# Laser Wakefield Acceleration:

# Channeling the Best Beams Ever

# *Simulations Performed at NERSC Grace Cover of Nature Magazine*

BERKELEY, CA \-- Researchers at the U.S. Department of Energy\'s
Lawrence Berkeley National Laboratory have taken a giant step toward
realizing the promise of laser wakefield acceleration, by guiding and
controlling extremely intense laser beams over greater distances than
ever before to produce high-quality, energetic electron beams.

![](media/image1.jpeg){width="4.5in" height="2.3784722222222223in"}The
experimental results were then analyzed by running the VORPAL plasma
simulation code on supercomputers at DOE\'s National Energy Research
Scientific Computing Center (NERSC) at Berkeley Lab. The results were
published in the Sept. 30 issue of Nature magazine, which chose an image
from the NERSC simulations for the magazine's cover.

For a quarter of a century physicists have been trying to push charged
particles to high energies with devices called laser wakefield
accelerators. In theory, particles accelerated by the electric fields of
laser-driven waves of plasma could reach, in just a few score meters,
the high energies attained by miles-long machines using conventional
radio-frequency acceleration. Stanford\'s linear accelerator, for
example, is two miles long and can accelerate electrons to 50 GeV (50
billion electron volts). Laser wakefield technology offers the
possibility of a compact, high-energy accelerator for probing the
subatomic world, for studying new materials and new technologies, and
for medical applications.\
\
In plasmas, researchers have generated electric fields a thousand to ten
thousand times greater than in conventional accelerators \-- but these
large fields exist only over the short distance that a laser pulse
remains intense; for tightly focused beams, that distance is typically
only a few hundred micrometers (millionths of a meter). The resulting
beams are of relatively poor quality, with particle energies so
widespread that fewer than one percent have enough punch for scientific
applications.\
\
The Berkeley Lab researchers achieve high-quality beams by first shaping
a channel through hydrogen gas with powerful, precisely timed laser
pulses, then accelerating bunches of electrons through the plasma inside
the channel. Because of the controlled accelerator length and the
characteristics of the channel, there are several billion electrons in
each bunch within a few percent of the same high energy, more than 80
MeV (80 million electron volts).\
\
The work was done by the L\'OASIS group (L\'OASIS stands for Laser
Optics and Accelerator Systems Integrated Studies), led by Wim Leemans
of the Center for Beam Physics in Berkeley Lab\'s Accelerator and Fusion
Research Division. To analyze their successful experiment, the group
collaborated with David Bruhwiler, John R. Cary and Chet Nieter of the
Tech-X Corporation of Boulder, Colorado, using the VORPAL plasma
simulation code to model their results on supercomputers at NERSC. This
modeling allowed the scientists to see the details of the evolution of
the experiment.

![](media/image2.jpeg){width="4.927777777777778in"
height="3.466666666666667in"}

"With VORPAL, one can see the laser pulse breakup and the injection of
particles into the laser-plasma accelerator," said Cary, CEO of Tech-X
Corp. and a professor of physics at the University of Colorado. "This
allows one to understand how the injection and acceleration occur in
detail so that the experiment's designers can figure out how to optimize
the process."

VORPAL, jointly developed by Tech-X Corporation and the University of
Colorado, is an object-oriented C++ framework for 1D, 2D and 3D parallel
electromagnetic simulation of charged fluids and particles on a
structured grid. VORPAL is a relativistic, arbitrary dimensional, hybrid
plasma and beam simulation code that contains utilities for data
analysis and scripts for data visualization.

\"Laser wakefield acceleration works on the principle that by sending a
laser pulse through a gas to create a plasma \-- separating negatively
charged electrons from positively charged ions in the gas \-- some of
the free electrons will be carried along in the wake of the plasma wave
created by the laser,\" Leemans explains. \"Imagine that the plasma is
the ocean, and the laser pulse is a ship moving through it. The
electrons are surfers riding the wave created by the ship\'s wake.\"\
\
Unfortunately, simply punching a laser pulse through a plume of gas
makes for a very short trip. \"The acceleration distance is limited to
what\'s called the Rayleigh length \-- the length over which the laser
remains focused \-- which is determined by the wavelength of light and
the size of the spot to which the beam is focused,\" says Leemans. \"For
optimum acceleration, you have to keep the wake going for many Rayleigh
lengths, until just before the electrons start to get ahead of the wave
and lose energy\" \-- a distance otherwise known as the dephasing
length.

To achieve this extended acceleration length, Leemans, working with his
graduate student Cameron Geddes and other colleagues, implemented what
he refers to as \"the plasma analogue of an optical fiber.\" The basic
principle of this technique was developed a decade ago, using less
powerful lasers, by Howard Milchberg from the University of Maryland,
then improved upon by Leemans and his former student Pavel Volfbeyn in
1998. But it took nearly another six years before the L\'OASIS lasers
and instrumentation were sufficiently advanced to try guiding extremely
intense laser pulses with the same method

One of the beams from the L\'OASIS laser is first sent as an igniter
pulse through the gas, to form what Leemans describes as a wire of
plasma. A second, heater pulse enters from the side and heats the plasma
wire; as the heated channel expands, it becomes denser at the edges and
less dense at the center. Five hundred picoseconds later, time enough
for the ions to move out of the way, an intense driver pulse, with peak
power approaching 10 terawatts (10 trillion watts), plows through the
plasma capillary.

By tailoring the plasma channel conditions and laser parameters, the
researchers are first able to achieve clean guiding of laser beams of
unprecedented high intensity, while suppressing electron capture. This
paves the way for using laser-powered plasma channels as
ultra-high-gradient accelerating structures.\
\
Next, by using even higher peak powers, plasma waves are excited that
are capable of picking up background plasma electrons, rapidly
accelerating them in the wake\'s electric field, then finally subsiding
just as the surfing electrons reach the dephasing length, when they are
on the verge of outrunning the wake.\
\
How is it that the electrons in the bunch are accelerated to nearly the
same energy? Following the experiments, Geddes and the other Berkeley
Lab researchers, with the help of their Tech-X collaborators, modeled
the evolution of the high-quality electron bunches on supercomputers at
NERSC. They found that the formation of large bunches of electrons with
nearly the same high energy is due to several factors. Most critical is
matching the long acceleration path to the dephasing length. Other
factors include plasma density and laser power, and how the laser pulse
evolved during its propagation in the plasma channel.\
\
\"The plasma-channel technique is an essential step toward developing
compact laser wakefield accelerators with multiple stages, which can
produce focused, ultrafast, high-energy bunches of electrons to compete
with state-of-the-art machines using conventional radio-frequency
acceleration,\" says Leemans. \"But the new technique has already
suggested novel sources of radiation, such as the efficient generation
of femtosecond x-rays\" \-- x-ray pulses measured in quadrillionths of a
second \-- \"and coherent terahertz and infrared radiation.\"\
\
\"High-quality electron beams from a laser wakefield accelerator using
plasma-channel guiding,\" by Cameron Geddes, Csaba Toth, Jeroen van
Tilborg, Eric Esarey, Carl Schroeder, David Bruhwiler, Chet Nieter, John
Cary, and Wim Leemans, appears in the 30 September 2004 issue of
Nature.\
\
Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research
and is managed by the University of California.\
\
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