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 Variable-Specific-Impulse Magnetoplasma Rocket

This rocket is expected to enable long-term human exploration of outer space.

Lyndon B. Johnson Space Center, Houston, Texas

Johnson Space Center has been leading the development of a high-power, electrothermal plasma rocket — the variable-specific-impulse magnetoplasma rocket (VASIMR) — that is capable of exhaust modulation at constant power. An electrodeless design enables the rocket to operate at power densities much greater than those of more conventional magnetoplasma or ion engines. An aspect of the engine design that affords a capability to achieve both high and variable specific impulse (Isp) places the VASIMR far ahead of anything available today. Inasmuch as this rocket can utilize hydrogen as its propellant, it can be operated at relatively low cost.

The design of the VASIMR is so original that a prototype is being developed in collaboration with the Department of Energy and with the Oak Ridge National Laboratory and its Center for Manufacturing Technology. The VASIMR is expected to be commercially useful for boosting communication satellites and other Earth-orbiting spacecraft to higher orbits, retrieving and servicing spacecraft in high orbits around the Earth, and boosting high-payload robotic spacecraft on very fast missions to other planets. Similarly, the VASIMR should make it possible for robotic spacecraft to travel quickly to the outer reaches of the Solar system and begin probing interstellar space. By far, the greatest potential of the VASIMR is expected to lie in its ability to significantly reduce the trip times for human missions to Mars and beyond. This reduction in times is expected to enable long-term exploration of outer space by humans — something that conventional rocket designs now preclude.

Because the VASIMR uses plasma to produce thrust, it is related to several previously developed thrusters; namely, the ion engine, the stationary plasma thruster (SPT) (also known as the Hall thruster), and the magnetoplasmadynamic (MPD) thruster [also known as the Lorentz-force accelerator (LFA)]. However, the VASIMR differs considerably from these other thrusters in that it lacks electrodes (a lack that enables the VASIMR to operate at much greater power densities) and has an inherent capability to achieve high and variable Isp. Both the ion engine and the SPT are electrostatic in nature and can only accelerate ions present in plasmas by means of either (1) externally applied electric fields (i.e., applied by an external grid as on an ion engine) or (2) axial charge nonuniformity as in the SPT. These ion-acceleration features, in turn, result in accelerated exhaust beams that must be neutralized by electron sources strategically located at the outlets before the exhaust streams leave the engines.

In the LFA, acceleration is not electrostatic but electromagnetic. A radial electric current flowing from a central cathode interacts with a self-generated azimuthal magnetic field to produce acceleration. Although LFAs can operate at power levels higher than those of either the ion engine or the SPT and do not require charge neutralization, their performances are still limited by the erosion of their electrodes.

An MPD plasma injector includes a cathode in contact with the plasma. Although the plasma at the location of contact is relatively cold, the cathode becomes eroded and the plasma becomes contaminated with cathode material (typically tungsten). The erosion and contamination can contribute to premature failure and to increased loss of energy through radiation from the contaminants in the plasma. An equal limitation on performance is exerted by nonionized propellant in a high-power amplifier cavity that is part of the MPD; the reason for this limitation is that neutral atoms and molecules in this region lead to charge-exchange losses, which, in turn reduce the overall efficiency of the engine and increase the unwanted heat load on the first wall (the liner) of the MPD thruster.

The design of the VASIMR avoids the aforementioned limiting features. The VASIMR contains three major magnetic cells — the forward, central, and aft cells. A plasma is injected into these cells, then heated, then expanded in a magnetic nozzle. (The magnetic configuration is of a type known as an asymmetric mirror.) The forward cell handles the main injection of propellant gas and an ionization system; the central cell serves as an amplifier to further heat the plasma to desired magnetic-nozzle-input conditions; and the aft cell acts as a hybrid two-stage magnetic nozzle that converts the thermal energy of the fluid into directed flow while protecting the nozzle walls and allowing efficient detachment of the plasma from the magnetic field. During operation of the VASIMR, a neutral gas (typically, hydrogen) is injected into the forward cell, where it is ionized. The resulting plasma is then heated in the central cell, to the desired temperature and density, by use of radio-frequency excitation and ion cyclotron resonance. Once heated, the plasma is magnetically and gas-dynamically exhausted by the aft cell to provide modulated thrust. Contamination is virtually eliminated and premature failures of components are unlikely.

The VASIMR offers numerous advantages over the prior art:

  • Its unique electrodeless design provides not only high thrust at maximum power but also highly efficient ion-cyclotron-resonance heating, and high efficiency of the VASIMR regarded as a helicon plasma source.
  • Because the VASIMR operates at relatively high voltage and low current, its mass is relatively low. This means that a one-ship human mission will not depend on a high-energy, complex rendezvous near Earth to achieve escape velocity. Instead, a rapid interplanetary transfer will be achieved with an adaptable exhaust, which will provide optimal acceleration throughout the mission.
  • The residual magnetic field of the engine and the hydrogen propellant will be effective as a shield against radiation.
  • Because of its continuous acceleration, the VASIMR will be able to produce a small amount of artificial gravitation, thereby reducing the physiological deconditioning produced by weightlessness.
  • The variability of thrust and Isp at constant power will afford a wide range of capabilities to abort.
  • Because hydrogen is the most abundant element in the universe, the supply of hydrogen could likely be regenerated in situ.
  • The VASIMR is flexible and adaptable to both fast transfers of humans and slower high-payload robotic missions; hence, there would be no need to develop separate propulsion systems for missions of each type, and costs would be held down accordingly.

Long-range benefits could be derived from the continued development of the VASIMR. The VASIMR can be expected to pave the way for fusion-driven plasma rockets. In addition, because the VASIMR is a high-Isp rocket, the VASIMR concept can be expected to lead to lower initial mass in low Earth orbit, relative to nuclear, thermal, and/or chemical rockets.

This work was done by Franklin R. Chang-Díaz of Johnson Space Center.

This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to the Patent Counsel, Johnson Space Center, (281) 483-0837. Refer to MSC-23041.

Record Set for Hottest Temperature on Earth: 3.6 Billion Degrees in Lab; Feb 2006

Scientists have produced superheated gas exceeding temperatures of 2 billion degrees Kelvin, or 3.6 billion degrees Fahrenheit.

This is hotter than the interior of our Sun, which is about 15 million degrees Kelvin, and also hotter than any previous temperature ever achieved on Earth, they say.

They don't know how they did it.

The feat was accomplished in the Z machine at Sandia National Laboratories.

"At first, we were disbelieving," said project leader Chris Deeney. "We repeated the experiment many times to make sure we had a true result."

Thermonuclear explosions are estimated to reach only tens to hundreds of millions of degrees Kelvin; other nuclear fusion experiments have achieved temperatures of about 500 million degrees Kelvin, said a spokesperson at the lab.

The achievement was detailed in the Feb. 24 issue of the journal Physical Review Letters.

The Z machine is the largest X-ray generator in the world. It’s designed to test materials under extreme temperatures and pressures. It works by releasing 20 million amps of electricity into a vertical array of very fine tungsten wires. The wires dissolve into a cloud of charged particles, a superheated gas called plasma.

A very strong magnetic field compresses the plasma into the thickness of a pencil lead. This causes the plasma to release energy in the form of X-rays, but the X-rays are usually only several million degrees.

Sandia researchers still aren’t sure how the machine achieved the new record. Part of it is probably due to the replacement of the tungsten steel wires with slightly thicker steel wires, which allow the plasma ions to travel faster and thus achieve higher temperatures.

One thing that puzzles scientists is that the high temperature was achieved after the plasma’s ions should have been losing energy and cooling. Also, when the high temperature was achieved, the Z machine was releasing more energy than was originally put in, something that usually occurs only in nuclear reactions.

Sandia consultant Malcolm Haines theorizes that some unknown energy source is involved, which is providing the machine with an extra jolt of energy just as the plasma ions are beginning to slow down.

Sandia National Laboratories is located by Albuquerque New Mexico and is part of the U.S.

Zero to 76,000 mph in a Second

By Leonard David LiveScience Senior Writer: 07 June 2005

Scientists at the Sandia National Labs in Albuquerque, New Mexico have accelerated a small plate from zero to 76,000 mph in less than a second. The speed of the thrust was a new record for Sandia’s "Z Machine" – not only the fastest gun in the West, but in the world too.

The Z Machine is now able to propel small plates at 34 kilometers a second, faster than the 30 kilometers per second that Earth travels through space in its orbit about the Sun. That’s 50 times faster than a rifle bullet, and three times the velocity needed to escape Earth’s gravitational field.

The ultra-tiny aluminum plates, just 850 microns thick, are accelerated at 1010 g. One g is the force of Earth’s gravity. Doing so without vaporizing the plates was possible because of the finer control now achievable of the magnetic field pulse that drives the flight.

Z’s hurled plates strike a target after traveling only five millimeters, or less than a quarter-inch. The impact generates a shock wave -- in some cases, reaching 15 million times atmospheric pressure -- that passes through the target material. The waves are so powerful that they turn solids into liquids, liquids into gases, and gases into plasmas in the same way that heat melts ice to water or boils water into steam.

One purpose of these very rapid flights is to help understand the extreme conditions found within the interiors of giant planets in our solar system. By creating states of matter extremely difficult to achieve on Earth, the flyer plates provide hard data to astrophysicists speculating on the structure and even the formation of planets like Jupiter and Saturn.

Didier Saumon, an astrophysicist at Los Alamos National Laboratory, noted that the internal structures of Jupiter and Saturn are composed mostly of hydrogen. So knowing its equation of state -- how hydrogen and its isotopes behave at pressures from one to 50 million atmospheres -- is highly relevant to how scientists infer the interior properties of these planets.

An upgrade of the Z Machine is planned for next year and is expected to achieve higher plate velocities.

An electrical storm lights up the surface of the Z machine, an accelerator built to simulate what happens during a nuclear explosion. The electrical discharges result from powerful electric fields that the experiment produces.

Housed at Sandia National Laboratories, the Z machine attracted a lot of attention eight years ago when its energy output more than quadrupled – raising hopes that the reactions in the Z could provide a new source of clean, abundant power. To help further progress towards this end, the machine is getting a $61.7 million upgrade, officials announced recently.

The Z uses a short burst of intense electricity – only a few 10 billionths of a second long – that forces an ionized gas to implode. The process is called a z-pinch because the pulse creates a magnetic field that squeezes particles in the vertical direction, which math books usually label as the "z-axis."

At the center of the z-pinch, in the space of a small soup can, gas particles race at each other at a million miles an hour. The collisions result in X-rays and extremely high temperatures.

Last year, when physicists placed a capsule of deuterium, or heavy hydrogen, at the focus of the z-pinch, they detected neutrons flying out from the implosion site – a signal that fusion reactions were taking place, as they do in the sun.

If researchers can learn to tame these fusion reactions, the setup can rely on a seemingly endless supply of deuterium fuel in seawater.