Plume Mode Research Articles

Hollow cathode discharge instability onset in electric thrusters

Hollow cathodes are an integral part of ion, and Hall thrusters are used for electric propulsion in deep space missions and in commercial communications satellites. Hollow cathodes are known to operate in a quiescent “spot mode” and in a noisy “plume mode” in which plasma instabilities generate erosive energetic ions. The onset of the plume mode in hollow cathodes has been defined historically as when the keeper voltage oscillation values exceed 5 Vpp (peak-to-peak). Using a LaB6 hollow cathode in a vacuum chamber setup that simulates operation in ion and Hall thrusters, a set of emissive and Langmuir probes have been used to investigate the plasma properties associated with plume mode onset as a function of discharge current and gas flow rate. We find that the plume mode onset occurs at even less than 2 Vpp of the keeper voltage for the 5–75 A hollow cathode investigated here and starts at higher gas flow rates than expected from the traditional 5 Vpp metric used by those in the field. Mode competition and coupling between three different instabilities observed in the near-cathode plume affect the overall plasma oscillation levels that are correlated to energetic ion production. We find that the plasma oscillation levels measured by in situ plasma diagnostics are more indicative of the presence of oscillations and the generation of energetic ions than indirect keeper voltage measurements.

Plume mode instability enhanced by emitter surface poisoning in hollow cathode

The unstable plume mode of hollow cathodes should be avoided in practical applications because it severely degrades the overall cathode lifetime. In this study, we investigate the spot-plume transition and plasma stability characteristics of an unused segmented lanthanum hexaboride emitter. The expansion of the unstable plume mode region is observed during a discharge experiment. Subsequently, the segmented emitter is retrieved, the inner surface of the emitter is observed, and the work function on the surface is measured at room temperature. The emitter surface exhibits color variations with oxygen and carbon detection. The downstream edge shows the original purple color and almost no degradation in the work function. The high temperature in this region promotes the desorption of carbon and oxygen. In the spot mode, this region mainly contributes to thermionic electron emission; therefore, the discharge voltage in the spot mode does not change during the discharge experiment. Carbon or carbide is detected in the middle of the axial direction on the emitter surface, where the surface temperature is not sufficiently high to desorb carbon during discharge. Based on the surface analysis results, the dominant substance in the region where carbon is detected was lanthanum carbide. An increase in the work function is indicated in the region, which appears to increase the plasma instability. According to previous studies, an increase in the work function results in a rise in the potential in the emitter, and an increase in the electron temperature in the outside plume region induces the plasma instability. Further investigation is needed to understand the mechanism connecting the rise in the work function and the rise in the electron temperature in the plume region.

Study on the plasma plume structures in a cylindrical Hall thruster

The structures of two typical plume modes (divergent and convergent plume) and their respective formation mechanisms in a cylindrical Hall thruster are investigated. The non-monotonic distribution of the two-dimensional ion energy distribution function map is obtained to segment the plume into 5 levels with different energy interval. The test results of the ion current fraction indicate that the ion beams in Level III and IV were the main components of the two plume modes. For Level IV (enhanced significantly in the convergent mode), these ions with focusing structure could only originate from the upstream E × B field due to their similar high potential and identical variation tendency in the mode transition, where the electric field converges towards the axis. This is mainly responsible for the formation of convergent mode. On the other hand, for Level III (dominating the divergent mode), these ions with hollow structure and less acceleration voltage are speculated to originate from the downstream magnetic mirror field, where the electric field manifests a defocusing characteristic along the diverging magnetic field lines. This is mainly responsible for the formation of divergent mode. Through the thrust equivalent calculation method, it is revealed that the upstream acceleration associated with the E × B field is the dominant contributor to the enhancement of thrust in the convergent mode. This study on the complex plume structure would facilitate the understanding of the CHT acceleration mechanism and provide useful ideas for the optimization of electric field structure and enhancement of plume efficiency.

Energetic ion and plasma oscillation measurements during plume mode operation of a hollow cathode

Hollow cathodes are important devices used for spacecraft electric propulsion. The hollow cathode has two operational modes. One mode is a stable mode called the spot mode, and the other is an unstable mode called the plume mode. Operation in plume mode should be avoided since the instability causes high-energy ions that sputter-erode the cathode parts. In this study, the relationship between discharge oscillations and ion energy distribution in plume mode was investigated using a triple Langmuir probe and retarding potential analyzer for a 40-A class xenon hollow cathode with a lanthanum hexaboride emitter. The triple probe can measure unsteady electron temperature and plasma density oscillations. The electron temperature was not so high, 1 to 2 eV. Some instabilities were observed in the plume mode. The ionization instability with a low frequency oscillation of 30 kHz was the dominant mode. A broad spectrum around 330 kHz due to ion acoustic turbulence was observed. In addition, in the downstream plume region, oscillations around 120 kHz were observed owing to temporal change in anomalous resistivity. The 95% ion population voltage found to be 20 and 30 eV in spot and plume modes, respectively. The magnitude of the low frequency ionization oscillation was found to be inversely proportional to ion energy in plume mode. This indicates that the resonant energy transfer from the oscillation to the ion energy through Landau damping probably plays an important role in high energy ion generation in plume mode. A clear correlation between discharge current and electron temperature waveforms was found. The larger the electron temperature fluctuation, the stronger the correlation between discharge current and electron temperature, and the larger the phase difference deviation from 180°.

Risk assessment of a cold atmospheric physical argon plasma jet on the skin, liver, and biochemical factors in an animal model.

In this study, an argon plasma jet was made for therapeutic purposes. Although cold plasma jet (CAPJ) is considered a safe device, its risk assessment is essential to be approved for a clinical trial. To this end, the safety parameters of the CAPJ were evaluated from both aspects associated with the instrument such as design, construction, and environmental conditions, and also function on living organisms. It was shown that the device is physically safe for the user and the animal. In continue, healthy rats were treated by the plasma at two modes of streamer and plume mode at two distances of 20 and 25 mm from the nozzle for 2, 5, and 10 minutes of exposure time. In the study of blood serum biochemistry, the strongest deviation from the average is observed at alkaline phosphatase in the plume mode in the 10min (P10) group. Also, the amount of Aspartate transaminase (AST) in the blood serum of rats is lower than the control group for all groups except streamer mode at 10 min exposure (S10). And, the mean ALT enzyme in the serum of the S5 group is significantly higher compared to the plume mode in the 5 min (P5) group. The histopathology of the liver is studied to support these results. Shrinkage of the nucleus, cell swelling, and hyperemia are observed in the liver tissue for streamer mode, while in the plasma plume mode, these changes are not tangible. No significant difference is observed in the mean value of total Bilirubin, Direct Bilirubin, Albumin, Na, and K factors in the different groups after the plasma treatment and the histopathology of kidney tissue is performed for all the groups to support this conclusion and it did not show any damage to the kidney tissue. As a result of risk assessment in this study, the treatment with a duration of 2 to 5 minutes is safe with the specifically designed CAPJ nozzle head at a distance of 25 mm from the nozzle.

Duration of Anode-Spot Mode Type 2 and Plume Mode in High-current Vacuum Arc

Duration of Anode-Spot Mode Type 2 and Plume Mode in High-current Vacuum Arc

Evidence of the ionization instability and ion acoustic turbulence correlation in sub-ampere hollow cathodes

Plasma instabilities in the plume of hollow cathodes have been extensively researched in particular for high-current operation. The rise of plume mode ionization-like instability leads to a degradation of cathode’s performance along with the emergence of highly energetic ions that can produce sputtering of various cathode’s surfaces. Numerical simulations using 2D fluid or hybrid codes brought forward an interesting correlation between the evolution of ion acoustic turbulence (IAT) and emergence of plume mode oscillations. Such numerical findings were proven to be true by experimental measurements of wave dispersion and plume mode-IAT correlation in the plume of cathodes emitting currents >10 A. This study brings forward evidence of the correlation between plume mode oscillations and IAT in the plume of low-current cathodes operating with Kr at sub-ampere current levels. It is shown that at <1 A the plume mode instability is highly correlated with the IAT and the anomalous electron collision drives the electron transport in the cathode plume. The fluctuations in IAT wave energy lead to large temperature oscillations which then drive fluctuations in the density via ionization.

Discharge and plasma plume characterization of a 100 A-class LaB6 hollow cathode

This article reports on the characterization of a laboratory model 100 A-class hollow cathode with a sintered lanthanum hexaboride (LaB6) emitter for high-power Hall thrusters. The cathode has been fired up to 70 A with xenon as working gas. The cathode architecture, test setup, ignition procedure, and power consumption are described first. The second part of this contribution comments on the current–voltage characteristics and the discharge modes obtained for discharge currents in the 30–70 A range and flow rates in the 15–30 SCCM range. The cathode operates in a spot mode at high discharge currents and in a plume mode with large oscillations at low currents and low gas flow rate. Spectral analysis shows that most frequencies reside in the 10–200 kHz range with flat and sharp distributions in plume and spot modes, respectively. Finally, we present electron temperatures and densities measured in the cathode plasma plume by means of incoherent Thomson scattering. The two quantities decrease along the axis. The density is large (up to ∼1019m−3) and increases with both the ion current and the gas flow rate. The electron temperature increases with the current and decreases with the gas flow rate. The temperature remains relatively low (&amp;lt;1.5 eV) in spite of large currents and applied powers.

Effect of freestream turbulence on the structure of boundary-layer flames

This study investigates the role of turbulence induced instabilities on three-dimensional boundary-layer flames. Boundary-layer diffusion flames under 0.99–2.43 m/s crossflow velocities were produced over a gaseous propane line burner. Incoming flows with turbulence intensities ranging from 0.5% to 16.8% were generated in a wind tunnel with fine wire meshes and perforated grids. It was observed that freestream turbulence initiated an earlier onset of visible coherent flame streaks, enlarged the initial streak spacing, and accelerated the growth of the streak spacing along the streamwise direction. Both spacing and fluctuation frequency of the flame streaks showed a nearly quadratic growth at high turbulence intensities. This instability promoted the transition of flames to a turbulent state, which ultimately modified the overall flame heating dynamics. The forward attachment length of the flame was found to be negatively correlated to the turbulence intensity; a dimensionless relationship was proposed to correlate the flame attachment length based on the Froude number, heat-release rate, and freestream turbulence intensity. Two heating modes, a momentum-dominated and a plume mode, were observed and found to be segregated by a critical Richardson number. The downstream heat flux was found to increase from 30 to 40 kW/m2 in the momentum-dominated regime, when flow turbulence intensity changed from less than 1% to a level of 14.9–16.8%. Finally, it was observed that placing a bar upstream of the burner tripped the flow to the point where the downstream flame structure closely resembled flames under the highest turbulence intensity investigated, suggesting a simplistic configuration for future study. Ultimately, these results may improve our understanding of the structure and heating dynamics of boundary-layer flames as they transition to turbulence.

Distinct discharge modes in micro Hall thruster plasmas

Two distinct discharge modes were observed in a 50 W-class micro-Hall thruster plasma under different operating conditions. A ball-shaped plasma with a broad plume (mode A) was observed at low mass flow rates (less than 0.37 mg s−1) over the entire operational anode voltage range (160–280 V). Raising the anode voltage beyond 200 V with the mass flow rate fixed (larger than 0.37 mg s−1) produced a narrow plume and stretched jet-like structure (mode B). In mode B, the thruster showed performance improvements in terms of thrust (3.8 mN vs 3.3 mN), specific impulse (913 s vs 800 s), and anode efficiency (28% vs 22%), with only a 2 W difference in the anode power (61 W in mode B and 59 W in mode A). This suggests that operation is more advantageous in mode B than in mode A for the utilization of such low-power Hall thrusters. Unique plume properties were observed in the two modes and considerable differences were measured in the Xe II ion acceleration structure, beam angle, and ionization rate as the mode changes, which were not reported in previous studies. Mode A exhibits an axially extended ion acceleration structure outside the discharge channel, where 75% of the final ion velocity is achieved at approximately 40 mm from the thruster exit, while most of the ion acceleration occurs within 10 mm from the thruster exit in mode B. Measurements show that the full width at half maximum of the Xe II ion energy distribution function, electron temperature, and Xe II emission intensity decreased after the plasma transitioned from mode A to mode B. Based on the optical emission spectroscopy, the ionization rate in the plasma plume decreased by 30%–41% after the mode change, which is likely related to the reduction of the beam angle and electron current by 24% and 30%, respectively.

Continent-scale Hiatus Maps for the Atlantic Realm and Australia since the Upper Jurassic and links to mantle flow induced dynamic topography.

Interregional geological maps hold important information for geodynamic models. Here, we use such maps to visualize major conformable and unconformable contacts at interregional scales and at the level of geologic series from the Upper Jurassic onward across North and South America, Europe, Africa and Australia. We extract hiatus information from these paleogeological maps, which we plot in a paleogeographical reference frame to link the maps to the plate and plume modes of mantle convection. We assume that interregional patterns of hiatus surfaces are proxy records of continent-scale mantle-induced vertical motion of the lithosphere. We find significant differences in the distribution of hiatus across and between continents at the timescale of geologic series, that is ten to a few tens of millions of years (Myrs). This is smaller than the mantle transit time, which, as the timescale of convection, is about 100-200 Myrs. Our results imply that different timescales for convection and topography in convective support must be an integral component of time-dependent geodynamic Earth models, consistent with the presence of a weaker upper mantle relative to the lower mantle. Additional geological constraints together with interregional geological maps at the resolution of stages (1-2 Myrs), are needed to assist in future geodynamic interpretations of interregional geologic hiatus.

Transient non-classical transport in the hollow cathode plume I: measurements of time-varying electron collision frequency

Electrostatic probes are employed to measure the time-variations in electron collision frequency due to a large-scale, low-frequency, plasma instability in a high-current hollow cathode plasma discharge (plume mode oscillation). Time-resolved measurements of ion acoustic turbulence are used to infer the effective collision frequency on the timescale of this underlying wave. Through a direct comparison, it is shown that the observed variation in the electron collision frequency cannot be described by classical collisional processes, i.e. Coulomb and neutral collisions, but rather is well represented by the changes in the anomalous collision frequency due to the turbulence.

Dynamics of a hollow cathode discharge in the frequency range of 1–500 kHz

We present results from 2D axisymmetric fluid simulations of the partially-ionized Xe gas in a 25-A cathode discharge, for a range of flow rates (5–20 sccm) in which transition between the so-called spot and plume modes has been observed in laboratory experiments. The simulations for the first time capture very well the characteristic rise of the peak-to-peak amplitude in the keeper voltage oscillations with decreasing flow rate—the so-called ‘plume margin’—thereby allowing for a closer interrogation of the processes that drive them. The oscillations are found to be due to plasma dynamics in the near-plume region that increase in amplitude as the flow rate is reduced. The cathode interior remains largely quiescent and changes in the emitter peak temperature do not exceed 2% across the entire range of flow rates examined. Along the cathode centerline the amplitude of the electron density oscillations peak at a location where the number density of the Xe atoms has diminished to its minimum value and the degree of ionization is close to one. The computed spectra reveal that most of the oscillation power resides in the frequency range of 100–300 kHz. These frequencies are much greater than the local ionization collision frequency ( < 10 kHz), and are associated with wavelengths of several centimeters. Such wavelengths are many orders of magnitude greater than λD. The waves are largely longitudinal with wave vector in the axial direction, and phase velocity ω/k > 10 km s−1, which is more than 6× greater than the ion acoustic speed. Though the computed and measured plume margins are in good agreement the measured spectra show that most of the power resides between ∼60 and 90 kHz, lower than the computed range (100–300 kHz), and exhibits a sharper distribution. It is argued this is due to the presence of a rotational mode in the experiment that is coupled with the longitudinal mode, yielding a complex 3D plasma motion that cannot be captured by the simulations due to the assumption of azimuthal axisymmetry, but that it is indeed the longitudinal plasma motion that drives the transition from spot to plume modes. An anomalous resistivity has been invoked in the simulations that is based on the formulations of Sagdeev and Galeev for the saturation of IAT which is long known to exist in these discharges. Though the simulations do not account for possible anomalous heating of the ions, the idealized model for the electrons is found to be sufficient in capturing the abovementioned dynamics, yielding a multiplication coefficient α of the anomalous collision frequency να = αωpe(Te/Ti)(ue/uTe) that is within a factor of two of the estimated theoretical value.

Discharge mode transition in a Krypton-fed 1 A-class LaB6 cathode for low-power Hall thrusters for small satellites

Thermionic cathodes are essential for the operation of various electrostatic propulsion devices. They strongly influence the performance and lifetime of the propulsion system. In this study, a 1 A-class LaB6 laboratory model hollow cathode has been tested with krypton in diode and triode configurations in order to assess the cathode discharge mode transition behavior. Measurements have been performed over a range of krypton mass flow rates (0.1, 0.15, and 0.21 mg/s, or 1.6, 2.4, and 3.4 sccm), keeper (0.1, 0.15, and 0.2 A), and anode currents (0.1–1 A) at a fixed cathode-to-anode distance. Seven criteria were used to distinguish between the spot and plume mode operations. The results show that the mode transition in low-current cathodes may be a nonlinear phenomenon, and only some of the existing mode transition criteria can be used to accurately predict the spot/plume discharge regions at low emission currents.

The effect of fuel bed height in grass fire spread: addressing the findings and recommendations of Moinuddin et al. (2018)

A recent numerical simulation study by Moinuddin et al. (2018) determined that over a specific range of Froude numbers defined by them as ‘plume mode’, grass fuel height has a strong inverse effect on the rate of fire spread in grasslands. They then suggested that a relationship for effect of fuel height derived from their simulation results could be used to support fire management decision-making. The present analysis used fire spread measurement data from two outdoor experimental burning studies in grass fuels where an explicit control of fuel height was imposed to verify the realism of their results. It was found that a reduction in grass height, with or without removal of the cut fuel and regardless of the Froude number, led to a significant reduction in rate of fire spread, a result opposite to the simulations obtained by Moinuddin et al. (2018).

Modeling of vacuum arc plasmas in anode spot or anode plume mode taking into account multiple ion components

This work investigates the arc behaviors with CuCr25 electrodes considering anode vapor using a magneto-hydro-dynamic model. Different kinds of components are considered including ions (Cu and Cr) with different charge numbers, electrons, and atoms (Cu and Cr). The effect of the anode sheath is also considered. The density distributions of these components are analyzed and compared with the experiments during the anode spot mode and the anode plume mode. Simulation results show that the anode vapor can enter the arc column forming a cool and poorly conducting region (i.e., neutral atom vapor area, NAVA) under high anode temperature. Atoms and single-charged ions mainly gather near each electrode. The highest double-charged ion density can be seen in front of the NAVA. Triple-charged ion density is negligibly low and reaches its maximum where the electron temperature is high. Cr is more likely to be ionized to a higher ionization level compared with Cu. Our results agree with experimental measurements of density distributions of different components and plasma temperature.

Identifying Plume Mode via WSR-88D Observations of Wildland Fire Convective Plumes and Proposed Tactical Decision Support Applications

To date, the use of Doppler radar (WSR-88D) in wildland fire operations has been limited, with tactical applications focused on analyzing ambient atmospheric features. This paper presents geographically diverse analysis of radar-observed wildland fire convective plumes to determine indicators of plume mode for tactical decision support. Through the visualization of buoyancy via thermal bubbles and vertical plumes, plume mode is revealed via WSR-88D interrogation of three Southern Great Plains grass/shrub fires and two timber fires in Texas and California. Analogous to thunderstorm convective modes, past research has identified two distinct plume modes of wildland fire: multicell and intense convective plume. Multicell plume mode is characterized by a series of shallow discrete cells that move away from the fire’s main buoyancy source, with successive cells rising, expanding, and replacing cells from the updraft source. This process, known as the thermal bubble concept, occurs most notably in strong vertical wind profile environments with a strong advection component. These cells or thermal bubbles are observed via WSR-88D data for three Southern Great Plains cases. Intense convective plumes are observed to be vertical with the low-level reflectivity maximum and maximum echo top juxtaposed and occurrence is confined to weak wind environments; these plume structures are identified in the two timber fire cases. An important WSR-88D signature, the back-sheared convective plume (hereafter BSCP), is identified in terms of transverse vortices and vortex rings, which may imply enhanced combustion rates due to increased turbulent mixing. Determination of plume convective mode via radar offers meteorologists the ability to detect changes in plume mode and to provide important tactical decision support information about fire behavior.

Plasma plume diagnostics of a LaB6 hollow cathode with triple Langmuir probes

High-power Hall Effect thrusters (HETs) represent an attractive propulsion option for the next-generation manned and robotic missions. The main challenging feature of these missions is a lifetime in the order of 104 hours required to the propulsion system. One of the most critical components of a HET is the cathode, used to emit electrons to sustain the discharge and to neutralize the ion beam. The main life-limiting factor of hollow cathodes designed for HETs is the material wear due to ion bombardment by the discharge plasma. The aim of this paper is to present the characterization of the electrical parameters and plasma plume properties of a hollow cathode. The effect of the cathode orifice geometry was also investigated, in terms of different length-to-diameter aspect-ratio (AR). Two ARs were analyzed to assess the effect of the orifice geometry on the plasma parameters and onset of unstable modes. The cathode analyzed, named HC20, is a hollow cathode developed for a mid-range power HET under development at SITAEL. The cathode was designed to provide discharge currents between 5 and 30 A, already characterized during previous experimental campaigns. The measurements in the plasma plume of the cathode were performed with triple Langmuir probes, one of which capable of scanning the plume along the cathode axis. The results show that a larger orifice AR can shift the transition to an unstable mode of operation of the cathode, named plume mode, at lower mass flow rate, increasing the cathode operative range.

Study of vacuum arc behavior under anode spot and anode plume modes

Anode spot (AS) and anode plume (AP) phenomena are widely observed in vacuum arc experiments and are related to anode melting and evaporation under strong heating from the arc column. Anode vapor will then strongly influence the arc column through ionization-recombination and energy exchange among atoms, ions, and electrons. This work investigated the characteristics of the vacuum arc with AS or AP using the two-dimensional magnetohydro dynamic model. Formation of the AS and AP modes was studied and analyzed using numerical simulation. Friction forces between ions and atoms were also taken into account. Simulation results show that anode vapor expansion depends on the pressure balance between the anode jet and cathode plasma. Higher anode temperature produces a larger neutral atom vapor area (NAVA), which was dominated by neutral atoms. Inside the NAVA, ion and electron temperatures were low in the AS and AP mode due to ionization and energy exchange. Electric conductivity in this area was also so low that the location of the maximal current density was near the edge of the anode instead of the anode center. The asymmetric appearance of an AP was mainly caused by the asymmetric anode temperature distribution with respect to the AS center. Qualitative comparisons show that the simulation results are consistent with the experimental results.

Triple Langmuir Probes Measurements of LaB6 Hollow Cathodes Plume

Lanthanum hexaboride hollow cathodes represent a viable option for high power Hall effect thruster applications, under development for the next generation of manned and robotic interplanetary missions. In this scenario, SITAEL and the University of Pisa are actively developing high current hollow cathodes capable of providing discharge current in the range 10-100 A to be coupled with high power Hall effect thrusters. The cathode design is based on an in-house theoretical model of the internal sections of the cathode, recently integrated with a simplified model of the cathode plume. Despite the application of hollow cathodes on flight and laboratory model Hall effect thrusters, many questions remain unsolved. In particular, issues related to onset of instabilities, due to plume mode or ion acoustic turbulence, are still unclear, while it is known that they can affect the overall performance of the cathode and thruster unit. This paper focuses on the experimental investigation of the cathode plume by means of measurements of the main plasma parameters, at different operating conditions and for different cathode geometry. Two cathodes were investigated, namely HC20 and HC60, designed to be coupled with SITAEL’s HT5k and HT20k (5 kW- and 20kW-class) Hall effect thrusters. The cathodes were mounted in stand-alone configuration with an auxiliary cylindrical anode. The experimental campaign was performed using triple Langmuir probes as plasma diagnostic system. The probes were mounted on scanning mechanisms to measure the plume parameters at various radial and axial distances from the keeper exit. General trends of electron temperature, plasma potential and plasma density are reported in terms of discharge current, mass flow rate and cathode orifice geometry. The results highlight that the cathode plate orifice selection affects the plume mode onset, giving the possibility to extend the stable mode of cathode operation in the current range required by the thruster.