A New Mechanism for Laser-Plasma Accelerators
In a paper which appeared in the June 2006 issue of “Physics of Plasmas” (PoP 13, 064506),
Professor Edison Liang of the Rice Physics and Astronomy Department
proposed and demonstrated, via computer simulations, a remarkable new
mechanism for accelerating electrons to ultra-high energies in
ultra-short distances using intense lasers. If his
results are eventually upheld by experiments, this can lead to a whole
new world of ultra-compact accelerators, with potentially far-reaching
applications ranging from medical applications to homeland security and
high energy physics. Even prior to its official
publication in the referred journal, this new concept of electron
acceleration has already attracted much attention from the scientific
community, as Dr. Liang gave numerous invited talks on his results at
major international conferences.
Conventional electron accelerators in use today require long distances to achieve high energies.
For example, using linear electrostatic fields, the Stanford Linear
Accelerator Center (SLAC) can reach a trillion electron volt10 in
roughly a kilometer. This corresponds to an “acceleration gradient” of a GeV (one billion electron volts) per meter.
New “laser-plasma accelerators” proposed by many groups, by propagating
an intense laser inside a low-density plasma to generate an
electrostatic “wakefield”, hope to reach acceleration gradients up to a GeV per millimeter.
A milestone was reached in 2005 when three experimental groups
independently demonstrated the viability of this “laser wakfield
accelerator”, or LWFA, concept. However, since the LWFA
plasma must have low enough density in order for lasers of optical or
infrared frequencies to propagate inside it, there are fundamental
limitations to both the acceleration gradient and the electron beam
The new mechanism proposed by Dr. Liang is radically different from the LWFA concept.
Instead of propagating a single laser pulse inside a low-density
plasma, he proposes to use two colliding laser pulses to compress a
thin slab of high-density plasma. When the plasma slab is
compressed to the point where the thickness becomes less than the “skin
depth”, the two laser pulses can “tunnel” through, despite the high
density. When the two laser pulses reemerge on the far
sides of the plasma, they pull out the surface electrons and continue
to accelerate them indefinitely with the full force of the laser field.
Computer simulations demonstrate that the electrons can reach GeV
energies in only a few hundred microns, for laser intensity equal to 1021 (or a billion trillion) Watts per square centimeter, the highest intensity achievable with the biggest existing lasers.
Most importantly, the acceleration gradient increases as the square
root of the laser intensity, so it can go even higher with future
generations of more intense lasers. However, it will be a
few more years before any laboratory will complete the construction of
two intense lasers in a “head-on”, or 180 degrees, configuration. So Dr. Liang’s proposal will have to wait a little longer for its validation in the laboratory.
Interestingly, this new acceleration mechanism was inspired not by
the systematic search for new laser applications, but by the study of
cosmic phenomenon. The underlying physical mechanism was originally
discovered in 2003 by Dr. Liang and his collaborators while studying
the radiation of cosmic gamma-ray bursts, the most powerful explosions
in the universe.