The new centerpiece
fusion experiment, called the National Spherical Torus Experiment (NSTX),
at the Princeton Plasma Physics Laboratory (PPPL) in New Jersey might
provide the answer to the question fusion researchers are asking in
an age of rising oil prices and greenhouse-gas levels: Can practical
fusion energy be developed at reduced cost?
The new experiment
will not break world records for plasma temperature and the amount of
carbon-free fusion energy produced, as did its predecessor at PPPL,
the Tokamak Fusion Test Reactor (TFTR). (TFTR produced a world-record
10.7 megawatts of fusion power in 1994.) But the much smaller NSTX aims
to provide new data for determining whether a subsequent, next-generation
spherical torus device could produce three times the power of TFTR at
one-third the cost.
NSTX passed its
first plasma test in February 1999 and resumed operation in September
1999. This machine fulfilled an important milestone in December 1999,
nine months ahead of schedule, by inducing an electric current of one
million amperes in its plasma. The plasma is an extremely hot state
of matter consisting of charged heavy hydrogen nuclei and free electrons
swirling around at very high velocities. This level of current was a
world record for a device of NSTX's design.
In January 2000,
a series of experiments on NSTX validated an idea about how to produce
and control its current. In one test, a current of 130,000 amps was
generated in the plasma by injecting current directly into the chamber,
thanks to a new technique developed in cooperation with the University
of Washington at Seattle.
torus concept, which was proposed in 1985 by Martin Peng, an ORNL scientist
now on assignment to PPPL, does not have a doughnut shape like TFTR.
Instead it is shaped like a cored apple, a more compact design. It was
generally believed in 1985 that a large hole in the doughnut was needed
to accommodate large magnets to confine the plasma and keep it from
crashing against the vessel wall, where it would lose its energy. But
Peng showed theoretically that only small magnets would be needed to
confine the spherical torus plasma. The spherical shape may overcome
the problem that has been prevalent in tokamaks: turbulence—and other
instabilities—that cause energy to leak from the hot plasma and undermine
fusion reactions unless a very strong magnetic field is applied.
researcher Martin Peng (shown here) conceived the National Spherical
Torus Experiment now in operation at Princeton Plasma Physics
"In the spherical
torus," Peng says, "the same plasma pressure can be maintained with
a much lower magnetic field, significantly reducing the size and cost
of the fusion device."
ORNL, Columbia University,
and the University of Washington joined PPPL in building the NSTX. ORNL
made major and key contributions in plasma design, including the plasma
shape (Dennis Strickler), plasma edge (Peter Mioduszewski et al.), radiofrequency
systems, and engineering design of the first wall components (Brad Nelson
et al.). Some of these and other ORNL researchers have since joined
the NSTX National Team of researchers from 15 national laboratories,
universities, and industries and contributed much to the success of
the initial experiments.
NSTX was built for $24
million two months ahead of schedule and within cost. "The equipment
already available at PPPL was used effectively to reduce much of the
construction cost of the NSTX facility," Peng says. "By using much smaller
electromagnets to confine the plasma, we have also reduced power requirements,
keeping operating costs down." Peng, who is the NSTX program director,
co-directs the spherical torus facility with PPPL physicist Masa Ono,
the NSTX project director.
In the next couple of
years, the NSTX team will apply powerful radiofrequency waves and neutral
particle beams to heat the plasma to very high temperatures (it is hoped
to more than 20 million degrees Celsius). Both of these techniques have
been areas of major strength for ORNL's Fusion Energy and Instrumentation
and Control divisions. In a recent test, Dave Swain and Phil Ryan of
ORNL, together with researchers from PPPL and General Atomics, coupledfor
the first timetwo of what will eventually be six radiofrequency
devices to heat the plasma in a new way, using a technique similar to
that employed in a kitchen microwave oven. John Wilgen and Greg Hanson
built and installed a radio wave interferometer system, which has already
produced high-quality measurements of plasma density near these new
system from ORNL was brought to NSTX by Tim Bigelow to pre-ionize the
fuel and ensure reliable plasma operation. Also, ORNL's Rajesh Maingi
was selected to serve as the deputy run coordinator, beginning in October
"We hope to achieve
high plasma performance in NSTX," Peng says. "The scientific knowledge
we will gain could pave the way for affordable development of economical
fusion power in the future."
Plasma Physics Laboratory