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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 X 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-ofthe-art computational results, identifies critical questions, and suggests directions for future research. Y. Raitses and I. D. Kaganovich gratefully acknowledge partial financial support by the AFOSR grant (No. FA9550–17-1–0010) and assistance in preparation and fruitful discussions with I. Romadanov, Eduardo Rodriquez, and J. B. Simmonds. The work of A. Smolyakov was supported in part by the Natural Sciences and Engineering Research Council of Canada (NSERC) Canada, by the AFOSR grant (No. FA9550-18-1-0132), and by Compute Canada, and he acknowledges fruitful discussions with O. Chapurin, S. Janhunen, M. Jimenez, O. Koshkarov, I. Romadanov, and D. Sydorenko. The contribution of E. Ahedo and M. Merino was supported by the Government of Spain, National Development and Research Program, Grant No. PID2019-108034RB-I00, and they thank J. Navarro and P. Fajardo for their contribution. M. Keidar and I. Schweigert gratefully acknowledge the AFSOR grant (No. FA9550-19-1-0166). I. Schweigert was partly supported by the Russian Science Foundation (Grant No. 17-19-01375). S. Tsikata acknowledges support from CNES (Centre national d’ etudes spatiales, France). F. Taccogna gratefully acknowledges financial support from the Italian minister of university and research (MIUR) under the project PON “CLOSE to the Earth” No. ARS ARS01–00141. R. Gueroult and N. J. Fisch were supported by Nos. DOE DESC0016072 and NSF PHY-1805316. A. Bourdon and P. Chabert gratefully acknowledge financial support of the French National Research Agency (L’Agence nationale de la recherche) ANR grant (No. ANR-16-CHIN-003-01) and Safran Aircraft Engines within the project POSEIDON Israel Science Foundation, Grant 1581/16.

Hall effect thrusters operating at power levels in excess of several hundreds of kilowatts have been identified as enabling technologies for applications such as lunar tugs, large satellite orbital transfer vehicles, and solar system exploration. These large thrusters introduce significant testing challenges due to the propellant flow rate exceeding the pumping speed available in most laboratories. Even with proposed upgrades in mind, the likelihood that multiple vacuum facilities will exist in the near future to allow long duration testing of high-power Hall thrusters operating at power levels in excess of 100 kW remains extremely low. In this article, we numerically explore the feasibility of testing Hall thrusters in a quasi-steady mode defined by pulsing the mass flow rate between a nominal and a low value. Our simulations indicate that sub-second durations available before the chamber reaches critical pressure are sufficiently long to achieve the steady-state current and flow field distributions, allowing us to characterize thruster performance and the near plume region.

The primary focus of this work is to study the ionization inside an air-breathing plasma thruster (ABPT) in low earth orbit applications. For this high-speed technology to work, a high degree of ionization needs to be achieved. This paper focuses on plasma chemistry simulation for air in low earth orbits (80–110 km) to explore the possibility of high ionization of the incoming air. The results of plasma chemistry simulation showed the variation of ionization degree and species densities concerning the mean input energy that contributed to the chemical reactions. This research is essential to understand ionization processes to develop a low earth orbit ABPT design. Our results have indicated the possibility of building ABPT without an external neutralizer. The neutralization is created by extracting negative and positive ions to obtain neutralization, thereby eliminating existing design complexity.

A new plasma source design that merges the main characteristics of capacitive dielectric barrier discharge (DBD) and cold atmospheric plasma jet (CAPJ) is discussed. The DBD system contains a flexible, porous matrix consisting of silica aerogel, which is comprised between two biased electrodes. The helium flow supply subjected to a sinusoidal voltage of around 5 kV in amplitude and 15 kHz in frequency provides a set of plasma jets that propagates more than 1 cm beyond the active DBD region. The studied plasma multi-jet system consists of an array of three aligned jets that flow in the laminar regime, and it is intended for treating the surfaces of 3D objects and large areas. CAPJ performance is discussed as a hypothetical morphing source in flat and bent configurations. Electrical characterization and optical emission spectroscopy diagnostics have provided current-voltage waveforms and the composition of the CAPJ through the aerogel layer, respectively. This novel source is promising for biomedical applications that require full adaptation of plasma parameters to delicate samples, such as wound healing and treatment of surgical margins in plasma-based cancer surgery.

The study of cold atmospheric plasma (CAP) began about a decade ago. Currently, multiple applications of CAP have been discovered including biomedicine, nanomaterials, agriculture, and water purifications. At the current stage of these research fields, it is obvious that the next move will be CAP optimization for each specific application. For example, in the field of plasma-based cancer treatment, due to the different responses of cell lines, CAP can activate different biological pathways in different cells, i.e., to be selective. One of the most commonly used CAP types is the cold atmospheric plasma jet (CAPJ). However, without a full understanding of CAPJ physics, it is impossible to optimize the plasma for every application condition. Moreover, since each research team is equipped with its own CAPJ generator, the hardware behaviors vary significantly across researchers. Therefore, a complete big picture of CAPJ control and parameters is a critical milestone for future CAPJ optimization in these research fields. This Review provides a summary of how CAPJ parameters can be manipulated with the control inputs and hardware design to extend that the chemical compositions can be modified by the gas flow rate, discharge waveform, target properties, and local environment. Based on the control map summarized in this work, CAPJ users can easily optimize their device for a certain specific purpose, such as maximizing OH and H2O2 for cancer treatment or maximizing O3 and ultraviolet for sterilization. Therefore, this study sheds light on the general theory of CAPJ control and can be a basis for future optimization of low-temperature plasma devices. Consideration of the plasma control based on machine learning methods has been receiving interest recently and certainly will become a future hot topic.

Glioblastoma is one of the most aggressive brain cancers. Chemotherapy is a standard modality for its therapy. Here, we demonstrated a novel strategy using a helium gas discharge tube as a tunable electromagnetic emission source to sensitize glioblastoma cells to the cytotoxicity of temozolomide (TMZ), a widely used drug for glioblastoma. After a single 7-min of treatment, the efficacy of TMZ was enhanced in two typical glioblastoma cell lines U87MG and A172 without affecting a normal human astrocyte cell line hTERT/E6/E7. The discharge tube is a non-invasive approach, which provides a safe, controllable, stable, and low-cost modality to improve the conventional chemotherapy.

The dynamics of nanoparticle growth in pulsed anodic arc discharge has been studied in time-resolved mode. To this end, a fast moving probe was employed to extract the material generated in a pulsed arc plasma held between two graphite electrodes. The probe motion was synchronized with the pulse phase and the exposure time to the plasma was set to 10 ms. The graphite anode was eroded in a helium atmosphere (300 Torr) by an arc plasma pulsed at 1 Hz with a 10% duty cycle and showing 250 A of peak current. The structure and morphology of the probe depositions were characterized by Raman spectroscopy, scanning electron microscopy, and focused ion beam. A maximal deposition rate of 260 μm/s was measured 5 mm away from the arc core during the active 0.1 s of the pulse. Such a rate yields a growth flux of 1.3 × 1021 cm−2 s−1, rich in carbon nanostructures (graphene platelets, nanotubes) with a characteristic aggregate size within 1–10 μm. The deposition during the inactive 0.9 s of the pulse was several orders of magnitude slower and consisted of amorphous carbon traces. Moreover, the nanoparticle distribution along the collecting probe is correlated with the pulse phase, thereby providing information on particle transport. Pulsed nanosynthesis can be modeled as a periodical growth process, where the volume and propagation velocity of the growth region can be adjusted through modulation of the pulse signal waveform. The proposed model constitutes a suitable framework to investigate the pulsed arc synthesis of nanomaterials with tailored physical and chemical properties.

Following the understanding of the cold atmospheric plasma jet control, the optimization of plasma parameters for biomedical applications has become an important area of research in the field of plasma-based cancer treatment. A real-time feedback signal is usually required by a control algorithm, such as a self-adaptive plasma jet, which is designed to automatically self-optimize its parameters to adapt to a variety of biomedical applications and situations. In this paper, we introduce the potential of replacing the cell viability or cell stress assay with electrochemical impedance spectroscopy (EIS) to provide a real-time feedback signal for a model predictive control (MPC) method aided by machine learning. The EIS frequency is in the kHz to GHz regime. Therefore, the MPC method is not only designed for minimizing the cancer cell viability, but also considered to optimize cell membrane behaviors and other chemical species dialing. Since these signals are in the range of GHz, we introduce alternatives for the impedance analyzer to measure the impedance spectrum, including a Fabry–Perot resonator and one of its scanning-array variations.

Pulsed anodic arc discharge is a novel synthesis method of nanomaterials by means of low-temperature atmospheric plasma. The technique consists in periodically supplying DC power to two vertically aligned electrodes in the form of short unipolar pulses with peak currents of a few hundred Amperes in a helium atmosphere. The pulsed arc plasmas are sustained at frequencies on the order of 1 Hz with around 10% of duty cycle. It constitutes a versatile technique thanks to a series of advantages compared to continuous DC arc processes, in particular, flexibility in the experimental conditions, process stability and repeatability, better utilization of ablating anode material, lower production of macroparticles, and lower thermal loads. Such features are discussed in this article. A brief overview concerning the recent accomplishments of pulsed arc discharge on deposition of carbon nanostructures (graphene and carbon nanotubes) and few-layer flakes of molybdenum disulphide and an outlook on future applications of this method for the discovery of new materials with tailored functional properties are provided.

Micropropulsion systems are rapidly gaining attention from the small satellite community as they can increase the mission lifetime and allow the satellite to perform complex maneuvers and precise attitude control. These systems need to be fully operational with the low power available on satellites. Various thruster concepts based on vacuum arcs are currently under development, predominantly in the pulsed regime due to the power constraints on small spacecraft. Pulsed vacuum arc thrusters are capable of efficiently producing highly-ionized supersonic plasma at very low average power. This Perspective article provides a critical analysis and a review of various aspects of electric propulsion technology based on vacuum arcs. Furthermore, we give a personal assessment of the present status and provide an outlook on the field, including the growing role in small satellites such as CubeSats. Vacuum arc micropropulsion systems could play an important role in mitigating the problem of space debris. Such a system could be integrated with a satellite so that, at the end of its mission and using metal components as solid fuel, it will lower the satellite’s orbit and accelerate reentrance into the atmosphere faster than by its natural decay rate.

Cold atmospheric plasma jet (CAPJ) is one of the latest and most promising techniques for potential cancer treatment and other biomedical applications. Due to the direct contact of air, the jet is sensitive to the parameters of the local environment such as relative humidity (RH). In a RH controlled chamber, the electron density of a helium CAPJ is measured using the Rayleigh microwave scattering method, and its optical emission spectroscopy (OES) is recorded using a spectrometer. A decreasing electron density along with the increasing RH is thus revealed, while the humidity effect on OES at a high discharge voltage is increased. These trends imply a shift of electron energy distribution function (EEDF) due to extra attachments of electrons as the physics behind such a phenomenon. This hypothesis is supported by a computation of the mean electron temperature and EEDF based on the electron density we measured and a plasma chemistry model. Therefore, this report is a basis of future CAPJ stabilization development, which is a necessity of reliable biomedical applications, such as an active control loop to make CAPJ immune to any accident environmental disturbance during a plasma-based cancer surgery.

This prospectus outlines some new ideas regarding plasma application in medicine. In particular, plasma parameter adaptation might allow for real time modification of the chemical composition of plasma in an effort to optimize the plasma effect on cancer and normal cells. Nowadays, there is convincing evidence that plasma effects might play an important role in cancer therapy. Among others, understanding plasma discharge self-organization, the mechanisms driving transition between different discharge patterns, and the development of the plasma devices having multiple discharge modes are very important aspects.

Rapid evolution of miniaturized, automatic, robotized, function-centered devices has redefined space technology, bringing closer the realization of most ambitious interplanetary missions and intense near-Earth space exploration. Small unmanned satellites and probes are now being launched in hundreds at a time, resurrecting a dream of satellite constellations, i.e., wide, all-covering networks of small satellites capable of forming universal multifunctional, intelligent platforms for global communication, navigation, ubiquitous data mining, Earth observation, and many other functions, which was once doomed by the extraordinary cost of such systems. The ingression of novel nanostructured materials provided a solid base that enabled the advancement of these affordable systems in aspects of power, instrumentation, and communication. However, absence of efficient and reliable thrust systems with the capacity to support precise maneuvering of small satellites and CubeSats over long periods of deployment remains a real stumbling block both for the deployment of large satellite systems and for further exploration of deep space using a new generation of spacecraft. The last few years have seen tremendous global efforts to develop various miniaturized space thrusters, with great success stories. Yet, there are critical challenges that still face the space technology. These have been outlined at an inaugural International Workshop on Micropropulsion and Cubesats, MPCS-2017, a joint effort between Plasma Sources and Application Centre/Space Propulsion Centre (Singapore) and the Micropropulsion and Nanotechnology Lab, the G. Washington University (USA) devoted to miniaturized space propulsion systems, and hosted by CNR-Nanotec—P.Las.M.I. lab in Bari, Italy. This focused review aims to highlight the most promising developments reported at MPCS-2017 by leading world-reputed experts in miniaturized space propulsion systems. Recent advances in several major types of small thrusters including Hall thrusters, ion engines, helicon, and vacuum arc devices are presented, and trends and perspectives are outlined.

Given the vast number of strategies used to control the behavior of laboratory and industrially relevant plasmas for material processing and other state-of-the-art applications, a potential user may find themselves overwhelmed with the diversity of physical configurations used to generate and control plasmas. Apparently, a need for clearly defined, physics-based classification of the presently available spectrum of plasma technologies is pressing, and the critically summary of the individual advantages, unique benefits, and challenges against key application criteria is a vital prerequisite for the further progress. To facilitate selection of the technological solutions that provide the best match to the needs of the end user, this work systematically explores plasma setups, focusing on the most significant family of the processes—control of plasma fluxes—which determine the distribution and delivery of mass and energy to the surfaces of materials being processed and synthesized. A novel classification based on the incorporation of substrates into plasma-generating circuitry is also proposed and illustrated by its application to a wide variety of plasma reactors, where the effect of substrate incorporation on the plasma fluxes is emphasized. With the key process and material parameters, such as growth and modification rates, phase transitions, crystallinity, density of lattice defects, and others being linked to plasma and energy fluxes, this review offers direction to physicists, engineers, and materials scientists engaged in the design and development of instrumentation for plasma processing and diagnostics, where the selection of the correct tools is critical for the advancement of emerging and high-performance applications.

Several interrelated phenomena near the surface ablated into a discharge plasma, such as ablation and ionization in accelerated plasma are studied. Two characteristic ablation modes are identified, namely, ablation mode with a velocity at the Knudsen layer edge smaller than the local sound speed and a velocity at the Knudsen layer edge close to the sound speed. The existence of these two ablation modes is determined by the current density in the acceleration region. The nonequilibrium ionization region in the presence of strong electromagnetic plasma acceleration is studied. In the subsonic regime, the ionization region thickness is proportional to the ionization rate and inversely proportional to the magnetic field. Conditions for ionization equilibrium in the accelerating plasma are determined. The specific example of a micropulsed plasma thruster is considered. It is concluded that both the equilibrium and nonequilibrium ionization regimes occur in this device.

An end-to-end model is presented of the transient plume created by a microlaser ablation plasma thruster. In this article, we describe a model of the plasma generation and expansion for a micro-laser plasma thruster operated in transmission-mode (T-mode). The laser ablation and plasma formation processes are modeled using a kinetic ablation model. This procedure provides boundary conditions at the target surface for the plume model that is based on a particle computational approach. The present study considers a 2.5–8 W diode-based laser irradiating a poly-vinyl chloride target for a pulse length of 3–10 ms. Laser beam shape full width at half maximum at the target is about 25×25 μm. The plume simulations reveal many details of the multicomponent plasma expansion. The results are compared with experimentally obtained plume signatures. Generally good agreement between experimental and calculated flux profiles is found.

Among Hall thruster technologies, the thruster with anode layer (TAL) has much wider technical capabilities, especially in the high-power regime of operation. In this paper, various aspects of the plasma flow in a high-power thruster with anode layer are studied. Based on a 2D hydrodynamic model, the formation of a space-charge sheath near the acceleration channel wall and the sheath expansion in the acceleration channel are calculated. It is found that the high-voltage sheath near the channel wall expands significantly and the quasineutral plasma region is confined in the middle of the channel. For instance, in the case of a 3 kV discharge voltage, the sheath thickness is about 1 cm, which is a significant portion of the channel width (which is typically a few cm). In addition, a simplified quasi-1D model is developed to study the anode acceleration layer, which is confined by channel walls. It is found that near-wall sheath expansion leads to an increase in current density along the channel, and this in...

Single walled carbon nanotubes (SWNT) produced by the anodic arc discharge over a range of constant background pressures of helium (100–1000 Torr) were examined under a high-resolution transmission electron microscope, and a Raman spectrometer. It was found that the average SWNT diameter is about 2 nm and fairly independent of the background pressure. Analysis of the relative purity of SWNTs samples suggests that highest SWNT relative concentration can be obtained at background pressure of about 200–300 Torr. Measured anode ablation rate increases linearly with background pressure. The model of the anodic arc discharge was developed. It was found that the predicted anode ablation rate agrees well with experiment suggesting that electron temperature in the anodic arc is about 0.5 eV.

Ion current distribution in a system with crossed magnetic and electrical fields for plasma immersion ion implantation has been investigated. It is found that the ion current to a target has a nonmonotonic behavior with bias voltage when a magnetic field is applied. For instance, the current density has a maximum of about 150 A/m2 at bias voltage of about 1 kV in the case of a magnetic field parallel to the target of about 0.035 T. These results are explained in terms of ionization by magnetized electrons in the E×B system. Our findings suggest that the system with crossed fields can be used for intense plasma immersed processing.

Segmented electrodes placed along a ceramic channel in a Hall thruster are shown to influence significantly the plasma potential distribution. Both the radial potential and the axial acceleration region are sensitive to the location of the segmented electrodes. The measured and theoretical potential profiles appear to be affected in detail by the electrode material ~graphite! having lower secondary electron emission than the ceramic channel walls. The measured plasma potential profile is shown as well to correlate with the observed and desirable narrowing of the plasma plume emanating from the thruster. © 2002 American Institute of Physics. @DOI: 10.1063/1.1510556#

Low temperature plasmas have been used in various plasma processing applications for several decades. But it is only in the last thirty years or so that sources generating such plasmas at atmospheric pressure in reliable and stable ways have become more prevalent. First, in the late 1980s, the dielectric barrier discharge was used to generate relatively large volume diffuse plasmas at atmospheric pressure. Then, in the early 2000s, plasma jets that can launch cold plasma plumes in ambient air were developed. Extensive experimental and modeling work was carried out on both methods and much of the physics governing such sources was elucidated. Starting in the mid-1990s, low temperature plasma discharges have been used as sources of chemically reactive species that can be transported to interact with biological media, cells, and tissues and induce impactful biological effects. However, many of the biochemical pathways whereby plasma affects cells remain not well understood. This situation is changing rather ...

The breakdown mechanism in the Micro-Cathode Arc Thruster has been studied to better understand the nature of the discharge ignition and to extend the ignition system lifetime. It has been found that optimal material selection of the insulator is an important factor during breakdown. Two opposite processes have been found to cycle during operation. The processes are degradation of the conductive film from the inter-electrode interface and re-deposition of the conductive film due to cathode spot erosion. Initial resistances were found to vary from hundreds of ohms to thousands of ohms based on the initial connectivity of the film to the electrodes. After initial breakdown however, resistances have been found to stabilize in a typical pattern. Materials capable of withstanding high temperatures, high pressures, and smooth surfaces are shown to be beneficial for extending thruster lifetime.

In this paper a model of the quasineutral plasma and the transition between the plasma and the dielectric wall in a Hall thruster channel is developed. The plasma is considered using a two-dimensional hydrodynamic approximation while the sheath in front of the dielectric surface is considered to be one dimensional and collisionless. The dielectric wall effect is taken into account by introducing an effective coefficient of the secondary electron emission (SEE), s. In order to develop a self-consistent model, the boundary parameters at the sheath edge (ion velocity and electric field) are obtained from the two-dimensional plasma bulk model. In the considered condition, i.e., ion temperature much smaller than that of electrons and significant ion acceleration in the axial direction, the presheath scale length becomes comparable to the channel width so that the plasma channel becomes an effective presheath. It is found that the radial ion velocity component at the plasma–sheath interface varies along the thr...

The vaporization of condensed materials in contact with high-current discharge plasmas is considered. A kinetic numerical method named direct simulation Monte Carlo (DSMC) and analytical kinetic approaches based on the bimodal distribution function approximation are employed. The solution of the kinetic layer problem depends upon the velocity at the outer boundary of the kinetic layer which varies from very small, corresponding to the high-density plasma near the evaporated surface, up to the sound speed, corresponding to evaporation into vacuum. The heavy particles density and temperature at the kinetic and hydrodynamic layer interface were obtained by the analytical method while DSMC calculation makes it possible to obtain the evolution of the particle distribution function within the kinetic layer and the layer thickness.

The new mode of Vacuum arc-Hot Refractory Anode Vacuum Arc-was studied experimentally using a Langmuir probe, two types of thermal probes, and film collection substrates. The plasma density, electron temperature, plasma energy flux, cathode erosion, mass deposition rate on a substrate, and macroparticle contamination in the deposited films were measured. The arc initially operated as a usual vacuum arc sustained by cathode spots, i.e., and the vapor and plasma source located at the cathode spot. At a later stage the anode heated up and metal vapor originating at the cathode was re-evaporated from the nonconsumable hot graphite anode. Initially, plasma density was about (3–4)⋅1020 m−3 but it increased with time, reaching about 2⋅1021 m−3 after 60 s in a 340 A arc. The electron temperature initially was about 1.6 eV and decreased with time to a steady-state value of about 1.1 eV after 20 s. The radial plasma energy flux generated by 175 and 340 A arcs was about 1 and 2 MW/m2, respectively, at 1.6 cm from the electrode axis. The deposition rate on substrates placed 110–120 mm from the electrode axis reached about 2 μm/min. The density of macroparticles found on substrates exposed during the first 60 s of arcing was ∼103 macroparticles per mm2, however, this density was reduced to about 1 macroparticle per mm2 on substrates exposed to only the second 30 s period.

We consider the case of energetic ion beam formation when the ion streaming velocity within the source plasma is substantial, i.e., when the ions have a drift speed (in the positive downstream direction) that is on the order of or greater than the ion acoustic speed in the plasma. Some interesting consequences can follow, including the capability of a negatively biased substrate located in the plasma stream to maintain high bias voltage, and of an ion source with no extractor or “conventionally poor” extractor providing a kind of plasma immersion ion implantation mode of operation. Here we summarize the kind of plasma geometry in which this situation can occur, and describe some experimental observations we’ve made of these effects, with reference to a simple theoretical basis for the mechanism.

An axial profile of the quasineutral plasma jet exhaust from a Hall thruster is studied. The plasma jet expansion is modeled using the sourceless steady-state hydrodynamic equations. It is considered that the plasma jet has a conical shape with a half angle of about 40°. The magnetic field surrounding the Hall thruster exit is included in the calculation. It is found that a magnetic field may significantly affect the axial profile of the plasma potential. For instance, in the case of zero magnetic field, the plasma potential is about −10 V at 1 m from the thruster exit, while in the case of a 0.1 T magnetic field, the plasma has a potential of about +25 V. Results predicted by the model are found to be in good agreement with experimental data for three different Hall thruster designs.

The free, steady state, two-dimensional radial plasma flow initiated between a pair of disk-shaped electrodes of a hot anode vacuum arc was analyzed in the hydrodynamic approximation. Studies include the influence of the self-magnetic field on the plasma density, velocity, radial spreading of the arc current and potential distribution. The free plasma boundary was calculated by solving the equations for the normal and tangential velocity components at the free boundary. It was found that the plasma significantly expands over a radial distance of about half of the interelectrode gap counted from the electrode edge and the plasma density in the center plane decreases by factor of 2, whereas the density of the fringe current decreases by a factor of 10. The self magnetic field does not influence the plasma flow and current spreading at radial distances larger than the interelectrode gap. The potential distribution is strongly nonsymmetric with respect to the central plane due to the influence of the plasma d...

The objective of the present work was to determine the influence of positive bias on plasma and macroparticle (MP) flow in curved magnetized plasma ducts. The plasma bulk and sheath regions were analyzed. In the plasma bulk, the current density and electrical field component normal to the wall were obtained and used as boundary conditions for the near wall sheath region. In the sheath, a nonstationary model for MP charging and motion was developed. The solution of the hydrodynamic equations in the plasma when a positive bias is applied to the wall result in a radial electrical current. The electric field in the plasma bulk is generated by the separation between the magnetically confined electrons, and the ions, which are thrown outwards by the centrifugal force. The field increases with increasing positive bias. It was shown that MPs traveling in the sheath accumulate a charge which depends on the potential distribution, in contrast to MP charging in the quasineutral plasma where the charge depends on plasma density and electron temperature. MP trapping in the near-wall sheath was found. MPs may move in the sheath region along the wall by a repetitive process of electrostatic attraction to the wall, mechanical reflection and neutralization, followed by MP charging and attraction, etc. For example, titanium MPs with a radius less than 0.4 μm and with a velocity component normal to the wall of about 20 m/s are trapped if the sheath potential drop exceeds 20 V. It was obtained that the MP transmission fraction through filter decreases by more than few orders of magnitude due to the trapping effect when a bias potential of +100 V is applied between the wall and the plasma.

The free boundary plasma arc jet expansion was analyzed based on a two-dimensional hydrodynamic model. The plasma jet expansion was modeled using the sourceless steady-state hydrodynamic equations, where the plasma free boundary was determined self-consistently. Due to the existence of an electric field in the quasineutral plasma the different charged ion species can be spatially separated. It was found that the mean charge state distribution is strongly nonuniform with a tendency for the highly charged species to appear near the plasma jet boundary region. Along the center line the density of singly charged ions falls off by about four times while the density of fourfold charged ions drops by more than 100 times. The radial charge state distribution becomes more nonuniform with increasing magnetic field. Good qualitative agreement between calculated and experimental radial distributions of different charged species was obtained.