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A large (+16 mm) pellet is undergoing testing for ITER disruption mitigation. The pellet, on the left, is exiting the guide tube just before hitting a simple target plate. It will shatter once it hits the plate. Photo: Combs, ORNL
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Heated to
extreme temperatures of up to 150 million C, the plasma in ITER's giant
experimental fusion reactor will be fed a fuel of frozen pellets of
deuterium-tritium, fired into the tokamak vacuum vessel by pellet injectors.
Testing of the most recent pellet injection design technology developed by Oak
Ridge National Laboratory and U.S. ITER is under way this fall at the DIII-D
research tokamak in San Diego,
operated by General Atomics for the Department of Energy through the Office of
Fusion Energy Sciences.
The design,
testing, and manufacture of this pellet injection system is one of the key
contributions of the United States to ITER, an international collaboration that
is committed to building the largest ever tokamak in southern France. Tokamaks
are doughnut-shaped machines that use magnetic fields to contain hot plasma.
The goal of ITER is to demonstrate the feasibility of fusion energy for
commercial-scale electricity; "first plasma" for the experimental
facility is planned for 2020.
Physicist David
Rasmussen serves as the lead for U.S. ITER's fueling team and as a group leader
in the Fusion Energy Division at Oak Ridge National Laboratory. He points out
that understanding of the plasma fuel in a fusion reactor has evolved over
several decades, and pellet injection is now seen as a compelling method to
control potential plasma instability inside the reactor. The U.S. ITER fueling
team, which includes physicists, engineers, technicians, and other experts,
collaborates closely with the international fusion science community to
integrate key research findings in plasma fueling and control.
The pellets are
made of deuterium and tritium, isotopes of hydrogen that are frozen at 11
degrees above absolute zero Kelvin. When they are fired into the tokamak by a
pulse of high-pressure gas, they vaporize and the particles are ionized,
becoming part of the plasma. Inside the tokamak, the deuterium particles are
heated up to as much as 200 million K, or more than 10 times the temperature
inside our sun.
"When we
send a frozen pellet into a high-temperature plasma, we sometimes call it a 'snowball
in hell'," Rasmussen says. "But temperature is really just the
measure of the energy of the particles in the plasma. When the deuterium and
tritium particles vaporize, ionize, and are heated, they move very fast,
colliding with enough energy to fuse." The energy released from the fusion
reaction, as energetic neutrons and helium, has potential as an abundant,
carbon-free energy source.
But pellets are
not just about fuel. Research has found that the pellets can also control
spontaneous instabilities which occur at the edge of the plasma, called edge
localized modes (ELMs). If the full energy of these ELMs is absorbed by the
machine, erosion of plasma-facing surfaces can occur. Small pellets can be used
to reduce the size of the ELMs into more frequent but less harmful events, a
task researchers like to call "burping the baby," Rasmussen says. In
addition, larger pellets about the size of a wine cork can be injected to break
down the plasma column altogether. This contingency comes into play should
operators need to stop the plasma racing around the reactor. The large pellets
can collapse the plasma "to give it a safe landing," says Rasmussen.
The fueling and
instability issues are caused by the nature of plasma itself. Plasma, the
fourth state of matter, is an ionized gas that includes positively and
negatively charged particles. "We apply very high magnetic fields, and the
plasma reacts to those," Rasmussen explains. "The plasma has its own
internal currents, and a whole menagerie of instabilities depends on those
internal currents. The ELMs are the ones that occur near the outside edge.
These are filaments, basically, that spiral around. Such bursts can be intense
and can damage the plasma-facing surfaces of the vessel."
Researchers are
experimenting with injecting small pellets, a couple of millimeters in
diameter, at the plasma edge. This approach imposes additional magnetic fields
that break up the ELM spirals into smaller events. "We put a little chaos
near the edge," notes Rasmussen. "That tends to make these things
unstable, so the edge releases these spirals in a more predictable way."
The researchers
have gone through a couple of iterations of the injector that provides these
small pellets. They have recently installed a new design on the DIII-D tokamak
that is now being tested. The test reactor DIII-D now uses only deuterium
pellets, but ITER will use both deuterium and tritium. The researchers are also
experimenting with pellets of neon and argon to control plasma events. Research
is also proceeding on the wine cork-sized pellet for collapsing a plasma.
"There are
certain conditions where the current gets interrupted and gets out of
control," Rasmussen says. "Instead of traveling in a controlled or
confined way around the tokamak, it shifts upward or downward in the chamber,
and comes into contact with the walls where it can potentially cause
damage."
Electromagnetic
effects that result from this current shift impact the entire tokamak. One
option for controlling such disruptions is to inject a massive amount of gas,
which collapses the plasma column and cools it. Large pellets are an option
being developed at ORNL, since they can cool the plasma even faster.
The gas to
control the plasma is frozen in the injector, then accelerated and bounced off
a number of metal plates to break it up into particles before entering the
plasma. The metal plates shatter the pellets so that shards are sprayed in at
several points. "What you see looking across the tokamak is a spray of
pellet particles, and then an intense emission of light. That means we are
converting the plasma back to gas," says Rasmussen.
One of the
things researchers are now trying to decide is how many large pellets they must
inject to collapse the plasma. "We think the number is perhaps 4 to 10 at
the same time. We are going to do experiments on DIII-D that will help us
identify just how much symmetry is required to make this work well." The
DIII-D in San Diego is the largest tokamak in
the United States
and a "good test bed for the system," Rasmussen says. "We have
identified four places on the ITER tokamak where we could do gas injection of
very large amounts of gas and also of these pellets."
Rasmussen is
very conscious that a major challenge remains: How to extend the physics from
the conditions at DIII-D to conditions orders of magnitude greater that are
expected in the ITER tokamak, which will be 10 stories high. "It is a
fairly big leap from DIII-D to ITER. We work with JET, a larger tokamak device
in the United Kingdom, as well on pellet injector experiments, so that is
closer to conditions on ITER, but it is not the full ITER conditions."
"In
addition to the experiments, we and our international colleagues do a lot of
modeling," Rasmussen says. "We have to really understand what effects
are going on in DIII-D to create accurate models. Then we can run that model
for ITER conditions.”
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