Field Emitter Array
Cathodes (FEAC) research
Introduction
The
Field-Emitter Array Cathodes (FEACs) offers the potential benefits
of low-power consumption, low-cost, no consumable requirements,
robustness, and high reliability. Space applications that could
benefit include: (1) electric propulsion (EP); (2) electrodynamic
(ED) tether propulsion, and (3) spacecraft charge control. Because
of their inherent weight, power, and cost advantages, FEACs can
provide the enabling technology in some new and significant
applications.
FEACs
utilize up to millions of miniature biased tips that individually
emit microamp levels of electron current using the field emission
process to generate amp-level currents with low bias potentials.
While the number of tips are large, they are cost-effectively
built using semiconductor fabrication technology. In recent years,
FEAC development has focused on applications such as flat-panel
displays and microwave tubes. Their ability to emit electrons
without heating or the need for an ionizable gas supply, coupled
with recent progress in operating in less than ideal vacuum,
suggest that the time is right to seriously consider use for space
applications as well.
Diverse FEAC space applications generally required devices
that: (1) emit on the order of 0.1 A/cm2; (2) operate
in the 10-5 to 10-6 Torr pressure range
(survive after operational exposure to 10-3 Torr); (3)
require bias potentials of 50-100 V or less; and, (4) have
sufficient life for multi-year space operations.
Figure 1 – Schematic of a Single Element of a Field Emitter Array Cathode (FEAC).
Figure 2 – SEM photograph of SRI Ring Cathode developed for the ARPA/NRL/NASA Vacuum Microelectronics Initiative (Emission gated rf amplifier), courtesy of Capp Spindt. These arrays were not resistively protected nor coated, but nevertheless produced 0.67 µA/tip @ a gate voltage of 70 V in a power tube (klystrode) environment.
Recent
investigations suggest that each of these requirements can be
achieved. What remains is to bring together the specific set of
FEAC fabrication technologies to validate and qualify the
technology for operation in the space environment. Three critical
FEAC development issues are: (1) arc suppression at each tip; (2)
minimization of bias (gate) potential; and (3) optimizing surface
coatings that lower work-function and withstand energetic ions,
oxygen flux, and undesirable adsorbates.
FEAC devices that meet these objectives will enable
miniaturized fuel-efficient EP systems such as the Closed-Drift
Hall thruster (CDT) that enhance performance of small satellites
by reducing spacecraft propellant mass. FEACs will also enable use
of “propellantless” ED tether
propulsion for efficient spacecraft deorbit, atmospheric
drag make-up, and orbit raising in the 250 to 2,000 km altitude.
Additionally, FEACs can provide a simple means to actively control
charging of spacecraft with large solar arrays.
An Improvement
for Tether Propulsion Systems
For the ProSEDS
mission, because FEAC technology is not yet developed for space
applications it is necessary to use a hollow cathode plasma
contactor similar to that described for EP (other less efficient
approaches include hot-filament electron gun emitters or ion
collection). State-of-the-art 1/4” hollow cathode plasma
contactors that can provide 5 A of current are estimated to
consume ~10-30 W (10-15 V, 1-2 A for Keeper) and
0.1 mg/s of xenon.
Note also that there are similar concerns as for EP with cathode
“poisoning” when exposed to an oxygen-rich environment
requiring an oxygen-free environment prior to launch. Also, this
cathode would probably need a radiator for thermal management.
Elimination of the gas and heater/keeper power requirements by
using FEACs represents enabling technology for several ED tether
applications and certainly mission enhancing for others.
Some Technical
Details
Operational
requirements are similar to EP applications, except that there
is no requirement to be near an energetic ion flux and the
charge must be ejected across a wider sheath to the ambient,
lower density space plasma. Needed requirements are:
•
Pressure Environment – The FEAC must be able to operate
with the expected ambient environment pressure and species plus
outgassing from the spacecraft (<10-7 Torr). It
must be able to survive pressure exposures to 10-3
Torr (e.g. due to attitude control thrusters such as hydrazine
and other spacecraft effluents).
•
Emission Currents – Arrays of FEACs will need to emit
peak current that can range from 1 to 10 A depending on the
specific application (1 to 6 A, typical). For example,
atmospheric drag make-up of a large spacecraft could require
less than 1 A while rapid deorbit of a spent stage or drag
make-up of the space station may benefit from currents as much
as 10 A. Emission of this level of current requires special
consideration of space charge effects which will limit maximum
allowed current densities while spacecraft surface area
utilization will place a lower limit. For example, for a small
satellite deboost application emission area might need to be
constrained to some number at or below 100 cm2 while
a large spacecraft boost system may be able to accept current
emission areas many times larger. For these emission areas,
operation should be below space charge limited current flow
levels (See Section 4.1 for further discussion).
•
Electron Emission Energy - In order to assure minimal
bias requirements which directly affects system efficiency,
extraction potentials (gate-to-tip plus anode-to-gate) less than
50-100 V or smaller are desired. The electron emission will
occur into a space plasma that will have densities ranging from
as low as 109 m-3 to 5x1012 m-3.
A low-potential plasma sheath will exist between the spacecraft
and the ambient plasma which the electrons must cross.
•
Leakage Current - Low leakage current of the FEAC is
required to ensure efficient cathode operation and since both EP
and ED tethers use similar systems the requirements here are
identical to that described above (e.g. FEAC gate current less
than 1% is best for conservative operation).
•
Lifetime – A typical ED tether thruster is expected to
operate for durations ranging from a few weeks of continuous
operation to several years at a 50% duty cycle. Translated into
hours of operation, this would equate to lifetimes requirements
between 1,000 hours and 13,000 hours depending on the
application. For some applications the unit would be expected to
remain dormant on the spacecraft for some 5 to 7 years before
operation.
A summary of the ARPA/NRL/NASA and NRL Programs in RF Vacuum
Microelectronics may be found in [Jensen, et al., (1998)]
and [Bandy, et al.,
(1998)]
Pre-start
heater power level will have similar power requirements
(5-10 V at 5-7 A).
An SRI FEAC sat vacuum sealed at Naval Research
Laboratory for 7 years before being returned and operated.
No performance degradation was seen in the data.
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