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Scientists at Los Alamos National Laboratory have observed, for
the first time, how a laser penetrates dense, electron-rich plasma to generate
ions. The process has applications for developing next generation particle
accelerators and new cancer treatments.
The results, published online in Nature Physics, also
confirm predictions made more than 60 years ago about the fundamental physics
of laser-plasma interaction. Plasmas dense with electrons normally reflect
laser light like a mirror. But a strong laser can drive those electrons to near
the speed of light, making the plasma transparent and accelerating the plasma
ions.
“That idea has been met with some skepticism in the field,” says
Rahul Shah of LANL’s plasma physics group. “We think that we’ve settled that
controversy.”
The team, which also included researchers from the Max Planck
Institute for Quantum Optics in Garching, Germany and Queens University in
Belfast, U.K., used the 200 trillion-watt short-pulse TRIDENT laser at Los
Alamos National Laboratory to observe the transparency phenomenon at 50
femtosecond resolution. Until now, those dynamics have been witnessed only in
computer simulations.
The team found close agreement between the model and their
experiments, which confirms what Los Alamos National Laboratory scientists have
long suspected—that directing a short-pulse laser at a very thin carbon foil
target will make the foil transparent to the laser.
“In a sense it also validates the simulation code that
researchers have been using for some time,” says Sasi Palaniyappan of LANL’s
plasma physics group. “At the same time it also tells us that we’re doing an
experiment that’s as close as possible to simulation.”
The results will help advance work to control the shape and
timing of laser pulses, precision that is necessary for developing
next-generation, laser-driven particle accelerators, he said. The researchers
have recently been awarded internal laboratory funding from the office of
Laboratory Directed Research and Development (LDRD) to pursue these
applications.
They now plan to add a second foil target, which could benefit
from further focusing and faster turn-on of the laser light transmitted through
the first foil. One application of the resulting ultra-short ion bunches is to
rapidly heat material and study the ensuing dynamics.
Particles accelerated by conventional accelerators aren’t fast
enough for such physics experiments. Also, energetic ions are applicable to
cancer therapy. A more compact, laser-driven ion source would make treatment
less expensive and more accessible to patients.
Source: Los Alamos National Laboratory
