Mini-magnetospheric plasma propulsion

Mini-magnetospheric plasma propulsion (M2P2) is a proposed form of spacecraft propulsion, a way to make a magnetic sail. Magnetic sails' efficiencies fall off as the square of their distance from the sun. To circumvent this problem, NASA has attempted to develop a system using a conductive plasma constrained by the magnetic field, a sort of synthetic magnetosphere. This automatically expands as the solar wind becomes less dense, and therefore was proposed to maintain the same thrust at all distances from the sun.

Subsequent analysis of the concept and the interactions of the solar wind with the plasma-inflated magnetic field appear to show that the concept is not feasible. The original concept assumes that the momentum of the intercepted solar wind momentum is completely coupled to the Lorentz force acting on the spacecraft. Subsequent analysis by Janhunen ([1]) and others indicate that most of the proposed benefit is lost due to momentum transfer to escaping plasma at the magnetopause, and also to near- spacecraft current effects.

In the original proposal, a coil in the magnetic M2P2 creates an electromagnetic loop or mini-magnetosphere extending out from the spacecraft, using a cloud of plasma to conduct the current to create the magnetic field. The solar wind is deflected (being made of protons, it has mass) and the reaction accelerates the spacecraft. The thrust can be in any direction at right angles to the solar wind, allowing efficient changes of orbit.

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One advantage is that no reaction mass is depleted or carried in the craft. The "propellant" is the gas needed to maintain the cloud. It therefore can generate thrust for long periods of time without refueling. One can calculate a "specific impulse" for this system by computing the amount of "propellant" consumed per newton of thrust. This is a figure of merit usually used for rockets, where the fuel is actually reaction mass. The "specific impulse" of such a system is about 200 kN·s/kg (200 times better than the space shuttle main engine). The system requires on the order of a kilowatt of power per newton of thrust, easily supplied by a nuclear reactor.

A unique advantage, not shared by magnetic or solar sails, is that the sail spreads automatically as the solar wind becomes less dense. In simulations, the system generates the same thrust anywhere within the heliopause. The solar and magnetic sails have a thrust that falls off as the square of the distance from the sun.

Another unique advantage is that the system requires no large mechanical system to catch solar protons or maintain the magnetic field. This reduces its total mass.

A spacecraft can also brake by diving into a stellar magnetopause.

However, M2P2 also requires a reliable source of electric power, and tanks to replenish the magnetosphere, which leaks.

In most ways M2P2 is a magnetic sail, and shares the same capabilities. Magnetic sails are not at all like rockets. They can maneuver using planetary magnetic fields and plasmas other than the solar wind. For information about other mission capabilities, including tacking in the solar wind, and maneuvers within a magnetic field, see magnetic sail.

Mechanism

The basic mechanism looks like a coffee can with both ends open. Gas is emitted from a valve in the coffee can. The coffee can confines the gas long enough for some electronics to strike an arc through the gas. Then a low voltage (low electrical potential or 'electromotive force') high-current (lots of electrons) electric current is pushed through the arc. The magnetic field from this current "traps" the plasma of the arc. The conductive plasma hardly moves in the magnetic field because whenever it cuts the magnetic field, eddy currents form opposing magnetic fields and stop the motion. The magnetic field naturally expands. This causes the arc to grow, and confines the ionized gas in the arc. This process is called "blowing the arc."

Blowing the arc is something like blowing a large soap bubble. The arc is fragile. If the electric current at some point becomes too low, the magnetic field decreases, gas deionizes and escapes, and the current and the magnetic field decreases even more.

A tank of gas is required to replace the ions of the cloud that leak from the magnetic field. The prototype uses argon for the plasma, but the researchers hope to develop helium plasmas as the technology improves. An argon plasma has more massive ions, which are easier to contain because they move more slowly for a given electric current and magnetic field. Radon gas would be still more massive and would have at least a slight tendency to be self ionizing. Helium is less massive, and using it therefore might yield more velocity per kilogram. However, more of it leaks from present designs.

Electric power is required to keep the cloud ionized, and keep the current flowing. The magnetic field and current requirements are small enough that they could be powered by solar cells on unmanned vehicles.

History

The M2P2 concept was proposed by geophysicist Robert Winglee. A prototype has been developed at the University of Washington.^[1] The expansion of the magnetic field using plasma injected has been successfully tested in a large vacuum chamber on Earth, but the development of thrust was not part of the experiment. A beam-powered variant, MagBeam,^[2] is also under development.

See also

• magnetic sail
• spacecraft propulsion

References

1. ^ http://www.ess.washington.edu/Space/M2P2
2. ^ http://www.ess.washington.edu/Space/magbeam/

• (September 1 2000) "Mini-magnetospheric Plasma Propulsion : Tapping the energy of the solar wind for spacecraft propulsion". Journal of Geophysical Research 105 (A9): 21,067-21,077.