The NCXX-II’s induction accelerator and neutralized drift compression system shape the ion pulse in such a way as to deposit about three million electron volts of kinetic energy per ion – the bulk of the energy—within one nanosecond duration, onto a spot just one millimeter in diameter on a thin aluminum foil. |
Berkeley
Lab, a partner in the Heavy Ion Fusion Sciences Virtual National
Laboratory (HIFS VNL) with Lawrence Livermore and the Princeton Plasma
Physics Laboratory, has been a leader in developing a special kind of
accelerator for experiments aimed at fusion power, called an induction
accelerator. The induction principle is like a string of transformers
with two windings, where the accelerator beam itself is the second
winding. Induction accelerators can handle ions with suitable kinetic
energy at higher currents (many more charged particles in the beam),
much more efficiently than RF accelerators.
“Choosing
the best kind of accelerator and the best kind of target are just the
start of the fusion-power challenge,” says Seidl. “To put the right
amount of energy on the target in the right pattern, scores of beams are
needed—and it must be possible to focus them tightly onto a target,
only a few millimeters wide, at a distance of several meters. New
targets have to be injected into the chamber five to ten times each
second, and the chamber has to be designed so the energy from ignition
is recovered. Meanwhile the final beam-focusing elements have to be
protected from the explosion debris, the energetic particles, and the
x-rays.”
Some
of these challenges would be easier to meet if the target didn’t have
to be hit from both sides at once. Researchers are encouraged by
indications that target burning, hot enough to spark and sustain
ignition, can be initiated with fewer beams illuminating the target from
only one side.
This side of fusion: warm dense matter
While
investigating approaches to heavy-ion fusion, Berkeley Lab and its
partners in the HIFS VNL are also tackling other scientific questions
related to heating matter to high temperatures with ion beams. The
current research program is designed to produce a state of matter that’s
on the way to fusion but not as hot—a state perhaps facetiously called
warm dense matter, which is “warm” (10,000 degrees Kelvin or so) only by
comparison to the millions of degrees typical of fusion reactions.
Not
a heavy-ion experiment, the Neutralized Drift Compression Experiment II
(NDCX-II) instead uses an induction linear accelerator to accelerate
and compress bunches of very light lithium ions to moderate energies.
NDCX-II confronts a problem common to all accelerators, the space-charge
problem, in which particles of the same charge—positive, in the case of
atomic ions – repel each other; the bunches try to blow themselves up.
For a given number of ions per bunch, this sets a lower limit on the
pulse length.
The NDCX-II accelerator is specifically designed to study warm dense matter. By using an induction accelerator and a neutralized drift compression system, the ion pulse can be shaped to deliver most of its energy to the target surface. |
After
acceleration in NDCX-II, the ion bunch enters a drift chamber where a
plasma (consisting of ions plus numerous free electrons) is injected to
neutralize the net charge of the pulse. A magnetic field focuses the
bunch radially; meanwhile the induction accelerator has manipulated the
ion velocity so that the rear end of the pulse catches up with the front
end, compressing it longitudinally. In this way the pulse is shaped to
deposit about three million electron volts of kinetic energy per ion,
within one nanosecond duration, onto a spot just one millimeter in
diameter on a thin foil target.
“A
high-energy accelerator, such as RHIC or LHC, would send an particle
beam through the target like a bullet through paper, with only a tiny
fraction of its energy lost,” says Joe Kwan of AFRD, the principal
investigator of NDCX-II. “Our ion beam is optimized to deposit most of
its energy in the thin target itself, heating it instantly to warm dense
matter conditions.”
Construction
of the NDCX-II accelerator began in 2009 and its first phase is
expected to be completed early in 2012, when experiments will begin. The
field of warm dense matter is an important research discipline in
itself, necessary to understand the state of matter inside giant planets
like Jupiter, and a variety of other astrophysical phenomena. Although
its targets are thin foils, not heavy-hydrogen capsules, NDCX-II will
make advances in acceleration, compression, and focusing of an intense
ion beam, which will inform driver concepts for heavy-ion fusion energy
production.
Beams,
targets, the architecture of reaction chambers, and the practical means
of capturing the available energy from inertial-fusion reactions
present a set of complex, interrelated problems that must be undertaken a
step at a time.
The
goal is well worth it, says Seidl. “If we can find a way to make
electricity with fusion, it will change the prospects for the health and
welfare of nations – a vast fuel supply that leaves little waste and
puts no carbon in the atmosphere. Success would signal positive changes
for the world’s future energy and environmental needs.”
For Part I of this feature, The Energy that Drives the Stars Comes Closer to Earth
More about warm dense matter and the NDCX-II experiment, link 1
More about warm dense matter and the NDCX-II experiment, link2
Updates on the National Academies report, “Prospects for Inertial Confinement Fusion Energy Systems
