Session DW1: Electric Propulsion III

Particle-in-Cell simulation of the flow in the VKI DRAG-ON facility

A facility for the reproduction of orbital flows, named DRAG-ON, has been successfully commissioned at VKI. The facility is designed to test intake-collectors for Air-Breathing Electric Propulsion applications. Relevant properties of intake models, such as their transmissivity and compression efficiency can be measured. The flow of fast neutrals encountered by a satellite in orbit is reproduced by a quasineutral plasma flow (of Ar or O₂/O ions), generated and electrostatically accelerated in a plasma source. Understanding the dynamics of the expanding plasma plume and its interaction with the intake model is fundamental for the correct interpretation of the experimental results. We use the Particle-in-Cell method to replicate numerically part of the experimental setup and assess these effects. An advanced implicit, energy-conserving scheme on unstructured grids is used to mitigate the numerical difficulties arsing from the multiscale nature of the plasma. Some undesired effects, such as the expansion of the plume due to the residual electron pressure and the deflection of ion trajectories due to the presence of plasma sheaths on the surface of the intake model are identified and analyzed.   

Ionospheric drag on spacecraft in low-Earth-orbit

Spacecraft in Low-Earth-Orbit (LEO) experience a range of perturbing forces that affect their trajectories, such as drag due to neutral gas in the residual atmosphere. A less well-studied drag mechanism is that from charged particles in the ionosphere, which is a plasma environment between altitudes of about 60-1000 km. Collection of such charged particles causes spacecraft charging that can generate unwanted electrostatic discharges or arcing, and the formation of a complex plasma sheath/wake structure. But, direct collection, and indirect deflection, of charged particles also results in momentum transfer and the formation of an additional drag force. While the neutral gas density is typically 1-2 orders of magnitude higher than the ionospheric plasma density, sheath expansion around a spacecraft can artificially increase its effective collecting area. Here, we present a model accounting for charged particle flow effects around objects in LEO and show that ionospheric drag can be significant, and in some cases, similar, or even higher, than drag due to the residual atmosphere. Aside from having important orbit prediction implications for space domain awareness and space traffic management, ionospheric drag can also potentially be exploited for active satellite control.      

Air-Breathing Magneto-Deflagration Propulsion for Sustained Very Low Earth Orbit

Satellites that operate at lower orbits have the potential to dramatically reduce the requirements, cost, and timelines that are involved in conventional launch processes. However, satellites at lower altitudes experience higher drag, must generate higher average thrust, and use more propellant to sustain orbit. These conditions lead to prohibitive requirements on the amount of propellant they must store and make long mission designs impractical. This talk describes a pulsed hydromagnetic plasma gun that operates in an air-breathing mode where propellant is harvested in orbit. We show how a deflagration mode enables the generation of T/P > 2 mN/kW with mass flow rates of air that can be harvested in very low Earth orbit (VLEO, ~250-350 km). We show this performance is sufficient to maintain altitude in VLEO and identify challenges other thrust architectures face in achieving this milestone.   

Characterization of the effect of plasma screen electrical configuration on gridded ion engine operation

Ground testing of electric propulsion thrusters is critical to the prediction of both in-space performance and lifetime. However, there may exist significant differences between interpreted engine performance and lifetime as compared with space operation particularly at higher power, higher throughput operation. This work focuses on facility effects as related to gridded ion thrusters. Here we investigate the effect of the potential of the engine's plasma screen on overall engine performance. Depending on its potential and local plasma conditions, the plasma screen can give rise to uncertainty in the calculated thrust, beam neutralization, and overall engine performance. The effect of the plasma screen electrical configuration can be amplified due to the increase of CEX plasma at higher power levels. This work focuses on effect of the electrical configuration of the plasma screen potential on ion beam plume profile, near wall CEX plasma properties, plasma properties within the plasma screen, and on thruster performance at various thruster throttle conditions.   

Experimental characterisation of the External Discharge Plasma Thruster

Hall thrusters have high thrust-to-power ratios and robust designs. However, plasma losses to the discharge channel walls can reduce their efficiency and lifespan at low power levels. To solve this problem, concepts have relocated the ionisation and acceleration regions away from the walls. The External Discharge Plasma Thruster (XPT) has been proposed as a solution to this problem by relocating these regions outside the cavity, potentially extending the lifetime of Hall thrusters and decreasing the mass of the thruster head. Previous research has shown the XPT has a low mass utilisation efficiency and a large plume divergence. This work investigates the XPT operated on xenon and krypton to identify plasma properties and ion beam characteristics using invasive and non-invasive diagnostic tools.

Optical Emission Spectroscopy (OES) is a non-invasive diagnostic tool used to estimate plasma properties such as electron temperature and density via the use of collisional-radiative models. However, the estimation is prone to uncertainty due to intrinsic factors, and the measurement is non-local. Langmuir probes are used to measure electron temperature, plasma density and potential at different near-field locations.  To measure the multiply charged ions and map the ion current density in the XPT, a Wien filter and a Faraday probe are used respectively. These diagnostics provide a comprehensive understanding of the plasma composition and propulsive performance, enabling design improvements and performance optimisation.