Relativistic Self-Focusing Gives Mid-IR Driven Electrons a Boost
August 15 2017 - 12:44PM
Business Wire
Researchers use long-wavelength low energy
femtosecond laser pulses to study relativistic self-collapse in
plasma, leading to relativistic electron beams
Conventional particle accelerators can range from large
room-sized devices to facilities multiple kilometers across. One of
the ways that scientists have looked to reduce the size and expense
of future accelerators is by developing laser –driven plasma
acceleration. Such accelerators, however, are growing in size and
complexity in order to maintain relevance for one of their
applications—high energy physics. However, there are many
applications that can use a lower energy and higher repetition rate
accelerated beam. For the first time, scientists have observed the
production of relativistic electrons driven by low-energy,
ultrashort mid-infrared laser pulses. A research team at the
University of Maryland, USA, with support from the Technical
University of Vienna, Austria, will present their group’s findings
at Frontiers in Optics + Laser Science APS/DLS (FIO + LS), held
17-21 September 2017 in Washington, DC.
“We’re trying to develop laser-driven accelerators that are
extremely compact and have a high repetition rate,” said Howard
Milchberg, Fellow of The American Physical Society (APS) and The
Optical Society (OSA), and professor of Physics and Electrical
Engineering at the University of Maryland. “That means using as low
a laser pulse energy as possible to generate relativistic
electrons. Such sources could have use in rapid scan imaging for
medical, scientific and security applications.”
Recently, the development of optical parametric chirp pulse
amplification (OPCPA) systems in the mid-infrared has enabled the
use of long wavelength pulses on the femtosecond scale. Until this
development, long wavelength laser pulses have primarily been
available from CO2 lasers, but they have a complicated multi-pulse
structure with pulse durations extending, at the shortest
durations, beyond several picoseconds, hundreds of times
longer.
Common laser-driven acceleration experiments depend on short
laser pulse interaction with a gas target. Compared to prior
experiments, the long driver wavelength used in this project
resulted in easy access to what is called the “critical density”
regime. Because the critical density varies inversely as the square
of the laser wavelength, gas targets used for mid-IR laser pulses
can be up to 100 times less dense than those used in the visible
and near-IR, making them far less difficult to engineer.
“When a few-millijoule femtosecond mid-IR laser pulses is
focused by a curved mirror into a hydrogen gas jet – a stream of
hydrogen puffing out of a nozzle – a collimated pulse of
relativistic electrons beams out the other side of the jet,”
Milchberg said, describing the experiment. “However, this can’t
happen unless the laser achieves an extremely high intensity – much
higher than achievable by focusing with the curved mirror alone. It
does so by relativistic self-focusing in the ionized hydrogen gas
so that it collapses to a size much smaller than its focal
spot.”
The importance of being in the critical density regime,
according to Milchberg, is that it promotes relativistic
self-focusing even for low energy laser pulses. This boosted
high-intensity interaction generates plasma waves that accelerate
some of the electrons from the ionized hydrogen into a
forward-directed relativistic beam.
The team found that electron beams were present for powers such
that the characteristic self-focusing length in plasma was shorter
than the gas jet width, showing that electron acceleration cannot
occur without relativistic self-focusing.
Relativistic self-focusing is an extreme example of the
well-known process of self-focusing in nonlinear optics, but now
with the bonus of accelerated relativistic particles generated from
the nonlinear medium.
Even with only 20 millijoules of mid-IR laser energy, the laser
in these experiments can significantly exceed the threshold for
relativistic self-focusing, giving rise to relativistic
multi-filamentation. The team observed multiple relativistic
electron beamlets associated with these filaments.
These innovations are among the beginning steps for the
development and applications of high repetition rate laser driven
accelerators. “In particular,” Milchberg said, “long wavelength
femtosecond lasers are especially promising, as they can access the
relativistic nonlinear regime of free electrons surprisingly
easily.”
About the Presentation
The presentation entitled "Laser wakefield acceleration with
mid-IR laser pulses," by Daniel Woodbury, will take place from on
Monday, 18 September at the International Ballroom East, Washington
Hilton, Washington DC, USA.
Media Registration
A media room for credentialed media and analysts will be located
on-site. Media interested in attending the event should register on
the FiO + LS website media center: Media Center.
About FiO + LS
Frontiers in Optics is The Optical Society’s (OSA) Annual
Meeting and held together with Laser Science, a meeting sponsored
by the American Physical Society’s Division of Laser Science (DLS).
The two meetings unite the OSA and APS communities for five days of
quality, cutting-edge presentations, in-demand invited speakers and
a variety of special events spanning a broad range of topics in
optics and photonics—the science of light—across the disciplines of
physics, biology and chemistry. The exhibit floor will feature
leading optics companies, technology products and programs. More
information at: FrontiersinOptics.org.
About The Optical Society
Founded in 1916, The Optical Society (OSA) is the leading
professional organization for scientists, engineers, students and
business leaders who fuel discoveries, shape real-life applications
and accelerate achievements in the science of light. Through
world-renowned publications, meetings and membership initiatives,
OSA provides quality research, inspired interactions and dedicated
resources for its extensive global network of optics and photonics
experts. For more information, visit osa.org.
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The Optical SocietyRebecca B. Andersen, +1
202-416-1443randersen@osa.orgorThe Optical SocietyJoshua Miller, +1
202-416-1435jmiller@osa.org