Developing Responsive and Agile Space Systems - Space-Library

Research Horizons
Independent R&D at Aerospace (continued)
for a small satellite indicates that air-breathing drag compensation
using an electrostatic thruster at an altitude of 200 kilometers
is feasible. Surveillance from 200 kilometers could improve image
resolution by a factor of 2 to 4 over today’s state of the art, or reduce
instrument volume/mass by perhaps a factor of 10 or more,”
Diamant said.
Work for this year will include development of a microwavepowered
plasma cathode. This type of cathode is expected to
tolerate operation in the oxygen-rich environment of the upper
atmosphere. After demonstrating extraction of sufficient electron
current, the cathode will be mated with a laboratory model lowpower
Hall thruster, and thruster performance will be measured
with relevant propellant mixtures.
Electric Propulsion Diagnostics and Modeling
Hall Current Thrusters (HCT) are
emerging as the leading propulsion
technology that will perform large
GEO (geosynchronous Earth orbit)
satellite orbit insertion and stationkeeping
because they significantly
increase spacecraft life and allow
delivery of heavier payloads to orbit
for a given booster size. In 2010
the first Advanced EHF satellite is
scheduled to be placed into GEO by
an HCT—a first for the Air Force.
With these advantages, however,
HCTs bring a new set of scientific
and engineering problems. Measurements
at The Aerospace Corporation,
for example, have found that
the electromagnetic emissions from
Hall thrusters can potentially interfere
with spacecraft communication
during orbit raising. The origin of
the troublesome strong emission
in the L, S, and C communication
bands (1–8 gigahertz) is yet to be
determined. “Because of its unique electromagnetic compatibility
facility and suite of diagnostics, Aerospace is positioned to study
this radiation and develop mitigation strategies,” said Rostislav
Spektor, senior member of the technical staff in the Propulsion
Science department. “Understanding these issues requires detailed
measurements and modeling of the plume and the plasma inside
the thruster.”
Spektor is principal investigator of an Aerospace research project
that aims to identify the origin of the Hall thruster emission in the
L, S, and C bands through a combination of measurements in the
Aerospace electromagnetic compatibility and near-field facilities.
“Aerospace operates the leading electric propulsion laboratory in
the United States specializing in the development and application
of thruster diagnostics, many of which define the state of the
art. While many research centers test electric propulsion devices,
Aerospace provides the only comprehensive noninvasive suite of
diagnostics,” said Edward Beiting, senior scientist in the Propulsion
Science department, who is coinvestigator of the study.
A second goal of the research is to measure distribution profiles
of ion and neutral velocities, plasma density, and electron energy
distribution functions inside the thruster and in the plume. This
will be done by using a suite of recently developed diagnostics in
the Aerospace near-field facility, which includes laser-induced
Rostislav Spektor explains the operation of the Hall thruster to Kara
Scheu, a summer undergraduate assistant.
fluorescence, Thomson scattering,
and a retarding potential analyzer.
“Success will allow, for the first time,
a quantitative measure of key plasma
properties in the discharge of a Hall
thruster,” Spektor said.
Significant progress has been
made since this project first began
in 2007, Spektor said. To investigate
the electromagnetic emissions,
Spektor and Beiting have developed
new technology that allows them
to measure the radiation with high
spatial precision. Using this technology,
they discovered that the L band
(1–2 gigahertz) emission originates
from the cathode. This has implications
for HCTs as well as the XIPS
(Xenon Ion Propulsion System) ion
thrusters used on the Wideband
Global Satcom spacecraft, since both
types of thrusters are using a cathode
for plume neutralization. Further
studies are being conducted to identify
the sources of the S (2–4 gigahertz) and C (4–8 gigahertz) band
emissions, and to understand the underlying physical processes that
cause this radiation.
The newly upgraded laser-induced fluorescence (LIF) diagnostic
has recently been used to study the Princeton Plasma Physics
Laboratory cylindrical Hall thruster—Princeton and Aerospace
have collaborated on the uses of this novel low-power thruster. The
thruster was successfully fired for the first time in the Aerospace
near-field facility. Spektor said that the two-dimensional velocity
profile inferred from the LIF measurements in the plume of the
cylindrical Hall thruster led to important insights into the physics
of this innovative device.
Also being researched is electron dynamics. “Electrons play
an important role in establishing operating parameters in a Hall
thruster,” Spektor said. He added that it has been recently proposed
that electrons are not thermalized in some regions of the plasma
discharge. He and Beiting are developing Thomson scattering diagnostics
and a miniature retarding potential probe to measure this
nonthermal electron behavior. The Thomson scattering method is
widely used to investigate fusion plasma, but has not yet been applied
to HCTs. “Verification of this behavior will be a major contribution
to the scientific community and may have practical implications
to HCT design,” Spektor said.
44 • Crosslink Summer 2009