By using an ultrapowerful laser to set off energy bursts lasting a tiny fraction of a second, scientists may finally be able to see--and perhaps control--what happens in the heart of an atom, its nucleus. This system, which its theorists call a "lasetron," could also briefly produce a massive magnetic field resembling that of a white dwarf star, opening the door to important new experiments in astrophysics.
A lasetron hasn't been built yet, but researchers Alexander Kaplan and Peter Shkolnikov proposed the idea and explained how and why the device should work in a recent paper published in Physical Review Letters. Kaplan is a professor of electrical and computer engineering in the Whiting School of Engineering. Shkolnikov, a former Johns Hopkins researcher, is now affiliated with the State University of New York at Stony Brook.
Since the paper appeared, researchers at centers equipped with very powerful lasers have contacted the authors to express interest in building a lasetron. "This would be a completely new tool for nuclear science," Kaplan says. "No one has ever seen the processes that occur within the nucleus of an atom. If we can produce very high intensity pulses, we could even control the nuclear reactions. It almost sounds like science fiction, but we might be able to slow down or accelerate nuclear fission. Using electromagnetic forces, we could try to control radioactivity, which no one has been able to do."
The lasetron tool is expected to produce bursts of light so swift they could illuminate nuclear events the way a camera flash in a dark room can "freeze" a moment of activity. Because nuclear movement takes place so quickly, scientists would need a pulse of light lasting just one zeptosecond to observe them. A zeptosecond is one-billion-trillionth of a second, or 10 (to the minus 21st power) second.
Kaplan and Shkolnikov say such super-fast pulses could be produced by using a circularly polarized petawatt, or 10 (to the 15th power) watt laser, one in which the light beams are set up so that the electric and magnetic fields move in a circular direction. The laser would be fired at a tiny target--a particle of material or an extremely thin wire. The laser would cause electrons in the target to break free. Some of these electrons would rotate rapidly within the laser light's electric field. As they spin, the researchers say, each electron would emit a burst of light in the shape of a tight cone. Seen from the edge, these spinning electrons would seem to flash on for a zeptosecond, like a tiny lighthouse. In theory, scientists could use these flashes to see activity in the nucleus of an atom.
When a lasetron sets electrons in motion, the researchers say, it would also create a magnetic field measuring about 1 million tesla, a field far more powerful than anything created on Earth and approaching the level found near white dwarf stars. Such fields would allow astrophysicists to test new theories about extreme conditions near those space objects. "This was a bonus we weren't expecting," Kaplan says.
Several hurdles exist before the Kaplan-Shkolnikov theory can be tested in a lab. First, no petawatt lasers, which require enormous amounts of power, exist yet. However, several petawatt lasers are being built in various nations, and some of these devices may be operational by the end of the year. The other key problem is that scientists currently have no means to detect and measure the zeptosecond pulses. The powerful magnetic fields would probably wreak havoc with existing types of detectors.
Nevertheless, within two years Kaplan believes less powerful lasers will be able to use the lasetron concept to produce electromagnetic fields for astrophysics research and possibly for use in advanced scanning devices that require the very short pulses generated by these magnetic fields. It may take quite a while, Kaplan says, before scientists are able to generate and characterize the zeptosecond pulses needed to peer inside a nucleus. "That doesn't bother me very much," Kaplan says. "Often, what theorists do is come up with ambitious ideas and spread the word about them. It's up to the experimentalists to take it from there."
Funding for this project was provided by the Air Force Office of Scientific Research.