|
 Phys. Rev. Lett. 95,
045002 (2005) |
| Jet plane. Electric and magnetic fields
applied to a disk and ring create a "spider" pattern of plasma. Over time, the
streams merge at the center into a single jet that is reminiscent of jets
shooting from quasars or the
sun. | | | | |
From the surface of the sun to the violent cores of quasars, many
astrophysical objects shoot plasma in sharply defined streams, guided by
magnetic fields. In the 22 July PRL, researchers present a
laboratory demonstration in which magnetic forces squeeze hot plasma into a
narrow tube without any extraordinary arrangement of conditions. This mechanism,
the scientists claim, may help explain why tightly confined jets arise in many
different astrophysical circumstances, although some astrophysicists find the
leap from lab to cosmos premature.
A plasma is a gas whose atoms have separated into a mix of charged ions and
electrons. Its behavior in a magnetic field is complex, but there are some basic
principles. Magnetic field lines cannot move through the plasma without
generating electric forces that resist the motion. So the charged particles are
"stuck" to the magnetic field lines and can only move along them, as when an
electric current flows in the plasma. In addition, just as two adjacent wires
carrying current are magnetically attracted to one another, so currents flowing
in a plasma try to squeeze together, pulling in magnetic field lines and plasma
with them.
Paul Bellan of the California Institute of Technology has theorized that
this tendency of currents to draw plasma into so-called magnetic flux tubes is
even stronger than others have assumed. When field lines flare like the bell of
a trumpet, he says, compressive magnetic forces not only narrow the bell, but
they also move plasma along the tube and create a more uniform, cylindrical
arrangement of plasma and field lines. Moreover, the same forces draw external
matter into the constricting tube, further increasing the plasma density.
In laboratory experiments, Bellan and his colleagues have now demonstrated
key elements of this theory. In a vacuum chamber, they placed a metal ring
around a metal disk, leaving a gap between the two. They generated an enveloping
magnetic field with a large coil and set up a voltage difference between the
disk and ring.
The researchers injected gas into 16 nozzles located at eight
equally spaced points around the ring and disk. A "spider leg" pattern of plasma
appeared, following the magnetic field's geometry, with eight arched tubes--each
one connecting a nozzle near the disk center with the adjacent one on the ring.
But the pattern rapidly evolved. Current flow along the "legs" narrowed their
cross section, while causing a dramatic increase in the plasma density. Then the
central portions of the eight arches merged into a single jet shooting outwards
from the disk, as the arches thinned. The experiment lasted some ten
microseconds, with the central jet narrowing further before succumbing to
instabilities.
Because there is evidence for current flows in many astrophysical
situations, Bellan argues that this mechanism for producing a tightly collimated
jet could apply widely. But Eric Priest, of St. Andrew's University in Scotland,
while admiring the Caltech team's demonstration, finds its astrophysical
applicability not at all obvious. He worries in particular that the timescale of
the experiment may not scale up appropriately to solar and astrophysical values.
Adam Frank of the University of Rochester, New York, cautions that the behavior
of astrophysical jets depends on additional factors, such as the ratio of
thermal energy to magnetic energy, that laboratory experiments may not
mimic.
Bellan responds that plasmas show consistent behavior under a broad range
of conditions, and that scaling arguments for the spatial extent, velocity, and
timescale of jets can be found. When he has lectured on this work, he says,
"astrophysicists don't give me a hard time."
--David Lindley
David Lindley is a
freelance writer in Alexandria, Virginia, and author of Degrees Kelvin: A
Tale of Genius, Invention, and Tragedy (Joseph Henry Press, March
2004).
Dynamic and Stagnating Plasma Flow Leading to
Magnetic-flux-tube Collimation
S. You, G. S. Yun, and P. M. Bellan
Phys. Rev. Lett. 95,
045002
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