Ablative Pulsed Plasma Thruster Research Articles

The plasma morphology of an asymmetric electrode ablative pulsed plasma thruster

The ablative pulsed plasma thruster (APPT) is a typical form of electric propulsion that is highly suitable for micro-satellites. However, its low performance has been a main drawback. Recently, an asymmetric segmented anode schematic was shown to enhance the performance of APPTs. The improved thrust performance obtained with the use of a segmented anode can potentially enable promising applications in future space tasks. To further understand the physical processes behind a segmented anode APPT, experiments using an ultra-high-speed camera, narrow bandpass filters, and a magnetic probe were conducted with both normal parallel-plate electrodes and a segmented anode on an experimental APPT. Successive images of light emission from C+, neutral C2, and broadband emission reveal the evolution of the plasma morphology on an APPT with a segmented anode. The magnetic field strength profiles show that the inter-electrode plasma propagation velocity of a segmented anode APPT is approximately 19 km s−1, which is slightly lower than that of a normal APPT at 23 km s−1. However, the similar order of magnitude suggests a similar downstream acceleration mechanism between the two. The arc morphology observed in the high-speed images shows that the segmented anode APPT with asymmetric electrodes has a restricted arc attachment point, possibly resulting in stronger arc current density at the anode corner. This should result in higher ionization and ablation when compared with a normal anode APPT. Based on the results here, we hypothesize that the segmented anode APPT may be able to better utilize the restrike energy in the propellant ionization process. This fundamentally changes the physical process behind APPT operation, meaning that the improvements should also be applicable to other APPT designs. It can also be adopted as an additional step in the overall optimization of an APPT design.

Early stage of the discharge in ablative pulsed plasma thrusters

Currently the renewed interest in ablative pulsed plasma thrusters (APPTs) mostly results from the needs of satellites of the nano and micro sectors. However, the efficiency of APPTs significantly decreases when they are scaled down. A better understanding of the physical processes involved in plasma generation and maintenance in this type of thruster may help to minimize APPTs’ performance degradation, when they are miniaturized. One of the issues, which up to now seems to be only marginally clarified, is an initial phase of discharge between the APPT’s electrodes. In this paper a novel analytical model of a flashover development is proposed. In the model, movement of electrons between the PPT’s electrodes is described relying on the concept of a saturated secondary electron avalanche. Two analytical approaches are considered. In the simplest approach, secondary electrons are assumed to be monoenergetic, whereas in the second, the Maxwellian distribution function is assumed. Simple, analytical formulas for surface current density, electron density and the thickness of the electron layer are derived. The predicted values are in a good agreement with measurements and computer simulations of other authors. The work was inspired by experimental examination of a micro APPT fed with polymeric nonvolatile propellant. For completeness, the thruster, known as an LμPPT (liquid micro pulsed plasma thruster), is described and its performance is presented.

Discharge voltage characteristic in ablative pulsed plasma thrusters

Ablative Pulsed Plasma Thruster (APPT), as the earliest electric thruster, has been applied in engineering applications over 50 years. The voltage characteristic, as the most direct parameter, is mostly measured in all APPT experiments to measure the performances. However, almost all researchers applied the capacitor voltages instead of the electrode voltages to estimate the performances (i.e., propellant utilization efficiency, plasma flow between electrodes). Due to the real-time variation of the plasma impedance, there were some deviations in these estimations. In this study, voltage characteristics of the accelerated electrodes were analyzed experimentally and theoretically: in the single discharge process, the trends of phase delay and amplitude difference between electrode voltage and capacitor voltage are related with the variations of the resistance and inductances of the plasma bulk; as the APPT discharge voltage increases, the phase delay and amplitude difference between electrode voltage and capacitor voltage increase in the whole trend, the estimated deviations become larger. This study provides a reference for the numerical model analysis in ablative pulsed plasma thrusters.

Inter-electrode discharge of an ablative pulsed plasma thruster with asymmetric electrodes

Ablative pulsed plasma thrusters (APPTs) are a form of propulsion that have been widely used in space tasks. In a recent study, an asymmetric segmented anode schematic was shown to be able to enhance the thrust performance of APPTs. To further understand the physical processes behind a segmented anode APPT, various research campaigns were carried out in this work: the discharge characteristics were studied, measurements were made of the total discharge resistance and inductance, and mapping was performed for the current density (along the side-view plane). From the discharge parameters and fitted total resistance and inductance profiles, we found a 10% lower discharge resistance for the segmented anode APPT compared with a normal anode APPT, which indicates a more optimized discharge process in the segmented anode APPT. The time-resolved current density maps offer two clear indications that verify the better performance of a segmented anode APPT. The first indication is the stronger current density on the surface of PTFE of the segmented anode during the main discharge, which shows that the segmented anode increases the discharge arc current density, which can then improve the ablation and ionization processes. The other indication is the more intense current density distribution between the electrodes during the follow-up discharge period (re-strikes) on the segmented anode APPT. This phenomenon implies that the segmented anode APPT may be able to better utilize the restrike energy in the propellant ionization process.

Investigation of the Initial Stage of the Discharge in an Ablative Pulsed Plasma Thruster

Discharge development in an ablative pulsed plasma thruster is considered. Data obtained experimentally are analyzed. The time of the discharge-development stages is determined. It is revealed that discharge development is determined by the initial stage of the discharge. The initial stage of the discharge in a certain ablative pulsed plasma thruster is investigated. The problem of carbonizing the working surfaces of propellant bars in this thruster is considered. A mathematical model for charged-particle motion in the ablative plasma thruster at the initial stage of the discharge is generated. The main simulation results are presented. The electric field generated by a system of electrodes corresponding to the thruster is plotted theoretically. How the electric field influences the initial stage of the discharge in the ablative pulsed plasma thruster is investigated. The behavior of charged particles under low currents and magnetic field initiation is studied.

Direct investigation of near-surface plasma acceleration in a pulsed plasma thruster using a segmented anode

The ablative pulsed plasma thruster (APPT) has been widely used on satellites due to its low electrical power requirements and structural simplicity. Recently, a segmented anode schematic has been proposed to increase the main discharge arc current density and the propellant mass ablation of APPTs while using the same input energy. In this work, a structurally similar multi-segmented anode was instead used for plasma measurements. The incentive behind the use of the multi-segmented anode was to measure the plasma propagation process and downstream current flow inside the discharge channel of the APPTs (volume between the electrodes). The multi-segmented anode divides the anode into multiple parts in order to directly measure and analyze the current flow for each anode segment as the APPT plasma moved downstream between the electrodes. This provided new information regarding the plasma propagation process between the electrodes. Local magnetic probe measurements were also conducted inside the APPT discharge channel. From the experimental results, distinct plasma progression towards the downstream direction was identified. The plasma reached a velocity of up to 25 km s−1 at only 5 mm from the propellant surface. In contrast, the downstream portion of the electrodes was found to contribute relatively little to the final velocity (an increase of ~16%). Furthermore, the base (1st) element of the multi-segmented anode (where the discharge arc is located) was found to carry over 5 times more current than downstream segments (which carry the maximum downstream plasma current). These results directly indicate that the downstream plasma carries relatively little current, thus resulting in a lower Lorentz force contribution and downstream acceleration. Instead, most of the plasma acceleration process occurs very near to the propellant surface.

Advanced Pulsed Plasma Thrusters and Their Application as a Part of Small Spacecraft Propulsion Systems

The paper presents the research results of the effect of a capacitor energy storage device configuration on the specific characteristics of advanced modern propulsion systems based on the ablative pulsed plasma thrusters (APPT). These thrusters are designed to perform specific tasks within the small spacecrafts with the onboard power capacity up to 200 W.

Ignition mechanism in ablative pulsed plasma thrusters with coaxial semiconductor spark plugs

Ignition process, as the initiation of the entire discharge, plays an important role in ablative pulsed plasma thrusters. While spark plug exactly how initiate discharge is achieved is still under review. This study did some experiments with two kinds of propellant surfaces (normal or inclined) and without propellant to explain the ignition process. The experimental results showed: when the thruster discharge without propellant, it is essentially a surface flashover process on ceramics; when the propellant was loaded, the main discharge occurs after the initial conductive path composed of electrons emitted by spark plug forming. This study provides a reference for the high performance pulsed plasma thrusters.

Continuous discharge in micro ablative pulsed plasma thrusters

The mechanism of continuous discharge in micro ablative pulsed plasma thrusters has always been ignored. A series of experiments on discharge completion rate were conducted, and theoretical analyses were performed to understand the results. These results showed that as initial voltage decreased to a certain value, a portion of energy was not released in a short time, which caused overall efficiency to greatly decrease. As the initial voltage decreased sequentially, this phenomenon occurred more frequently. This study provides a reference for high-frequency micro ablative pulsed plasma thrusters.

Optimization of the Energy Distribution in Ablative Pulsed Plasma Thrusters

Regarding space missions, electric propulsion efficiency is a hot topic due to the development of microsatellites. However, ablative pulsed plasma thrusters, which were the first electric propulsion devices, suffer from low efficiency (less than 10%). In this study, a double-discharge ablative pulsed plasma thruster that allocates the total energy to two capacitors and discharges at different locations was designed to improve the efficiency of these thrusters. The performance of the double-discharge ablative pulsed plasma thruster was calculated based on experimental results under four different working conditions. The results showed that the double-discharge ablative pulsed plasma thruster performance was significantly improved relative to that of a regular ablative pulsed plasma thruster, in which the total energy is stored in a single capacitor. When the total energy remains constant (2.25 J), as the energy allocated to the secondary capacitor increases, the thrust efficiency and specific impulse increased significantly, respectively. Finally, theoretical analysis combined with the experimental results revealed the successful separation of the ablation–ionization and acceleration processes for the double-discharge ablative pulsed plasma thruster, contributing to the improvement of its performance. This research provides a reference for the design of high-performance ablative pulsed plasma thrusters.

An ablative pulsed plasma thruster with a segmented anode

An ablative pulsed plasma thruster (APPT) design with a ‘segmented anode’ is proposed in this paper. We aim to examine the effect that this asymmetric electrode configuration (a normal cathode and a segmented anode) has on the performance of an APPT. The magnetic field of the discharge arc, plasma density in the exit plume, impulse bit, and thrust efficiency were studied using a magnetic probe, Langmuir probe, thrust stand, and mass bit measurements, respectively. When compared with conventional symmetric parallel electrodes, the segmented anode APPT shows an improvement in the impulse bit of up to 28%. The thrust efficiency is also improved by 49% (from 5.3% to 7.9% for conventional and segmented designs, respectively). Long-exposure broadband emission images of the discharge morphology show that compared with a normal anode, a segmented anode results in clear differences in the luminous discharge morphology and better collimation of the plasma. The magnetic probe data indicate that the segmented anode APPT exhibits a higher current density in the discharge arc. Furthermore, Langmuir probe data collected from the central exit plane show that the peak electron density is 75% higher than with conventional parallel electrodes. These results are believed to be fundamental to the physical mechanisms behind the increased impulse bit of an APPT with a segmented electrode.

Characteristics of plasma properties in double discharge ablative pulsed plasma thrusters

Ablative pulsed plasma thrusters, the earliest electric space propulsion devices, create highly transient plasmas in short discharges that are expelled to create thrust. In recent years, the double-discharge ablative pulsed plasma thruster design has been proposed to improve the low-thrust efficiency. In this study, optical emission spectroscopy was applied to investigate the plasma properties in different regions and energy distributions. The electron temperature and electron density of the plasmas are derived and discussed. This study provides a physical mechanism for double-discharge pulsed plasma thrusters.

Discharge characteristics of an ablative pulsed plasma thruster with non-volatile liquid propellant

Pulsed plasma thrusters (PPTs) are a form of electric spacecraft propulsion. They have an extremely simple structure and are highly suitable for nano/micro-spacecraft with weights in the kilogram range. Such small spacecraft have recently experienced increased growth but still lack suitable efficient propulsion systems. PPTs operate in a pulsed mode (one discharge = one shot) and typically use solid polytetrafluoroethylene (PTFE) as a propellant. However, new non-volatile liquids in the perfluoropolyether (PFPE) family have recently been found to be promising alternatives. A recent study presented results on the physical characteristics of PFPE vs. PTFE, showing that PFPE is superior in terms of physical characteristics such as its resistance to carbon deposition. This letter will examine the electrical discharge characteristics of PFPE vs. PTFE. The results demonstrate that PFPE has excellent shot-to-shot repeatability and a lower discharge resistance when compared with PTFE. Taken together with its physical characteristics, PFPE appears to be a strong contender to PTFE as a PPT propellant.

Discharge reliability in ablative pulsed plasma thrusters

Discharge reliability is typically neglected in low-ignition-cycle ablative pulsed plasma thrusters (APPTs). In this study, the discharge reliability of an APPT is assessed analytically and experimentally. The goals of this study are to better understand the ignition characteristics and to assess the accuracy of the analytical method. For each of six sets of operating conditions, 500 tests of a parallel-plate APPT with a coaxial semiconductor spark plug are conducted. The discharge voltage and current are measured with a high-voltage probe and a Rogowski coil, respectively, to determine whether the discharge is successful. Generally, the discharge success rate increases as the discharge voltage increases, and it decreases as the electrode gap and the number of ignitions increases. The theoretical analysis and the experimental results are reasonably consistent. This approach provides a reference for designing APPTs and improving their stability.

Characteristics of a non-volatile liquid propellant in liquid-fed ablative pulsed plasma thrusters

In the past several decades, the use of electric propulsion in spacecraft has experienced tremendous growth. With the increasing adoption of small satellites in the kilogram range, suitable propulsion systems will be necessary in the near future. Pulsed plasma thrusters (PPTs) were the first form of electric propulsion to be deployed in orbit, and are highly suitable for small satellites due to their inherent simplicity. However, their lifetime is limited by disadvantages such as carbon deposition leading to thruster failure, and complicated feeding systems required due to the conventional use of solid propellants (usually polytetrafluoroethylene (PTFE)). A promising alternative to solid propellants has recently emerged in the form of non-volatile liquids that are stable in vacuum. This study presents a broad comparison of the non-volatile liquid perfluoropolyether (PFPE) and solid PTFE as propellants on a PPT with a common design base. We show that liquid PFPE can be successfully used as a propellant, and exhibits similar plasma discharge properties to conventional solid PTFE, but with a mass bit that is an order of magnitude higher for an identical ablation area. We also demonstrate that the liquid PFPE propellant has exceptional resistance to carbon deposition, completely negating one of the major causes of thruster failure, while solid PTFE exhibited considerable carbon build-up. Energy dispersive X-ray spectroscopy was used to examine the elemental compositions of the surface deposition on the electrodes and the ablation area of the propellant (or PFPE encapsulator). The results show that based on its physical characteristics and behavior, non-volatile liquid PFPE is an extremely promising propellant for use in PPTs, with an extensive scope available for future research and development.

Development of Russian Next-generation Ablative Pulsed Plasma Thrusters

The development of ablative pulsed plasma thrusters (APPT) continues at RIAME MAI. In 2014 one of them, APPT-45-2, was mounted on board the small spacecraft MKA-FKI PN2 developed by the S.A. Lavochkin Scientific and Production Association. Now in cooperation with the Scientific Technical Innovation Center “TECHCOM” the electric propulsion system (EPS) APPT-250 of the next generation is being developed, that is intended for the orbit maintanance of the constellation of small communication satellites. Power consumption of EPS is from 60 to 120W, the total thrust impulse is 15.6 kN•s. APPT-250 differs from the earlier APPT-based propulsion systems by substantially lower mass. A digital power processing unit (PPU), developed by “TECHCOM”, and power capacitors with high specific energy intensity are used for the first time.

Simulation of the Motion of Charged Particles in an Ablative Pulsed Plasma Thruster at the Initial Stage of the Discharge

A basic scheme of an ablation pulsed plasma thruster with rail geometry and a side feed of the propellant bar is considered. For such geometry, a theoretical description of the initial stage of the vacuum discharge, which would serve to explain the experimental results, does not exist. It was demonstrated experimentally that the configuration of the discharge channel depends essentially on the initial stage of discharge. A model of the motion of charged particles in the inhomogeneous electric and magnetic fields was developed. Due to the low plasma density at the initial stage of discharge, the model of the charged particles motion in electric and magnetic fields is described well by differential equations.

Modeling of gas ionization and plasma flow in ablative pulsed plasma thrusters

A one-dimensional model to study the gas ionization and plasma flow in ablative pulsed plasma thrusters(APPTs) is established in this paper. The discharge process of the APPT used in the LES-6 satellite is simulated to validate the model. The simulation results for the impulse bit and propellant utilization give values of 29.05μNs and 9.56%, respectively, which are in good agreement with experimental results. To test the new ionization sub-model, the discharge process of a particular APPT, XPPT-1, is simulated, and a numerical result for the propellant utilization of 62.8% is obtained, which also agrees well with experiment. The gas ionization simulation results indicate that an APPT with a lower average propellant ablation rate and higher average electric field intensity between electrodes should have higher propellant utilization. The plasma density distribution between the electrodes of APPTs can also be obtained using the new model, and the numerical results show that the plasma generation and flow are discontinuous, which is in good agreement with past experimental results of high-speed photography. This model provides a new tool with which to study the physical mechanisms of APPTs and a reference for the design of high-performance APPTs.

Numerical studies of wall–plasma interactions and ionization phenomena in an ablative pulsed plasma thruster

Wall–plasma interactions excited by ablation controlled arcs are very critical physical processes in pulsed plasma thrusters (PPTs). Their effects on the ionization processes of ablated vapor into discharge plasma directly determine PPT performances. To reveal the physics governing the ionization phenomena in PPT discharge, a modified model taking into account the pyrolysis effect of heated polytetrafluoroethylene propellant on the wall–plasma interactions was developed. The feasibility of the modified model was analyzed by creating a one-dimensional simulation of a rectangular ablative PPT. The wall–plasma interaction results based on this modified model were found to be more realistic than for the unmodified model; this reflects the dynamic changes of the inflow parameters during discharge in our model. Furthermore, the temporal and spatial variations of the different plasma species in the discharge chamber were numerically studied. The numerical studies showed that polytetrafluoroethylene plasma was mainly composed of monovalent ions; carbon and fluorine ions were concentrated in the upstream and downstream discharge chamber, respectively. The results based on this modified model were in good agreement with the experimental formation times of the various plasma species. A large number of short-lived and highly ionized carbon and fluorine species (divalent and trivalent ions) were created during initial discharge. These highly ionized species reached their peak density earlier than the singly ionized species.

Study of breakdown in an ablative pulsed plasma thruster

Breakdown in ablative pulsed plasma thrusters (APPTs) must be studied in order to design new types of APPTs and measure particular parameters. In this paper, we studied a parallel-plate ablative pulsed plasma thruster that used a coaxial semiconductor spark plug. By operating the APPT about 500 times with various capacitor voltages and electrode gaps, we measured and analyzed the voltage of the spark plug, the voltage between the electrodes, and the discharge current. These experiments revealed a time delay (∼1–10 μs) between spark plug ignition and capacitor discharge, which may affect the performance of high-pulsing-rate (>10 kHz) and double-discharge APPTs, and the measurements of some of the APPT parameters. The delay time decreased as the capacitor voltage increased, and it increased with an increasing electrode gap and increasing number of ignitions. We explain our results through a simple theoretical analysis.