Focus: Plasma Jets on Earth
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 is a freelance science writer in Alexandria, Virginia.