goal of fusion energy research is to develop the technology and
firm scientific understanding to warrant construction of an electric
power plant. One route to commercial fusion energy is inertial fusion,
and a leading means of creating inertial fusion is with high-energy
lasers. In this technique, laser pulses either directly compress
a BB-sized capsule, or target, of fusion fuel (deuterium and tritium
ions), causing the fuel to ignite; or the pulses are converted first
into x rays inside a metal case, called a hohlraum, that contains
the fuel capsule, and the x rays compress the capsule, leading to
ignition of the fuel. The first method is called direct drive, and
the second is termed indirect drive. In both methods, the goal is
to have the fuel maintain compression long enough for its deuterium
and tritium nuclei to fuse and liberate more energy than is required
to drive the reaction. The ratio of energy in to energy out is called
gain. The energy produced will be used to boil water to drive the
electric turbines of a commercial power plant.
In practice, however, achieving
inertial fusion requires enormous energy delivered uniformly to
the capsule. For years, scientists have explored ways to achieve
inertial fusion that reduced the cost and are compatible with a
power plant. Two conceptsheavy-ion fusion and fast ignitionare
being explored by Lawrence Livermore physicists and collaborators
as attractive candidates for producing commercial electricity through
A team of Lawrence Livermore
physicists led by Max Tabak is exploring target designs for both
concepts as part of the Department of Energys Inertial Fusion
Energy Program. Tabak notes that the feasibility of an inertial
fusion energy power plant is strongly affected by the requirements
of the target for achieving ignition and high gain. We want
targets that will contribute to lower system costs, he says.
That means targets that are easy to fabricate, that minimize environmental
hazards produced during the fusion reaction, and that permit higher
leading target design for heavy-ion-beam fusion are so-called
distributed radiator targets in which a metal hohlraum contains
radiation converters to stop the ion beam and symmetrically
convert its energy into x rays that compress and ignite a plastic
capsule containing fusion fuel.
Heavy Ions Instead of Photons
The heavy-ion-fusion concept,
first discussed in 1975, replaces lasers with induction accelerators
that produce intense beams of heavy ions such as lead. Accelerators
that produce ion beams for high-energy physics research have demonstrated
20- to 40-percent operating efficiency, as opposed to the 5- to
10-percent efficiency of lasers. The difference is important because
driver efficiency determines how much of the electricity produced
must be fed back to power the driver. In addition, scientists have
ample experience using accelerators at about 10 hertz (repetitions
per second). Scientists believe that 10 hertz, the approximate firing
rate of a car engine at idle, is about the rate at which an accelerator
would need to fire at an inertial fusion power plant.
Heavy-ion beams are
potentially a better means of carrying energy and power to a target
than are the photons of a laser, Tabak says. Heavy ions are
preferable to light ions because their current would be tens of
kiloamperes lower. Lower beam currents make focusing the beam, done
with magnetic fields, easier. In contrast, beams of protons, the
lightest ions, would generate enormous currents that would be more
difficult to focus.
However, scientists are unsure
whether powerful ion beams could be focused easily onto targets.
The problem is that unlike photons, which are electrically neutral,
ions feel their electrical charge and, as a result,
tend to move away from each other. This self-repulsion could make
precise focusing difficult. Heavy-ion beams present major
scientific challenges, says Tabak, but many experts
believe they are surmountable.
The team is working on a
broad range of target designs to satisfy both accelerator builders,
who want designs that best couple the ion energy to the fusion fuel,
and target builders, who desire designs that are cost-effective
and easy to mass-produce. Were giving both groups a
lot of options, says Tabak.
hybrid design for heavy-ion beam fusion features a thick metal
shield that blocks the path of the heavy-ion beam. The energy
deposited behind the shield radiates through the hohlraum
to compress the capsule of fusion fuel. Iron radiation shims
are used to enhance symmetry of the fuels compression.
the Leading Candidates Are . . .
The leading target
candidates are so-called distributed radiator targets in which a
metal hohlraum contains carefully located radiation converters to
stop the ion beam and symmetrically convert its energy into x rays
that compress and ignite a plastic fuel-filled capsule. One variant
is a close-coupled target, designed by Lawrence Livermore physicist
Debra Callahan-Miller, that features a smaller hohlraum. This design
permits halving the heavy-ion beam energy required to obtain fusion.
However, the close-coupled target also requires a smaller beam focal
spot than conventional distributed radiator targets. Simulations
using Livermores LASNEX code show gains of 130 (energy liberated
by the fusion reaction divided by energy put into the target) at
3.3 megajoules of ion beam energy and 90 at 1.75 megajoules of ion
option is a hybrid design by Callahan-Miller that features a thick
metal shield to block the path of the heavy-ion beam. The energy
deposited behind the shield radiates through the hohlraum to the
capsule. Because this design alone does not produce adequate symmetry
of the fuel capsule, iron radiation shims are used to remove the
last 1 to 2 percent of asymmetry.
teams heavy-ion target designs are part of a wider effort
of the Lawrence Livermore Heavy Ion Fusion group that is working
to understand better the physics of intense ion beams and their
interactions with fusion targets. The group is a part of a national
inertial fusion effort that includes fusion researchers at Lawrence
Berkeley and Sandia national laboratories, Princeton Plasma Physics
Laboratory, General Atomics, Massachusetts Institute of Technology,
and other centers.
In this design for a fast-ignition target, a gold cone is attached
to the berylliumcopper spherical shell enclosing deuteriumtritium
fusion fuel. The cone penetrates almost to the center of the
capsule to allow a petawatt laser to directly deposit its energy
to the compressed fuel. (b) A computer simulation of the target
fuel being ignited by a petawatt laser. The fuel has a hollow
core and is located about 100 micrometers from the tip of the
Ignition Adds Second Driver
Fast ignition was conceived
by Tabak and other researchers in 1990. Since publication of the
first paper in 1994, research on the concept has spread from Livermore
to other national laboratories and to research centers in Europe,
Japan, and Russia.
Fast ignition can be used
with laser-driven direct or indirect drive, greatly relaxing the
efficiency requirement on the driver and providing an attractive
pathway to fusion energy. In fast ignition, the capsules deuteriumtritium
fuel is first compressed to high density by a standard laser pulse
lasting 1 to 10 nanoseconds. Then, an extremely short, 10- to 100-picosecond,
high-intensity pulse from a second laser, presumably a petawatt
laser, ignites the fuels dense plasma core with enormous currents
(1 billion amperes) of super-hot electrons. (The first petawatt
laser was developed by Lawrence Livermore researchers in the mid-1990s
to test the fast ignition concept. See S&TR, March
2000, The Amazing
of the Petawatt.) A hybrid fast-ignition concept has been explored
in which target compression is accomplished with an ion beam and
ignition is achieved with a petawatt laser.
ignition offers the prospect of significantly reduced driver energy
and the compression symmetry needed to achieve ignition. For example,
various models show that the required energy of the compression
beams could be reduced from 3 to 5 megajoules to less than 1 megajoule.
Even with the added cost of the ignition laser, such relaxed driver
requirements might provide capital cost savings of 30 to 40 percent
for a fusion power plant. Tabak says that fast ignition should also
allow lower target-fabrication-finish requirements.
Livermore team has also explored several different target geometries
for fast ignition. Tabak notes that fast ignition may not need a
hohlraum. The overriding design requirement is to ensure that the
energy from the petawatt laser couples efficiently to the ignition
region of the compressed fuel.
novel design features a gold cone attached to the spherical shell
enclosing the deuteriumtritium fuel. The cone penetrates almost
to the center of the capsule. In this way, the petawatt pulse has
direct access to the ignition region. The cone provides a
clear path for the petawatt laser so that its energy can be deposited
within about 100 micrometers or less of the high-density core,
explains Tabak. The design team is exploring variations in cone
designs to reduce the distance between the capsules ignition
region and the apex of the cone.
simulations, combined with recent experiments in Japan and on the
Omega laser at the University of Rochester, continue to show considerable
promise for the concept. The experiments on Omega use prototype
capsules designed by Lawrence Livermore physicist Steve Hatchett
and manufactured by General Atomics. One series of experiments is
showing scientists how the presence of a cone on the target affects
the compression of fusion fuel.
target designs continue to evolve as the design team gains insight
from experiments, simulations, and advances in the theoretical underpinnings
of fast ignition and heavy-ion beams. The team is motivated by the
steady progress its work is making toward eventual deployment of
a fusion power plant. Whatever inertial fusion method is ultimately
selected for commercial development, it will be using minuscule
targets that are precisely designed.
fast ignition, heavy-ion fusion, hohlraum, inertial fusion energy,
laser fusion, Omega laser, petawatt laser.
information contact Max Tabak (925) 423-4791 (firstname.lastname@example.org).