Hitching a Ride on a Magnetic Bubble

NASA An artist's concept of a space probe riding a solar-wind driven magnetic bubble past Jupiter.

"The magnetic field for our magnetosphere comes from a 1-ft diameter coil of 16 gauge enameled wire. We run 5 to 30 amp currents through the coil; that creates a 300 gauss field at the mouth of the solenoid" - about 3 times stronger than a typical refrigerator magnet.

Normally, the intensity of such a magnetic field diminishes rapidly with increasing distance from the coil. "It's similar to a dipole field that falls off as the cube of the distance," explains Gallagher. "Dipolar magnetic bubbles are a problem, though, because they don't present the cross section we need to intercept plenty of solar wind power."

To make the bubble bigger, Gallagher and his colleagues blew up the magnetic field - much like inflating a balloon - by injecting ionized gas near the coil. The innovative use of ionized gas (called plasma) to blow up the magnetic bubble is what gives the project its name: Mini-Magnetospheric Plasma Propulsion or M2P2 for short.

"The thing that makes M2P2 special is that we inflate the field from the inside with low-energy plasma," says Gallagher. "Earth's magnetosphere is inflated with plasma, too, but it's not as dense as the plasma inside the M2P2 bubble. Jupiter's magnetosphere comes closer - the sources of plasma there are active volcanoes on Io."

The Marshall scientists use a more down-to-Earth plasma source for their M2P2 experiments - a helicon plasma generator, which ionizes gaseous argon and helium with high-power radio waves. "Helicons are fairly common," noted Gallagher. They are routinely used for fundamental plasma research and to etch commercial semiconductors.

"Last week's test was a success. We were able to completely fill the vacuum chamber with a magnetic bubble. The only thing that stopped the expansion was the presence of the chamber walls. In space this same experiment might create a mini-magnetosphere 15 km across."

Maintaining such a bubble in space would require about 1 kW of power and less than 1 kg per day of helium propellant for the plasma source. In return, the bubble would intercept about 600 kW of solar wind power.

"One of the advantages of M2P2 is that it requires no new technology," says Gallagher. "The plasma sources and solenoids at the heart of the bubble are off-the-shelf devices."

"M2P2 is a 'constant-force' device," he added. "And that's another big advantage. If you sail the spacecraft far from the Sun, you won't lose thrust." How can that be?