PROTONS, the positively charged,
subatomic particles discovered by Lord Rutherford nearly 100 years
ago, are still surprising scientists. Lawrence Livermore researchers
are discovering that proton beams created by powerful, ultrashort
pulses of laser light can be used to create and even diagnose plasmas,
the superhot state of matter that exists in the cores of stars
and in detonating nuclear weapons. The proton-beam experiments
promise new techniques for maintaining the nation’s nuclear
arsenal and for better understanding how stars function.
proton beams used in the Laboratory’s experiments are
produced by pulses of laser light lasting only about 100 femtoseconds
(a femtosecond is 10–15 seconds, or one-quadrillionth
of a second) and having a brightness, or irradiance, up to 5 x
1020 watts per square centimeter. When such fleeting pulses are
onto thin foil targets, as many as 100 billion protons are emitted,
with energies up to 25 megaelectronvolts. The
protons come from a spot on the foil about 200 micrometers in diameter,
and the beam’s
duration is a few times longer than the laser pulse. The highest-energy
protons diverge 1 to 2 degrees from the perpendicular, while the
lowest-energy protons form a cone about 20 degrees from perpendicular.
Funded by the Laboratory Directed Research and Development Program,
the Livermore experiments are led by physicists Pravesh Patel and
Andrew Mackinnon. Patel, who works in the Laboratory’s Physics
and AdvancedTechnologies Directorate, is researching new ways to
create and better understand plasmas. Mackinnon, from Livermore’s
National Ignition Facility (NIF) Programs Directorate, is developing
new ways to measure the plasmas created in NIF experiments. Both
physicists are collaborating with colleagues from Queen’s
University in Belfast, Northern Ireland; Heinrich-Heine-Universität
in Düsseldorf, Germany; the LULI laser facility at l’Ecole
Polytechnique in France; Rutherford Appleton Laboratory in the
United Kingdom; and the University of California (UC) at Davis.
“Plasmas are often referred to as the fourth state of matter,” says
Patel. “They are abundant in the universe but relatively
uncommon on Earth. Plasmas are extremely hot, highly transient
objects and thus are difficult to control or to accurately probe.”
Diagnostic setup for imaging proton beams produced by a 10-joule
pulse of laser light that lasts only 100 femtoseconds. The
pulse irradiates a thin aluminum foil. The generated protons
are then “pulled” from the back side of the foil
by electrons traveling close to the speed of light. Protons
with the highest energy diverge the least from an angle perpendicular
to the back foil surface. (b) Images from a multilayer film
pack taken over a broad energy range show that the number of
protons decreases with increasing energy.
team wants to develop new methods for creating plasmas in the laboratory,
so they can study them at temperatures ranging from
a few electronvolts to hundreds of electronvolts and at the high
energy densities (more than 100,000 joules per gram) that exist
in stars. The current generation of high-power lasers makes such
studies possible because they can compress and heat matter to these
scientists want to measure plasmas in a uniform-density, single-temperature
state. As a material is heated to several electronvolts,
the pressure in it increases to more than a million times atmospheric
pressure. This increased pressure causes the plasma to expand hydrodynamically,
as in a violent explosion. Under these conditions, measuring plasma
properties is extremely difficult.
way to overcome these problems is to use what scientists call isochoric
heating—heating at constant volume. With isochoric
heating, plasmas don’t expand during the time they are heated,
and their energy can be relatively uniform. Established methods
of isochoric heating, such as laser-driven shock heating, x-ray
heating, and ion heating, are relatively fast (10–6 to 10–9
seconds), but these timescales are still longer than those during
which significant hydrodynamic expansion can occur (10–11
to 10–12 seconds). Another method, direct heating with intense
subpicosecond (10–12 seconds) laser pulses, creates a highly
nonuniform heating pattern. The laser energy is absorbed within
less than 100 nanometers of material, and the heat localization
creates a large temperature and density gradient.
approach to isochoric heating, discovered at Livermore, uses laser-produced
proton beams to generate fleeting, dense plasma
states at constant volume and density. The heating period is shorter
than the time needed for significant hydrodynamic expansion to
occur, so the material is heated to a plasma state in a few picoseconds.
In effect, says Patel, the proton beam dumps a huge amount of energy
almost instantaneously and suddenly increases a target’s
temperature to millions of degrees.
Particle-in-cell code showing how a thin foil target expands
after laser irradiation. (b) Hybrid code calculation of the
transport of high-energy electrons through a solid density
foil. The calculation shows the complex electron trajectories
and large electric fields at the rear surface of the foil that
create the protons.
Layout for the proton-beam experiments using the Janus ultrashort-pulse
(JanUSP) laser. When JanUSP irradiates an aluminum foil, it
creates a proton beam that then heats a second foil. Both foils
are 10 micrometers thick, and the second foil is placed 250
micrometers behind the first. (b) The plasma created in the
second foil is almost 200 micrometers
in diameter. (c) Layout for the experiments with a curved target. (d) When
the target’s rear surface is curved, the laser produces a focused
proton beam that is 50 micrometers in diameter and almost 10 times more
powerful than the beam produced with a flat target.
JanUSP Makes It Possible
their experiments, the researchers rely on Livermore’s
Janus ultrashort-pulse (JanUSP) laser, one of the brightest lasers
in the world. (See S&TR, May 2000, JanUSP Opens New World of Physics Research.) JanUSP
produces a beam with an average intensity of 5 x 1020 watts
per square centimeter that lasts about 100 femtoseconds. The
laser operates at a wavelength of 800 nanometers and delivers 10
joules of energy.
In one set of experiments, the laser pulse produced a proton beam
from a 10-micrometer-thick sheet of aluminum foil. The proton beam
then heated a second 10-micrometer-thick aluminum foil that was
placed 250 micrometers directly behind the first. Within a few
picoseconds, the heating created a 4-electronvolt plasma almost
200 micrometers in diameter—too short for much hydrodynamic
expansion to occur.
The discovery that intense, highly directional proton beams could
be generated from an ultrashort laser pulse heating a solid target
was made by Livermore researchers several years ago while conducting
experiments with the Laboratory’s Petawatt laser. The Petawatt
laser operated on 1 of the 10 beam lines of Livermore’s Nova
laser, which was decommissioned in 1999. (See S&TR, March 2000,
The Amazing Power of the Petawatt.) Experiments by Livermore physicist Richard Snavely
and others to characterize the proton beams revealed a unique combination
of properties, including peak proton energies of 55 megaelectronvolts
and conversion efficiencies (of laser energy to proton energy)
up to 7 percent.
The scientists also discovered that the protons in the beam originated
in hydrocarbons found in surface contamination on the foil’s
back surface. Livermore theoretical physicists, led by Steve Hatchett
and Scott Wilks, used computer simulations to study this behavior.
They found that the pulse from an ultrashort laser accelerates
electrons from the interaction region at the front of the target
with relativistic energies; that is, the electrons travel close
to the speed of light. The electrons emerging at the foil’s
rear surface induce a large electrostatic charge field, which in
turn accelerates protons from hydrocarbon contaminants on the rear
surface. The protons accelerate from 0 to 20 megaelectronvolts
at 20 percent the speed of light and travel in a well-defined,
highly directional beam perpendicular to the target. X rays, in
contrast, are emitted at random angles.
Simulations by Wilks showed that by curving the laser target’s
rear surface, the proton beam could be focused to a far higher
state of energy density. To test this design, the team asked General
Atomics in San Diego, California, to manufacture aluminum hemispheres
that are 10 micrometers thick, 320 micrometers in diameter,
and almost perfectly smooth on the inside to ensure a high-quality
proton beam. With the shaped targets, the proton beam was almost
10 times more powerful than the beam produced from flat targets.
The proton beam was focused on a 50-micrometer-diameter area of
a foil placed behind the target, which was then heated to 23 electronvolts.
the first time, the experiments showed that we can focus proton
beams,” says Patel. He notes that when the techniques of
proton heating and focusing can be applied with more powerful lasers,
scientists may be able to isochorically heat plasmas to much higher
temperatures and pressures. This advance would provide many opportunities
in high-energy-density physics and fusion energy research.
The first proton probing experiments
of laser-driven implosions were conducted using the 100-terawatt
Vulcan laser system at Rutherford Appleton Laboratory in
the United Kingdom. The targets were plastic microballoons,
500 micrometers in diameter. Six beams from the Vulcan
laser were focused onto a microballoon while an ultrashort-pulse
laser beam was used to make either the diagnostic proton
or x-ray beam.
Protons for Radiography
The team is also using proton beams for radiographic applications
to diagnose plasma conditions generated by high-power lasers at
picosecond timescales. The first proton probing experiments of
a laser-driven implosion were conducted by Mackinnon in 2002 using
the 100-terawatt Vulcan laser at Rutherford Appleton Laboratory.
This experiment was conducted in collaboration with scientists
at Queen’s University and UC Davis. “We wanted to investigate
the suitability of proton radiographs to diagnose an implosion
capsule in inertial confinement fusion experiments,” says
Plastic microballoons, 500 micrometers in diameter—or about
one-fourth the size of the targets planned for NIF—were used
as targets. Each of the Vulcan laser’s six long-pulse beams
was fired for 1 nanosecond at a wavelength of 1 micrometer
and an irradiance of 10 terawatts per centimeter. Each beam’s
energy was 100 to 150 joules, so the maximum energy on the target
was up to 900 joules. The six laser beams illuminating the
target arrived from six orthogonal directions, a setup designed
to provide the best symmetry for this number of beams.
addition, an ultrashort laser beam was used to make either a diagnostic
proton beam of about 7 megaelectronvolts or a diagnostic
x-ray beam of about 4.5 kiloelectronvolts. The proton beam was
obtained by focusing a 100-joule laser pulse with an irradiance
of about 5 x 1019 watts per square centimeter for 1 picosecond
onto a tungsten foil 25 micrometers thick. To image the implosion,
the team used a multilayer pack of dosimetry film in which each
piece of film was filtered by the preceding piece. In this way,
the film pack gave a series of images from each shot with an energy
ranging from 3 to 15 megaelectronvolts.
radiographs showing the evolution of a laser-driven implosion
of a 500-micrometer-diameter balloon: (a) prior to implosion
and at (b) 2 nanoseconds and (c) 3 nanoseconds after the laser
pulse. When the laser beams are slightly mistimed, the radiographs
show asymmetries in the target: (d) Only four beams (shown
by red arrows) strike the target, all at 4 nanoseconds
after the laser pulse. (e) Beam arrival is staggered from 1
to 5 nanoseconds after the laser pulse.
team took radiographs of microballoons both before and during implosion.
One image, of a 500-micrometer-diameter microballoon
with a 7-micrometer wall thickness, showed good contrast at a resolution
of 5 to 10 micrometers. A series of radiographs (shown above),
which were taken by varying the delay between the implosion beams
and the beam used to produce the proton or x-ray beam, revealed
how the implosion process evolved.
one experiment, the beams were set to converge on the target asymmetrically—that is, the six beams arrived at the target
at slightly different times. The laser beams on the left-hand side
arrived 1 to 2 nanoseconds before the laser beams on the right-hand
side. This asymmetry led to significant distortions. For example,
the shell traveled much farther inward on the left-hand side than
it did on the right.
more symmetric drive conditions, the target remained nearly spherical
during the implosion. However, even when the beams arrived
at the same time, the proton radiographs revealed some plasma asymmetries.
For example, in one experiment, the upper part of the shell traveled
almost twice the distance traveled by the lower part of the shell.
proton radiographs were the first taken of a laser-driven implosion
with picosecond resolution. The team found that the temporal
and spatial resolution remained high throughout all stages of the
“The images show the promise of proton radiography for diagnosing
early time distortions in the implosion process with high resolution
and very good image contrast,” says Mackinnon. “The
x-radiographs also had good resolution, but the image contrast
was high only when the density was high.”
to Mackinnon, proton beams with energies from 50 to 100 megaelectronvolts,
produced by an ultrashort-pulse laser, could one day be used to
probe the cores of NIF targets as they are compressed by laser
light. Lower-energy protons also could be useful, for example,
to diagnose electric and magnetic fields inside hohlraums, the
metal cases that enclose many NIF targets. More experimental and
theoretical work is under way to fully investigate this promising
In one particle-deflection technique for diagnosing the transient
electric and magnetic fields, a proton beam passes through
two identical gratings. The gratings are separated by a small
distance, and their rulings are rotated at slight angles to
each other. (b) The proton beam is imprinted with a grating
pattern called proton moiré. A proton beam passing through
the plasma causes a shift in the moiré pattern, which
can be used to infer the strength of the electric and magnetic
fields. (c) Proton moiré is similar to (d) the more
common optical moiré.
notes that another kind of proton radiography is being studied
by researchers at Lawrence Livermore and Los Alamos national
laboratories. But the protons created in those studies are much
more energetic—about 800 megaelectronvolts. (See S&TR, November
2000, Protons Reveal the Inside Story.)
That research centers on beams of extremely high-energy protons
focused with magnetic lenses
is designed to image deep inside larger exploding objects.
physicists Mike Key and Richard Town are also studying whether
proton beams, instead of electron beams, can be used to
drive fast ignition on NIF. (See S&TR, March 2000, The Amazing Power of the Petawatt.)
In fast ignition, at the moment of maximum compression, a laser
pulse plows through the plasma to make a path for another very
short, high-intensity pulse (presumably, of electrons) to ignite
the compressed fuel. In theory, fast ignition reduces both the
laser energy and the precision requirements for achieving ignition.
Another particle-deflection technique uses a single grid to
subdivide protons into hundreds of small proton beamlets. (b) Proton
radiograph of plasma created by a 300-picosecond laser pulse
on a 125-micrometer-thick wire shows the plasma magnified and
distorted because of the plasma’s electric and magnetic
fields. The pattern is similar to (c) a hybrid code simulation
of protons propagating through a spherical region that contains
a radial electric field, which reproduces the essential features
of the experiment.
Field Strength and Geometry
In collaboration with Marco Borghesi from Queen’s University
and Oswald Willi and G. Pretzler from Heinrich-Heine-Universität,
the Livermore team is investigating another aspect of proton radiography:
diagnosing the transient electric and magnetic fields directly
through particle-deflection measurements. Unlike x rays, protons
are electrically charged, so they interact with electric and magnetic
fields in plasmas. Proton probing would provide a new method to
visualize and measure fields in laser plasma experiments, which
are not well understood.
For these experiments, the researchers are using the JanUSP, Vulcan,
and LULI lasers. Developing such proton radiography diagnostics
supports the Laboratory’s stockpile stewardship mission by
helping scientists better understand hot, dense plasmas.
In one technique, the proton beam passes through two identical
gratings. The gratings are separated by a small distance, and their
rulings are rotated at slight angles to each other. In effect,
the proton beam is imprinted with a pattern of the gratings, called
proton moiré. When the beam passes through the plasma, the
electric and magnetic fields can cause shifts in the moiré pattern.
The change in pattern can then be used to infer the strength of
the electric and magnetic fields.
A related technique uses a single, two-dimensional grid to subdivide
protons into hundreds of small proton beamlets. A hybrid code that
simulated proton propagation through a plasma containing a radial
electric field essentially reproduced the main features of the
Livermore team expects protons to complement x rays as a diagnostics
tool, not replace them. The team is confident that its pioneering
use of protons to create and diagnose plasmas will advance a host
of research projects, both at Livermore and at plasma research
Key Words: Janus ultrashort-pulse (JanUSP) laser, National
Ignition Facility (NIF), Petawatt laser, plasma, protons.
For further information contact Pravesh Patel (925) 423-7450
or Andy Mackinnon (925) 424-2711 (firstname.lastname@example.org).
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