Ion-induced charge emission from unpolished surfaces bombarded by an [Emim][BF₄] electrospray plume

Electrospray propulsion is a promising candidate for small satellite propulsion. In electrospray thrusters, ions and/or droplets from room-temperature ionic liquids (ILs) are electrostatically extracted and accelerated to high velocities (>10 000 m/s) to provide thrust. Recent development has focused on porous-glass electrospray thrusters,^1–3 which passively feed the IL through a porous-glass substrate with machined tips toward a charged extraction grid. These thrusters typically emit in a pure-ion regime in which the plume mainly consists of single ions (monomers) or ions attached to a neutral pair (dimers), resulting in very high specific impulse (>1500 s).

An important aspect of any electric propulsion (EP) systems is accurate modeling and understanding of the plume ion–surface interactions. High energy ions impinging on a surface cause sputtering (removal of surface atoms) and ion-induced electron emission (IIEE). Many spacecraft designers were initially hesitant to use ion or Hall thrusters for their missions, and the EP community dedicated decades to experimental and modeling research to understand these surface interactions for xenon plasma ions. The data from these experiments are used in plume–surface interaction models to estimate thruster lifetime and model spacecraft contamination and charging. Results are used by spacecraft designers and the effects of xenon EP systems are now understood well enough to be used on low-risk commercial and government missions. While xenon-based EP plume ion–surface interactions are relatively well understood, at least to the point where models are being used to integrate EP onto spacecraft, the study of ion–surface interactions for electrospray plumes is very much in its infancy.

Early research on electrospray plume ion–surface interactions has focused on sputtering yields from electrospray plumes impinging on surfaces and has found that yields up to six atoms per impinging molecular ion species are possible and that chemical reactions between the ion and surface may be an important sputtering mechanism.^4,5 However, each of these studies focused on capillary electrospray emitters that emit primarily charged droplets consisting of hundreds to thousands of molecules and not the pure-ion monomer and dimer plumes of porous-glass electrospray thrusters. No experimental work has been performed to study sputtering of electrospray plumes in the pure-ion regime and attempts at molecular dynamics models of electrospray plume sputtering have largely not agreed with the existing experimental data for droplet-surface sputtering.⁶ 

Recent literature identifies anomalous thruster test results that may be due to plume–surface interactions. Testing of electrically isolated thruster systems, similar to a spacecraft configuration, has shown anomalous spacecraft charge loss⁷ when the thruster is operated in anion mode and then turned off. Possible explanations of this charge loss are plume ions returning to the charged spacecraft and causing IIEE, or plume ions impinging on facility surfaces and generating secondary positive charges that return to the spacecraft. The University of Southampton reported possible IIEE interference in time-of-flight measurements when negative currents were measured during cation mode experiments and in anion mode the currents did not return to zero after the plume was impeded.² Experiments at Massachusetts Institute of Technology suggest IIEE from a Faraday cup explains anomalous retarding potential analyzer (RPA) traces of anion energy distribution.⁸ The University of Southampton has performed an electron emission suppression study using a negatively biased nickel grid to suppress electron emission from an aluminum plate that collects current from a porous-glass electrospray thruster plume.⁹ The method of using a negatively biased grid to suppress electron emission is common in electrospray diagnostics, but, as discussed in Sec. IV E, may not provide a full picture of the ion-surface charge emission mechanism and the experimental results obtained may be more complicated than commonly assumed. Uchizono et al. have investigated the effect of a suppression bias voltage applied to an electrospray target and found cation and anion yields of 0.311 and 0.29 secondary species per incident ion, respectively.¹⁰ Recent modeling work at UCLA has calculated a theoretical electron emission yield vs ion energy relationship for a stainless steel surface bombarded by an electrospray plume.¹¹ As discussed in Secs. IV and V, a wider range of data is needed from both modeling and experimental groups to make useful comparisons and more advanced models are needed to capture all aspects of the complicated experimental systems. To understand and adequately account for plume–facility interactions in future experimental electrospray research, more information is needed about the charge emission behavior of surfaces bombarded by electrospray plumes.

The following sections describe experiments that investigate the charged species emitted from common thruster, facility, and spacecraft materials bombarded by an electrospray plume operating in the pure-ion regime. Charge collecting surfaces are often biased positive, or a grid is biased negative, to suppress electron emission from the collector, and the experiments described here provide information on the charge emission mechanisms taking place in these systems and prompt questions regarding unknown effects that have yet to be investigated. Results here for the first time show the effect of emitter voltage, operating polarity, and surface material on the yield of charged species, including evidence of emitted secondary ions along with emitted electrons. The experimental setup is described in Sec. II followed by the experimental results, analysis, and conclusions.

The electrospray experiments are performed in a 24 in. dia.  × 27 in. vacuum chamber equipped with one CTI Cryo-Torr 8 cryopump powered by a Brooks 9600 helium compressor and roughed by a dry mechanical pump. The chamber has a base pressure of 2 × 10⁻⁶ Torr, and all experiments are performed at an operating pressure below 3 × 10⁻⁵ Torr. Inside the chamber, a Newmark Systems RM-3 rotary stage is mounted on an optical breadboard plate with a mount for an electrospray emitter/extractor assembly.

A single etched 0.5-mm-diameter tungsten wire externally wetted emitter is used as the electrospray source in all experiments. This type of emitter is well characterized in the literature^12,13 and typically emits in the pure-ion regime similar to a porous-glass electrospray thruster.² The emitter is electrochemically etched following the procedure in Ref. 12 with further details given here. The tip is dipped in a 1N NaOH solution at 50 V until there is a smooth, concave curvature from the wire to a rounded point. A 0.25 mm diameter tungsten wire is spot welded orthogonally ∼3 mm from the tip to provide an IL reservoir. The tip/reservoir assembly is submerged for 45 s in a 2N NaOH solution saturated with K₃Fe(CN)₆ at 90–95 °C to roughen the surface of the emitter for better fluid transport to the tip. The resulting emitter has a tip radius of curvature of ∼32 mm and is shown in Fig. 1(a). The emitter with the crossbar attached and mounted in its copper holder is shown in Fig. 1(b).

Before testing, the emitter is ultrasonically cleaned in distilled water for 10 min, then in ethanol for 10 min to remove the water. After drying with compressed air, a heat gun is used to heat the tip/reservoir assembly for 1 min to increase the wettability of the surface. A syringe is then used to drag a drop of [Emim][BF₄] over the tip and back to the reservoir while ensuring that the emitter is wetted on all sides. The [Emim][BF₄] is stored and loaded into the syringe in a humidity-controlled glovebox at approximately 2% humidity and the emitter is loaded in atmosphere. The emitter is mounted on a copper block that serves as an electrical lead and heat sink. The temperature of the emitter is not monitored or controlled, and we assume the emitter remains at room temperature. Due to the small size of the droplet and approximately 2 h of exposure to vacuum during facility pump down, we assume that any volatiles have boiled off the propellant before experiments begin. A 1 mm thick stainless steel extractor plate has a 1.4 mm diameter hole centered on the emitter tip and is located 0.1 mm downstream of the emitter tip. The entire assembly is mounted in the vacuum facility as described above and the chamber is pumped down to operating pressure.

A schematic of the experimental setup is shown in Fig. 2. The emitter and target are each connected to a Matsusada AMS-5B6 high-voltage amplifier. The extractor is grounded for all experiments here. The emitter is operated in a 1 Hz square-wave AC mode with a peak-to-peak value of 2× the emitter bias voltage. A custom high-voltage current monitor is connected between each amplifier and the emitter and target. This current monitor consists of a TI AMC1311 isolation amplifier that amplifies the voltage drop across a $1MΩ$ shunt resistor on the high-voltage side and outputs a differential signal at ground with a gain of one. Because the isolation amplifier is a single-polarity device, a diode is used to direct the current toward different amplifiers for positive and negative emission mode measurements. Each of these custom current monitors was tested by being placed in series with a $1GΩ$ resistor followed by a Keithley 6514 electrometer. A 0.5, 1, 1.5, and 2 kV magnitude square wave was then applied across the circuit, and the currents measured by the electrometer were compared to the currents measured by each of the custom current monitors and the maximum difference was ±9.62%. This error is illustrated in the error bars of all current measurements as well as quantities calculated from current measurements. The current to the extractor is measured using a Keithley 6514 electrometer.

A 10 mm aperture Kimball Physics FC-71A Faraday cup with retarding potential analyzer (RPA) grids is mounted to one side of the emitter. Current to the Faraday cup is measured by a Keithley 6514 electrometer and the retarding grid is biased using a Matsusada AMS-5B6 high-voltage amplifier. The Faraday cup is used to measure the plume current density distribution and plume ion energy distribution, and the results are compared with literature data for other emitters to verify a similar or typical operation. A National Instruments (NI) USB-6211 data acquisition unit measures analog signals from all instruments and controls the high-voltage amplifiers via analog output. Data are recorded on a PC using the NI DAQExpress software.

For plume–surface interaction experiments, the emitter is aimed at a 15 × 15 cm² target of the material of interest. The target is mounted 8 cm from the extractor resulting in a 43° capture angle of the plume—far greater than the measured 17° divergence half-angle—such that the entire plume impinges on the target. For the experiments using tungsten carbide, molybdenum, and gold targets, the size of the target was limited by material cost and so the target was placed 1 cm from the extractor to capture the entire electrospray plume. The emitter is operated in AC mode at 1 Hz, and the currents to the emitter and extractor are measured as described above. Current is measured at the target, emitter, and extractor as the DC bias voltage on the target plate is adjusted from −85 to +85 V.

It is important to the interpretation of the data in these experiments to understand the possible sources (and sinks) of charge and the corresponding effect on the measured currents, as illustrated in Fig. 2. The high voltage of the alternating bipolar supply creates a strong electric field between the emitter and extractor, and positive or negative (cation or anion) charge is emitted from the emitter $(IEmission)$⁠. Most of the emission current passes through the extractor orifice and creates the ion plume $(IPlume)$⁠, while a small fraction (on average 3.6%) impinges on the upstream side of the extractor grid $(IImpinging)$⁠. These impinging ions may cause secondary charge emission from the upstream side of the extractor and the secondary charge may migrate upstream to be collected at the emitter $(IExt,Em)$⁠. The emitter current measured is then the emission current $(IEmission)$ minus this secondary charge emission current between the extractor to emitter $(IExt,Em)$⁠, as shown in Eq. (1). From our experiments here, it is not possible to directly measure this secondary current between extractor and emitter,

Emission current that leaves through the extractor orifice without impinging on the extractor makes up the plume current, as given by Eq. (2). All of the plume current is incident on the target and may give rise to secondary charge emission from the target, $(ISecondary)$⁠, as shown in Eq. (3). In the absence of secondary current, all the plume current is collected at the target and the two currents will be equal $(IPlume=ITarget)$⁠. In most experiments though, this is rarely the case because of secondary emission current from the target. For example, electrons, or other negative charges, leaving the target surface result in a more positive current being measured at the target (because $ISecondary<0$⁠), whereas positive charges leaving the surface result in a more negative current being measured at the target (because $ISecondary>0$⁠). For example, electron emission from the target caused by incident plume cations results in a larger positive current being measured at the target than the total plume cation current $(ITarget>IPlume)$⁠,

Secondary charge emitted from the target can travel upstream and be collected by the extractor or leave the system by flowing to the facility walls. We define these as the return current $(IReturn)$ and wall current $(IWall)$⁠, respectively, and the secondary current is given as Eq. (4). The measured extractor current is then the sum of the impinging current from the emitter and the return secondary current from the target, minus any secondary current on the upstream side between the extractor and emitter, as shown in Eq. (5). In some cases (when the target is close to the extractor), nearly all the secondary current is collected at the extractor $(ISecondary≈IReturn)$⁠, but in other cases (when the target is farther away) much less secondary current is collected at the extractor $(IReturn≠ISecondary)$⁠. In this latter case, we assume that the secondary current is finding its way to ground through the facility walls or other grounded components of the setup whose current is not measured,

Additional secondary charge emission may also be present, especially at the extractor, but we neglect these effects. For instance, the return current from the target to the extractor consists of charge that bombards the extractor after being accelerated through the potential difference between the target and extractor (up to ±85 V) and could generate secondary emissions from the downstream surface of the extractor.

Finally, we note that combining equations 1, 2, and 5, and assuming all extractor-to-emitter secondary current $(IExt,Em)$ is collected by the emitter, shows that the plume current can be calculated as the emitter current minus extractor current when there is no target downstream $(IReturn=0)$⁠, as shown in Eq. (6). This is a good assumption because the strong electric field between the extractor and emitter guides secondary charge emission toward the emitter. When there is no target downstream, the plume is incident on the chamber wall 61 cm from the source, and our results suggest that the return current is small or negligible,

In the following experiments and analyses, we measure $IEmitter,IExtractor,andITarget$⁠. We calculate $IPlume$ using Eq. (6) and $ISecondary$ using Eq. (3). We determine positive and negative secondary charge emission yields for each anion and cation emission mode (four different yields) by dividing $ISecondary$ by $IPlume$⁠, and this is described in detail in Sec. IV. We use Eq. (5) to determine the quantity $IImpinging−IExt,Em$ by measuring $IExtractor$ when no target is present $(IReturn=0)$⁠. Then, when a target is present, we use Eq. (5) to calculate $IReturn$ by subtracting the previously measured quantity $IImpinging−IExt,.Em$ from the measured $IExtractor$⁠. We assume the quantity $IImpinging−IExt,Em$ does not change with or without a target present because impingement and extractor–emitter secondary charge emission are phenomena in the high electric field region upstream of the extractor and isolated from changes downstream in the plume.

This section describes the results obtained using the experimental setup described above. The performance of the emitter is quantified by emission current, plume current distribution, and plume energy distribution. The surfaces used as targets in the experiments are thoroughly characterized by roughness, composition, and surface layers. Finally, the measured emitter, extractor, and target currents are presented and explained in terms of emitted secondary charges.

The performance of the emitter used here (UIUC emitter) is compared in Fig. 3 to similar emitters in the literature and in use at Air Force Research Lab (AFRL) Kirtland for [Emim][BF₄] electrosprays.

Figure 3(a) illustrates the measured emitter current in the negative polarity mode. The current linearly increases in magnitude from 150 to 500 nA for emitter voltages between 1.5 and 2.9 kV. The emitter used in these experiments was cleaned, reloaded with IL, and realigned with the extractor between some experiments, resulting in some variation in starting voltage and emitter current. However, the operating voltages stay in the range of 1.5–2.9 kV and emitter currents stay in the range of 200–1000 nA for these experiments, which is typical in the operation of externally wetted emitters and thrusters.^9,12 Figure 3(b) illustrates the current density distribution of the electrospray plume and the beam half-cone angle of the emitter used in these experiments is 17°. The measurements for each emitter are normalized by the centerline current density and, although the half-cone angles vary between 17° and 25°, the angular profile for each emitter follows the same general trend.

An RPA trace at the centerline of the plume and approximately 3 cm from the emitter is shown in Fig. 4(a). This trace observes the same trends indicating plume fragmentation as data in the literature for [Emim][BF₄] plumes from both tungsten wire emitters and porous-glass electrospray thrusters.^8,9 Figure 4(b) illustrates the derivative of the normalized current in Fig. 4(a) with respect to energy. This yields an approximate energy distribution of the plume with peaks at the “zero-field fragmentation” energy and at the full acceleration energy. Chengyu et al. explained how the fragmentation process yields these energy peaks. The anomalous peak and drop in anion current at retarding potentials less than the zero-field fragmentation energy in Fig. 4(a) is explained as an effect of negative charge emission from the negatively biased RPA grids and is likely not a measure of anion current (our results presented next support this assertion by showing negative charge emission due to anion bombardment significantly increases when a surface is biased negatively). While this emitter has not been tested with more advanced diagnostics such as time-of-flight mass spectrometry, we assume based on these favorable comparisons with literature data that the composition of the plume is comparable to other tungsten wire emitters and porous-glass electrospray thrusters (i.e., our secondary charge emission data are relevant to these types of electrospray sources).

Eight industrial materials were used in these experiments. 6061 aluminum, carbon graphite, 400 nickel, and 316 stainless steel were selected for their common use in ground-based facilities and thruster/spacecraft construction. Grade 5 titanium, tungsten carbide, and molybdenum were selected for the study of refractory metals. Gold is selected due to its potential as a low sputtering material for coatings on extractor grids or other thruster and spacecraft components. The materials used here are not pure sputtering targets but rather commonly used alloys or industrial materials that are found in ground-based test facilities, thrusters, and diagnostics. All materials were purchased from McMaster-Carr and cleaned with water and ethanol, but no other surface modification or treatment was performed, e.g., no polishing. The focus here is on the charge emission properties of as-received surfaces that are commonly used in electrospray experiments.

The aluminum, stainless steel, carbon graphite, titanium, and nickel targets are 15 × 15 cm² square plates, the tungsten carbide target is a 7.5 × 7.5 cm² plate, and the gold and molybdenum targets are foils of thickness 25 and 127 μm, respectively, applied to a stainless steel plate via an adhesive at the edges. The molybdenum foil covers a 3 × 3 cm² area, and the gold foil covers a 2.5 × 2.5 cm² area.

The structure and composition of the material surface or surface layers is known to affect the plume–surface interaction properties¹⁴ and, therefore, it is important to thoroughly characterize both the structure and composition of the surface to better quantify the surface properties. The Keyence VK-X1000 laser microscope at the UIUC Materials Research Laboratory (MRL) is used to take detailed images of the surface and measure the roughness (RMS), average peak height (SPK), and average valley depth (SVK) with an accuracy of 0.5 nm.

Table I displays the surface structure properties measured from each of the images shown in Fig. 5. Except for the carbon graphite and tungsten carbide, each of the RMS values falls into the range of 0.1–0.5 μm representing relatively smooth surfaces with minor ridges and scratches. The carbon graphite and tungsten carbide have higher RMS values of 4.009 and 1.393 μm, respectively, and both have larger SVK values of 9.184 and 1.763 μm, respectively. The carbon graphite and tungsten carbide have especially rough surfaces with especially large valleys or craters.

The Kratos Axis ULTRA x-ray photoelectron spectroscopy (XPS) machine at MRL is used to measure the atomic composition at the surface. XPS is a surface measurement technique that only penetrates a few atomic layers into the sample such that the data are only representative of the surface layer. Data on the tungsten carbide surface could not be collected due to the inability of the XPS machine to pump down to operating pressure because of outgassing from gases trapped in the material during the manufacturing sintering process. Table II displays the atomic composition measured by XPS. The data show that these surfaces contain large amounts of carbon and oxygen, indicative of metal oxide layers and hydrocarbon contaminants present on the surface.

Additionally, XPS provides measurements of electrons from individual orbitals for each element, allowing for the distinction between metal and metal oxides. Using the XPS energy spectra and the relative peak heights of metal and metal oxides, and carbon and oxygen, the thickness of the metal oxide layer and the hydrocarbon layer of each surface can be calculated using the method described in Ref. 15. These results are shown in Table III. The hydrocarbon layer for each surface ranges from 0.82 nm for molybdenum to 2.71 nm for nickel. These thicknesses are typical of contamination layers for untreated surfaces.¹⁵ Except for titanium, the thicknesses of the oxide layers range from 1.52 nm for nickel to 4.42 nm for molybdenum. Titanium has an oxide layer thickness of nearly twice the thickness of the next largest oxide layer at 8.63 nm. It is well known that titanium readily oxidizes when exposed to atmosphere,¹⁶ which explains why the titanium oxide layer is thicker than the other surfaces.

An externally wetted emitter is operated in a 1 Hz AC square-wave mode to provide both cation and anion electrospray plumes of [Emim][BF₄]. The currents of the emitter, extractor, and target are measured as the DC bias on the target is increased from −85 to +85 V. Figure 6 shows an example of the raw measurements for molybdenum target material. In this example, the externally wetted emitter is operating alternately at ±1.8 kV. The target bias is −85 V. When the emitter voltage is positive, cations are emitted and the emitter current is +450 nA and the target current is +600 nA, which is larger than the emitter current. The extractor current is −100 nA. When the emitter voltage is negative, anions are emitted and the emitter current is −450 nA and the target current is −200 nA, which is more positive than the emitter current. The extractor current is −130 nA. The onset delay and current overshoot when the emitter voltage polarity is switched are typical of externally wetted tungsten emitters and these effects have been characterized by Lozano and Martínez-Sánchez.¹³ Data like that shown in Fig. 6 are collected for several minutes as the target bias is increased in steps of 5 V every 5 s. Each current is averaged for a given operating polarity and target bias voltage. The measurements taken during the onset delay are not included in the averaged currents as the time of the onset delay increases with emitter voltage and varies with the alignment of the emitter and extractor. The averaged currents are plotted as a function of target bias for different target materials and emitter voltages, examples of which are shown in Fig. 7.

Figure 7 displays measured currents as a function of target bias voltage for some of the target materials and emitter voltages tested. The same general trends are found for all materials although there are quantitative differences that are elaborated on below. Considering Fig. 7(a), positive currents are collected in the cation operating mode when the emitter is biased positive. The emitter current is measured at 425–450 nA and does not change with the target bias. The target current decreases from 700 to 400 nA as the bias on the target increases from −85 to +85 V. Negative currents are collected in the anion operating mode when the emitter is biased negative. The emitter current is measured at −425 to −450 nA and does not change with the target bias. The target current decreases from roughly −200 to −500 nA as the bias on the target is increased from −85 to +85 V. It is important to note that the target bias of ±85 V is negligible in comparison to the emitter voltages of ≥1.5 kV, and the RPA data in Fig. 4 show that very few, if any, ions present in the plume have energies below 85 eV. In other words, the target bias is not repelling or attracting anions or cations emitted by the electrospray emitter.

Still considering Fig. 7(a), the plume currents $(ICationandIAnion)$ are calculated as the emitter current minus the extractor current when no target is present, and the plume current is approximately 96%–97% of the emitter current. When the target is biased negative, the current measured at the target in both operating polarities is more positive (less negative) than the plume current. This suggests that at a negative target bias there is net negative secondary current being emitted from the surface. This negative secondary current is the difference between the target and plume currents, and we denote it as $Ication−andIanion−$ for cation and anion modes, respectively. When the target is biased positive, the current measured at the target in both operating polarities is more negative (less positive) than the plume current. This suggests that at a positive target bias there is net positive secondary current being emitted from the surface. This positive secondary current is the difference between the target and plume currents and we denote it as $Ication+andIanion+$ for cation and anion modes, respectively. While we have focused this description of the results on Fig. 7(a), the same general trends can be seen in Figs. 7(b)–7(d) and are present for all materials and all emitter voltages tested.

For each Figs. 7(a)–7(d), and in all experiments performed, the cation mode extractor current is more positive (less negative) than the anion mode extractor current. The extractor current in Fig. 7(a) increases from −180 to 80 nA in cation mode and −280 to 80 nA in anion mode. This change in extractor current is attributed to the collection of emitted charges from the target surface. Secondary emitted charge from the biased target is traveling upstream and being collected at the grounded extractor.

Figure 8(a) shows the return current as a fraction of the secondary current for the ±85 V target bias point in each cation and anion mode averaged over all tests of the targets at 1 and 8 cm distance. The data show that for targets tested at 1 cm from the extractor the fraction of current returning to the extractor is on average 60%–90% and this is 20%–50% larger than targets tested at 8 cm from the extractor. This is also illustrated in Figs. 7(b) and 7(c) where the changes in extractor current do not track as closely with the changes in target current for 8 cm targets. As illustrated in Fig. 8(a), the return current fraction at 1 cm distance is approximately 30% larger for negative secondary charges than for positive secondary charges. This could be due to a difference in trajectory and mobility in electrons vs ions. While the composition of positive secondary charges inherently must be all ions, the composition of negative secondary charges could be a mix of anions and electrons. To further verify the presence of return current, the electrospray plume was pointed away from any nearby targets or diagnostics such that emitted ions would impinge upon the grounded chamber surface 61 cm away. Figure 8(b) shows an example dataset for this configuration. The extractor current is below ±30 nA while the emitter current is ±700 nA. In this configuration, the extractor current over all experiments is measured to be on average 3.6% and at all times less than 5% of the emitter current and is always the same polarity as the emitter current, suggesting that this extractor current is due to emitted ions from the emitter impinging on the extractor $(IImpinging)$ and extractor–emitter secondary current $(IExt,Em)$ and not due to secondary emission current from the far downstream chamber surface.

The data presented in Fig. 7 can be interpreted as the secondary charge emission yield from a given target material with the surface morphology (roughness, oxide, and hydrocarbon layers) measured and described above. The emission yield can be calculated using the experimental data by dividing the magnitude of the secondary current by the magnitude of the plume current [using the analysis of Eqs. (1)–(6), followed by Eq. (7)]. Given the steady-state behavior at both ends of the target bias range in Fig. 7 and the typical energy of secondary electrons and ions from Ar⁺ bombardment of materials is typically only a few eV,^17,18 it is unlikely that any emitted charges of the opposite polarity remain unsuppressed at the ±85 V target bias. There are two electrospray operating modes (cation, anion) and two possible types of emitted current (positive, negative), so there are four yields calculated using Eqs. (8)–(11): yield of positive charges per incident cation $(γcation+)$⁠, yield of negative charges per incident cation $(γcation−)$⁠, yield of positive charges per incident anion $(γanion+)$⁠, and yield of negative charges per incident anion $(γanion−)$⁠,

where $ICation$ is the plume current calculated in cation mode and $IAnion$ is the plume current calculated in anion mode. These yields are calculated and presented in Fig. 9 for emitter operating voltages from 1.5 to 2.9 kV for different target materials with different surface morphology.

The yields for most material illustrated in Fig. 9 exhibit a linearly increasing yield with increasing emitter voltage for all four emission yields. $γcation+$ ranges from roughly 0.05 for molybdenum to 0.55 for gold with all other materials falling somewhere in between. $γcation−$ ranges from roughly 0.05 at the low end of the carbon graphite, nickel, and gold materials to 0.7 at the high end of the aluminum yield data. $γanion+$ ranges from 0 for nickel, titanium, and stainless steel to 0.4 for gold, although the yield for gold decreases greatly as the emitter voltage increases. Molybdenum, however, reaches 0.35 at large emitter voltages and exhibits a faster linear increase with emitter voltage with a slope of 0.4 kV⁻¹ whereas gold displays a linearly decreasing trend, suggesting that molybdenum would have the highest value of $γanion+$ as emitter voltage continues to increase. $γanion−$ exhibits by far the largest yields ranging from 0.3 at the low end of carbon graphite to nearly 1.3 at the high end of aluminum and stainless steel. For negative secondary charges, carbon graphite, molybdenum, and gold display among the highest yields in both cation and anion plumes and aluminum, titanium, and stainless steel display among the lowest in both cation and anion plumes. For positive secondary charges, aluminum displays the highest yields and carbon graphite displays the lowest yields in both cation and anion plumes.

The experimental implications of the data presented in Fig. 9 are that secondary charge emission from surfaces bombarded by electrospray plumes is unavoidable by applying a bias in either polarity to the surface. For example, a tungsten carbide surface could be used for charge collection in a cation plume and be positively biased to suppress emission of negative charges from the surface $(γcation−)$⁠, but the measured current will still need to be adjusted by 5%–10% to account for the emission of positive charges from the surface $(γcation+)$ depending on the emitter voltage. Using the current vs target bias traces in Fig. 7, a surface could be biased at the point where the target current is equal to the emitter current. However, this is not necessarily the point where there is no secondary charge emission, but rather there could be equally positive and negative secondary charge being emitted and this cannot be determined without further characterization of the secondary charges at different target biases. Additionally, most electrospray diagnostics involve more complicated systems than a single charge collecting plate and one electrospray operating polarity. Often, as in these experiments, the emitter or thruster is operated in an alternating polarity mode and most diagnostics include additional biased surfaces, grids, and other components, which may be subject to impingement from the plume. Even with proper experimental design, it will quickly become difficult to verify the source of the currents being collected. This challenge would be greatly eased by the development of higher fidelity electrospray modeling, which utilizes secondary charge yield data to model an experimental system or a thruster–spacecraft configuration more accurately.

Figure 10 illustrates the negative secondary charge emission yields of an [Emim][BF₄] electrospray plume vs ion-induced electron emission (IIEE) from a Xe⁺ ion beam for aluminum, molybdenum, and gold surfaces.^19–22 For aluminum, the yields of [Emim][BF₄] cations and anions increase from 0.2 to 0.75 and 0.6 to 1.2, respectively, over the 1.5–2.5 kV emitter voltage range. For molybdenum, the yields of [Emim][BF₄] cations and anions increase from 0.25 to 0.7 and 0.55 to 0.9, respectively, over the 1.7–2.4 kV emitter voltage range. For gold, the yields of [Emim][BF₄] cations and anions range from 0 to 0.15 and 0.75 to 1, respectively, over the 2.1–2.9 kV emitter voltage range. Xenon requires much higher ion energies to induce negative charge emission with the yield reaching 0.1 at 4–5 keV and increasing to 1 at 23–42 keV for all three materials. Figure 10 clearly illustrates that, at least for aluminum, molybdenum, and gold, the negative charge emission is more significant at lower energy for electrospray plumes than for electric propulsion systems utilizing xenon propellant.

As discussed earlier, previous studies involving charge emission from surfaces bombarded by electrospray plumes have focused on IIEE caused by impinging cations. The evidence of positive charge emission presented in Figs. 7 and 9 strongly suggests that electrons are not the only charged particles emitted from surfaces bombarded by electrospray plumes and possibly not the only negative charges emitted. It is also clear that charge emission is an equally important consideration for anion plumes. Sections IV B–IV F will explore previous work on IIEE in electrosprays as it relates to the data presented here and the feasibility of ions being emitted from surfaces bombarded by electrospray plumes.

As discussed previously, thin surface layers and surface morphology are both known to affect secondary charge emission behavior of surfaces. It is, therefore, possible that surface characteristics may affect secondary charge emission caused by electrospray plume ions. To investigate this, each of the four yields was averaged over the entire range of emitter voltages and plotted against target surface roughness [Fig. 11(a)], hydrocarbon layer thickness [Fig. 11(b)], and oxide layer thickness [Fig. 11(c)]. In all cases except one, the yields show no clear dependence on the surface parameter. The one possible exception is the effect of roughness and hydrocarbon layer thickness on negative charge yield due to anion bombardment $(γanion−)$⁠. The lack of clear trends between secondary charge emission yield and surface parameters suggests that the underlying target material is affecting the yield, not just the surface features. In other words, the results of Fig. 11 suggest that the variation in secondary charge emission yields of Fig. 9 cannot be attributed solely to surface features of the targets but must also be due to the underlying target material. In these experiments, it is likely that both the underlying material and surface features are contributing to the results, but the independent effects cannot be isolated here.

As Fig. 11(a) shows, negative charge yield due to anion bombardment $(γanion−)$ appears to follow an exponential decay relationship with surface roughness. In general, the emission is expected to decrease with increasing surface roughness as emitted secondary particles are more likely to be reabsorbed when surface features are relatively large. This is what Nishimura et al. describe.²³ They investigated secondary electron emission from aluminum as a function of surface roughness and found that for surfaces with a feature aspect ratio (height/width ratio) greater than 0.3, the secondary emission decreases exponentially with the increasing aspect ratio.²³ Modeling by Cao et al. predict the same trend.²⁴ Their models predict the effect of roughness on secondary electron yield for a random rough surface and illustrate an exponentially decaying trend. Their models use similar roughness (0.1–1.0 μm) and predict similar yields (1.5–0.4) as the results here. The main mechanism for lesser electron yields at higher surface roughness is the reentering of electrons into the surface. This effect will be present independent of the impinging species (i.e., the behavior will be comparable for impinging electrons and impinging molecular ions).

As illustrated in Fig. 11(b), negative charge yield due to anion bombardment $(γanion−)$ appears to follow an increasing linear trend with hydrocarbon layer thickness. Wilson and Dennison investigated the effect of carbon layer thickness on a gold substrate and found that in the low energy range (<2 keV) the incident electrons interact mostly with the surface layer and thicker carbon layers produced increased electron yield. The typical yield increase was about 10%–20% from a yield value of 1–1.2.²⁵ Our targets have hydrocarbon layers of 0.8–3 nm thickness and it is, therefore, possible this hydrocarbon layer has a similar effect on the secondary charge emission as the carbon layer. More investigation is needed to determine the mechanism of the increased yield, as the literature is lacking on the effect of hydrocarbon layers on secondary yields.

As illustrated in Fig. 11(c), there does not appear to be a clear trend regarding oxide layer thickness. Literature data suggest that an oxide layer affects secondary emission, but only for thin layers (<5 nm), and when the thickness reaches a critical value the emission is no longer affected. For example, Marcak et al. studied the secondary electron emission from aluminum with varying oxide layer under argon ion bombardment.²⁶ They found that after a minimum oxide layer thickness is established (about 5 nm in their experiments) the secondary emission is no longer a function of layer thickness and is a constant factor of two larger than for the pristine surface. Our surfaces here all have between 2 and 8 nm of an oxide layer. [EMIM][BF₄] has an estimated molecular diameter of 710 pm²⁷ while a xenon atom has an effective diameter of 432 pm based on the van der Waals radius. Due to the size of the molecular ions, they will likely have a lower penetration depth, making it more likely to interact solely with the oxide layer. It is likely that we do not see the effect of the oxide layer due to a lower penetration depth of polyatomic ions compared to monatomic ions.²⁸ Finally, it is important to note that additional relationships between secondary emission yields and surface parameters may not be evident here because surface roughness, oxide layer thickness, hydrocarbon layer thickness, and underlying substrate material all change with each target.

Our results suggest there is no easy way to eliminate secondary charge emission. Biasing a target positive eliminates negative charge emission, but promotes positive charge emission. Biasing a target negative eliminates positive charge emission, but promotes negative charge emission. Perhaps then, if there is always going to be secondary charge emission, it may in some cases be beneficial to at least emit positive and negative secondary charge at the same rate such that there is no net secondary current. The target potential at which positive and negative secondary charge is emitted at the same rate we call the target bias for net zero secondary current. Figure 12(a) shows the target bias voltage required such that there is zero net secondary current for cation (blue) and anion (red) mode. This target bias voltage is denoted the zero point. For all materials tested, the zero point voltage is in the range of ±10 V. For all materials in anion mode, the zero point is at a positive target bias from 2.5 to 9 V and for all materials in cation mode the zero point is at a negative target bias from −1 to 8.5 V, except for nickel which has a cation mode zero point of +0.5 V. The error bars on these measurements represent one standard deviation in each direction of the zero points for the set of measurements collected at each emitter voltage.

Applying a bias to the target in either polarity suppresses the emission of secondary charges of the opposite polarity. As Fig. 7 shows, as the target bias magnitude is increased, the magnitude of the current measured at the target plateaus and reaches a steady state. This constant target current indicates full suppression of opposite polarity charge emission and that there is only secondary emission of charges in the same polarity of the target bias. We define the target bias voltage at which the target current becomes <5% of its steady state as the bias voltage for full suppression of opposite polarity charge emission. Using the data of Fig. 7 for all test cases, the full suppression bias voltage is calculated for each positive and negative target bias in cation and anion modes and is averaged over all emitter voltages tested. The target bias for full suppression of opposite polarity charge emission is plotted for each material in Fig. 12(b). Generally, a target bias of <±0 V is required to reach full suppression. However, the aluminum surface requires −58 V for a negative target bias in anion mode. It is unclear why a much larger target bias is required for full suppression in this instance. The error bars on these measurements represent one standard deviation for the set of measurements collected at each emitter voltage.

The primary mechanism of charge emission considered in the literature is ion-induced electron emission (IIEE), where a high energy primary ion impinging on a surface results in an electron being emitted from the surface. This process is primarily attributed to the mechanism of kinetic electron emission (KEE), where some of the kinetic energy of the incident ion is transferred to the electron and causes it to migrate to and then leave the surface. A more detailed explanation of electron emission mechanisms can be found in Ref. 11. The data presented in this work are compared in this section to existing theoretical and experimental data focusing on IIEE.

Recent work by Magnusson et al. in modeling electron emission from incident polyatomic ions using the software TRansport of Ions in Matter (TRIM) has been applied to Emim⁺ monomer ions. Figure 13(a) compares this theoretical dataset to the data presented in Fig. 9(b) for the negative charge yield of a cation plume impinging on a stainless steel target. The theoretical work assumes a monoenergetic, single species ion beam impinging on a pristine stainless steel surface. The theoretical yields (UCLA, orange) increase from 1.2 to 1.9 over the ion energy range of 1.6–2.8 keV and are on the order of 6–9 times larger than the experimentally measured yields (UIUC). Further, the theoretical predictions display an increasing trend with emitter voltage while the experimental results are constant.

Closer agreement is obtained when one considers the ion energy distribution of the experimental plume is not monoenergetic. The theoretical yields are modified based on the energy distribution of Fig. 4. This adjustment is done by numerically integrating the change in normalized current in Fig. 4(a) multiplied by the emission yield energy distribution of the unadjusted data in orange (denoted as UCLA) as shown in Eq. (12),

where $γadj$ is the adjusted electron emission yield as a function of emitter voltage, $INorm$ is the normalized current of the RPA measurement, $γtheo$ is the theoretical electron emission yield as a function of energy, and E is the energy of the impinging ion. The data presented in red in Fig. 13(a) (denoted as UCLA-Adj) are the result. The adjusted emission yield increases from 0.8 to 1.2 over an energy range of 1.8–2.9 keV, and is still 3–4× larger than the experimental measurements and still displays an increasing trend. These remaining differences may be because the experimental plume consists not just of monomers, but also of dimers and neutrals (resulting from fragmented dimers) and that the real surface of the experiment has finite thickness oxide and hydrocarbon layers and is not polished.

Figure 13(b) compares the experimental data presented in Fig. 9(b) of the negative charge yield of a cation plume impinging on an aluminum surface to a similar measurement performed utilizing a porous-glass electrospray thruster at the University of Southampton (SOTON).⁹ It is important to note that the experiment at SOTON is typical of electrospray community experiments wherein “as-received” materials are used as plume and other diagnostics. The surface morphology of these materials was not measured or reported, but it is very likely that the surface has a significant surface roughness (not polished) and oxide and hydrocarbon layers. Their experiment was intended to suppress IIEE, but is interpreted here as the suppression of all negative charges from the surface. In the experiment performed at SOTON, a nickel mesh grid is positioned in front of the aluminum current collecting surface and biased negative to suppress the emission of negative charges from the aluminum surface similar to how we bias the surface itself positive in these experiments. The difference in current collected between the 0 V grid bias and bias voltage required to fully suppress secondary charge emission (−80 V) is used to calculate the negative charge emission yield for impinging cations as shown in Eq. (7). The negative secondary emission yield from the University of Southampton increases from 0.5 to 2.4 over a range of emitter voltages from 2.3 to 3 kV. While a more complete overlap of energy range is needed to make a full comparison between datasets, the experimental data presented in this paper agree well in the very narrow emitter voltage range of 2.3–2.5 kV, with yields in both datasets increasing from approximately 0.55–0.75. The combination of UIUC and SOTON data in Fig. 13(b) suggests a change in slope of the emission yield for aluminum at about 2.5 kV, but this has not been verified within a single experiment. The anion mode data collected from SOTON do not agree with the results collected here. When the SOTON nickel mesh grid was biased negatively in anion mode, no change in the collected current was observed, which suggests a negative secondary charge yield of 0 for impinging anions, whereas in our data of Fig. 9(d) the negative secondary charge yield for anions impinging on aluminum ranges from 0.6 to 1.2. It may be that due to the higher current density of a porous-glass electrospray thruster, the space-charge effect of the negatively charged plume suppresses negative secondary charges from being emitted from the surface.

Section IV E discussed work in electrospray charge emission that has assumed IIEE is the only form of charge emission from surfaces. The data presented in Figs. 7 and 9 illustrate a more complicated picture of charge emission in electrospray plume–surface interactions. The existence of the positive charge emission yields and their dependence on emitter voltage strongly suggest that positive ions are being emitted from the surface and prompt the possibility that negative ions may be emitted from the surface along with electrons. The experiments performed in this work do not allow for a distinction between different species of the same charge emitted from the surface. It is well known in time-of-flight secondary ion mass spectrometry (TOF-SIMS) experiments that in addition to sputtered neutral particles, positive and negative ions can be sputtered from surfaces. Lundquist¹⁸ found that secondary cations contribute 50% of sputtered species from pure copper bombarded by a 3 keV Ar⁺ beam and emission of secondary cations from pure surfaces is much more common than emission of secondary anions.^18,29,30 Further, sputtering of the oxide layer of aluminum at incident ion energies below 500 eV has been shown to result in O⁻ yields of similar magnitude to the electron emission yield.¹⁴ Gries³¹ found that secondary cations contribute 10% of sputtered species from pure aluminum sputtered by a 5 keV Ar⁺ beam. While the sputtering mechanisms would certainly be different for an electrospray plume, it is possible that secondary ions contribute to the secondary currents measured in Fig. 7. Uchizono et al. have made similar experimental observations of changing current measurements at the target and other surfaces as the target bias is changed, including the observation of both negative and positive secondaries.¹⁰ They also observe that secondary ion return current to the thruster can cause physical changes in propellant quality, anomalous neutral mass loss, and a glow discharge during thruster operation. Understanding and mitigating these types of ground testing effects is critical to the development of electrospray systems.

Another complication of these experiments with [Emim][BF₄] plumes is that the IL may condense and build up into a thin film on the surface.^11,32 We did not see direct evidence of this in our experiments. The time dependence of this buildup has yet to be studied. It is reasonable to assume that such an IL thin film would itself be sputtered by the electrospray plume and result in charged propellant ions or droplets being emitted from the surface. Each target surface tested here is impinged upon by the electrospray plume for approximately 1 h. If every ion incident on the surface contributed to an evenly distributed IL layer, based on the 17° plume divergence, it would result in a 1.6 nm layer by the end of an experiment with an 8 cm distance between the emitter and target and a 98.9 nm layer for a 1 cm distance between the emitter and target. In contrast, the maximum oxide or hydrocarbon layer thickness we measured was less than 9 nm. Because of the plume mass distribution, IL layer thickness would vary from thicker in the center of the target and thinner as the angle from centerline increases. IL layers of this order of thickness could entirely change the plume–surface interaction to a point where the plume is no longer interacting with the target surface itself. Film formation could also depend on the surface morphology (interactions with existing surface layers) and future experiments are needed to determine how IL thin films accumulate and how this mechanism could affect the secondary charge emission properties.

Emitted secondary charge yields have been measured for eight different materials bombarded by [Emim][BF₄] electrospray plumes for emitter voltages between 1.5 and 2.9 kV. The target materials were not pristine or polished, but rather were typical of “as-received” surfaces used by experimentalists within the electrospray community and had surface roughness of a few micrometers and oxide and hydrocarbon layers a few nanometers thick. We find that both positive and negative secondary charges can be emitted from a surface bombarded by both cations and anions. The secondary positive charge yield during cation bombardment $(γcation+)$ ranges from 0 to 0.55, the secondary negative charge yield during cation bombardment $(γcation−)$ ranges from 0 to 0.75, the secondary positive charge yield during anion bombardment $(γanion+)$ ranges from 0 to 0.4, and the secondary negative charge yield during anion bombardment $(γanion−)$ ranges from 0.3 to 1.3. While common interpretation in the literature suggests that ion-induced electron emission is the only important charge emission mechanism, evidence presented here supports the possibility of sputtered ions. The yields presented here should be used by experimentalists to inform material choice and experimental design and correct current measurements in experiments. These yields and future work in this area should be used by modeling groups to develop high fidelity electrospray models capable of predicting facility effects in experiments and more accurately predicting thruster performance and thruster–spacecraft interactions. Future experimental work in this area should measure energy and mass distributions of secondary emitted electrons and ions to better characterize the plume–surface interaction and should also filter the electrospray plume by energy and mass to measure the charge emission yields dependence on incident energy and species. Experiments should also characterize IL accumulation on surfaces and the effect an IL layer has on sputtering and secondary charge emission.

The authors thank the NASA Early Stage Innovation program, Federal Award Identification No. 80NSSC19K0215, for funding the experimental work for this project and Dr. Thomas Liu at NASA Glenn Research Center for his contributions as an advisor for the project. Additionally, the authors thank Dr. Benjamin Prince and Dr. Shawn Miller for hosting Matthew Klosterman at the Air Force Research Laboratory at Kirtland to conduct preliminary externally wetted electrospray experiments.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.