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We present an approach to identifying topological edge states in two dimensional (2D) problems. Such scattering-immune surface waves are typically identified using the Berry concept, which evaluates Chern numbers of bulk transmission bands. Instead, here, we approach the task from an analytical electromagnetic scattering perspective, which enables the study of a wide class of configurations in which waves are confined to propagate at the interface between a topologically non-trivial magnetized plasma and a topologically trivial medium such as air. Although it is of theoretical significance to classify interfacial states by their topological invariants, we believe that this approach enables the engagement of a broader range of researchers, particularly experimentalists who seek to design devices that exploit the ensuing non-reciprocal and scattering-immune properties.

We investigated the use of plasma-fixated nitrogen, which produces nitrates (NO3−) in water, as a possible nitrogen fertilizer for recreational turf such as rye grass and bent grass. Experiments were carried out to study the effects of nitrate concentration on growth, the further effects of adding phosphorous (P) and potassium (K) to the plasma nitrated solution to make an N–P–K complete fertilizer, and to compare the efficacy of plasma-fixated nitrogen to sodium nitrate (NaNO3) and potassium nitrate (KNO3). The results indicate that the growth and biomass of the plants were strongly dependent on the concentration of the plasma-fixated nitrogen. Adding P–K to the plasma-fixated nitrogen improved grass growth. Grass that was supplied plasma-fixated nitrogen had improved growth compared to those supplied with equal amounts of NaNO3 and KNO3. This work highlights the potential use of plasma-fixated nitrogen as a fertilizer source for commonly used turf grass.

The Quad Confinement Plasma Source is a novel plasma device developed for space propulsion applications, whose core is an E×Bdischarge with open electron drift. The magnetic field is produced by independently powered electromagnets able to generate different magnetic field topologies with the ultimate aim of manipulating the ion flow field for achieving thrust vectoring. In this work, we map the ion velocity in the plasmaejected from the Quad Confinement Thruster with different magnetic configurations using non-intrusive Laser-Induced Fluorescence diagnostics. Measurements show a steep ion acceleration layer located 8 cm downstream the exit plane of the discharge channel, detached from any physical boundary of the plasma source. In this location, the ion velocity increases from 3 km/s to 10 km/s within a 1 cm axial region. The ion acceleration profile has been haracterised under multiple testing conditions in order to identify the influence of the magnetic field intensity and topology on this peculiar ion acceleration layer.

We present a theoretical analysis and experimental demonstration of a microwave circulator that uses a magnetized plasma as the gyrotropic element. Unlike traditional circulators that exploit the anisotropic permeability of ferrite, here, we exploit the anisotropic dielectric constant of a magnetized plasma to achieve non-reciprocity. The advantage of a plasma-loaded circulator is that it allows for tunability, and modulation that is limited in speed by the ionization/recombination time of the plasma. The theoretical analysis treats the problem of electromagnetic scattering from a heterogeneous gyrotropic plasma rod to confirm scattering behavior and to guide in defining the design parameter space covered by more detailed computational simulations. Proof-of-concept experiments are carried out using a custom-fabricated low-frequency gas discharge tube as the plasma element. Here, we confirm the plasma-frequency dependent tunability predicted by the simulations, demonstrating this tunability in the experiments by varying the applied discharge voltage. These initial experiments suggest that isolation in excess of 25 dB is achievable, in reasonable agreement with the theory and simulations for studies in the S and C ranges of the microwave spectrum.

In an expansion of a previous study [1], we apply inverse design methods to produce two-dimensional plasma metamaterial devices with realistic plasma elements which incorporate quartz envelopes, collisionality (loss), non-uniform density profiles, and resistance to experimental error/perturbation. Backpropagated finite difference frequency domain simulations are used to design waveguides and demultiplexers operating under the transverse magnetic polarization. Optimal devices with realistic elements are compared to previous devices with idealized elements, and several parameter initialization schemes for the optimization algorithm are explored. Demultiplexing and waveguiding are demonstrated for microwave-regime devices composed of plasma elements with reasonable space-averaged plasma frequencies ~10 GHz and a collision frequency ~1 GHz, allowing for future in-situ training and experimental realization of these designs., 9 pages, 9 figures

We apply inverse design methods to produce two-dimensional plasma metamaterial (PMM) devices. Backpropagated finite difference frequency domain (FDFD) simulations are used to design waveguides and demultiplexers operating under both transverse electric (TE) and transverse magnetic (TM) modes. Demultiplexing and waveguiding are demonstrated for devices composed of plasma elements with reasonable plasma densities ~7 GHz, allowing for future in-situ training and experimental realization of these designs. We also explore the possible applicability of PMMs to nonlinear boolean operations for use in optical computing. Functionally complete logical connectives (OR and AND) are achieved in the TM mode.

We report on time-resolved measurements of electron number density by continuous-wave laser absorption in a low-energy nanosecond-scale laser-produced spark in atmospheric pressure air. Laser absorption is a result of free-free and bound-free electron excitation, with the absorption coefficient modeled and evaluated using estimates of the time-variation in electron temperature and probe laser absorption path length. Plasma electron number densities are determined to be as high as $n_\text{e}=7\times10^{19}$ cm$^{-3}$, and decay to $1/e$ of their peak values over a period of about 50 ns following plasma formation using a 20 mJ, 10 ns pulse width frequency-doubled Nd:YAG laser. The measured plasma densities at later times are shown to be in reasonable agreement with Stark broadening measurements of the 3s[$^5S{^o}$]-3p[$^5P$] electronic transition in atomic oxygen at 777 nm. This study provides support for the use of such continuous wave laser absorption for time resolved electron density measurements in low energy spark discharges in air, provided that an estimate of the electron temperature and laser path length can be made by accompanying diagnostics., 9 pages and 7 figures

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 experimentally demonstrate the controlled scattering of incident transverse-electric electromagnetic waves from a gyrotropic magnetized plasma cylindrical discharge. Scattered electromagnetic waves can bend left and right by changing the external magnetic field of a plasma rod. Measured scattered wavefronts are in good agreement with electromagnetic simulations. A gyrotropic response is observed for incident wave frequencies ranging from 3.5 to 5.6 GHz for conditions corresponding to a ratio of cyclotron frequency to plasma frequency, [Formula: see text] 0.16. The observation of a gyrotropic response from cylindrical plasma discharges paves the way for their use as building blocks for future devices such as magnetized plasma photonic crystals, topological insulators, plasma metamaterials, non-reciprocal waveguide structures, and other devices, which require a tunable gyrotropic response from centimeter to meter-scale materials with application-specific geometry.

The manner in which magnetized plasma jets evolve remains key to better understanding the behavior of hydromagnetic systems and providing new insights into how they can be dynamically controlled. In this work, we present the underlying theory, apparatus, and optical features of a schlieren diagnostic capable of cinematically visualizing dense plasma jets. We identify the range of plasma density over which the diagnostic is sensitive and describe ways to further improve image contrast. This diagnostic features the unique ability to simultaneously resolve both the characteristic Alfvenic timescales and spatial flow features with continuous acquisition over the lifetime of a jet. We use this diagnostic to visualize the formation and evolution of hydromagnetic jets produced from a plasma gun device. Dynamic coherent flow features are identified and tracked over time throughout the evolutionary progression of plasma jets. Finally, the process by which these coherent features translate into perturbations of magnetized bow shocks is visualized.

We report on the results of an experimental campaign to measure time-varying velocity distributions in the near-field of a low power Hall thruster. We employ a sample-hold technique, enhanced by parallelizing the measurement hardware into several signal processing channels that vastly increases the data acquisition rate. The measurements are applied to study flow field dynamics in a commercial BHT-600 Hall thruster undergoing unforced breathing mode oscillations in the 44–49 kHz range. A very detailed experimental picture of the near-field emerges from these studies. The results indicate that velocity fluctuations lessen further downstream of the exit plane. Along the thruster axis where there is a general appearance of a central jet, there is evidence of a low velocity ion population in between the periodic bursts of high velocity ions, indicative of local ionization of neutrals outside of the thruster. One possible source of this residual ionization may be background chamber gas, which is not unexpected with the limited pumping capacity of ground test facilities.

We report here on initial studies of a pulsed hydromagnetic plasma gun that can operate in either a pre-filled or a gas-puff mode on demand. These modes enable agile and responsive performance through tunable thrust and specific impulse. Operation with a molecular nitrogen propellant is demonstrated to show that the hydromagnetic thruster is a candidate technology for air-harvesting and drag compensation in the very low Earth orbit. A dual mode operation is achieved by leveraging propellant gasdynamics to change the fill fraction and flow collisionality within the thruster. This results in the formation of distinct modes that are characterized by the current-driven hydromagnetic waves that they allow, namely, magneto-deflagration and magneto-detonation, respectively. These modes form the basis of using gasdynamics to enable responsive thruster performance. Using time-of-flight emission diagnostics to characterize near-field flow velocities, we find that a relatively dramatic transition occurs between modes as gas is allowed to expand in the thruster, with exhaust velocities ranging from 10 to 55 km/s in the deflagration and detonation regimes, respectively. Simulations of the processed mass bit offer the first glimpse into possible thruster performance and trade-offs between specific impulse and thrust. An impulse bit tunability of ∼22% is predicted, with differing propellant fill fractions when operating in a burst mode.

Air-breathing electric propulsion is a promising concept that may allow sustained access to very low earth orbit by guaranteeing continuous drag compensation through in situ harvesting of propellant. For earth observation, such low orbits benefit image resolution and reduce signal latency and transmission power requirements, easing link budgets. Moreover, not having to carry typical electrostatic thruster propellants such as xenon and tanks positively impact overall mission costs. However, the low molecular mass and high first ionization energies of principal atmospheric constituents hinder efficient thruster operation and result in strongly degraded performance. In this study, we examine the performance of an extended channel Hall thruster design for air-breathing operations. An extended channel compensates for the reduced residence times of lighter neutrals. The longer channel, coupled with an extended radial magnetic field region, allows for an increased length of the ionization zone, enhancing molecular ionization probability. Here, we present direct thrust measurements of this thruster operating with a pure nitrogen flow. Analysis of the results show performance in the 500–800 W anode power range, with thrust, specific impulse, and total thrust efficiency ranging from 17 to 22 mN, 1000 to 1100 s, and 14% to 18%, respectively, with a constant 2 mg/s pure molecular nitrogen propellant flow. Performance tends to degrade with increased voltage, suggesting increased contributions of electron-molecule kinetics to performance losses, contrary to what is typically seen in the Hall thrusters operating with propellants such as xenon.

We present a microstrip photonic bandgap (PBG) device with a switchable plasma element integrated into the patterned ground plane. A pulsed laser plasma is generated in one of the PBG cavity structures creating a medium having an effective negative permittivity in the 8–10 GHz frequency range due to the high plasma density ne = 1017 cm−3. The PBG structure consists of a linear array of seven cavities along the length of the microstrip manufactured by removing cylindrical portions of copper in the ground plane. The plasma is produced in the central cavity of the PBG array and recombines over times of several microseconds allowing for the characterization of the microstrip transmission over a range of plasma density. The results confirm a switchable transmission due to plasma interactions with the electromagnetic field within the PBG structure.

This paper presents the design of a low-mass RF power inverter at moderate to high power levels (e.g., tens of watts and above) for weight critical applications such as CubeSat plasma thrusters. Our approach for mass reduction includes resonant switching operation at tens of MHz, and the 3-D printing of lightweight scaffolds that form air core inductors after plating with a thin layer of copper. Specifically, we present a 14.2-MHz 50-W resonant dc-RF power inverter implemented with 3-D printed and copper plated air core toroidal inductors. As a demonstration of design flexibility of the 3-D printing process, these toroidal inductors are designed and implemented with optimal cross sections [1] to improve quality factors. The weight of the proposed inverter is reduced by 50% when compared to a 40-W state-of-the-art counterpart in which all the inductors were implemented within the printed circuit board. The inverter achieves 91% electrical efficiency when operated on a 50- $\Omega $ resistive load at an input voltage of 50 V. To prove its capability of driving plasma loads, the inverter was first tested with a matching network running an inductively coupled plasma (ICP) in a low-pressure argon ampoule outside vacuum environment. The inverter drew 50 W to run the ICP at an input voltage of 20 V. Then as an example application, the inverter was tested with another specifically tuned matching network to drive a helicon double layer (HDL) plasma intended for CubeSat thruster applications. The HDL plasma was formed in a vacuum chamber and was fed with argon. The combined inverter and matching network outputs 40 W and achieves 86% dc input to plasma efficiency at an input voltage of 40 V.

The replication of astrophysical flows in the laboratory is critical for isolating particular phenomena and dynamics that appear in complex, highly-coupled natural systems. In particular, plasma jets are observed in astrophysical contexts at a variety of scales, typically at high magnetic Reynolds number and driven by internal currents. In this paper, we present detailed measurements of the plasma parameters within deflagration-produced plasma jets, the scaling of these parameters against both machine operating conditions and the corresponding astrophysical phenomena. Using optical and spectroscopic diagnostics, including Schlieren cinematography, we demonstrate the production of current-driven plasma jets of ∼100 km/s and magnetic Reynolds numbers of ∼100, and discuss the dynamics of their acceleration into vacuum. The results of this study will contribute to the reproduction of various types of astrophysical jets in the laboratory and indicate the ability to further probe active research areas such as jet collimation, stability, and interaction.

International audience

International audience; We describe the use of a plasma-functionalized vacancy defect in a one-dimensional microwave photonic crystal to experimentally measure the electron number density of glow discharges at 5--40 torr. The photonic crystal consists of spaced alumina plates with a built-in void defect that breaks the repeating symmetry of the layers, resulting in narrow defect transmission peaks within relatively deep bandgaps. We exploit the sensitivity of the defect transmission at 28 GHz to varying plasma density to measure electron number densities as low as 2 × 109 cm−3. Defect energy shifts are proportional to plasma density, in reasonable agreement with theoretical predictions of photonic crystal performance. At higher discharge current densities and discharge pressure, we see a departure from the model predictions, largely attributable to the heating of the alumina structure, causing expansion and changes in the lattice parameter that counteract the effect of the increased plasma density on the defect state frequency.

Instabilities play a prominent role in determining the inherent structure and properties of magnetized plasma jets spanning both laboratory and astrophysical settings. The manner in which prominent unstable modes dynamically evolve remains key to understanding plasma behavior and control. In astrophysical phenomena, self-similar jets are observed to propagate over vast distances while avoiding breakup caused by unstable mode growth. However, the production of stable dense plasma jets in the laboratory has been limited by the onset of unstable modes that restrict jet lifetime, collimation, and scalability. In this work, we visualize the formation of stable laboratory-generated, dense, super-magnetosonic plasma jets in real time, and we identify an underlying mechanism that contributes to this behavior. The current-driven plasma jets generated in our experiments form a flowing Z-pinch, which is generally unstable to the m = 1 kink instability. Our results indicate that a stable dense plasma jet can be maintained for timescales over which a steady pinch current can be sustained, even at levels which would otherwise lead to rapid unstable mode growth and resultant pinch disassembly.

The introduction of 3D printing has enabled fabrication of photonic crystal devices with complex crystal structures that would be challenging to construct using other fabrication methods. In this paper, we construct and characterize a photonic crystal consisting of two 4 × 8 × 8 cubic lattices composed of spherical silicon nitride elements straddling a layer of 8 plasma discharge tubes, creating a 3D hybrid plasma photonic crystal device. Integrating under-dense gaseous plasma elements provides a unique coupling dynamic between the dielectric spheres and the cylindrical plasma discharges, creating a monolithic hybrid photonic crystal with solid state and reconfigurable elements. The device has resonant modes that have attenuation peaks that are either switchable, tunable in amplitude, or tunable in frequency with variations in plasma density. The response of these bands seen with varying plasma density is confirmed through simulations when effects due to the heating of the photonic crystal from the gaseous plasma elements are accounted for in the experiments. We discuss how this reconfigurable device may be used and expanded upon for applications in photonic artificial neural networks and optical computing systems.

We present experimental studies of inverted-corona targets as neutron sources at the OMEGA Laser Facility and the National Ignition Facility (NIF). Laser beams are directed onto the inner walls of a capsule via laser-entrance holes (LEHs), heating the target interior to fusion conditions. The fusion fuel is provided either as a wall liner, e.g., deuterated plastic (CD), or as a gas fill, e.g., D2 gas. Such targets are robust to low-mode drive asymmetries, allowing for single-sided laser drive. On OMEGA, 1.8-mm-diameter targets with either a 10-μm CD liner or up to 2 atm of D2-gas fill were driven with up to 18 kJ of laser energy in a 1-ns square pulse. Neutron yields of up to 1.5 × 1010 generally followed expected trends with fill pressure or laser energy, although the data imply some mix of the CH wall into the fusion fuel for either design. Comparable performance was observed with single-sided (1x LEH) or double-sided (2x LEH) drive. NIF experiments tested the platform at scaled up dimensions and energies, combining a 15-μm CD liner and a 3-atm D2-gas fill in a 4.5-mm diameter target, laser-driven with up to 330 kJ. Neutron yields up to 2.6 × 1012 were measured, exceeding the scaled yield expectation from the OMEGA data. The observed energy scaling on the NIF implies that the neutron production is gas dominated, suggesting a performance boost from using deuterium–tritium (DT) gas. We estimate that neutron yields exceeding 1014 should be readily achievable using a modest laser drive of ∼300 kJ with a DT fill.

Transient events in fusion power plants such as DEMO and ITER are known to pose a severe threat to plasma facing components (PFCs) due to melting and erosion after repeated edge localized mode (ELM) loads. In situ experimental testing of potential PFC materials at fusion relevant conditions is difficult and expensive, and as a result plasma devices capable of replicating the desired transient conditions are of increasing interest. At Stanford University, an experimental facility designed to mimic the heat flux, particle fluence, and other key characteristics of ELMs and disruption events in a controlled setting has been developed. A pulsed plasma accelerator operating in the deflagration mode is used to generate high-velocity (40–100 km/s) directed plasma jets that are stagnated on target material samples. In this paper, we present probe data characterizing the plasma parameters of the accelerated plume using hydrogen as the working gas, as well as preliminary target studies of silicon and copper witness plates exposed to pulses at various total and peak shot energies. Results from the probe analysis indicate achieved energy fluxes and heat flux parameters that are ELM-like, and the observed linearly correlated damage morphologies on the witness plates indicate that initial surface roughness plays a significant role in the growth characteristics of surface damage patterns. These results lay the groundwork for future studies of ELM-like loading on reactor-relevant materials using the Stanford plasma accelerator facility.

We report on numerical simulations of laser-driven convergent plasma fusion targets. These “inverted corona” fusion targets are useful for the study of counter-streaming and converging rarefied plasma flows, and previous experiments have demonstrated their potential as neutron sources. The scheme consists of a fuel layer lined along the interior surface of a hollow plastic shell that is laser-ablated and expands inward towards the target center. The plasma streams generated in these targets are initially nearly collisionless as they converge, leading to wide interaction length scales and long interaction time scales as the jets interpenetrate. Such kinetic effects impact mixing of constituent ions - a phenomenon not properly captured by single-fluid hydrodynamic simulations. Here we conduct numerical simulations using two different methods: (1) single-fluid simulations in HYDRA, and (2) kinetic-ion, fluid-electron hybrid particle-in-cell (PIC) simulations in the code Chicago. It is shown that the initially nearly collisionless plasma fronts interpenetrate deeply and lead to broader interaction regions in space and time resulting in significant beam-beam fusion. The two approaches make different, testable predictions for the effect of fuel-layer thickness on neutron yield.

We report on the dynamic behavior of gradient-driven drift waves in a strongly obstructed magnetron discharge. The magnetron has a magnetic topology that results in a toroidal plasma within the gap and supports the development of very coherent modes of rotating plasma structures. The modes and their rotation are present over a wide range of conditions, and the rotation is retrograde to the usual externally imposed E × B direction. This feature seems to be unique to this device and is attributed to a field reversal due to the strong anode-directed electron diffusion that arises from large axial plasma density gradients. A multi-fluid model is proposed, and a Fourier analysis of the linearized equations results in the identification of conditions that support the growth of these instabilities and their transitions across mode symmetries, controlled experimentally by varying the discharge voltage. The model also provides insight on the possible mechanism driving cross-field particle transport. Experiments are carried out with a segmented anode to confirm the localized current flow concomitant with the presence of a coherent structure. These segment currents together with high speed videography unambiguously confirm the direction of plasma rotation and reveal the existence of a stochastic regime between voltage-controlled mode transitions. An analysis of the segment currents in this regime indicates that the lower frequency state decays into a spectrum of coherent higher frequency states that exhibit features consistent with a three-wave nonlinear parametric mixing process.

We report on dual-polarization Dirac cones in a simple square lattice two-dimensional (2D) photonic crystal (PC) based on transmission at accidental degeneracies centered at the k = 0 symmetry ( Γ ) point. Finite difference time domain simulations are used to identify the material and geometric parameters for Dirac-like dispersion. A configuration that produces a Dirac-like point for both transverse electric and transverse magnetic polarizations at the same frequency is presented. The PC dispersion shows the expected threefold degenerate linear branch crossings at the Dirac-like point. Full-field electromagnetic wave simulations exhibit some common behaviors of devices based on Dirac-like dispersion, such as cloaking and waveguiding. The configuration works for a considerable range of the parameter space, and thus is experimentally realizable with a wide range of materials.

Extraordinary wave transmission is demonstrated through a double-negative composite comprised of a negative-permeability array of double split ring resonators and a negative-permittivity array of plasma discharge tubes at microwave frequencies. A transmission peak emerges in a double-negative band and controlling the electron density inside the plasma tubes dynamically regulates the transmission properties. By performing experiments and theoretical calculations, we verify that the composite permits wave propagation with negative permeability and controllable permittivity, which indicates that a tunable negative-refractive-index device is achieved.

Degradation of first wall materials due to plasma disturbances severely limit both the lifetime and longevity of fusion reactors. Among the various kinds of disturbances, type I edge localized modes (ELMs) in particular present significant design challenges due to their expected heat loading and relative frequency in next step fusion reactors. Plasma gun devices have been used extensively to replicate ELM conditions in the laboratory, however feature higher density, lower temperatures, and thus higher flow collisionality than those expected in fusion conditions. This work presents experimental visualizations that indicate strong shocks form in gun devices over spatial and temporal scales that precede ablation dynamics. These measurements are used to validate detailed magnetohydrodynamic simulations that capture the production of plasma jets and the shielding effect collisionality plays in particle transport to material surfaces. Simulations show that self-shielding effects in plasma guns reduce the free streaming heat flux by up to 90% and further reduce the incoming particle kinetic energy impinging on material surfaces. These simulations are performed over a range of operating conditions for gun devices and a discussion is provided regarding how existing experimental measurements can be interpreted when extrapolating to fusion conditions.

This paper reports on the study of repetitive nanosecond-pulsed discharge splitting of carbon dioxide (CO2) for the production of CO. Gas chromatography is used to analyze the composition of the reformed gas when CO2 is exposed to high-voltage (15 kV) very short (10 ns) electrical discharges that deposit as much as 0.4 mJ of energy at a rate of 30 kHz. Conversion rate and energy efficiency are obtained while the discharge pressure is varied between 2.4 and 5.1 atm. At the tested conditions, the maximum conversion rate and energy efficiency are found to be 7.3% and 11.5%, respectively. The energy efficiency drops slightly with increased pressure because of the decreased electric field and electron energy per molecule. An energy balance analysis of a set of CO2 plasma reactions reveals that the dominant dissociation pathway under these conditions passes through the excitation of CO2 (10.5 eV) followed by autodissociation into CO and O, which are often in excited states.

The reduction of a mechanism describing plasma chemistry for dry air has been carried out for conditions of nanosecond-pulsed discharge plasmas. The discharge conditions include both diffuse glow and filamentary spark modes. A reduced set for glow discharges is found to have inelastic electron-impact reactions of N2 and O2, quenching of excited N2, electron attachment to O2−, ion conversion between N2+, N4+, O4+, O2+·N2, and O2+, and ion–ion and electron–ion recombination. When the discharge is in a filamentary mode, additional reactions are required beyond those for the glow discharge as a result of the increased electron number density and high levels of molecular dissociation. Those additional reactions include electron-impact excitation of N and O, quenching of excited N2 by N and O, and associative ionization by collisions between excited N2. A significant reduction in the number of species and reactions is obtained without compromising the predicted species’ number densities and temperature. These results provide researchers with reduced kinetic mechanisms that high-fidelity simulations could rely on to save computational time.

A 2-D axial–azimuthal model of an annular Hall thruster discharge is developed. We use a hybrid fluid–particle-in-cell (PIC) treatment to model a Xe-propellant Hall thruster. Using a PIC approach, the positive Xe+ ions are modeled as collisionless, nonmagnetized, discrete super-particles, whereas the electrons are treated as a magnetized 2-D fluid. The model includes a continuously replenished Xe neutral background, with an imposed radial magnetic field and an applied axial electric potential. Motivated by experimental evidence of anomalously high electron mobility across the magnetic field that has been attributed to quasi-coherent fluctuations in the plasma properties, we use the numerical model to resolve azimuthal electron dynamics and study the impact of fluctuations on electron transport. Representative low-voltage (100 V) operating condition simulations predict localized plasma properties similar to those observed in experiments. The simulations predict coherent azimuthally propagating disturbances which appear to contribute to enhanced electron transport beyond that due to classical scattering.

An entropy closure formulation for Hall thrusters was implemented in a radial–axial hybrid simulation. We modeled the entropy production and its scaling with effective collision frequency and magnetic field. This entropy source was used in an entropy transport equation resulting in a differential equation for the effective electron mobility. With this formulation, the effective electron mobility is a computed variable. This paper presents the results of 2-D hybrid simulations of a laboratory Hall thruster that incorporate this model and compares these results to simulations using both Bohm and experimental measurements for the mobility. Simulations were also performed using this model for varying discharge voltage and background gas density. The background gas density is found to have a significant effect on the computed discharge current. The entropy closure model is found to better capture the current–voltage characteristics that are measured experimentally than other models used for electron transport.

Measurements are presented which examine the use of gaseous plasma elements as highly-tunable resonators. The resonator considered here is a laser-induced plasma kernel generated by focusing the fundamental output from a Q-switched Nd:YAG laser through a lens and into a gas at constant pressure. The near-ellipsoidal plasma element interacts with incoming microwave radiation through excitation of low-order, electric-dipole resonances similar to those seen in metallic spheres. The tunability of these elements stems from the dispersive nature of plasmas arising from their variable electron density, electron momentum transfer collision frequency, and the concomitant e↵ect of these properties on the excited surface plasmon resonance. Experiments were carried out in the Ku band of the microwave spectrum to characterize the scattering properties of these resonators for di↵erent values of electron density. The experimental results are compared with results from theoretical approximations and finite element method electromagnetic simulations. The described tunable resonators have the potential to be used as the building blocks in a new class of all-plasma metamaterials with fully three-dimensional structural flexibility.

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.

Two successive focused laser pulses are employed to experimentally simulate laser-induced breakdown plasmas at high repetition rates. We find that energy absorption of the second laser pulse by the plasma produced by the first laser pulse is enhanced slightly when the time interval between the pulses is shorter than several tens of nanoseconds but falls to almost zero when the time interval is between a few hundreds of nanoseconds and several tens of microseconds. This behavior is attributed to gas heating by the first breakdown event. In premixed ethane–air mixtures, we identify another strong reduction in the second laser pulse absorption when this pulse coincides with the heat released by combustion, typically milliseconds after the first laser pulse. The fuel–air equivalence ratio (ϕ) and base flow speed are also varied in this study. The results show that the window of reduced absorption coinciding with heat release due to combustion is narrowed when the base flow speed is increased, and also under fuel lean and fuel rich conditions. These results suggest that the use of pulsed high frequency laser breakdowns for premixed combustion stabilization is optimized when laser pulse repetition rates below a certain frequency (e.g., 500 Hz at the conditions that ϕ is 1 and the base flow speed is 4.9 m/s) to maximize laser energy coupling and for improved anchoring of the flame base.

The ion plume of a 72mm diameter Hall Effect Thruster operated on mixtures of xenon/nitrogen and xenon/air is investigated by means of a Wien filter (or E x B probe). The dependence of the velocities of the plume ions (Xe⁺, Xe²⁺, Xe³⁺, O₂⁺, O⁺, N₂⁺ and N⁺) on the operating parameters of the thruster (anode voltage, anode power, mass flow rate and magnetic field) is explored. The most probable ion acceleration voltages, the ion current and density fractions of the multi-propellant, multi-species ion beam, are computed from the Wien filter spectra through a dedicated post-processing analysis. The knowledge of these properties is fundamental for understanding the contribution of each ion species to the propulsive performance metrics of the thruster when operated on these molecular gas mixtures.

A low-cost, fully automated, electromagnetic field probing robot is described in this paper. The platform, called OpenEM Field Robot, is capable of mapping the spatial electromagnetic fields in free space. The system supports multiple antenna configurations and is scalable to large measurement domains. 3D printed antenna holders are also incorporated in order to allow for the near field measurements with minimal scattering and interference with the measurement. The goal of the system is to design a fully integrated platform for measuring electromagnetic fields that can be used with various other RF measurement devices and is configurable and easily modified for various applications in scientific research and education. Keywords: Electromagnetic field measurement, Microwave, RF, Field measurement

A dual-frequency driven plasma actuator is studied with respect to the influence of changing the position of radio frequency 13.56 MHz pulses within the phase of a low-frequency (LF) 5 kHz sinusoidal voltage. Fast imaging and 2-D particle image velocimetry are used to study the optical emission from the discharge and the generation of an induced flow, which can be useful for aerodynamic flow control, in ambient air. The results show that the propagation of the discharge away from the powered electrode and the velocity of the induced flow are maximized when the pulses are positioned at the peaks of the LF waveform. In contrast, the most intense optical emission is found to occur when the pulses are positioned close to the zero crossings for positive LF waveform voltages. The reasons behind these observations are discussed.

The characteristics of vortices induced by spanwise forcing using a streamwise-oriented dielectric barrier discharge (DBD) vortex generator (VG) are investigated with and without a boundary layer bleed slot, the flow of which is also driven by a DBD actuator. The velocity fields of the induced flows are characterized by ensemble averaged particle image velocimetry. A stronger downward motion and proximity of the induced vortex to the forcing electrode and the wall are observed when the DBD VG is used in conjunction with a DBD-active bleed slot. The effect of varying the applied voltage and freestream velocity on the induced vortex is examined. Researches are carried out on the ability for this DBD VG to control the separation on an inclined flat-plate and diverging ramp. We find that the DBD VG, together with DBD-activated bleed slots, leads to more robust separation control in these adverse pressure gradient configurations.

A dielectric barrier discharge (DBD) actuator is used to stabilize the base of a lifted ethane turbulent jet diffusion flame by modifying the coflow velocity field. The velocity field and flame base are measured by particle image velocimetry and unfiltered flame chemiluminescence imaging, respectively. An axisymmetric DBD actuator, integrated onto the nozzle body and driven by 8 kHz, 10-12-kV peak-to-peak sinusoidal voltage, generates the directional flow that opposes the coflow that surrounds the jet nozzle. This flow induces a separation bubble that retards and diverts the surrounding fluid further away from the fuel jet. The turbulent jet diffusion flame liftoff height is found to be significantly reduced by this plasma actuation.

This paper examines the stabilization of premixed and jet diffusion flames of methane, ethane, and propane by nanosecond repetitive pulsed plasma discharges. Combustion products are measured using gas chromatography while laser-induced breakdown spectroscopy (LIBS) is used to characterize the local equivalence ratios. We find that in premixed flames, although plasma-assisted flame holding takes place under fuel-lean conditions, propagation of combustion occurs at/or above the known lean flammability limits. In jet diffusion flames, the flames are found to be anchored best to the discharge at jet speeds that are much higher than the normal blow-off speed when the discharge is placed where the local fuel–air equivalence ratio is in a limited flammable regime.

A plasma metasurface that works as a tunable THz wave reflector was designed, generated, and experimentally characterized. Theoretical calculations based on the Lewin effective medium model were used to determine the required plasma properties and corresponding reflection bands1. The laser-induced breakdown plasma was generated by focusing the light from a 750 mJ/p Q-switched Nd:YAG laser through a multi-lens array. A gated CCD camera coupled to a high-resolution spectrometer was used to obtain chord-averaged broadening data for the cross section of a single plasma element, and the data was Abel inverted to derive the radial plasma density distribution. The resulting values were used in 3D finite-difference time-domain simulations, which confirmed the metasurface's function as a THz wave reflector.