Drop erosion and the life of electrospray thrusters

J. Fernandez de la Mora, M. Gamero-Castaño, A. GomezYale University

University of California Irvine

Electrospray propulsion with nanodropsContinuous control of specific impulse, from 100 s to a few 1000 sSTTR Alameda-Yale, funded by AFOSR (Dr. Birkan)

General field of energetic nanodrops

• Produced by EHD from bulk liquids• Useful in electrical propulsion. Sizes from 5-10

nm to microns. Speeds even above 10 km/s• Materials: Liquid, low volatility, high eectrical

conductivity. All relevant materials developed for propulsion

• Other applications of brams of nanodrops

Extractor erosion after tens of hours of operation

G. Lenguito, J. Fernandez de la Mora and A. Gomez: AIAA propulsion conference 2010

Eroding a silicon surface with a nanodrop beam?

• How fast does it proceed? • Erosion will probably limit thruster life in

electrospray propulsion. • New opportunities for microfabrication• Controlled erosion of the edges of extractor

electrodes • How does it depend on drop size, energy? Extension

to heavy ion beams enabled also by new electrospray ion sources of heavy substances

M. Gamero-Castaño, M. Mahadevan, Sputtering yields of Si, SiC, and B4C under nanodroplet bombardment at normal incidence, J. App. Phys. 106, 054305, 2009

What do we know about nanodrop erosion?

M. Gamero-Castaño, M. Mahadevan, Sputtering yields of Si, SiC, and B4C under nanodroplet bombardment at normal incidence, J. App. Phys. 106, 054305, 2009

The maximum receding rates of the substrates’ surfaces were 448, 172, and 170 nm/min resp.• So an extractor electrode 25 um thick would last less than an hour in the core of the beam.• Or the edge of the electrode would be readily beam-cut in the region intersecting the beam

EMI-Im ionic liquid, profiles measured at a r plane 102 mm from the emission point

The large angle tail cannot be explained by space charge repulsion; the tail likely contains scattered particles

The trajectories of most droplets are driven by the beam’s space charge

M. Gamero-Castaño, The Structure of Electrospray Beams in Vacuum. Journal of Fluid Mechanics, 604, 339-368 (2008)

Is it possible to entirely avoid extractor erosion?

• Probably yes for the main space-charge dominated beam core. Avoiding it will nonetheless limit the power density, as greater current/area will widen the beam)

• Perhaps not for the tail of scattered (?) particles, so it pays to understand its origin. If the tails are ions scattered by collisions with individual drops, then the solution is avoiding ion emission (also desirable for high propulsion efficiency)

• Fine-tune extractor fabrication via nanodrop erosion: array edge effects; relaxing need for precise fabrication (smaller features)

• Complete fabrication of extractor to precisely fit non MEMS nozzle arrays (CAC’s Holey fibers, etc.)

• Etc.

Beneficial aspects of drop erosion in electrospray propulsion