Emission measurements from planar arrays of porous ionic liquid ion sources

Abstract

We present current emission measurements from arrays of 480 porous ionic liquid ion source emitters for applications in space propulsion and in the processing and analysis of materials. The emitters are fabricated with a top-down approach from 10 × 10 × 1 mm3 bulk porous nickel and are housed within a microetched silicon frame and stainless-steel aperture-matching extraction grids. Measurements of emitted and collected currents are reported using two ionic liquids (ILs), EMI-BF4 and EMI-Im. Total beam currents up to about 400 µA can be sustained through passive feeding with IL fed from the rear of the bulk porous substrate. These currents are in agreement with previous work where currents of about 1 µA were observed from single emitters. Ion transparencies as high as 0.95 are achieved with manually assembled grids. Current emission displays reasonable symmetry between polarities, although slight differences could yield to the accumulation of one type of ion in the source leading to electrochemical reactions despite voltage alternation. Strong current decays with time and evidence of surface contamination reinforce this hypothesis. Future work should address this issue through closed-loop control of charge emission.

Comparing Direct and Indirect Thrust Measurements from Passively Fed Ionic Electrospray Thrusters

“…Arrays of externally wetted [17] and porous [2,5] ILIS have, encouragingly, been demonstrated to support a few hundred μA per cm 2 of highly ionic emissions. Porous substrates are particularly advantageous in that they inherently provide a simple and integral flow path from a rear mounted and hydraulically coupled reservoir; enabling simple capillary driven and zero-g compatible passive feeding.…”

“…Typical high current passively fed IL electrospray sources operate with V em ∼ 1-2 kV [2,5,22], corresponding to an idealized specific impulse of ∼ 4000-6000 s for particles of mass m EMI . However, we demonstrate in this report that lower values, ∼ 1000-3000 s, may result once all the listed parameters and both emission polarities are considered.…”

“…The minimum voltage to start emission can be estimated through recognizing that the applied electrostatic pressure must overcome the liquid surface tension [38,42] and, particularly relevant in porous devices [2], the negative Laplace pressure due to the porous reservoir. The cited relations indicate a minimum voltage which scales with the liquid surface tension γ and length scales f and r p for the emitter field enhancement and capillarity length scales respectively; V start ∼ f πγ 2rp [42].…”

“…Interest in electrospray propulsion has seen a resurgence over the past decade with the advent of techniques for fabricating high current density emission sources [1][2][3][4][5] and the performance benefits identified when employing room temperature Ionic Liquids (ILs) as propellants [6][7][8]. IL electrosprays can yield highly ionic and monoenergetic beamlets, leading to high specific impulse and propulsive efficiency from inherently miniature emission sites (Taylor cones [9]).…”

“…The potential benefits of ILIS include: i) the low surface tension of ILs leads to low operating voltages compared with LMIS, ii) ILs are liquid at room temperature and therefore may not require heating, iii) many ILs have negligible vapour pressure, enabling storage at vacuum, iv) positive and negative ion emissions can be sustained, enabling charge neutralization without a dedicated neutralizer, v) high specific impulses, in excess of 3000 s [16], and propulsive efficiencies nearing 90 % [8] and finally, vi) the low flow rates inherent can be accommodated through passive feeding alone [2,7,17]. The later feature could greatly reduce the mass, power and volume requirements of a thruster by obviating the need for pumps, valves and propellant pressurization systems.…”