Jets and accretion disks have been observed to accompany widely varying types of astrophysical objects, ranging from proto-star systems to binary stars to galactic nuclei. While the mechanism for jet formation is the subject of much debate, many of the proposed theoretical models predict that jets form as the result of magnetic forces.
Now, a team of applied physicists at the California Institute of Technology have brought this seemingly remote phenomenon into the lab. By using technology originally developed for creating a magnetic fusion configuration called a spheromak, they have produced plasmas that incorporate the essential physics of astrophysical jets. (Plasmas are ionized gases and are excellent electrical conductors; everyday examples of plasmas are lightning, the northern lights, and the glowing gas in neon signs.)
Reporting in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, Caltech professor of applied physics Paul Bellan and postdoctoral scholar Scott Hsu describe how their work helps explain the magnetic dynamics of these jets. By placing two concentric copper electrodes and a coaxial coil in a large vacuum vessel and driving huge electric currents through hydrogen plasma, these scientists have succeeded in producing jet-like structures that not only resemble those in astronomical images, but also develop remarkable helical instabilities that could help explain the "wiggled" structure observed in some astrophysical jets.
"Photographs clearly show that the jet-like structures in the experiment form spontaneously," says Bellan, who studies laboratory plasma physics but chanced upon the astrophysical application when he was looking at how plasmas with large internal currents can self-organize. "We originally built this experiment to study spheromak formation, but it also dawned on us that the combination of electrode structure, applied magnetic field, and applied voltage is similar to theoretical descriptions of accretion disks, and so might produce jet-like plasmas."
The theory Bellan refers to states that jets can be formed when magnetic fields are twisted up by the rotation of accretion disks. Magnetic field lines in plasma are like elastic bands frozen into jello. The electric currents flowing in the plasma (jello) can change the shape of the magnetic field lines (elastic bands) and thus change the shape of the plasma as well. Magnetic forces associated with these currents squeeze both the plasma and its embedded magnetic field into a narrow jet that shoots out along the axis of the disk.
By applying a voltage differential across the gap between the two concentric electrodes, Bellan and Hsu effectively simulate an accretion disk spinning in the presence of a magnetic field. The coil produces magnetic field lines linking the two concentric electrodes in a manner similar to the magnetic field linking the central object and the accretion disk.
In the experiment an electric current of about 100 kiloamperes is driven through the tenuous plasma, resulting in two-foot-long jet-like structures traveling at approximately 90 thousand miles per hour. More intense currents cause a jet to become unstable so that it deforms into a theoretically predicted helical shape known as a kink. Even greater currents cause the kinked jets to break off and form a spheromak. The jets last about 5 to 10 millionths of a second, and are photographed with a special high-speed camera.
"These things are very scalable, which is why we're arguing that the work applies to astrophysics," Bellan explains. "If you made the experiment the size of Pasadena, for example, the jets might last one second; or if it were the size of the earth, they would last about 10 minutes. But obviously, that's impractical."
The importance of the study, Bellan and Hsu say, is that it provides compelling evidence in support of the idea that astrophysical jets are formed by magnetic forces associated with rotating accretion disks, and it also provides quantitative information on the stability properties of these jets.
The work was supported by a grant from the U.S. Department of Energy.
Contact: Robert Tindol (626) 395-3631