The extraction and acceleration of ions from a plasma source is often performed electrostatically using a set of direct-current (DC) biased grids. To ensure that equal currents of positive and negative charges are ejected, and to compensate the downstream ion beam space charge, a separate electron-emitting neutraliser is required. If instead the grids are biased with a radiofrequency (RF) voltage, then both electrons and ions can be extracted from the same source and a neutraliser is no longer needed. Using a combination of theory and particle-in-cell simulations, we demonstrate the fundamental physics of RF biasing and identify important electron and ion temporal dynamics. By including a capacitor in the external circuit, a DC self-bias voltage forms across the grids due to initial particle charging. This self-bias voltage develops self-consistently and ensures equal ion and electron currents are ejected from the plasma source, while also leading to ion acceleration. A potential well is observed to form downstream of the last grid which leads to electron trapping and the formation of a quasi-neutral plume. Because of their larger mass, ions are continuously extracted from the plasma source, while electrons are only extracted in pulses synchronised with the RF cycle. As long as the RF period is less than the ion transit time between the grids, ions can be efficiently extracted with no direct grid impingement and otherwise similar behaviour to DC biased grids.

We propose an alternative method to accelerate ions in classical gridded ion thrusters and ion sources such that co-extracted electrons from the source may provide beam space charge neutralization. In this way there is no need for an additional electron neutralizer. The method consists of applying RF voltage to a two-grid acceleration system via a blocking capacitor. Due to the unequal effective area of the two grids in contact with the plasma, a DC self-bias is formed, rectifying the applied RF voltage. As a result, ions are continuously accelerated within the grid system while electrons are emitted in brief instants within the RF period when the RF space charge sheath collapses. This paper presents the first experimental results and a proof-ofprinciple. Experiments are carried out using the Neptune thruster prototype which is a gridded Inductively Coupled Plasma (ICP) source operated at 4 MHz, attached to a larger beam propagation chamber. The RF power supply is used both for the ICP discharge (plasma generation) and powering the acceleration grids via a capacitor for ion acceleration and electron extraction without any DC power supplies. The ion and electron energies, particle flux and densities are measured using retarding field energy analyzers (RFEA), Langmuir probes and a large beam target. The system is operating in Argon and N 2. The DC self-bias is found to be generated within the gridded extraction system in all the range of operating conditions. Broad quasi-neutral ion-electron beams are measured in the downstream chamber with energies up to 400 eV. The beams from the RF acceleration method are compared with classical DC acceleration with an additional external electron neutralizer. It is found that the two acceleration techniques provide similar performance, but the ion energy distribution function from RF acceleration is broader while the floating potential of the beam is lower than for the DC accelerated beam.

The plasma propulsion with electronegative gases (PEGASES) thruster is a gridded ion thruster that accelerates alternately positively and negatively charged ions to provide thrust. Over the last few years, various prototypes have been tested, adequate diagnostics have been developed, and analytical models and simulations are made to better understand and control the physics involved. Here, we present the state-of-the-art in the PEGASES development as of 2013, and discuss its possible future in space.Index Terms-Electric propulsion, electronegative plasmas, ion-ion plasmas, negative ion thrusters.

Propulsion is a critical subsystem of many spacecraft1–4. For efficient propellant usage, electric propulsion systems based on the electrostatic acceleration of ions formed during electron impact ionization of a gas are particularly attractive5,6. At present, xenon is used almost exclusively as an ionizable propellant for space propulsion2–5. However, xenon is rare, it must be stored under high pressure and commercial production is expensive7–9. Here we demonstrate a propulsion system that uses iodine propellant and we present in-orbit results of this new technology. Diatomic iodine is stored as a solid and sublimated at low temperatures. A plasma is then produced with a radio-frequency inductive antenna, and we show that the ionization efficiency is enhanced compared with xenon. Both atomic and molecular iodine ions are accelerated by high-voltage grids to generate thrust, and a highly collimated beam can be produced with substantial iodine dissociation. The propulsion system has been successfully operated in space onboard a small satellite with manoeuvres confirmed using satellite tracking data. We anticipate that these results will accelerate the adoption of alternative propellants within the space industry and demonstrate the potential of iodine for a wide range of space missions. For example, iodine enables substantial system miniaturization and simplification, which provides small satellites and satellite constellations with new capabilities for deployment, collision avoidance, end-of-life disposal and space exploration10–14.

Electric space propulsion is an intensively developing field addressing new demands and challenges for long-term spacecraft operation. Many novel plasma propulsion concepts aim to find new acceleration principles, use alternative propellants, upscale or downscale thrusters for large thrust or for very small spacecrafts etc. In this work we review the neutralizerfree concepts, where both positive and negative particles are extracted and accelerated from plasmas. We can divide these concepts into three main categories, defined by their acceleration principle: (i) neutral beam generation, (ii) plasma acceleration/expansion and (iii) bipolar beam acceleration. We describe the basic physical principles and evaluate the main advantages and drawbacks in view of general space applications. We also present here further detail on a recent concept where RF voltages are used to accelerate quasi-simultaneously positive ions and electrons from the same source.

The results of investigations on the simultaneous extraction of positive ions and electrons from a single-grid ICP source are reported. It is shown that the single-grid ion source is capable of generating coincident ion and electron flows in contrast to the more common two- or three-grid sources. Electron and ion emission characteristics of the source are presented for the cases of DC and RF acceleration bias. A method of ion/electron current ratio control is proposed allowing to change the current compensation state of the ion beam from non-compensated to highly over-compensated. The researches were conducted in the ion energy range 10–250 eV with a high ion current density of 5 mA/cm2, so the presented results may be useful for modern technology applications.

International audienceThe PEGASES (Plasma Propulsion with Electronegative Gases) thruster is a gridded ion thruster, where both positive and negative ions are accelerated to generate thrust. In this way, additional downstream neutralization by electrons is redundant. To achieve this, the thruster accelerates alternately positive and negative ions from an ion-ion plasma where the electron density is three orders of magnitude lower than the ion densities. This paper presents a first experimental study of the alternate acceleration in PEGASES, where SF6 is used as the working gas. Various electrostatic probes are used to investigate the source plasma potential and the energy, composition, and current of the extracted beams. We show here that the plasma potential control in such system is key parameter defining success of ion extraction and is sensitive to both parasitic electron current paths in the source region and deposition of sulphur containing dielectric films on the grids. In addition, large oscillations in the ion-ion plasma potential are found in the negative ion extraction phase. The oscillation occurs when the primary plasma approaches the grounded parts of the main core via sub-millimetres technological inputs. By controlling and suppressing the various undesired effects, we achieve perfect ion-ion plasma potential control with stable oscillation-free operation in the range of the available acceleration voltages (±350 V). The measured positive and negative ion currents in the beam are about 10 mA for each component at RF power of 100 W and non-optimized extraction system. Two different energy analyzers with and without magnetic electron suppression system are used to measure and compare the negative and positive ion and electron fluxes formed by the thruster. It is found that at alternate ion-ion extraction the positive and negative ion energy peaks are similar in areas and symmetrical in position with /− ion energy corresponding to the amplitude of the applied acceleration voltage

By applying a square-wave voltage with frequencies between 10 kHz to 1 MHz to a set of grids terminating an ion-ion plasma source, we experimentally demonstrate the alternate extraction and acceleration of high energy (100's of eV) positive and negative ion beams. In addition, the ratio of positive-to-negative ion beam current can be controlled by adjusting the applied square-wave duty cycle. Temporallyresolved floating potential measurements of a target show that the downstream potential can be controlled and sufficiently reduced at high applied frequencies (∼ 200 kHz), indicating that space-charge compensation can be achieved to prevent beam stalling.