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To Go Beyond PUBLIC ACCESS

Experiments With an Advanced Electromagnetic Engine Aim for High-Speed, Long-Distance Transportation to Reach Farther into Space.

[+] Author Notes

Associate EditorAl Reisz is a Fellow of ASME and president of Reisz Engineers in Huntsville. Ala.

Mechanical Engineering 130(11), 42-45 (Nov 01, 2008) (3 pages) doi:10.1115/1.2008-NOV-2

This article discusses experiments with an advanced electromagnetic engine that aims for high-speed, long-distance transportation to reach farther into space. Experimental work at Marshall Space Flight Center in Alabama is attempting to develop an electromagnetic engine designed to achieve higher velocities than current space-engine options and to last longer, too. Space engines with higher specific impulse will sense new science from deep space exploration quicker. In a way, higher specific impulse quickens our intelligence acquisition. Reisz Engineers and the University of Michigan are investigating the propulsive performances of an experimental advanced electromagnetic engine configuration. This electromagnetic propulsion configuration has a magnetic nozzle and the engine performance can be throttled. Electromagnetic propulsion systems can also be configured for operations in Earth space environment, and for lunar robotic and lunar mapping missions. Electromagnetic and fusion space engines promise fast and reliable propulsion systems, which will be needed if mankind is to pursue its exploration of the outer realms of our solar system and beyond.

The Soviet Union launched the artificial moon Sputnik, the first man-made satellite to circle Earth. The United States followed four months later with the launch of Explorer 1. On America's first foray into space, we discovered the Van Allen belt of charged solar and cosmic particles surrounding Earth. Since entering the space age, mankind's knowledge and understanding of the universe has increased at an exponential rate. With ray-sensing telescopes outside our atmosphere, we now see more clearly into our solar system and beyond it. We see a cosmos far more enigmatic than any of us ever imagined.

The farther into space that we send exploratory missions, the more we will continue to learn. To go farther, future space exploration will require propulsion systems with very high velocities to traverse gigantic distances.

Experimental work at Marshall Space Flight Center in Alabama is attempting to develop an electromagnetic engine designed to achieve higher velocities than current space-engine options and to last longer, too.

To attain the velocities needed for exploration, space engines must exhibit high specific impulse, a measure-ment of a propulsion system's ability to convert propellant into speed. In engineering terms, specific impulse, Is' is thrust generated per unit of propellant weight consumed over time (Is, = F/W, where F is thrust and W is weight of propellant consumed per second). Specific impulse is expressed in seconds.

Space engines with higher specific impulse will sense new science from deep space exploration quicker. In a way, higher specific impulse quickens our intelligence acquisition.

Space engines use electric and magnetic forces to eject plasma particles to attain reactive forward-propelling forces, as in current ion engines and Hall effect thrusters. Very hjgh velocities are generated by the efficient direct conversion of input energy into directed kinetic energy of the plasma particles being exhausted. Such in-space engines are currently being used for near-Earth space environment missions, such as satellite placement and maintenance, and for interplanetary exploration.

Ion engines have an electrode that creates an electric field and works with an electric grid to drive propellant ions into space. Hall effect thrusters, fi rst developed by the Russians, use ring magnets to create a magnetic field perpendicular to an electric field created by an elec trode. Interac tion of the magnetic and electric fi elds generates an additional electric current, the Hall current. The operating life for both types of engines is limited because their electrodes erode with operation as does the grid in the ion engine. The grid in the ion engine also hampers the ejection of ions.

Electromagnetic engines are another option. They have magnets that ring a cylinder into which propellant is injected. The propellant is brought to a plasma state by radio frequency waves and controlled by a diverging axial magnetic field created by the magnets. After the propellant is heated to the plasma state, its particles interact to create more energy that can be converted into thrust. Electromagnetic engines of this type have no electrodes or grids and generate very high velocities for long-life operation, propulsion characteristics required for deep space missions.

Reisz Engineers and the University of Michigan are investigating the propulsive performances of an experimental advanced electromagnetic engine configuration. This electromagnetic propulsion configuration has a magnetic nozzle and the engine performance can be throttled. This research is being conducted under the NASA Small Business Technology Transfer Research program.

The project has designed and tested an experimental electromagnetic engine apparatus at NASA's Marshall Space Flight Center.

The experimental engine has an electron cyclotron resonance (ECR) plenum cylindrical chamber ringed with magnets. The magnetic field strength is configured so that stronger magnets are at the ends of the cylinder. They act as magnetic mirrors that reflections and electrons to keep the plasma in the cylinder, although sufficiently high-velocity particles can escape through the end magnetic mirror. When particles escape, the magnetic field at the end of the cylinder extends to a magnetic nozzle by using extended ring magnetics.

The experimental engine has an electron cyclotron resonance (ECR) plenum cylindrical chamber ringed with magnets. The magnetic field strength is configured so that stronger magnets are at the ends of the cylinder. They act as magnetic mirrors that reflections and electrons to keep the plasma in the cylinder, although sufficiently high-velocity particles can escape through the end magnetic mirror. When particles escape, the magnetic field at the end of the cylinder extends to a magnetic nozzle by using extended ring magnetics.

In this experiment, argon flows into the plenum. The argon atoms are subjected to the electromagnetic radiation at frequencies of 2.45 or 10 GHz. The waves are launched into the plenum cylinder by an RF generator, either a 10 GHz klys tron or a 2.45 GHz magnetron. The electromagnetic waves launched from the generator are transported by a wave guide. The guide is rectangular leaving the generator, but changes configuration along the way to the ECR chamber so as to transition the waves to circul arly polarized radiation. This experiment finds that right circul arly polarized waves are more completely absorbed in the plasma than are the left circularly polarized waves, which are mostly reflected.

The microwaves enter the plenum through a dielectric window. The magnetic field strength in the ECR chamber is adjusted so that the electron resonant frequency of the propellant gas/plasma matches the RF waves being injected, 2.45 or 10 GHz in this experiment. Electrons of the propellant gas atoms absorb the radiation energy and are freed from the argon atoms, ionizing the gas. Thermal energy is converted to kinetic energy.

Some faster and much smaller electrons escape the strong end magnetic mirror of the ECR cylinder. The evacuation of electrons out of the cylinder end creates an axial elec tric potential inside the cylinder. With this electric potential, ions are accelerated axially with higher velocities that impel them through the strong end magnetic mirror and through the nozzle. As the plasma expands through the nozzle, the mean free path between collisions increases so that the plasma particles become nearly collisionless. The thermal expansion through the nozzle also contributes to ion velocity.

Because the ions and electrons escape at the same rate, the electric potential developed in the gas dynamic state remains and continues to contribute to ion acceleration with the axial magnetic field.

The interaction of the swirling electrons with the axial magnetic field causes a Lorentz force inward toward the ECR center that tends to contain the plasma plume and keep it from expanding outward and touching the cylinder wall. A Lorentz force is a resulting force perpendicular to the plane of the movement of an electric charge across an axial magnetic field. Where the field lines are parallel with the axis, the Lorentz force is directly inward. As the field lines converge and diverge with the magnetic strength along the axis, the Lorentz force vector tilts accordingly to decelerate or accelerate the ions.

Taking measurements: A photo of the electromagnetic engine's exhaust plume shows some of the test devices in place.

Grahic Jump LocationTaking measurements: A photo of the electromagnetic engine's exhaust plume shows some of the test devices in place.

In our experiment, radio waves of 2.45 GHz and 10 GHz from a magnetron and a klystron, respectively, powered by 2 to 3 KW of electricity, are injected into the ECR chamber. Argon propellant, at flow rates from 0.2 to 2.31 milligrams per second, is injected into the chamber. Magnetic coils are energized by direct current ranging from 0 to 1,000 amps at 0 to 50 volts.

Propulsive performance measurements are made with a Langmuir probe, measuring nozzle exhaust temperatures and particle densities. We calculated velocities and thrusts from known and sensed information. Recent sensing instrumentation includes a laser induced fluo-rescence (LIF) probe that directly measures ion velocity from the nozzle.

Measurements show that the shape of the magnetic field strength along the axis has strong influence on the velocity of the exhaustingjet stream. The magnetic field shape varies along the axis in accordance with the strength of the ring magnets. Strong magnetic fields, at the mirror magnets in this experiment, yield more compact exhaust plumes with higher velocities.

Velocity measurements with the more accurate LIF instrument were made at slower velocities in order to sense the exhausting ions. An unexpected discovery was that ions were exhausted at two distinct velocities at the same time. LIF measurements across the nozzle found ions in the outer region escaping at a velocity of 5,000 meters per second and those in the center at 3,000 mps.

We believe that the exhausting ions are therefore collisionless or nearly so, because otherwise the ion collisions would slow the faster ions and ions would exit at average velocities.

The cause of the bimodal velocity may be that the center magnetic field lines are straighter and, thus, ions enter the magnetic mirror center on a straight path. Therefore, the magnetic mirror does not reflect relatively slower ions, resulting in a slower velocity measurement at the center. The outer realms of the plasma have curved magnetic field lines. The magnetic mirror here reflects slower ions more effectively, resulting in higher ion velocities penetrating and being measured. Also, the higher number of ions going through the central part of the magnetic mirrors may render it less effective in reflecting relatively slower ions than at the outer portions of the mirror.

Another possible explanation is that the Lorentz force concentrates ions toward the center of the chamber. Higher-velocity ions may be more defiant of this force concentration and be in the outer realm of the chamber. Further research of this electromagnetic engine will give clues to obtaining significantly higher velocities and higher specific impulses.

The velocities measured in this research were limited by the available laboratory power to magnets with 17 and 33 turns, controlled propellapt flow, and available RF wave sources. With this configuration of electromagnetic engine using the 2.45 GHz RF waves and 2 kW of electricity, energy balance calculations determine velocities up to 50,000 meters per second or 5,000 seconds of specific impulse. Thrust is about 70 mN.

The potential for significantly higher velocities can be obtained with stronger magnetic fields and more precise shaping of the magnetic field along the axis for a greater magnetic gradient. Designing an ECR chamber so that ions will be generated in areas contributing to higher velocities will improve performance. With continued research, design, and experimentation, velocities on the order of 100,000 meters per second or 10,000 seconds of specific impulse may be attained, an order very fworable for deep space missions.

Electric power can be supplied by photovoltaic cells for Earth proximity missions and perhaps for missions to Mars. Beyond that, a small nuclear electric power plant may be needed, such as the type NASA planned for the Prometheus program.

More efficient methods of generating electric power for space propulsion are being developed. A plasma stream moving through a magnetic field will generate an electric potential from which electric current can be obtained. Some of the plasma flow could be diverted to generate electricity to power the magnetics. A thermionic converter to produce electricity from the plasma is also a possibility.

Terry Kammash, professor emeritus of nuclear engineering and radiological sciences at the University of Michigan and a principal participant in this work, believes that fusion propulsion is possible in the electromagnetic engine with the gas dynamic mirror. Kammash, who is the editor of the book Fusion Enelgy in Space Propulsion, published by the American Institute of Aeronautics and Astronautics, believes that, when the magnetic field in the cylinder is strong enough to cause more horizontal collisions with sufficient force, fusion can result.

According to Kammash, "When the magnetic field in the chamber is sufficiently strong, it can confine deuterium- tritium plasma of high density and temperature to allow these hydrogen isotopes to undergo fusion reactions, thereby producing a great deal of energy. This energy manifests itself in the velocity of the charged particles that escape from the end of the GDM device, which serves as a magnetic nozzle, thereby producing very high specific impulse. Moreover, because of the high density of the confined plasma, the escaping particles will also have high density, which in turn manifests itself in large thrust. In contrast to other fusion-type devices, the GDM is uniquely suited for fusion propulsion applications because of its ability to confine high-density, hightemperature plasma long enough to allow fusion reactions to take place, thereby heating the plasma to the desired temperatures before it is ejected to generate very impressive propulsive capabilities."

Attempts to do so would require deuterium and tritium, two isotopes of hydrogen, to enter the gas dynamic mirror mode with the strong end magnets . If nuclei of deuterium and tritium collide with sufficient energy in a plasma state, they fuse to create a helium atom, an expelled neutron, and-given E=mc2-17.6 MeV of energy.

In 2029, the asteroid Apophis will pass in the vicinity of Earth. Current predictions are that it will miss Earth if it stays on its current trajectory. Even so, concern is that Earth's gravity will alter Apophis's trajectory so that when it returns in 2036, it may be on a collision course with our planet. If that happens, one solution being proposed is to send a nuclear bomb by a space engine to knock the asteroid off a collision course with Earth. This electromagnetic engine, with continued development, will be the quickest and the most reliable space engine for this mission.

Electromagnetic propulsion systems can also be configured for operations in Earth space environment, and for lunar robotic and lunar mapping missions. Electromagnetic and fusion space engines promise fast and reliable propulsion systems, which will be needed if mankind is to pursue its exploration of the outer realms of our solar system and beyond.

A schematic diagram of the experimental engine at Marshall. The plasma column and exhaust plume are shaped by the interaction of electrical and magnetic forces.

Grahic Jump LocationA schematic diagram of the experimental engine at Marshall. The plasma column and exhaust plume are shaped by the interaction of electrical and magnetic forces.

As the magnetic field lines diverge and converge as influenced by the coils, the inward Lorentz force tilts to accelerate or decelarate the plasma particles.

Grahic Jump LocationAs the magnetic field lines diverge and converge as influenced by the coils, the inward Lorentz force tilts to accelerate or decelarate the plasma particles.

Copyright © 2008 by ASME
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