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Super-brief pulses for nuclear control

Published 2/11/2002 1:38 PM

BALTIMORE, Feb. 6 (UPI) -- Energy bursts lasting only zeptoseconds -- among the briefest time spans theorized -- may in principle grant scientists the power to look inside an atom and control nuclear fusion.

"I'm not talking about exciting or suppressing atomic explosions -- yet," said lead researcher Alexander Kaplan of Johns Hopkins University in Baltimore. "But this could possibly mean we could touch the nucleus -- and one of the ultimate ideas would be trying to control nuclear reactions, slow them down or the other way around."

These energy bursts and the unbelievably powerful magnetic fields used to create them -- of a magnitude found normally only in collapsed stars -- may in future help scientists do anything from scan human bodies to generate experimental new states of matter and investigate the origin of the cosmos.

"Zeptosecond pulses and astrophysical scale magnetic fields would be a significant achievement," said laser expert Mike Perry, director of the photonics division at General Atomics in San Diego. "Any ability to perform astrophysical scale experiments in the laboratory offers the promise to test rapidly unfolding theories of the early universe."

The pulses and fields could theoretically be created by an super-powerful laser-driven electron accelerator called a "lasetron." This new application of existing technologies is proposed by Kaplan and his colleague Peter Shkolnikov of the State University of New York in Stonybook in the scientific journal Physical Review Letters.

In theory, a lasetron requires a petawatt laser, which can deliver laser beams that pack a quadrillion-watt punch -- more than 1,200 times the entire U.S. electrical generating capacity -- in less than trillionths of a second. About three petawatt lasers are currently under construction around the world, and one once operated at Lawrence Livermore National Laboratory in California under Perry's leadership.

A petawatt laser fired at wires only a few hundred atoms in diameter -- the mightiest of hammers hitting the tiniest of anvils -- could drive the electrons in those targets to incredibly near-light speeds. These electrons are actually 100 times more massive than electrons at rest, because kinetic energy actually transforms into mass at such "ultrarelativistic" speeds.

Kaplan and Shkolnivov theorize a circularly polarized petawatt laser beam -- one in which all the light rays are aligned to move circularly -- would cause electrons it hit to spin in extremely tight orbits only 3,000 angstroms in diameter, or roughly 3,000 times the diameter of a hydrogen atom.

A rotating electrical charge creates a magnetic field and these electrons would generate a magnetic field of about 1 million tesla, which is a far, far more powerful field than any made on Earth so far.

"It's unbelievable," Kaplan said. "The best we can get in the lab nowadays is a few hundred tesla, maybe 1,000 if we were generous with fields developed by explosions ... This is a magnetic field more powerful than any that normally exist in the sun, and closer to the field found in a white dwarf."

This million-tesla field is powerful enough for the electrons to bottle themselves into one huge electron, and such bottled-up electrons fire energy bursts. The faster the electrons go, the shorter the pulse, and in theory, these electrons would generate bursts lasting only zeptoseconds. Light can travel around the Earth in a tenth of a second -- but in a zeptosecond, light can only travel the distance of an atomic nucleus.

With zeptosecond pulses, it may be possible to look inside a nucleus, "see almost a movie, look at internal nuclear motion or a fast process like fusion or fission, or stimulate and control nuclear reactions," Kaplan said. "It is possible to start doing crazy things."

This experimental application may remain on the horizon for a while -- no sensor is as yet sensitive enough to detect zeptosecond pulses, and a million-tesla field would likely blow out any detectors so far, Kaplan said. Perry added the edges of a petawatt pulse could explosively vaporize its target before the pulse's core drives the wire's electrons to near-light speeds.

Still, Kaplan said weaker lasers could still generate powerful fields for advanced magnetic scanners in an X-ray emulating system "that would allow you to look into people at an airport, for example."

"It's one of those wild shots that produce a whole chain of ideas," Kaplan said.


(Reported by Charles Choi in New York.)

Copyright 2002 United Press International
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