William H. Goldstein
Associate Director of Physics and Advanced Technologies
Let There Be Light Sources
LAWRENCE Livermore continues to be a leader and innovator in the
development and application of light sources, building the most powerful
lasers in the world and demonstrating the first x-ray laser. As light
sources become brighter, faster, and more energetic, their role in
the Laboratory’s future is as important as ever. In fact, the
Laboratory’s long-range science and technology plan identifies
new light sources as crucial to progress in many of our core science
and technology areas: stockpile stewardship; high-energy-density
physics; nuclear and radiative science; and chemical, biological,
and materials research.
Two articles in this issue look
at applications of the new generation of intense light sources. The first,
An Extraordinarily Bright Idea, discusses
the Linac Coherent Light Source (LCLS), an x-ray laser being built
at the Stanford Linear Accelerator Center by a consortium of institutions
that includes Livermore. The second article, Using Proton Beams to
Create and Probe Plasmas, describes exciting developments using the Laboratory’s
Janus-pumped ultrashort-pulse (JanUSP) laser.
The LCLS is what is called
a single-pass, free-electron laser. A very short bunch of high-energy
electrons is injected into an undulating
magnetic field where they emit bremsstrahlung—literally, braking
radiation—as they are accelerated. Under carefully designed
conditions, the emitted radiation interacts with the electron bunch
and builds in intensity. The resulting x-ray beam is 10 billion times
brighter than currently available light sources. Its copious photons
are coherent, with energies more than 10 times that needed to ionize
any atom. Interactions between this beam and atoms are different
from those produced by even the most intense optical lasers. X-ray
pulses are tunable from 0.8 to 8 kiloelectronvolts, may be less
than 100 femtoseconds long, and may have wavelengths as small as
0.1 nanometer. These photon intensities, pulse lengths, and wavelengths
will allow scientists to make measurements on atomic scales.
One particularly exciting use of the LCLS will be to examine the
structure and function of such large biomolecules as proteins. With
current x-ray light sources, structure can be determined only for
those molecules that can be formed into a crystal pattern, a process
that invariably destroys the protein’s functionality. With
its ultrabright, ultrashort pulses, the LCLS will be used to image
single molecules, without the need to crystallize or immobilize them.
Many challenges remain to meet this goal, but the payoffs for understanding
the mechanisms of life are enormous.
We will also be able to use LCLS’s x-ray pulses to heat material
to conditions replicating those inside weapons, stars, and planets.
By splitting the x-ray beam, we can use part of it to heat a material
and the other part to take measurements. Once the beam is split,
one or more of its parts can be delayed with respect to the others,
allowing us to look at what happens as a function of time. Using
this technique, we can perform dynamic studies of materials fast
enough to see molecular motion taking place during chemical reactions.
We will also be able to measure the interactions of complex systems,
such as protein folding and crystalline phase transitions.
JanUSP produces laser pulses as short as those produced by the LCLS
but in visible light. When JanUSP’s laser energy is focused
onto a thin metal target, a plasma forms. Electrons in this plasma
are accelerated and escape, which sets up an intense electric field
that pulls a short-pulse, high-energy proton beam out of the target.
This effect, discovered at Livermore, has created an entirely new
field of science at laser laboratories around the world.
The JanUSP proton beam can
be focused to heat material just as the x rays from the LCLS will
be. The proton beam also can be used for
radiography, providing time-frozen pictures with spatial resolution
of 1 micrometer. Because the protons are charged, they respond
to electric and magnetic fields, so they can be used to measure these
fields on very small time and space scales.
This new, exciting proton beam may even find its way into the National
Ignition Facility, providing a novel way to ignite inertial confinement
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December 3, 2003