Research Horizons
Low-Power Electric Propulsion
The concept of operationally responsive access to space is leading to an increased interest in small, low-power spacecraft. Research scientist Kevin Diamant of The Aerospace Corporation's Propulsion Science department said that these spacecraft—severely limited in mass and power—could benefit from the use of low-power electric propulsion, increasing useful payload mass and enhancing mission duration and flexibility.
"The most important figure of merit for satellite propulsion is the velocity at which propellant exits the thruster. For a given amount of momentum imparted to the exhaust stream, and thereby to the spacecraft, larger exhaust velocity results in reduced exhaust mass. Reduced propellant mass can translate to reduced launch cost or additional payload. Or, for a given propellant load, larger exhaust velocity permits a greater total impulse (momentum change), which equates to greater maneuverability or longer life on station," Diamant explained.
 Rostislav Spektor and Xuan Eapen, senior research associate, investigate the origin of Hall thruster electromagnetic emission in the EMI facility. |
The vast majority of satellites have relied on chemical thrusters, but chemical rocket exhaust velocity is limited by the heat released by the combustion or decomposition of the propellant. Diamant, principal investigator of an Aerospace team exploring the Hall thruster—a type of plasma-based propulsion system—said that this limitation can be removed by supplying power to the propellant from an external source, which in electric propulsion is any external electrical power source. Rostislav Spektor, senior member of the technical staff, Propulsion Science department, is a coinvestigator on the team.
"The Hall thruster may be an attractive option due to its compactness and relatively simple construction," Diamant said. "Typical Hall thruster exhaust velocities range from 15,000 to 20,000 meters per second; whereas practical values for chemical thrusters lie in the range of 2000 to 4500 meters per second."
The Hall thruster is an electrostatic thruster—a type of ion thruster that operates on the principle that a charged particle accelerates in an electric field, Diamant explained. Hall thrusters typically consist of an annular (ring-shaped) ceramic discharge channel with an electrode (anode) at one end. Propellant (usually xenon) enters through ports in the anode, and is ionized in a high-voltage discharge struck between the anode and another electrode (cathode) placed externally to the channel. The ionized gas—plasma—consists of neutral atoms, free electrons, and positively charged ions. A radial magnetic field is applied close to the channel exit. This magnetic field impedes the motion of electrons to the anode, resulting in the presence of a large electric field in the plasma. Ions are accelerated by this field, producing thrust.
Hall thrusters in the 0.4–1.4-kilowatt power range have extensive flight heritage. Approximately 250 Hall thrusters are in space, mostly on Russian satellites launched since the early 1970s, but also on several recently launched satellites built by Space Systems/Loral. The AEHF (Advanced Extremely High Frequency) satellites will carry 4.5-kilowatt Hall thrusters for orbit raising, stationkeeping, and repositioning. A 200-watt Hall thruster recently performed drag compensation for the Air Force's TacSat-2 spacecraft.
Diamant pointed out, however, that scaling Hall thrusters to power levels below a few hundred watts while preserving high average exhaust velocity and efficiency is challenging because of the need to reduce channel size to preserve ionization efficiency. Small size leads to difficulty in generation of magnetic fields with appropriate magnitude and topology, and to increased power loss from the plasma due to the larger surface area-to-volume ratio.
"The cylindrical Hall thruster (CHT), invented by researchers at Princeton University Plasma Physics Laboratory, eliminates most, or all, of the inner wall of the annulus [area between two concentric circles], with the intent of boosting efficiency and life at low power," Diamant said. "A drawback of the CHT is its relatively wide plume divergence. Ions accelerated at high angles detract from thruster performance and can potentially heat and erode nearby structures."
"In an ongoing Aerospace collaboration with Princeton in the use of CHTs, we have verified that the CHT plume divergence can be reduced by approximately 25 percent and ion energy increased by 10 percent by operating an auxiliary discharge to an electrode (known as a 'keeper') placed just in front of the cathode," Diamant said. The researchers also found that efficiency from 30 to 40 percent may be achievable at power levels from 100 to 200 watts. Measurements of multiple charged ions in the CHT plume found them to be correlated with the presence of thruster erosion products.
Diamant said that in the coming year, the researchers will examine the feasibility of using a low-power Hall thruster to perform drag compensation in low orbit using propellant ingested from the atmosphere. "By decreasing altitude, smaller, lower-power, and less massive instruments can deliver capability similar to that achieved by larger satellites in higher orbits. For example, the resolution of Earth imagery is proportional to the ratio of orbital altitude to optical aperture diameter. Lower orbits enable finer resolution, or smaller, and therefore lighter and cheaper, optics."
Today's commercial Earth-observing satellites operate at altitudes from 400 to 800 kilometers and are able to resolve objects less than a meter in size. "An analysis based on assumptions appropriate 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 microwave-powered 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 low-power 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."
Rostislav Spektor explains the operation of the Hall thruster to Kara Scheu, a summer undergraduate assistant. |
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 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.
