THIS WEEK IN

CREDIT: JEFF CANDY/GENERAL ATOMICS CORP.
turns out that plasmas are not quiescent or
classical at all, but are always quivering
with small-scale fluctuations in all of their
parameters—density, temperature, and
even the local microscopic magnetic and
electric fields. It was clear by the late 1970s
that these turbulent fluctuations in a tokamak
were driven by the high-pressure
plasma in the center expanding into the lowpressure
outer regions. The fluctuating, turbulent,
electric and magnetic fields move
the particles across the mean field in convective
eddylike motions—much more
quickly than classical diffusion. The transport
of heat and particles across the field is
effectively a random walk with an enhanced
step length and short correlation time. The
density fluctuations in a computer simulation
of a tokamak can be seen in the figure.
The eddies are characteristically elongated
along the field lines and have small-scale
lengths perpendicular to the field.
The complexity of this turbulence is
immense. For example, the shape of the
magnetic bottle, the profiles of plasma density
and temperature within the bottle, and
the microscopic dynamics of both ions and
electrons all play critical roles (2–4).
Furthermore, the turbulence can exist on
many different spatial scales simultaneously.
Indeed, it is also possible that turbulence
excited in one region of the plasma
can produce transport in another. A concerted
effort to measure, understand, and
model the turbulence has brought significant
results in recent years. One of the most
important is the development of sophisticated
computational models (5–8)—the
figure, for example, was produced with
such a model. These models make quantitatively
accurate predictions in many situations.
However, some of the most exciting
experimental results, in which turbulence
has been observed to be substantially
reduced, cannot yet be accurately predicted
by the codes.
The first such observation occurred in
1982 at the ASDEX experiment at the Max-
Planck Institute for Plasma Physics near
Munich, Germany—where the so-called H-
mode (or high-confinement mode) for tokamak
operation was discovered (9). It was
found that when the heat leaking out of the
plasma reached a critical threshold, a radial
electric field, a plasma velocity gradient,
and a steep pressure gradient spontaneously
arose at the edge of the plasma. This led to a
dramatic enhancement of the confinement
time and was presumed to be the result of a
reduction in turbulent transport at the edge.
This was consequently termed an “edge
A new twist. Gyro-kinetic simulation of plasma density fluctuations in a shaped tokamak. Image
shows a cut-away view of the density fluctuations (red/blue colors indicate positive/negative
fluctuations). The blue halo is the last closed magnetic flux surface in the simulated tokamak.
transport barrier,” which kept plasma particles
and thermal energy from escaping to
the material walls of the device. It was surprising
and gratifying to the researchers
that—almost by itself—the plasma had
found a way to reduce the turbulence, if
only in a small region of the plasma.
It was subsequently discovered, in the
1990s, that such transport barriers do not
only exist at the edge of the plasma, but can
also be produced in the interior regions of
the plasma (so-called internal transport barriers,
ITBs) (10). These transport barriers in
the core can be induced by rapid localized
changes to the “twist” of the magnetic
fields of the tokamak (reversed magnetic
shear) or to the plasma drift velocity (velocity
shear). ITBs have enabled access to
enhanced performance regimes of these
tokamaks and have generated much excitement
throughout the research community.
However, precise measurements of the
reduction of turbulence in one of these ITBs
near the core region has been very difficult.
In fact, only recently have experimental
P ERSPECTIVES
techniques been developed to determine
which components of the turbulence are
being suppressed within a transport barrier.
New measurements at the JT-60U tokamak
in Japan by a team led by Nazikian from
the Princeton Plasma Physics Laboratory
have used microwave reflectometry to determine
that the density correlation length of
the plasma fluctuations is reduced substantially
during the establishment of an internal
transport barrier—from about 20 cm to 4
mm—and that this corresponds to a reduction
in turbulence (11). The radius of the
plasma is about 1 m in JT-60U.
The recent discoveries about turbulence
and control of turbulent transport through
transport barriers and progress in both
experimental measurements and improved
(predictive) simulation capabilities are now
leading to renewed optimism for fusion as
an energy source and confidence that new
designs for fusion experiments will indeed
work. However, the large-scale “reactor-relevant”
experiments in which many of these
discoveries were made—the Joint European
Torus (JET) in the United Kingdom, JT-
60U in Japan, and the Tokamak Fusion Test
Reactor (TFTR) in the United States—were
constructed more than 20 years ago. The
next critical step in fusion research is the
International Thermonuclear Experimental
Reactor (ITER), a US$5 billion project,
which is planned to be operational in 2015
in Cadarache, France (12–16). Edge transport
barriers are key to ITER achieving a
“burning” fusion plasma. Advanced operational
regimes using internal transport barriers
are also planned for ITER.
Such experiments may enable fusion
power to be economical sooner than anyone
has previously thought. For the past 40
years, some have criticized the fusion community
for perpetually claiming to be only
30 years away from realizing success. As of
today, fusion power may be much closer
than that.
References and Notes
1. Original patent reproduced in M. G. Haines, Plasma
Phys. Control. Fusion 38, 643 (1996).
2. W. Horton, Rev. Mod. Phys. 71, 735 (1999).
3. T. L. Rhodes et al., Phys. Plasmas 9, 2141 (2002).
4. X. Garbet et al., Plasma Phys. Control. Fusion 46,B557
(2004).
5. A. Dimits et al., Phys. Rev. Lett. 77, 71 (1996).
6. Z. Lin et al., Science 281, 1835 (1998).
7. W. Dorland et al., Phys. Rev. Lett. 85 5579 (2000).
8. J. Candy, R. E.Waltz, Phys. Rev. Lett. 91, 045001(2003).
9. F. Wagner et al., Phys. Rev. Lett. 49, 1408 (1982).
10. Y. Koide et al., Phys. Rev. Lett. 72, 3662 (1994).
11. R. Nazikian et al., Phys. Rev. Lett. 95, 135002 (2005).
12. R. Aymar, P. Barabaschi, Y. Shimomura, Plasma Phys.
Control. Fusion 44, 519 (2002).
13. X. Litaudon et al., Plasma Phys. Control. Fusion 46, A19
(2004).
14. The ITER Web page is at www.iter.org.
15. D. Clery, D. Normile, Science 309, 28 (2005).
16. D. King, Nature 428, 891 (2004).
10.1126/science.1113477
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