Magnetic Fusion Energy (MFE)

 

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Magnetic confinement devices use extremely strong magnetic fields to trap the ions of the plasma, forcing them to follow circular paths about the magnetic field lines. This prevents the ions from contacting and penetrating the walls of the containing vessel.


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The plasma needs to reach at least 100 million degrees Celcius before the hydrogen atoms can overcome nuclei repulsion and start to fuse. To reach such temperatures, several different heating methods need to be combined. Ohmic, or resistive, heating by passing a current through the plasma can create temperatures of 20 to 30 million degrees. Then high-energy neutral atoms are injected into the plasma, where they become ionized and transfer their energy to the plasma as they collide with plasma particles, heating the plasma further. High frequency radio waves can next be used to increase the energy of the neutral ions and thus the plasma. One other technique is to increase the magnetic field of the plasma along a gradient, compressing the plasma and therefore raising its temperature.

Current Status of MFE

The highest energy output to input ratio achieved to date is 65%. This occurred at the Joint European Torus (JET) in England. At the Princeton Plasma Physics Laboratory (PPPL), an output that was 27% of the energy input is the highest that has been achieved. A chief way to increase this percentage is to increase the scale of the system; in fact the JET is significantly larger than the Tokamak Fusion Test Reactor (TFTR) at PPPL. MFE is more developed than IFE and thusfar it has achieved better results.


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Picture of the JET.
However, no sustaining reactions longer than one second have yet been achieved, and therefore the plasma continually needs to be reheated externally to the starting temperature. [1]

Future Research Directions

There are many different projects in the planning and construction stages that should help to bring magnetic fusion energy closer to realization. Currently, there are three different "burning plasma" experiments in the design phase (shown below): Fusion Ignition Research Experiment (FIRE), Ignitor, and International Thermonuclear Experimental Reactor (ITER). Through these experiments, researchers would be able to develop, study and improve "burning plasmas," those that are able to maintain their own temperatures by heat transfer from the alpha particles created by the fusion reactions. This would also be the first opportunity to study the practical problems likely to occur in an actual fusion power plant, such as the accumulation of helium ash and methods for controlling the burning rate. All of these experiments would be performed in a tokamak, which is the only structure currently capable of containing a burning plasma. It is hoped that ITER will surpass the 100% energy output to input ratio, largely through the use superconducting magnets.


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The major area of MFE research is the exploration of new plasma confinement configurations. Currently there are many under study. These include a renewed interest in the stellarator through the National Compact Stellarator Experiment (NCSX) at PPPL, which is in the beginnings of its design phase, and the development of the spherical torus. The National Spherical Torus Experiment (NSTX) at PPPL and the Mega Amp Spherical Tokamak (MAST) in the UK are two such reactors that have already been constructed.

There is also research looks at improving the materials used in the fusion chamber. The Advanced Power Extraction (APEX) Study, which began in 1998 at UCLA, is the best example of chamber technology research. They are looking at the feasibility of a liquid first wall, an all-liquid wall, and a solid wall, among other ideas. These walls are the closest to the plasma (right) in the confinement device.


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The Levitated Dipole Experiment (LEX) at M.I.T. is another area of MFE research. Researchers are testing the idea of confining the plasma by adiabatic compression. This would involve levitating a one-half ton ring of superconducting magnets.

Learn about: inertial confinement.

Go on to other technical challenges: Theoretical Research or Cold Fusion

Sources

[1] Ratios obtained from personal communication with Tony DeMeo of Information Services at PPPL, April 25, 2002.