POWER June 2020 - 37

NUCLEAR TECHNOLOGY
2. The deuterium-tritium reaction is considered
by most scientists to be the most
promising for fusion energy. When a deuterium
nucleus (2H) and a tritium nucleus (3H)
combine, the result is a helium nucleus and
a very energetic neutron. Under the right circumstances,
the energy released is sufficient
to induce further fusion reactions. Source:
Creative Commons/Wykis
laboratory-feasible reactions. However,
the challenges involved in handling radioactive
tritium and dealing with the
copious amounts of energy and fusion
products produced, means most present
fusion experiments study the fusion
of deuterium to deuterium (which has a
lower fusion reaction rate) and then extrapolate
the results to DT.
DT fusion produces an extremely
energetic (14.1-MeV) neutron, which
is potentially useful for breeding more
tritium but also creates challenges for
the surrounding materials in the tokamak
(more on both of these below).
The reaction also generates a helium
nucleus that carries away about onefifth
of the reaction energy (3.5 MeV).
In a fusion reactor, this nucleus shares
its energy with the surrounding ions,
keeping them hot and sustaining the
fusion process.
The first DT reaction was achieved in
1932 at Cambridge University by Australian
physicist Mark Oliphant. Though
there are a number of methods by which
fusion can be induced, consensus research
into practical fusion energy has
settled on magnetic confinement as the
most promising for electricity generation.
In this method, magnetic fields are
used to confine hydrogen gas, which
is heated until electrons separate from
their nuclei, forming a plasma-an ionized
" soup " of charged particles.
Most fusion research employs a
magnetic confinement device known
as a tokamak. First developed by Russian
physicists Andrei Sakharov and
Igor Tamm during the 1950s, a tokamak
is a toroidal (that is, torus- or donutJune
2020 |POWER
shaped) vacuum vessel that uses powerful
electromagnets to confine and
shape the plasma. (The word tokamak
is a Russian acronym for " toroidal confinement
machine. " )
Creating and maintaining the confining
magnetic fields in a tokamak requires
three arrays of magnets (Figure
3). External coils around the ring of the
tokamak produce the toroidal magnetic
field, parallel to the circumference of
the torus. The central solenoid uses a
powerful pulse of energy that generates
a toroidal current within the plasma. The
movement of ions with this current in
turn creates a second poloidal (parallel
to the poles) magnetic field. Finally, poloidal
coils around the circumference of
the torus are used to control the position
and shape of the plasma.
Rather than a single magnetic field,
the arrangement results in an array of
nested flux surfaces that confine the
ionized particles in a variety of orbits
in and around the tokamak. Because
the particles in the plasma are tied to
the magnetic fields, this field structure
keeps the hottest parts of the plasma
away from the walls, creating an insulating
effect that allows very high temperatures
to be achieved.
The induced plasma current provides
a critical element of the magnetic confinement
as well as a certain amount
of heating, but this alone is not enough
to induce fusion. Additional heating is
typically provided by microwaves and
particle beams.
Several dozen tokamaks are now in
operation around the world. The first to
demonstrate fusion at significant scale
(10 MW) was the Tokamak Fusion Test
Reactor (TFTR) device at the Princeton
Plasma Physics Laboratory, though it
has since shut down. The largest current
device is the Joint European Torus
(JET) in the UK, which has a radius of
about 3 meters (m) and has made 16
MW of fusion power. A similar device in
Japan is being upgraded to study techniques
for future facilities and plasma
sustainment. Smaller devices, like the
DIII-D National Fusion Facility, which is
operated for the Department of Energy
by General Atomics in San Diego, are
used to explore the physics for future facilities
and develop techniques to raise
fusion performance.
Some fusion research employs a device
similar to a tokamak known as a
stellarator. Rather than having a large
plasma current,
a stellarator uses a
twisted array of helical windings around
the torus to create the poloidal field externally.
As a result, it does not need
to generate a plasma current, which
means it is capable of steady-state operation
rather than needing a pulse of
energy from the central solenoid. However,
stellarators have more complicated
geometry and are more difficult to
build because of these additional windings.
The approach is thought to offer
considerable promise, and though past
stellarators have encountered more
problems with plasma confinement
than tokamaks, the technology continues
to draw research attention. The
Wendelstein 7-X, an advanced stellarator
that went online in Germany in 2015
(also the largest so far) is studying how
well stellarators can contain energy and
reach fusion conditions.
No current device has been able to
generate more fusion power than the
heating energy required to start the reaction.
Scientists measure this assessment
with a value known as fusion gain
(expressed as the symbol Q), which is
the ratio of fusion power to the input
power required to maintain the reaction.
Q = 1 represents the breakeven
point, but because of heat losses, burning
plasmas are not reached until about
Q = 5. Current tokamaks have achieved
around Q = 0.6 with DT reactions. Fu3.
A tokamak consists of a vacuum chamber surrounded by an array of electromagnetic coils
that work together to confine the plasma inside strong magnetic fields (left). The DIII-D National
Fusion Facility (interior shown at right) in San Diego is the largest operating tokamak in the U.S.
Courtesy: General Atomics
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POWER June 2020

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