Physicists Play Ring-Around-the-Atom
NORMAL, Ill. -- Electrons racing about their atomic nuclei at nearly
the speed of light bring new perspectives to the development of x-ray
lasers. High-power, short-pulse lasers can coax electrons to yield
x-rays, but they excite the electrons so much that the atoms can no
longer hold onto them. A team at Illinois State University's Intense
Laser Phys-ics Theory Unit, however, has theorized that a strong
magnetic field and a laser orders of magnitude less intense can produce
the same effects in a more stable system.
The theory suggests that there is an alternative to the study of
relativistic atomic electrons, but one that requires a large, static
magnetic field, said Rainer Grobe and Qichang "Charles" Su, directors
of the unit. They and their fellow researchers simulated the
interaction of pulses from a 100-TW/cm2 CO2
laser and atomic electrons in a 1-kT magnetic field aligned to that of
the laser. They predict in their report, which appeared in the April 10
issue of Physical Review Letters, that such a system could
impel the electrons to speeds approaching 80 percent of the speed of
light after a few cycles. It also could increase harmonic generation.
A theoretical model developed by
researchers predicts that the stimulation of atomic electrons in a powerful magnetic field by a CO2
laser will accelerate them to relativistic speeds, forming the ringlike
structure seen here, which they call a cycloatom. Courtesy of Illinois
State University, Intense Laser Physics Theory Unit.
Particle accelerators can coerce electrons into relativistic
conditions, but they carry a high price tag and need space. Tabletop
lasers with intensities on the order of petawatts per square centimeter
have yielded soft x-rays. They also are very expensive, if less so than
the traditional accelerator, and the unwanted ionization effects of
these lasers limit their usefulness.
The electrons in Grobe and Su's model do not break away from the atomic
nuclei because the magnetic field produces an additional binding force
that keeps them in their orbits. The electrons do display unexpected
behavior, including the formation of structures that the researchers
have dubbed "cycloatoms."
As the laser excites electrons in the field, they coalesce into a
crescent-shaped charge cloud that grows to encircle the nucleus. The
rings have an average kinetic energy of several hundred thousand
electron volts. The researchers suggest that the collision of these
cycloatoms in a high-density gas could produce novel energy bursts.
Although Grobe acknowledged that there could be challenges in
developing a strong field, both researchers said that the work opens
doors and that they are excited by the prospect of producing real
"The dynamics of relativistic atomic electrons are not very well
understood," said Su. "Our initial studies leading to the discovery of
cycloatoms are just a first step in investigating the effects of
relativity." The interaction of cycloatoms could shed light on
high-order harmonic generation, antimatter and astrophysical processes.
by Daniel S. Burgess
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