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Re-Generable Field-Emission Cathodes for Low-Power Electric Propulsion

American Institute of Aeronautics and Astronautics

1
Re-generable Field Emission Cathodes for Low-Power
Electric Propulsion
Jason M. Makela
*
and Lyon B. King
Michigan Technological University, Houghton, MI, 49931, USA
The research reported here explores the possibility of field-emission cathodes for use in
EP that have the ability to be re-generated when the emitter tip becomes damaged. The
method for re-generation takes advantage of Taylor cone formation in an effort to solidify,
or quench, an operating liquid-metal-ion-source (LMIS) to preserve the sharp Taylor cone
tip for use as a field-emission cathode. Electron emission I-V curves were taken after Taylor
cones were formed by quenching the LMIS at different discharge currents. It is shown that
quenching the LMIS at as low as 2 µ
µ
µ
µA produced an increase in electron discharge current as
compared with the unquenched emitter, 53 nA as compared with 25 nA at an extraction
voltage of 2.7 kV. When the ion emission current at quench was increased to 3 µ
µ
µ
µA and then
25 µ
µ
µ
µA, the discharge that was measured increased to 210 nA for the 3 µ
µ
µ
µA emitter and 1.02
µ
µ
µ
µA for the 25 µ
µ
µ
µA emitter at an extraction voltage of 2.7 kV. Fitting the electron emission I-V
characteristics using Fowler-Nordheim theory indicated tip radii as small as 80 nm were
formed during the LMIS quenching process.
Nomenclature
a

= Fowler-Nordheim term (see Equation 2)
b
= Fowler-Nordheim term (see Equation 3)
I

= discharge current (I)
k
= empirical relation relating tip radius and gap spacing
V

= extraction voltage (V)
r
= emitter tip radius (m)

= Nordheim image-correction term

= work function (eV)
µ

= Fowler-Nordheim term

I.
Introduction
IQUID-METAL ion sources (LMIS) have found extensive use as ion sources of high brightness in focused ion
beam materials processing applications
1
and, more recently, as EP thrusters via a technology commonly known
as field-emission electric propulsion (FEEP).
2,3,4
In an LMIS or FEEP thruster, an intense electric field is created
near the surface of a low melting-temperature liquid metal, such as In, by a downstream electrode. A balance
between the liquid surface tension and electrostatic forces causes a structure known as a Taylor cone to form in the
liquid. The mechanisms of Taylor cone formation are by now well understood.
5
Because the Taylor cone has a very
sharp tip, geometric enhancement of the local electric field at the cone tip is sufficient to extract metal ions directly
from the liquid.
2
The ions emerge from a very narrow (few nanometer diameter) liquid jet at the cone apex and are
subsequently accelerated by the electric field to either produce thrust (FEEP) or for materials processing applications
(LMIS). Other applications and areas of interest for the use of focused ion beams include lithography,
semiconductor doping, sample preparation for TEM imaging, circuit repair, scanning ion microscopy, and scanning
ion mass spectroscopy.
6

The research reported here takes advantage of Taylor cone formation in an effort to solidify, or quench, an
operating LMIS to preserve the sharp Taylor cone tip for use as a field-emission cathode for EP. The resulting metal

*
PhD Candidate, Mechanical Engineering, 1018 RL Smith Building, 1400 Townsend Drive Associate Professor, Mechanical Engineering, 1014 RL Smith Building, 1400 Townsend Drive
L
43rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit
8 - 11 July 2007, Cincinnati, OH
AIAA 2007-5171
Copyright © 2007 by Jason M. Makela. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.
American Institute of Aeronautics and Astronautics

2
structure will have a tip radius of 10s to 100s of nanometers, which is ideal for Fowler-Nordheim emission. By
reversing the polarity of the LMIS, the solid-metal tip will then function as a cold electron emitter. The motivation
for this is due to the limited lifetime of current micro-fabricated field emitters. As electron discharge is continued
for long durations, the emitter tip begins to wear and blunt. As the sharpness of the emitter tip decreases, the local
electric field decreases. This circumstance is unfavorable and eventually renders the emitter tip useless as an
electrode source. Where current field emitters would be destroyed at this point, the process described here of
applying heat to re-melt the In and switching the polarity back to obtain ion emission allows for the re-generation of
a sharp Taylor cone. Another advantage the In coating has is that tips coated with In have a lower work function
than W, 3.5 eV for In as opposed to W at 4.5 eV.
7
So once a sharp tip has been formed, it can once again be used for
electron emission.
While never applied to EP or space-based applications, the idea to use a liquid-metal Taylor cone as a combined
electron/ion source is not new. The earliest documentation of a liquid-metal electron source (LMES) was the work
of Swanson and Schwind.
8
Because the formation of a Taylor cone is independent of field polarity, Swanson and
Schwind applied electron-extracting fields to a liquid metal in an effort to coax electron emission from the
(nonsolidified) cone. Their early paper reports repetitive pulsed electron emission from liquid Ga-In Taylor cones
formed on the tip of a W needle electrode with current pulses as high as 250 A for 10 msec at ~50,000 pulses-per-
second. A field-emission-initiated explosive emission process during which a small portion of the liquid metal is
expelled was proposed to describe the behavior. The phenomena responsible for pulsed emission were supported by
Gomer the following year.
9
Later on, using Ga and In, Rao et. al. found that it is possible to obtain dc electron
emission if the LMES is first operated as an LMIS and then the Taylor cone is frozen in.
10
It is now understood
that, during operation as an LMIS, the Taylor cone forms a jet-like protrusion at the cone apex that solidifies when
the cone is quenched by removing heater power. It is the protrusion that is responsible for the stable electron
emission when the polarity is changed to emit electrons. Formation of the protrusion was determined to be reversible
and reproducible.
This research focuses on using a single field emitter that can function as both an ion and an electron source in an
attempt to solve the problem of tip degradation by allowing for the possibility of tip re-generation. The primary goal
of the research reported here was to determine if it was possible to use a quenched Taylor cone from an ion emitting
tip to obtain electron emission. Further testing of different quenching conditions and emitter heating currents was
also investigated to determine what type of effect both have on electron discharge I-V characteristics.
II.
Description of Apparatus
Sharp W needles were formed by electrochemically etching W wires in a 2M NaOH solution. The etching
procedure utilized was similar to the method used and described in further detail by Ekvall.
11
Using this etching
technique it was possible to obtain reproducible tip diameters from the 100s of nanometers range up to a few
microns, depending on the etch conditions.
The sharpened W tips were then coated with In by dipping the heated filament in a liquid crucible of In. The
etched and coated tips were then inserted into a fixture that served as both a heater as well as an In reservoir. An
emitter fixture schematic can be seen in Figure 1. A planar stainless-steel extraction electrode was positioned
downstream of the tip. Typical gap spacing between emitter tip and extraction electrode was 1.0 to 1.5 mm.

American Institute of Aeronautics and Astronautics

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Figure 1. LMIS/electron emitter fixture.

To operate the tip as an LMIS, the emitter heater was used to maintain the In reservoir above the melting
temperature of In, which is 156.6 deg. C. For electron emission the emitter heater was un-powered, solidifying the
In in the reservoir as well as on the emitter tip. The experimental setup for ion and electron emission is shown in
Figures 2 and 3, respectively. A current amplifier with gain of 10
5
V/A was