The interelectrode breakdown mechanism and discharge characteristics of the electrospray thruster

In the actual working process of electric thrusters based on high-voltage electric fields, the discharge breakdown phenomenon is universal and complex, and such phenomena will have a significant impact on the thruster structure, working state and spacecraft system. In order to study the interpolar discharge breakdown characteristics of ionic liquid electrospray thrusters, a basic electrospray model and test system were constructed, and the change curves of discharge characteristic parameters such as breakdown voltage, threshold current, breakdown voltage frequency and so on in the range of 7×10−3~105 Pa with air pressure and transmitter inner diameter were obtained, and the air pressure range that the electrospray model could work in normally was calibrated. The results show that the breakdown voltage characteristic curve of the electrospray model has typical minimum characteristics, and the minimum values all appear around 80 Pa. Lowering the air pressure below 10−2 Pa can effectively increase the breakdown threshold between the poles and the emission current, thereby obtaining a larger voltage regulation range, and when the air pressure is reduced to 7×10−3 Pa, the breakdown can reach more than 3200 V. The 60-μm inner diameter emitter performed better in the discharge experiment, and the breakdown threshold, emission current and operating area range were better than the slightly larger inner diameter emitter under the same working conditions.


Introduction
The Ionic Liquid Electrospray Thruster (ILET), based on the principle of electrostatic injection, is a new type of electric propulsion technology [1,2].Compared with the traditional propulsion method, ILET has the advantages of high specific impulse (1000~5000 s), small and accurate thrust (0.1~5000 μN), high efficiency (50%~80%), beam self-neutralization, low power consumption, simple structure, and low cost [3,4].Based on the above characteristics, ILET has become one of the best candidate propulsion methods for micro-nano satellite formation networking and space gravitational wave detection.ILET relies on a high-intensity electric field.Under the action of the electric field force and the liquid surface tension, the spraying medium forms a Taylor cone at the top of the emitter and accelerates the ejection of micro-scale droplets or ions at the tip of the Taylor cone to produce a reaction thrust [5][6][7][8].
However, while the various electric thrusters based on high-intensity electric fields are widely used in space propulsion, unexpected discharge breakdowns always exist.During the 2000-h durability test of ARC Seibersdorf's In-FEEP thruster, the high-voltage discharge between the residual droplets on the extraction pole and the emitter caused the emitter of the In-FEEP thruster to melt due to excessive local Joule heat, which changed the structural parameters of the thruster [9].The XIPS-13 ion thruster from Boeing has caused electrode discharge breakdown on many satellites, which significantly shortens the service life of satellites [10].In the ground test phase, many thrusters or prototypes have also experienced the situation of discharge chamber flameout due to the electrical breakdown of interpole discharge [11,12].The above examples show that the phenomenon of discharge breakdown in the highvoltage electric field will not only affect the actual propulsion performance, operating state, and working environment of the thruster, but also cause hidden dangers such as short circuits, damage, and pollution to the spacecraft, and even endanger the progress of the space mission.
The breakdown effect of ILET is also universal and complex.When the electrode breakdown occurs, the actively applied electric field distorts, resulting in an unstable working state of the electrospray system.In addition, the starting voltage of the electrospray system is determined by the characteristics of the spray working medium itself (conductivity, surface tension, etc.), temperature, and polar distance.Because the voltage applied in a wide pressure range is insufficient to reach the starting voltage of the spray working medium, the electrospray system ceases to function.As a result, in the field of electrospray propulsion, in addition to concentrating on propellant selection, performance enhancement, and structure simplification, it is also important to accurately comprehend the discharge characteristics of the electrospray electric field at the level of its fundamental physical model.At present, there are abundant theoretical achievements related to the physical processes of electrospray and discharge physics, but there are few studies on the intersection of the two fields, and quite a few gaps in the discharge mechanism and experimental research under the electrospray field have not been filled.
Based on the discharge mechanism of a non-uniform electric field, a set of electrospray basic model that are equal to ILET and a test system were established in this study.The variation rules of various discharge characteristic parameters (breakdown voltage, threshold current, breakdown voltage frequency, etc.) during the process of reducing the pressure to a high vacuum were investigated, and the theoretical explanations of the relevant rules were discussed.

Experimental system
The basic model of an electrospray system consists of emitter (anode), extraction electrode (cathode), propellant, and supply system.A typical electrospray thruster has a pole spacing of less than 5 mm and an emitter size of 10 to 500 μm [13][14][15][16][17]. Therefore, the characteristics of an electrospray system operating in a strong electric field and its structural features constitute the basic models of short-gap low-pressure gas discharge and vacuum discharge under a non-uniform electric field.According to the working principle of the electrospray system, a set of active supply equivalent desktop experiment system was built, which were composed of a vacuum system, an electrospray model, and a test system.The schematic diagram is shown in Figure 1.
The structural parameters of the electrospray model referred to the actual ILET or the scale of the principle prototype of each research unit [13,14,18,19].Capillary tubes with an inner diameter of 60 μm, 110 μm and 160 μm were selected as the emitter.A stainless steel plate with a center aperture of 5 mm and a thickness of 1 mm was selected as the extractor, and the pole spacing was 2 mm.The ionic liquid 1-ethyl-3-methylimidazole tetrafluoroborate (EMI-BF 4 ) that is commonly used in ILET was selected as the propellant for the electrospray model.The vacuum system was composed of a vacuum chamber body, a molecular pump, a mechanical pump, a ceramic electrode flange, an ionization gauge, and a resistance gauge.The chamber body was a cylindrical structure with a size of φ60×60 cm, and the working vacuum was 7×10 -3 Pa.The working high-voltage source transmitted high voltage by weak current control the strong current, and connected the circuit by connecting the emitter to high-voltage and drawing the electrode to ground, so that the inter-electrode voltage could be adjusted in the range of 0~15 kV.The test system was composed of a high-voltage test device (Xi'an Weisman, V6VL15) and a digital oscilloscope.The high-voltage test device is specially used to test the amplitude of the voltage between the poles and displays the specific voltage waveform and frequency through the oscilloscope.The physical diagram of the experimental system is shown in Figure 2.

Experimental method
The experimental pressure range was 6×10 -3 ~10 5 Pa, and 4~5 characteristic pressure values were selected for each pressure level to discharge.The breakdown threshold voltage under each working condition was measured by the uniform voltage boost method with a step size of 15 V.In order to eliminate the residual charge and influence between the electrodes after discharge, the same two discharge intervals were 100 s.When the waveform displayed by the oscilloscope changes from a straight line to a pulse waveform, the minimum value read is the breakdown voltage, and the current value stabilized for more than 5 s before the breakdown is the effective value.In order to eliminate the effect of the partial air dissolved in the vacuum on the ionic liquid spilling out at the emission electrode, the ionic liquid should be placed in the vacuum before the experiment starts.The specific static method is as follows: filling the syringe with ionic liquid, controlling the liquid level under the emitter, then starting the vacuum equipment and placing the syringe in vacuum for 24 hours.After completion, the ionic liquid level is in a vacuum state, and pushing the piston handle by the injection pump to makes the liquid evenly fill the entire syringe and emitter, and the emitter is in a state of infiltration.
During the experiment, the ambient temperature was 18~22 °C.So as to reduce the influence of water vapor on the experimental results, the relative humidity of the air in the laboratory was ensured to be below 15%.Under different inner diameters of the transmitter, by changing the air pressure in the vacuum chamber, the breakdown voltage characteristic curve, threshold current curve, and waveform frequency curve were obtained.

Breakdown voltage characteristic curves
Figures 3(a) and (b) show the breakdown voltage characteristic curves of three emitters with different inner diameters in the low-pressure and vacuum discharge ranges respectively.The breakdown voltage corresponding to the three inner diameters has a similar change rule to the Paschen curve when the air pressure is gradually decreasing [20][21][22].The curve is generally a "U-shaped" distribution and exhibits typical minimum value characteristics.The minimum points corresponding to the three inner diameters all appear near 80 Pa, and the minimum breakdown voltages between electrodes are 570 V, 555 V, and 555 V, respectively.
As can be seen from Figure 3(a), in the range of 10 4 ~10 5 Pa, the breakdown voltage drops sharply from 5 kV to about 1.5 kV as the air pressure decreases.At the same distance and pressure, the breakdown voltage is inversely correlated to the inner diameter of the emitter.In the range of 10~10 4  Pa, the breakdown voltage threshold is below 1500 V, the breakdown voltage changes with the pressure are small, and the three curves have a high coincidence degree but still show the minimum characteristics.As can be seen from Figure 3(b), in the pressure range of 7×10 -3 ~10 Pa, as the pressure continues to decrease, the breakdown voltage is inversely correlated with the pressure and is insensitive to the change of the emitter inner diameter.Under the same pressure conditions, the diversity of the three curves is weak.When the pressure drops below 10 -2 Pa, the breakdown voltage between electrodes is about 3200 V.The electrospray system can work normally, and with the increase in vacuum degree, the larger the adjustable range of voltage and the wider the thrust range.According to the streamer theory, electron impact ionization and space photoionization play a major role in sustaining self-sustained discharge when the discharge field is at a greater atmospheric pressure (about 5000~10 5 Pa) [23].In the range of 10 4 ~10 5 Pa, the main mechanism of breakdown is streamer discharge.When the air pressure decreases, the density of air molecules in the vacuum chamber decreases, and the average free path of colliding particles increases.Although the number of collisions between particles decreases, the increase in the average free path of collisions causes the kinetic energy obtained by particles before collisions to increase sharply.The inhibiting effect of the former on discharge development is less than the promoting effect of the latter.Therefore, the breakdown threshold is greatly reduced in this pressure range.At the same time, the length of the plasma stream channel of the smaller inner diameter emitter breakdown is slightly larger than that of the large inner diameter emitter, and the breakdown of the former is relatively difficult, so the breakdown threshold of the three inner diameter emitters has the rule of "60-μm>110-μm>160-μm".
According to the Townsend discharge theory, the primary reasons of self-sustaining discharge at low-pressure (about 1.33~5000 Pa) are collision ionization between electrons and gas molecules in the gap and secondary electron emission from the cathode [24].In the range of 10~10 4 Pa, the Townsend discharge is the main discharge mechanism, and the impact of collision frequency and collision mean free path on the discharge threshold is close to the equilibrium state.At the same time, the flow channel gradually disappears, and the inner diameter of the emitter has little influence on the discharge threshold, so the coincidence degree of the three curves is high.
When the air pressure is less than 1.33 Pa, vacuum discharge replaces low-pressure gas discharge across the transition zone as the discharge form in the electrospray field gap, and the breakdown threshold is significantly raised.Due to the extremely low air density in a high vacuum environment, the average free range of air molecules is significantly greater than the gap distance, resulting in very little collision ionization in the gap and insufficient energy accumulation to cause the gap to breakdown, which is what causes the fundamental difference in mechanism between vacuum discharge and lowpressure gas discharge.Field emission theory is currently thought to be the primary breakdown mechanism [25][26][27].

Starting voltage and operating range of calibration
The electrospray system makes the charged particles gain kinetic energy by applying a high-voltage.When the electric field force on the charged particles is greater than the liquid surface tension, the charged particles are extracted from the jet medium and accelerated, thus gaining thrust.With the increase in voltage, the ion density in the jet increases, and the emission mode of ILET gradually changes from droplet mode to jet mode and ion mode, which has a higher specific impulse.In other words, the emission voltage is one of the decisive factors in ILET performance [16].When the electrode breakdown occurs, the applied electric field distorts.At this time, if the breakdown voltage is less than the starting voltage, the charged particles cannot accelerate from the emitter to the extraction pole and form a stable thrust due to the influence of the distorted electric field.The starting voltage and approximate operating interval calibration results of the electrospray system in this experiment are shown in Table 1.
Table 1.Starting voltages and operating range.

Threshold current curves
The threshold current curves of the three inner diameter emitters in the low-pressure and vacuum discharge ranges are shown in Figure 4(a) and (b).As can be seen from Figure 4(a), with the decrease in air pressure, the current value drops sharply in the range of 10 4 ~10 5 Pa, which is positively correlated with air pressure and inversely correlated with inner diameter.In the range of 200~10 4 Pa, the current value decreases slowly, the slope of the curve changes obviously, and the inflection point values are all within 10 4 ~2×10 4 Pa.Continuing to decrease the air pressure, the current difference between the three inner diameters gradually decreased, and the threshold current value was maintained at a low level from 10 to 200 Pa, varying only from 0 to 1 μA.According to the starting voltage calibration results, the starting voltage of the electrospray system is about 2000 V.When it is less than the breakdown voltage, the electrospray system can work normally, so it can be approximately determined that 2×10 4 ~10 5 Pa is the working zone, and 10~2×10 4 Pa is the failure zone.The threshold current in the working zone is the emission current of the electrospray, because the greater the air pressure, the stronger the suppression of corona discharge on the surface field strength [28,29].The threshold current in the failure zone is the discharge current, and the reason for the curve inflection point is the transformation process of the emission current to the discharge current.In the range of 10~200 Pa, the minimum of threshold current generated by non-self-sustaining dark discharge is also around 80 Pa.As can be seen from Figure 4(b), the threshold current is still below 1 μA in the range of 3~10 Pa.When the pressure drops to 2 Pa, the current value slowly rises to 10 μA.Continue to reduce the pressure, when the breakdown voltage is greater than the starting voltage, the electrospray model can work normally.At this time, the threshold current is the emission current of the electrospray model in the vacuum operating zone.Due to the lack of corona discharge inhibiting the field intensity on the surface of the Taylor cone, the emission current in the vacuum operating zone is much higher than that in the near-atmosphere operating zone, which can reach more than 250 μA, and the emission current tends to rise slowly with the further reduction of the air pressure.When the other parameters are the same, the larger the emission current, the better the performance of the electrospray propulsion system.Therefore, increasing the vacuum degree can effectively improve the propulsion performance.

Breakdown voltage frequency curves
The breakdown between the two poles of the electrospray field distorts the active electric field, and the waveform of the breakdown voltage changes periodically.Figure 5 shows the frequency curve of the breakdown voltage waveform in the low-pressure gas discharge range.It can be seen from the figure that within 400~10 5 Pa, with the decrease of the air pressure, the frequency slowly increases from 100 Pa to around 2000 Pa, and the frequency of the wide inner diameter emitter is slightly larger than that of the narrow inner diameter emitter.In the range of 40~400 Pa, the intersection point of the three curves occurs in the range of 300~400 Pa, and the frequency is inversely correlated with the inner diameter, and the peak value appears around 80 Pa.When the air pressure is 5~60 Pa, the breakdown voltage is not periodic.When the pressure is reduced to less than 5 Pa, the breakdown voltage reappears periodically, but the frequency value changes are more complex and the repeatability is low.

Conclusion
We constructed an identical electrospray model and test equipment with the intention of addressing the discharge breakdown issue in ILET, and then got the operating area and characteristic curves of discharge parameters as a result of lowering air pressure from the atmosphere to a high vacuum.According to the analysis and discussion of the results, the conclusions are as follows: (1) The characteristic curve of the breakdown voltage of the electrospray electric field and the Paschen curve of the uniform electric field have similar variation rules, and both have typical minimum characteristics.The minimum pressure point of each inner diameter appears to be near 80 Pa.The breakdown voltage changes periodically, and its frequency has good regularity in the low-pressure gas discharge range.
(2) When the breakdown voltage is less than the starting voltage, the electrospray model cannot work properly.The breakdown threshold between electrodes in the electrospray model can be effectively increased by reducing the pressure to a high vacuum.The higher the vacuum degree, the wider the adjustable range of the applied voltage and the wider the corresponding thrust range.
(3) Under the same working conditions, the small inner diameter emitter has a larger emission current, a higher breakdown threshold, and a wider working pressure range.
(4) In the near-atmosphere operating zone, the threshold current is positively correlated with the air pressure and inversely correlated with the emitter inner diameter.Due to the suppression of the field intensity on the surface of Taylor cone by corona discharge, the emission current in the high vacuum operating zone is much larger than that in the near-atmosphere operating zone under the same working conditions.
When the air pressure is within the pressure range from 5 Pa to the right edge of the vacuum working area, the breakdown voltage is also periodic, but the breakdown voltage frequency is greatly affected by random error, and the repeated experiment results have a large gap.Therefore, due to the above factors, the study has not mastered the frequency variation curve in this interval.

Figure 1 .
Figure 1.Schematic diagram of the experimental system.

Figure 2 .
Figure 2. Physical diagram of the experimental system: A micro-syringe pump; B emitter; C extractor; D low voltage DC power supply; E high voltage power supply; F digital oscilloscope; G vacuum chamber.

Figure 3 .
Figure 3. U-P curves of three internal diameters under: (a)10~10 5 Pa, (b)7×10 -3 ~10 Pa.According to the streamer theory, electron impact ionization and space photoionization play a major role in sustaining self-sustained discharge when the discharge field is at a greater atmospheric pressure (about 5000~10 5 Pa)[23].In the range of 10 4 ~10 5 Pa, the main mechanism of breakdown is streamer discharge.When the air pressure decreases, the density of air molecules in the vacuum chamber decreases, and the average free path of colliding particles increases.Although the number of collisions between particles decreases, the increase in the average free path of collisions causes the kinetic energy obtained by particles before collisions to increase sharply.The inhibiting effect of the former on discharge development is less than the promoting effect of the latter.Therefore, the breakdown threshold is greatly reduced in this pressure range.At the same time, the length of the plasma stream channel of the smaller inner diameter emitter breakdown is slightly larger than that of the large inner diameter emitter, and the breakdown of the former is relatively difficult, so the breakdown threshold of the three inner diameter emitters has the rule of "60-μm>110-μm>160-μm".According to the Townsend discharge theory, the primary reasons of self-sustaining discharge at low-pressure (about 1.33~5000 Pa) are collision ionization between electrons and gas molecules in the

Figure 5 .
Figure 5. Frequency curves of the breakdown voltage under the 20~10 5 Pa.Figure5shows the frequency curve of the breakdown voltage waveform in the low-pressure gas discharge range.It can be seen from the figure that within 400~105 Pa, with the decrease of the air