Lifetime Considerations for Electrospray Thrusters

Abstract

Ionic liquid electrospray thrusters are capable of producing microNewton precision thrust at a high thrust–power ratio but have yet to demonstrate lifetimes that are suitable for most missions. Accumulation of propellant on the extractor and accelerator grids is thought to be the most significant life-limiting mechanism. In this study, we developed a life model to examine the effects of design features, operating conditions, and emission properties on the porous accelerator grid saturation time of a thruster operating in droplet emission mode. Characterizing a range of geometries and operating conditions revealed that modifying grid aperture radius and grid spacing by 3–7% can significantly improve thruster lifetime by 200–400%, though a need for explicit mass flux measurement was highlighted. Tolerance analysis showed that misalignment can result in 20–50% lifetime reduction. In addition, examining the impact of electron backstreaming showed that increasing aperture radius produces a significant increase in backstreaming current compared to changing grid spacing. A study of accelerator grid bias voltages revealed that applying a reasonably strong accelerator grid potential (in the order of a kV) can minimize backstreaming current to negligible levels for a range of geometries.

Polyatomic Ion-Induced Electron Emission (IIEE) in Electrospray Thrusters

“…The material constant Λ, as presented by Schou, is difficult to compute analytically, relying on stopping powers for low-energy electrons, whose energy loss functions are far from certain for calculations just above the target's Fermi energy [58,59]. Thus, the most viable technique is to determine Λ empirically by dividing γ a by the total energy distribution into the target per Equation (7). Many previous studies on IIEE have utilized this technique.…”

“…The Electrospray Life Model (ELM) [6,7] developed by the Plasma and Space Propulsion Laboratory (PSPL) at University of California, Los Angeles (UCLA) demonstrated [5] that thruster lifetime can be severely compromised due to propellant over-spray and off-axis emission, resulting in undesirable impingement of the plume on the extractor and accelerator grids of the thruster assembly (Figure 1). Such impingement leads to propellant accumulation [8], which in the presence of a strong electrostatic field diminishes efficiency and causes more serious failures, such as shorting.…”

“…Such impingement leads to propellant accumulation [8], which in the presence of a strong electrostatic field diminishes efficiency and causes more serious failures, such as shorting. While particular attention has been paid to propellant back-spray [7], secondary electron generation due to grid impingement may also contribute to decreased thruster lifetime [9]. Current density measurement errors can become excessive due to the stochastic generation of electrons from grid surfaces and the beam dump by ion, and perhaps even droplet bombardment [10].…”

“…In addition, electron emission due to predominantly ion bombardment becomes increasingly significant in thruster configurations without an accelerator grid where the angular current distribution is wide [11], causing increased electron emission from chamber surfaces unless suppressive measures are taken. Such electrons could also back-stream through the plume towards the emitter, neutralizing, fragmenting, and otherwise disrupting the beam profile or instigating decomposition chemistry at the emitter [6,7]. Secondary electrons generated from even low-energy ion bombardment can still accelerate to significant velocities in an electric field and eject additional electrons from chamber surfaces (tertiaries).…”

“…As electrospray thrusters continue seeking prolonged lifetime applications, such as the LISA mission, more attention must be paid to understanding and mitigating secondary currents from device surfaces. Electrospray thruster schematic with extraction and acceleration potentials shown (modified from [7]). Charged droplets and ions impinging the electrodes and beam dump, as shown, can cause ion-induced electron emission (IIEE), increased measurement uncertainty, and possible electron back flow.…”