What is Fusion?
- The nucleus of an atom consists of protons, which have
a positive electrical charge, and neutrons which
have absolutely no charge at all. These nucleons are held
together by a powerful force, known as the strong force. The amount
of energy you would have to expend to overcome the
strong force and dismantle the nucleus is called
binding energy, and each chemical
element has a unique binding energy associated with its
atomic nuclei. Fusion reactions release surplus binding
energy when light nuclei fuse, forming heavier nuclei.
|A single Helium nucleus contains
less energy than separate deuterium nuclei. Fusing them releases
surplus binding energy.
- In the Alcator C-Mod experiments, two deuterons --
nuclei of heavy hydrogen -- fuse, creating helium, a natural
The Fusion Challenge
- The adage “opposites attract'' is
true of electrically charged particles as well as people, and is the
reason why fusion reactions are so difficult to achieve. Since all
atomic nuclei have a positive electrical charge, they tend to repel
rather than attract each other. Consequently, a tremendous amount of
energy is required to bring the
nuclei close enough for the strong nuclear force
to bind them. For this reason, fusion reactions generally occur at
temperatures of the order of twenty to one hundred million degrees.
At these temperatures, matter exists not as a solid, liquid, or gas,
but in a fourth state called “plasma''. The
task of confining even a wisp of superheated plasma fuel has proven
to be the greatest challenge in fusion research.
The Alcator Solution— Compact Size, High Magnetic Field Intensity
A plasma is a gaseous sea of positively charged atoms,
called ions, and free, negatively charged atomic particles called
electrons. The behavior of plasmas is extremely
complex, and the task of controlling a superheated plasma with
magnetic fields has been compared to suspending a piece of jello
between rubber bands.
- The Alcator C-Mod experiment and its predecessors,
Alcator C and Alcator A, belong to a class of devices
called tokamaks, which use magnets to confine the
plasma in a donut shape, called a torus. There are major tokamak
experiments all over the world working at the leading edge of
controlled fusion research, and dozens of smaller, less powerful
devices. Alcator C-Mod is the newest, most advanced world-class
built to explore the physics of plasmas in a compact, high
|Video shot of confined plasma.
- The word Alcator is an acronym derived from the Italian
words Alto Campo Torus, meaning high field torus, and the magnetic
fields developed inside the Alcator tokamaks are among the highest
- Alcator's high confining fields let researchers
experiment with plasmas hotter and denser than those in machines of
similar size. In fact, Alcator C was the first device to produce the
density and confinement of hot plasma necessary for a useful fusion
Key to the Machine
|Numbering is clockwise beginning at
the top left.
- The blue poloidal field magnets control the plasma's
shape and position. Molybdenum tiles protect the vacuum vessel's
plasma facing wall.
- Three of 20 toroidal field magnet arms join the
tokamak’s central core. The horizontal windings around the core
create a maximum ohmic heating field of 22 Tesla.
- The wedge plate forms channels that support the
toroidal field magnet arms.
- Twenty vertical legs complete the magnet, which can
produce a 9 Tesla toroidal field.
- The top and bottom covers, made of solid stainless
steel, are 10' in diameter and 26'' thick. Each cover weighs 35 tons
and bends 1/8'', during a maximum performance pulse.
- Draw bars, made of a super strong alloy, hold the covers
|The fields of Alcator.
The Fields of Alcator
The pulsed ohmic heating field moves plasma particles along
toroidal field lines, producing a maximum heating current of
3,000,000 amps. The current also produces the poloidal field, which
combines with the toroidal field to produce a stable magnetic
Major Research Directions
Alcator C-Mod has a unique divertor system that uses specially shaped
magnetic fields to “scrape'' away the cooler, outer edge
of the plasma, draw it into an isolated channel on the bottom of the
vacuum vessel, then pump it out of the machine. This is necessary
because some ions escape from Alcator C-Mod's “magnetic
bottle,'' and collide with the wall of the vacuum
vessel, where they deposit their energy and become
neutralized. Efficient divertor systems will be key elements in
future fusion plants. Alcator C-Mod, which employs divertor channels
along the top and bottom of the vessel in conjunction with a unique
baffle design, has set the standard for the next generation of
|The baffled divertor
channel’ is outlined in red. The bright area in the bottom left
represents the divertor magnet and its surrounding field.
|Video shot of confined plasma.
The toroidal field, almost a quarter of million times stronger than
the earth’s magnetic field, confines plasma so hot, only its
outer edge emits visible radiation.
For fusion to occur, plasma must be kept well away from the
walls of the vacuum vessel. The confinement is never perfect,
however, and some plasma eventually leaks through the confining
fields. Since this process is turbulent and chaotic, predicting the
amount of heat and numbers of particles lost is a major theoretical
challenge. At present, scientists use experimental
results from different tokamaks, to estimate these losses in future
machines. However, deeper insight is being gained from experiments
based on manipulation of fueling techniques, optimal use of the
divertor, and variation of toroidal and poloidal fields. The
objective is to increase our understanding of the principles
governing the transport of particles and energy in
high temperature plasmas. With this knowledge we will be able
to improve confinement, and build better tokamaks.
The shape and position of the plasma is important. The
edges of the plasma should be held away from the wall. Touching the
wall cools the tenuous plasma and impairs the cleaning action of the
divertor system. An elongated plasma shape, taller than it is wide,
produces superior heating and confinement. The "Physics
Operator" uses graphical computer tools to program
the plasma's shape (and other parameters) before each shot.
Fast feedback systems control magnet currents, gas valves, and other
equipment during the shot to produce the desired results.
Radio Frequency Heating
Ions in the center of the plasma circulate around the magnetic field
lines 80,000,000 times each second.
When radio waves at 80 MHz, the ''ion cyclotron'' frequency, are
injected into the plasma, the central ions absorb the wave energy,
heating the plasma. Alcator C-Mod employs two antennae, and a total
of four million watts of power for this type of plasma heating. The
objective is to discover how to optimize RF heating in compact,
high field tokamaks, and to use the flexibility of this technique to
study confinement and control.
|Two antennae, pumping four million watts
at 80 NHz, can increase plasma temperature more than ten million
Diagnostics Paint a Picture of Plasma Evolution
To a plasma scientist,
diagnostics are sophisticated experiments that
measure key magnetic field, or plasma parameters. Some examples are:
The hot, central portion of the plasma is invisible to the
naked eye, but glows brightly in the X-ray spectrum. Measuring the
quantity of X-ray emissions at varying wavelengths reveals plasma
|Ultraviolet and X-ray tomography
produce different images of a single event. In graphs such as these,
brilliance equals power; the snaking bright lines trace the evolution
of a plasma perturbation.
X-Ray & Ultraviolet Tomography
These diagnostics work like a CAT scan. X-ray and
ultraviolet detectors, placed around a D-shaped cross section
of the vacuum vessel, reveal the amount of power radiated at
each point in the cross section.
|A sophisticated Michelson
interferometer, using RF modulated HeNe and CO2 lasers, measures plasma density.
Two broadened laser beams, traveling separate but
equally long paths, create a pattern of light and dark regions when
combined. Mathematical analysis yields a picture of plasma density.
A laser beam, fired through the plasma, is
scattered by fast moving electrons, which shift the beam's ''color''
in the same way that the motion of a speeding locomotive alters the
pitch of its whistle. Analysis of this “Doppler shift'' and the
amount of scattering, provides detailed information on temperature
and density, at many different
points in the plasma.
frozen fuel and ''puffing'' gaseous fuel into the plasma is an
important part of the research. High magnetic fields allow Alcator to
produce relatively high-density plasmas. High plasma densities have
been key to many of Alcator's achievements, including low impurity
levels, good confinement of the plasma's energy, and world record
color represents a different value of magnetic flux. Plasma
parameters such as temperature and density are constant along
surfaces of constant hue.
A Day in the Life
What is it like to be involved in
experimental fusion research? To answer this, imagine that you are a
fusion scientist. Today, it's your turn to run Alcator C-Mod, so
you'll call all the shots.
invigorating tension fills the control room as physicists study the
last shot’s diagnostics. They can fire another shot after the
magnets cool for 15 minute.
You, like all of Alcator C-Mod's scientists and graduate
students, must have a specialty. Some are RF heating experts, others
are Thomson scattering specialists. Imagine that you specialize in
magnetohydrodynamics, focusing on the collective behavior of plasmas.
Today you will try to make a new kind of plasma, one that has never
been made before.
You will have thirty or thirty-five chances to achieve this,
before the day is over. Each try, or “shot,'' lasts only a few
seconds, because the electrical currents which create the tokamak's
huge fields heat its magnets, despite the -196 degree F liquid
nitrogen flowing around them all the time. Between shots, you'll have
about fifteen minutes of recooling time to evaluate
the last shot's diagnostics, see what happened, and figure out what
to do next. During this period, you'll consult, perhaps even argue,
with other specialists, whose diagnostics you're studying. Networking
is standard procedure, no matter which specialist is in charge, But
when you've reached a decision -- you'll call the next shot!