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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.

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.

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.

A short-pulse repetitive discharge is used to ignite hydrogen jet flames in supersonic crossflows. Nonequilibrium plasma is produced by repetitive pulses of 7-kV peak voltage, 20-ns pulsewidth, and 50-kHz repetition rate. Sonic or subsonic hydrogen jets are injected into a pure-oxygen supersonic free-stream flow of Mach numbers M = 1.7-2.3. The fuel injection nozzles and electrodes are mounted flush with the surface of a flat plate that is oriented to be parallel to the flow to minimize stagnation pressure losses associated with generated shock waves. A configuration combining an upstream subsonic oblique jet and a downstream sonic transverse jet serves to provide an adequate flow condition for jet flame ignition. The flow pattern and shock waves induced by the dual hydrogen jets are characterized by Schlieren imaging. Planar-laser-induced fluorescence and emission spectroscopy are employed for imaging the distribution of OH radicals. The OH fluorescence image of the region in the vicinity of the discharge confirms jet flame ignition by the plasma.

We report on a study of the use of repetitive ultrashort pulsed plasma discharges in stabilizing a lifted methane jet diffusion flame in elevated temperature (855 K-975 K) vitiated coflow. CH chemiluminescence images are used to record the flame liftoff height, which serves as a measure of the flame stability. The results show that, for the same reduced electric field (E /n), the stabilizing ability of the discharge in the investigated temperature range diminishes with increasing coflow temperature. Based on the results of a zero-dimensional transient chemistry simulation, it is conjectured that the reduced propensity for stabilization at elevated temperature is caused by the rapid depletion of H2 and CO formed in the postplasma gas, under high ambient temperature conditions.

An electron cross-field transport model based on instantaneous simulated plasma properties is incorporated into a radial-axial hybrid simulation of a Hall plasma thruster. The model is used to capture the reduction of fluctuation-based anomalous transport that is seen experimentally in the region of high axial shear in the electron fluid. Similar transport barriers are observed by the magnetic confinement fusion community due to shear suppression of plasma turbulence through an increase in the decorrelation rate of plasma eddies. The model assumes that the effective Hall parameter can be computed as the sum of the classical term, a near-wall conductivity term, and a fluctuation-based term that includes the effect of shear. A comparison is made between shear-based, experimental, and Bohm-type models for cross-field transport. Although the shear-based model predicts a wider transport barrier than experimentally observed, overall, it better predicts measured plasma properties than the Bohm model, particularly in the case of electron temperature and electric potential. The shear-based transport model also better predicts the breathing-mode oscillations and time-averaged discharge current than both the Bohm and experimental mobility models. The plasma property that is most sensitive to adjustment of the fitting parameters used in the shear-based model is the plasma density. Applications of these fitting parameters in other operating conditions and thruster geometries are examined in order to determine the robustness and portability of the model. Without changing the fitting parameters, the simulation was able to reproduce macroscopic properties, such as thrust and efficiency, of an SPT-100-type thruster within 30% and match qualitative expectations for a bismuth-fueled Hall thruster.

A 2-D radial-axial (r-z) hybrid fluid/particle-in-cell (PIC) model has been developed to model energetic particle-induced channel-wall erosion in coaxial Hall discharge plasma thrusters. The discharge model geometry corresponds to that of a so-called stationary plasma thruster with an extended dielectric channel, and the computational domain extends from the anode at the base of this channel through the channel interior and into the near-field plume region. A model of the wall-erosion process has been added to the simulation in order to assess thruster degradation due to ion and energetic-neutral-induced sputtering of the channel walls. The effect of ion-neutral collisions, including momentum and charge-exchange collisions, on the erosion process is examined. These models are used to simulate the long-term wall-erosion history. For the specific Hall-thruster-configuration modeled, collisions were found to have less than a 10% effect on wall erosion. The erosion rate is seen to decrease with time, in good agreement with experimental measurements of long-term erosion in similar thrusters, resulting in a wall recession of as much as 2 mm after 4000 h of simulated operation.

The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

Experimental results are presented for studies of low (2-20 kHz) and intermediate-frequency (20-100 kHz) oscillations in crossed-field closed electron-drift Hall discharges. Conditional sampling using two electrostatic probes is used to identify and extract properties of coherent structures associated with the propagation of azimuthal and longitudinal instabilities within the discharge channel. The azimuthal component phase velocities are determined for a wide range of wave frequencies and over characteristic regimes of operation of these devices. A variety of propagation modes are observed and analyzed, including the appearance of an induced mode due to the presence of the probes themselves. This later result is believed to be the first direct evidence of how fluctuations can be influenced in these Hall discharges using relatively simple actuation methods.

A model for a high-pressure thermal plasma flowing onto an electrode in stagnation flow geometry based on a continuum description of the flow is used to establish anode current density limitations. These limitations can be overcome at high anode current densities when ionization in the nonequilibrium boundary layer supplies the required ion current to the distant plasma. Two modes of anode operation (low and high current density modes) are identified and interpreted to be a difference between a diffuse and constricted mode of arc attachment. Although the flow field is highly idealized, the results provide insight as to the coupling of the ionizational nonequilibrium boundary layer to the flow field and the concomitant effect that this coupling has on anode current limitation. The quasi-neutral approximation used is expected to accurately describe the dynamics within the ionization nonequilibrium boundary layer. The anode saturation current (current density above which the anode must emit ions) strongly increases with increasing free stream velocity and anode temperature. These results have important implications for the understanding of high-power arc jet operation, and the use of high-power arc jets in low thrust space propulsion. >

A two-temperature, multifluid model of a plasma in stagnation flow against a cooled, electrically biased surface is presented. The model couples bulk fluid motion, species diffusion and convection, electron and bulk energy equations, and net finite-rate ionization with Poisson's equation for the electric field in a generalized formulation. Application of the model to argon flow reveals important interactions between thermal, hydrodynamic, chemical and electrical boundary layers, with implications for current-limiting regimes of arcjet operation. The response of a planar Langmuir probe in contact with a collisional, flowing plasma is examined. Determinations of current-voltage behavior compare well with simple theory, including dependence on incident plasma velocity. Departures from this theory arise from boundary-layer perturbations near the electrode surface, away from free-stream conditions. The computational model incorporates a finite-rate catalytic recombination of ions and electrons at the electrode surface together with a specified current. >

Nanosecond repetitively pulsed (NRP) spark discharges have been generated in room-temperature air at atmospheric pressure. Single-shot image sequences of NRP spark discharges show that the path of the spark discharge channel can change with each pulse as well as during a single pulse, suggesting that several filaments can be generated within a single discharge event.

We present high speed images from E times B discharges in microscale magnetically confined plasmas. The images depict strong and highly ordered drift waves and underlying smaller scale turbulence near the plasma edge, with characteristic length scales larger than the electron cyclotron radius. The phase velocity of the large-scale disturbances is in agreement with that for nablan-driven isothermal drift waves.

Ion current oscillations in a linear geometry Hall discharge with an open electron drift are studied using a planar Langmuir probe. The bulk discharge behavior is reconstructed from time-resolved measurements of the local ion current density. These ion beam images reveal the same fluctuating "breathing" behavior found in closed-drift Hall thrusters.

The structure of a magnetized, collisionless crossed-field coaxial Hall discharge is investigated using low-impedance, negatively biased Langmuir probes. We present images depicting frequency-position renderings and cross-correlation plots, which identify coherent and random fluctuations in plasma density. These fluctuations are found to depend on the discharge operating conditions, and are believed to greatly affect cross-field electron transport.