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Plasma Science and Fusion Center

Massachusetts Institute of Technology

 
 

Alcator c-mod

What is Fusion?

  • The nucleus of an atom consists of protons, which have a positive electrical charge, and neutrons which
    fusion rx
    A single Helium nucleus contains less energy than separate deuterium nuclei. Fusing them releases surplus binding energy.
    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.
  • In the Alcator C-Mod experiments, two deuterons -- nuclei of heavy hydrogen -- fuse, creating helium, a natural atmospheric gas.

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 tokamak,
    video shot of confined plasma
    Video shot of confined plasma.
    built to explore the physics of plasmas in a compact, high field environment.
  • 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 ever achieved.
  • 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 reaction.

 

 

 

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Key to the Machine

cutaway of alcator
Numbering is clockwise beginning at the top left.
  1. The blue poloidal field magnets control the plasma's shape and position. Molybdenum tiles protect the vacuum vessel's plasma facing wall.
  2. 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.
  3. The wedge plate forms channels that support the toroidal field magnet arms.
  4. Twenty vertical legs complete the magnet, which can produce a 9 Tesla toroidal field.
  5. 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.
  6. Draw bars, made of a super strong alloy, hold the covers in place.

 

fields of alcator
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 bottle.

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Major Research Directions

Divertor Physics

Alcator C-Mod has a unique divertor system that uses specially shaped
divertor channel
The baffled divertor channel’ is outlined in red. The bright area in the bottom left represents the divertor magnet and its surrounding field.
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 tokamaks.

 

video shot of confined plasma
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.

Plasma Confinement

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.

 

Plasma Control

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.
small antennae
Two antennae, pumping four million watts at 80 NHz, can increase plasma temperature more than ten million degrees centigrade.
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.

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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:

 

X-Ray Spectroscopy
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 temperature.

small snake
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.

small laser
A sophisticated Michelson interferometer, using RF modulated HeNe and CO2 lasers, measures plasma density.

 

Laser Interferometry

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.

 

(top)Thomson Scattering

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.

 

Fueling Experiments

Injecting 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 levels

Here, each 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.

physicists study shot
An 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!

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