points in the NDCX-II beamline and for optimizing the trajectory of the beam at the entry of the plasma-filled drift section. Keywords: Charged-particle beams, Induction accelerators, Beam dynamics, Beam emittance, Ion beam diagnostics, PACS Codes: 41.75.-i, 41.85.Ja, 52.59.Sa, 52.59.Wd, 29.27.Eg

We performed integrated modelling of the chemical pathways of formation for boron nitride nanotube (BNNT) precursors during high-temperature synthesis in a B/N2 mixture. Modelling includes quantum chemistry, quantum-classical molecular dynamics, thermodynamic, and kinetic approaches. It is shown that BN compounds are formed in the interaction of N2 molecules with small boron clusters (N2 molecule fixation) rather than with less reactive liquid boron. We demonstrate that the transformation and consumption of liquid boron proceeds through the evaporation of clusters, Bm with m less than or equal to 5 and their subsequent conversion into BmNn chains. The production of such chains is crucial to the growth of BNNTs because these chains form the building blocks of bigger and longer BN chains and rings, which are themselves the building blocks of fullborenes and BNNTs. Moreover, kinetic modelling revealed that B4N4 and B5N4 species play a major role in the N2 molecule fixation process. The formation of these species via reactions with B4 and B5 clusters is not adequately described under the assumption of thermodynamic equilibrium because the accumulation of both B4N4 and B5N4 depends on the background gas pressure and the gas cooling rate. Long BN chains and rings, which are precursors of the fullborene and BNNT growth, form via self-assembly of component B4N4 and B5N4. Our modelling results (particularly the increased densities of B4N4 and B5N4 species at higher gas pressures) explain the experimentally observed effect of gas pressure on the yield of high-quality BNNTs. The catalytic role of hydrogen was also studied; it is shown that HBNH molecules can be the main precursor of BNNT synthesis in the presence of hydrogen., Comment: 20 pages, 11 figures, 5 tables

International audience; The effect of plasma turbulence on the trajectories of dust particles is investigated for the first time. The dynamics of dust particles is computed using the ad-hoc developed Dust Injection Simulator code, using a 3D turbulent plasma background computed with the TOKAM3X code. As a result, the evolution of the particle trajectories is governed by the ion drag force, and the shape of the trajectory is set by the Stokes number St ∝ a d /n 0 , with a d the dust radius and n 0 the density at the separatrix. The plasma turbulence is observed to scatter the dust particles, exhibiting a hyperdiffusive regime in all cases. The amplitude of the turbulent spread of the trajectories ∆r 2 is shown to depend on the ratio Ku/St, with Ku ∝ u rms the Kubo number and u rms the fluctuation level of the plasma flow. These results are compared with a simple analytical model, predicting ∆r 2 ∝ (Ku/St) 2 t 3 , or ∆r 2 ∝ (u rms n 0 /a d) 2 t 3. As the dust is heated by the plasma fluxes, thermionic emission sets the dust charge, originally negative, to slightly positive values. This results in a substantial reduction of the ion drag force through the suppression of its Coulomb scattering component. The dust grain inertia is then no longer negligible, and drives the transition from a hyperdiffusive regime towards a ballistic one.

Tracing magnetic field-lines of the Earth's magnetosphere using beams of relativistic electrons will open up new insights into space weather and magnetospheric physics. Analytic models and a single-particle-motion code were used to explore the dynamics of an electron beam emitted from an orbiting satellite and propagating until impact with the Earth. The impact location of the beam on the upper atmosphere is strongly influenced by magnetospheric conditions, shifting up to several-degrees in latitude between different phases of a simulated storm. The beam density cross-section evolves due to cyclotron motion of the beam centroid and oscillations of the beam envelope. The impact density profile is ring shaped, with major radius $\sim 22$ meters, given by the final cyclotron radius of the beam centroid, and ring thickness $\sim 2$ meters given by the final beam envelope. Motion of the satellite may also act to spread the beam, however it will remain sufficiently focused for detection by ground-based optical and radio detectors. An array of such ground stations will be able to detect shifts in impact location of the beam, and thereby infer information regarding magnetospheric conditions.

The Sun's connection with the Earth's magnetic field and atmosphere is carried out through the exchange of electromagnetic and mass flux and is regulated by a complex interconnection of processes. During space weather events, solar flares, or fast streams of solar atmosphere strongly disturb the Earth's environment. Often the electric currents that connect the different parts of the Sun-Earth system become unstable and explosively release the stored electromagnetic energy in one of the more dramatic expressions of space weather—the geomagnetic storm and substorm. Some aspects of the magnetosphere-ionosphere connection that generates auroral arcs during space weather events are well-known. However, several fundamental problems remain unsolved because of the lack of unambiguous identification of the magnetic field connection between the magnetosphere and the ionosphere. The correct mapping between different regions of the magnetosphere and their foot-points in the ionosphere, coupled with appropriate distributed measurements of plasma and fields in focused regions of the magnetosphere, is necessary to establish unambiguously that a given magnetospheric process is the generator of an observed arc. We present a new paradigm that should enable the resolution of the mapping ambiguities. The paradigm calls for the application of energetic electron beams as magnetic field tracers. The three most important problems for which the correct magnetic field mapping would provide closure to are the substorm growth phase arcs, the expansion phase onset arcs and the system of arcs that emerge from the magnetosphere-ionosphere connection during the development of the early substorm expansion phase phenomenon known as substorm current wedge (SCW). In this communication we describe how beam tracers, in combination with distributed measurements in the magnetosphere, can be used to disentangle the mechanisms that generate these critical substorm phenomena. Since the application of beams as tracers require demonstration that the beams can be injected into the loss cone, that the spacecraft potentials induced by the beam emission are manageable, and that sufficient electron flux reaches the atmosphere to be detectable by optical or radio means after the beam has propagated thousands of kilometers under competing effects of beam spread and constriction as well as effects of beam-induced instabilities, in this communication we review how these challenges are currently being addressed and discuss the next steps toward the realization of active experiments in space using relativistic electron beams.

Using electron beam accelerators attached to satellites in Earth orbit, it may be possible to measure arc length and curvature of field-lines in the inner magnetosphere if the accelerator is designed with the capability to vary the beam energy. In combination with additional information, these measurements would be very useful in modeling the magnetic field of the inner magnetosphere. For this purpose, a three step data assimilation modeling approach is discussed. The first step in the procedure would be to use prior information to obtain an initial forecast of the inner magnetosphere. Then, a family of curves would be defined that satisfies the observed geometric attributes measured by the experiments, and the prior forecast would then be used to optimize the curve with respect to the allowed degrees of freedom. Finally, this approximation of the field-line would be used to improve the initial forecast of the inner magnetosphere, resulting in a description of the system that is optimally consistent with both the prior information and the measured curvature and arc length. This article details the method by which a family of possible approximations of the field-line may be defined via a numerical procedure, which is central to the three step approach. This method serves effectively as a pre-conditioner for parameter estimation problems using field-line curvature and arc length measurements in combination with other measurements.

International audience; The increasing need to demonstrate the correctness of computer simulations has highlighted the importance of benchmarks. We define in this paper a representative simulation case to study low-temperature partially-magnetized plasmas. Seven independently developed Particle-In-Cell codes have simulated this benchmark case, with the same specified conditions. The characteristics of the codes used, such as implementation details or computing times and resources, are given. First, we compare at steady-state the time-averaged axial profiles of three main discharge parameters (axial electric field, ion density and electron temperature). We show that the results obtained exhibit a very good agreement within 5% between all the codes. As ExB discharges are known to cause instabilities propagating in the direction of electron drift, an analysis of these instabilities is then performed and a similar behaviour is retrieved between all the codes. A particular attention has been paid to the numerical convergence by varying the number of macroparticles per cell and we show that the chosen benchmark case displays a good convergence. Detailed outputs are given in the supplementary data, to be used by other similar codes in the perspective of code verification. 2D axial-azimuthal Particle-In-Cell benchmark for low-temperature partially ..

Magnetosphere-ionosphere coupling is a particularly important process that regulates and controls magnetospheric dynamics such as storms and substorms. However, in order to understand magnetosphere-ionosphere coupling it is necessary to understand how regions of the magnetosphere are connected to the ionosphere. It has been proposed that this connection may be established by firing electron beams from satellites that can reach an ionospheric footpoint creating detectable emissions. This type of experiment would greatly aid in identifying the relationship between convection processes in the magnetotail and the ionosphere and how the plasma sheet current layer evolves during the growth phase preceding substorms. For practical purposes, the use of relativistic electron beams with kinetic energy on the order of 1 MeV would be ideal for detectability. However, Porazik et al. \cite{Porazik2014} has shown that, for relativistic particles, higher order terms of the magnetic moment are necessary for consideration of the ionospheric accessibility of the beams. These higher order terms are related to gradients in the magnetic field and curvature and are typically unimportant unless the beam is injected along the magnetic field direction, such that the zero order magnetic moment is small. In this article, we address two important consequences related to these higher order terms. First, we investigate the consequences for satellites positioned in regions subject to magnetotail stretching and demonstrate systematically how curvature affects accessibility. We find that curvature can reduce accessibility for beams injected from the current sheet, but can increase accessibility for beams injected just above the current sheet. Second, we investigate how detectability of ionospheric precipitation of variable energy field-aligned electron beams could be used as a constraint on field-line curvature, which would be valuable for field-line reconstruction and/or stability analysis.

A high-yield production of high-quality boron-nitride nanotubes (BNNTs) was reported recently in several publications. A boron-rich material is evaporated by a laser or plasma in a nitrogen-rich atmosphere to supply precursor gaseous species for nucleation and growth of BNNTs. Either hydrogen was added or pressure was increased in the system to achieve high yield and high purity of the synthesized nanotubes. According to the widely-accepted "root grow" mechanism, upon the gas cooling, boron droplets form first, then they adsorb nitrogen from surrounding gas species, and BNNTs grow on their surfaces. However, what are these precursor species that provide nitrogen for the growth is still an open question. To answer this question, we performed thermodynamic calculations of B-N mixture composition considering broad set of gas species. In enhancement of previous studies, the condensation of boron is now taken into account and is shown to have drastic effect on the gas chemical composition. B2N molecules were identified to be a major source of nitrogen for growth of BNNTs. Presence of B2N molecules in a B-N gas mixture was verified by our spectroscopic measurements during a laser ablation of boron-rich targets in nitrogen. It was shown that the increase of pressure has a quantitative effect on the mixture composition yielding increase of the precursor density. The hydrogen addition might open an additional channel of nitrogen supply to support growth of BNNTs. Nitrogen atoms react with abundant H2 molecules to form NH2 and then NH3 precursor species, instead of just recombining back to inert N2 molecules, as in the no-hydrogen case. In addition, thermodynamics was applied in conjunction with agglomeration theory to predict the size of boron droplets upon growth of BNNTs. Analytical relations for identification of crucial species densities were derived.

Self-organization and anomalous transport in gradient-drift driven turbulence in partially magnetized plasmas with crossed electric and magnetic fields is demonstrated in two-dimensional fluid simulations. The development of large scale structures and flows is shown to occur as a result of the inverse energy cascade from short wavelength instabilities. The turbulence shows complex interaction of small scale modes with large scale zonal flow modes, vortices, and streamers resulting in strongly intermittent anomalous transport that significantly exceeds the classical collisional values. The turbulence driven secondary instabilities and large scale structures are shown to dominate the anomalous electron current. Such anomalous transport and structures are consistent with a number of experimental observations in laboratory plasmas.

The spoke instability in an $E\times B$ Penning discharge is shown to be strongly affected by the boundary that is perpendicular to $B$ field lines. The instability is the strongest when bounded by dielectric walls. With a conducting wall, biased to collect electron current from the plasma, the spoke becomes faster, less coherent and localised closer to the axis. The corresponding anomalous cross-field transport is assessed via simultaneous time-resolved measurements of plasma potential and density. This shows a dominant large-scale $E\times B$ anomalous character of the electron cross-field current for dielectric walls reaching $40-100$% of the discharge current, with an effective Hall parameter $\beta_\mathrm{eff}\sim10$. The anomalous current is greatly reduced with the conducting boundary (characterised by $\beta_\mathrm{eff}\sim10^2$). These experimental measurements are shown to be qualitatively consistent with the decrease of the $E$ field that triggers the collisionless Simon-Hoh instability.

Graphite ablation by an electric arc or a laser/solar flux is widely used for the synthesis of carbon nanomaterials. Previously, it was observed in multiple arc experiments that the ablation rate is a strong nonlinear function of the arc current and it drastically increases at some threshold current. We developed an analytical model explaining this transition in the rate of ablation by an electric arc or a laser/solar flux. The model not only explains the observations but can also accurately predict the experimentally observed ablation rates. The model takes into account redeposition of carbon back to the ablated surface, which is the key process responsible for the observed effects.

A novel regime of the saturation of the Pierce-type ion-sound instability in a bounded ion-beam-plasma system is revealed in 1D particle-in-cell simulations. It is found that the saturation of the instability is mediated by the oscillating virtual anode potential structure. The periodically oscillating potential barrier separates the incoming beam ions into two groups. One component forms a supersonic beam, which is accelerated to an energy exceeding the energy of the initial cold ion beam. The other component is organized as a self-consistent phase space structure of trapped ions with a wide energy spread—the ion hole. The effective temperature (energy spread) of the ions trapped in the hole is lower than the initial beam energy. In the final stage, the ion hole expands over the whole system length.

Ion beam charge neutralization by electron injection is a complex kinetic process. Recent experiments show that the resulting self-potential of the ion beam after neutralization by plasma is much lower than the temperature of plasma electrons [Stepanov et al., Phys. Plasmas 23, 043113 (2016)], indicating that kinetic effects are important and may affect the neutralization of the ion beam. We performed a numerical study of the charge neutralization process of an ion beam making use of a two-dimensional electrostatic particle-in-cell code. The results show that the process of charge neutralization by electron injection is composed of two stages. During the first stage, the self-potential of the beam is higher than the temperature of injected electrons (Te/e) and all injected electrons are captured by the ion beam. During the second stage, hot electrons escape from the ion beam and the beam self-potential (φ) decreases because cold electrons slowly accumulate resulting in the beam self-potential φ to become much lower than Te/e in agreement with previous experimental observations at Princeton Advanced Teststand. We also determined that the resulting φ scales as φ ∼ T e, in agreement with previous experimental observations from Gabovich's group. In addition, the results show that the transverse position of the electron source has a great impact on ion beam neutralization. A slight shift of the electron source as relevant to the ion thrusters leads to a large increase in the beam self-potential because of an increase in potential energy of injected electrons.

© 2018 Author(s). The interaction of ion beams with matter includes the investigation of the basic principles of ion stopping in heated materials. An unsolved question is the effect of different, especially higher, ion beam fluences on ion stopping in solid targets. This is relevant in applications such as in fusion sciences. To address this question, a Thomson parabola was built for the Neutralized Drift Compression eXperiment (NDCX-II) for ion energy-loss measurements at different ion beam fluences. The linear induction accelerator NDCX-II delivers 2 ns short, intense ion pulses, up to several tens of nC/pulse, or 1010-1011 ions, with a peak kinetic energy of ∼1.1 MeV and a minimal spot size of 2 mm FWHM. For this particular accelerator, the energy determination with conventional beam diagnostics, for example, time of flight measurements, is imprecise due to the non-trivial longitudinal phase space of the beam. In contrast, a Thomson parabola is well suited to reliably determine the beam energy distribution. The Thomson parabola differentiates charged particles by energy and charge-to-mass ratio, through deflection of charged particles by electric and magnetic fields. During first proof-of-principle experiments, we achieved to reproduce the average initial helium beam energy as predicted by computer simulations with a deviation of only 1.4%25. Successful energy-loss measurements with 1 μm thick silicon nitride foils show the suitability of the accelerator for such experiments. The initial ion energy was determined during a primary measurement without a target, while a second measurement, incorporating the target, was used to determine the transmitted energy. The energy-loss was then determined as the difference between the two energies.

The Neutralized Drift Compression eXperiment (NDCX-II) at Lawrence Berkeley National Laboratory is an induction accelerator designed to deliver intense nano-second pulses of ions, up to several tens of nC/pulse, with ion energies up to 1.2 Me V1. Voltage pulses of different amplitudes and lengths accelerate and rapidly compress the ion bunch. The compression is achieved through an induced head-to-tail velocity tilt, which leads to a pulse length of 2 ns FWHM at the target position. A final solenoid focusses the beam to a minimum spot size of 2 mm FWHM on target.

The Electron Cyclotron Drift Instability (ECDI) driven by the electron $E\times B$ drift in partially magnetized plasmas is investigated with highly resolved particle-in-cell simulations. The emphasis is on two-dimensional effects involving the parallel dynamics along the magnetic field in a finite length plasma with dielectric walls. It is found that the instability develops as a sequence of growing cyclotron harmonics demonstrating wave breaking and complex nonlinear interactions, being particularly pronounced in ion density fluctuations at short wavelengths. At the same time, nonlinear evolution of fluctuations of the ion and electron density, as well as the anomalous electron current, shows cascade toward long wavelengths. Tendency to generate long wavelength components is most clearly observed in the spectra of the electron density and the anomalous current fluctuations. An intense but slowly growing mode with a distinct eigen-mode structure along the magnetic field develops at a later nonlinear stage enhancing the tendency toward long wavelength condensation. The latter mode having a finite wavelength along the magnetic field is identified as the Modified Two-Stream Instability (MTSI). It is shown that the MTSI mode results in strong parallel heating of electrons., 14 pages, 19 figures, 2 tables

We have synthesized boron nitride nanotubes (BNNTs) in an arc in presence of boron and nitrogen species only, without transition metals. We find that BNNTs are often attached to pure boron nanoparticles, suggesting that root-growth is a likely mechanism for their formation. To gain further insight into this process we have studied key mechanisms for root growth of BNNTs on the surface of a liquid boron droplet by ab initio molecular dynamics simulations. We find that nitrogen atoms reside predominantly on the droplet surface where they organize to form boron nitride islands below 2400 K. To minimize contact with the liquid particle underneath, the islands assume non-planar configurations that are likely precursors for the thermal nucleation of cap structures. Once formed, the caps are stable and can easily incorporate nitrogen and boron atoms at their base, resulting in further growth. Our simulations support the root-growth mechanism of BNNTs and provide comprehensive evidence of the active role played by liquid boron., Comment: supporting inofrmation inlcuded

This work presents a study of the region of nanoparticle growth in an atmospheric pressure carbon arc. The nanoparticles are detected using the planar laser-induced incandescence technique. The measurements revealed large clouds of nanoparticles in the arc periphery bordering the region with a high density of diatomic carbon molecules. Two-dimensional computational fluid dynamic simulations of the arc combined with thermodynamic modeling show that this is due to the interplay of the condensation of carbon molecular species and the convection flow pattern. These results show that the nanoparticles are formed in the colder, peripheral regions of the arc and describe the parameters necessary for coagulation.

Complex structures on a material surface can significantly reduce the total secondary electron emission yield from that surface. A foam or fuzz is a solid surface above which is placed a layer of isotropically aligned whiskers. Primary electrons that penetrate into this layer produce secondary electrons that become trapped and not escape into the bulk plasma. In this manner the secondary electron yield (SEY) may be reduced. We developed an analytic model and conducted numerical simulations of secondary electron emission from a foam to determine the extent of SEY reduction. We find that the relevant condition for SEY minimization is $\bar{u} \equiv AD/2 >>1 $, where $D$ is the volume fill fraction and $A$ is the aspect ratio of the whisker layer, the ratio of the thickness of the layer to the radius of the fibers. We find that foam can not reduce the SEY from a surface to less than 0.3 of its flat value.

Atmospheric pressure arcs have recently found application in the production of nanoparticles. Distinguishing features of such arcs are small length and hot ablating anode characterized by intensive electron emission and radiation from its surface. We performed one-dimensional modeling of argon arc, which shows that near-electrode effects of thermal and ionization non-equilibrium play important role in operation of a short arc, because the non-equilibrium regions are up to several millimeters long and are comparable with the arc length. The near-anode region is typically longer than the near-cathode region and its length depends more strongly on the current density. The model was extensively verified and validated against previous simulation results and experimental data. Volt-Ampere characteristic (VAC) of the near-anode region depends on the anode cooling mechanism. The anode voltage is negative. In case of strong anode cooling (water-cooled anode) when anode is cold, temperature and plasma density gradients increase with current density resulting in decrease of the anode voltage (absolute value increases). Falling VAC of the near-anode region suggests the arc constriction near the anode. Without anode cooling, the anode temperature increases significantly with current density, leading to drastic increase in the thermionic emission current from the anode. Correspondingly, the anode voltage increases to suppress the emission - and the opposite trend in the VAC is observed. The results of simulations were found to be independent of sheath model used: collisional (fluid) or collisionless model gave the same plasma profiles for both near-anode and near-cathode regions., 29 pages, 16 figures

Cross-field anomalous transport is an important element of the operation and performance of E × B discharges. The anomalous electron current is driven by instabilities which may be excited in such plasmas due to presence of the strong E × B electron current. Using 1D particle-in-cell simulations we study the development of the the E × B drift cyclotron instability 1 driven exclusively by the E × B flow, in absence of density and magnetic field gradients. In this case, the instability occurs due to the resonance between the ballistic $kv _{E} \times _{B}$ modes and electron cyclotron harmonics $m\omega _{ce}$. Initially, in the linear stage we observe the growth of the most unstable $m^{th}$ harmonic, consistent with the linear theory. At a later stage, the intense electron heating is observed due to phase-space mixing of the electron distribution function and the mode robustly reorganizes into the fundamental mode that propagates with the acoustic speed in the ion frame 2, 3, with a sub-dominant spectrum of higher order cyclotron side-bands. A progressive inverse cascade to a larger scale structure (with a size larger than the the length scale of the fundamental mode) in the anomalous current is also observed in the non-linear stage.

We investigate operation of the flat probe for measurements of the anisotropic ion or electron velocity distribution function (IVDF or EVDF). Particular attention was given for the limiting case of very narrow VDFs. A new algorithm for analysis of the flat probe data was developed. Method was verified for EVDF in low pressure, low voltage beam discharge. Big departure from previous traditional method was demonstrated. Experimental measurements of EVDF were performed 1, 2 and compared with Monte Carlo simulations. Good agreement between two was observed.

Long-time evolution of the two-stream instability of an ion beam pulse propagating through a background plasma is investigated using a large-scale, one-dimensional kinetic simulation. After initial saturation of the two-stream instability due to particle trapping in the plasma wave, significant portion of the initially trapped in the wave field electrons become detrapped and move ahead of the ion beam. It is found that particles can be accelerated in front of the ion beam at a potential amplitude lower than predicted because the cold background electrons are moderately heated as they travel through the plasma wave generated due to the initial two-stream instability. The potential wells in the plasma wave is non-sinusoidal because of the modulations in both ion and electron densities. The detrapped electron population forms an electron beam that propagates faster than the ion beam and starts to interact with the background plasma ahead of the beam pulse. Secondary two-stream instability between the streaming electrons and background plasma electrons develops ahead of the ion beam pulse leading to heating of the background electrons. The heating of plasma electrons and beam ions eventually extinguish the instability.

The left branch of the Paschen curve for helium gas is studied both experimentally and by means of particle-in-cell/Monte Carlo collisions (PIC/MCC) simulations. The physical model incorporates electron, ion, and fast atom species whose energy-dependent anisotropic scattering on background neutrals, as well as backscattering at the electrodes, is properly accounted for. For the range of breakdown voltage 15 kV ≤ V br ≤ 130 kV, a good agreement is observed between simulations and available experimental results for the discharge gap d = 1.4 cm. The PIC/MCC model is then used to estimate the Paschen curve at higher voltages up to 1 MV, based on the availability of input atomic data. We find that the pd similarity scaling does hold, and that above 300 kV the value of pd at breakdown begins to increase with increasing voltage.

Results are presented from two-dimensional (radial-azimuthal), electrostatic Particle-In-Cell simulations of a Penning discharge, where an electron beam is injected along the axis, parallel to the magnetic field, into a cylindrical plasma radially bounded by grounded metal. In preliminary, one-dimensional (radial) simulations, the anomalous electron transport is solely due to the instability of the pure lower-hybrid mode. With also the azimuthal direction numerically resolved, the Simon-Hoh mode becomes active and the lower-hybrid mode is destabilized by density-gradient effects. In the more realistic, twodimensional simulations, the lower-hybrid and Simon-Hoh modes both contribute to the anomalous transport. These latter simulations demonstrate the excitation of an azimuthally asymmetric, spoke-like structure rotating in the direction of ExB and electron diamagnetic drifts and persisting throughout the remainder of this simulation. The spokes are characterized by an increase in plasma density and are shown to channel most of the radial leakage current. The spoke appears similar to the one seen in experiments and it exhibits the same scaling with ion mass. The Hall parameter in the simulations is also in good agreement with experiment.

Self-consistent model of atmospheric pressure carbon arc discharge in helium atmosphere was developed in the framework of the nanoparticle synthesis project1 and implemented into the 3D CFD-code ANSYS CFX, which was highly customized for this purpose. Arc discharge model consists of fluid model for non-equilibrium plasma coupled with models of heat transfer in electrodes, ablation of anode, carbon deposition at cathode and space charge sheathes.

Propagation of charged particle beams in background plasma as a method of space charge neutralization has been shown to achieve high degrees of charge and current neutralization and therefore can enable nearly ballistic propagation and focusing of charged particle beams. Correspondingly, use of plasmas for propagation of charged particle beams has important applications for transport and focusing of intense particle beams in electric propulsion, inertial fusion and high energy density laboratory plasma physics. However, the streaming of beam ions through a background plasma can lead to development of the two-stream instability between the beam ions and the plasma electrons [1, 2]. The electric and magnetic self-fields enhanced by the two-stream instability can lead to defocusing of the ion beam and fast scattering of an electron beam. Using particle-in-cell (PIC) simulations, we study the scaling of the instability-driven selfelectromagnetic fields and consequent defocusing forces with the background plasma density and beam ion mass. We identify plasma parameters where the defocusing forces can be reduced.

Propagation of charged particle beams in background plasma as a method of space charge neutralization has been shown to achieve a high degree of charge and current neutralization and therefore enables nearly ballistic propagation and focusing of charged particle beams. Correspondingly, use of plasmas for propagation of charged particle beams has important applications for transport and focusing of intense particle beams in inertial fusion and high energy density laboratory plasma physics. However, the streaming of beam ions through a background plasma can lead to development of the two-stream instability between the beam ions and the plasma electrons. The beam electric and magnetic fields enhanced by the two-stream instability can lead to defocusing of the ion beam. Using particle-in-cell (PIC) simulations, we study the scaling of the instability-driven self-electromagnetic fields and consequent defocusing forces with the background plasma density and beam ion mass. We identify plasma parameters where the defocusing forces can be reduced., Comment: 28 pages, submitted to Phys. Plasmas

We find that negative charges on an armchair single-walled carbon nanotube (SWCNT) can significantly enhance the migration of a carbon adatom on the external surfaces of SWCNTs, along the direction of the tube axis. Nanotube charging results in stronger binding of adatoms to SWCNTs and consequent longer lifetimes of adatoms before desorption, which in turn increases their migration distance several orders of magnitude. These results support the hypothesis of diffusion enhanced SWCNT growth in the volume of arc plasma. This process could enhance effective carbon flux to the metal catalyst.

I Adamovich et al 2017 J. Phys. D: Appl. Phys. 50 323001, Journal of Physics D: Applied Physics published the first Plasma Roadmap in 2012 consisting of the individual perspectives of 16 leading experts in the various sub-fields of low temperature plasma science and technology. The 2017 Plasma Roadmap is the first update of a planned series of periodic updates of the Plasma Roadmap. The continuously growing interdisciplinary nature of the low temperature plasma field and its equally broad range of applications are making it increasingly difficult to identify major challenges that encompass all of the many sub-fields and applications. This intellectual diversity is ultimately a strength of the field. The current state of the art for the 19 sub-fields addressed in this roadmap demonstrates the enviable track record of the low temperature plasma field in the development of plasmas as an enabling technology for a vast range of technologies that underpin our modern society. At the same time, the many important scientific and technological challenges shared in this roadmap show that the path forward is not only scientifically rich but has the potential to make wide and far reaching contributions to many societal challenges.

An independent control of the flux and energy of ions impacting on an object immersed in a plasma is often desirable for many industrial processes such as microelectronics manufacturing. We demonstrate that a simultaneous control of these quantities is possible by a suitable choice of a static magnetic field applied parallel to the plane electrodes in a standard single frequency capacitively coupled plasma device. Our particle-in-cell simulations show a 60% reduction in the sheath width (that improves control of ion energy) and a four fold increase in the ion flux at the electrode as a consequence of the altered ion and electron dynamics due to the ambient magnetic field. A detailed analysis of the particle dynamics is presented and the optimized operating parameters of the device are discussed. The present technique offers a simple and attractive alternative to conventional dual frequency based devices that often suffer from undesirable limitations arising from frequency coupling and electromagnetic effects., Comment: 6 pages, 4 figures

The optimal pitch angle which maximizes the penetration distance, along the magnetic field, of relativistic charged particles injected from the midplane of an axisymmetric field is investigated analytically and numerically. Higher-order terms of the magnetic moment invariant are necessary to correctly determine the mirror point of trapped energetic particles, and therefore the loss cone. The modified loss cone resulting from the inclusion of higher-order terms is no longer entirely defined by the pitch angle but also by the phase angle of the particle at the point of injection. The optimal orientation of the injection has a nonzero component perpendicular to the magnetic field line, and is in the plane tangential to the flux surface. Numerical integration of particle orbits were carried out for a relativistic electron in a dipole field, showing agreement with analytic expressions. The results are relevant to experiments, which are concerned with injection of relativistic beams into the atmosphere from aboard a spacecraft in the magnetosphere.

A barium titanate ferroelectric cylindrical plasma source has been developed, tested and delivered for the Neutralized Drift Compression Experiment NDCX-II at Lawrence Berkeley National Laboratory (LBNL). The plasma source design is based on the successful design of the NDCX-I plasma source. A 7 kV pulse applied across the 3.8 mm-thick ceramic cylinder wall produces a large polarization surface charge density that leads to breakdown and plasma formation. The plasma that fills the NDCX-II drift section upstream of the final-focusing solenoid has a plasma number density exceeding 10 10 cm −3 and an electron temperature of several eV. The operating principle of the ferroelectric plasma source are reviewed and a detailed description of the installation plans is presented. The criteria for plasma sources with larger number density will be given, and concepts will be presented for plasma sources for driver applications. Plasma sources for drivers will need to be highly reliable, and operate at several Hz for millions of shots.

The streaming of an intense ion beam relative to background plasma can cause the development of fast electrostatic collective instabilities. The plasma waves produced by the two-stream instability modify the ion beam current neutralization and produce non-linear average forces which can lead to defocusing of the ion beam. Recently, a theoretical model describing the average de-focusing forces acting on the beam ions has been developed, and the scalings of the forces with beam-plasma parameters have been identified (Startsev et al. in press [1] ). These scalings can be used in the development of realistic ion beam compression scenarios in present and next-generation ion-beam-driven high energy density physics and heavy ion fusion experiments. In this paper the results of particle-in-cell simulations of ion beam propagation through neutralizing background plasma for NDCX-II parameters are presented. The simulation results show that the two-stream instability can play a significant role in the ion beam dynamics. The effects of velocity tilt on the development of the instability and ion beam compressibility for typical NDCX-II parameters are also simulated. It is shown that the two-stream instability may be an important factor in limiting the maximum longitudinal compression of the ion beam.