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August 19, 2013
A Torn Seam Magnetic Field Container that Exceeds Perfection: The Perturbed Magnetic Field Effect

In the Large Helical Device (LHD) the “container” braided by the magnetic field lines confines extremely high-temperature plasma exceeding several tens of millions degrees. The LHD container, through precise manufacturing of the superconducting coils that generate the magnetic field and continued fundamental research over many years, is approaching a condition of near “perfection.” However, according to recent research, we have learned that if we cause changes in the surface of the container and dare to cause a small open seam, the penetration of impurities into the plasma can be suppressed and we then can gain “delicious effects” in the characteristics of heat and particles of the plasma edge. Here we will introduce the characteristics that such an “even better torn-seam container” can show.

Extreme high-temperature plasma is confined by a magnetic field container that cannot be seen. Because this container, unlike steel and concrete, does not melt under high temperatures, for plasma, this may be said to be a chamber sturdier than one made of steel. The magnetic field container is a layered-type structure of material woven in a different way, as each layer is systematically bound together. In order to strongly confine the extremely-high-temperature plasma, it is necessary that the material for the multi-layered magnetic field lines be prepared without damage. However, while such a perfect magnetic field container certainly is good for confining extremely-high-temperature core plasma, when raising the temperature of the plasma edge in a container in which low temperatures are preferred, at times “perfection” may also bring results not sought.

Adding but minor disturbance to the magnetic field, the surface of a perfectly bound magnetic field container brings changes to the binding, and we are attempting to create a container with a small tear. A small change is called a “perturbation,” and we call this a “perturbed magnetic field.” In the LHD, separate from the superconducting coils that construct the magnetic field container, in order to generate a perturbed magnetic field in the container, above and below the vacuum vessel each ten small circular coils are installed. The strength of the magnetic field created by these coils is extremely weak at 1/1,000 of the strength of the magnetic field created by the superconducting coils. By overlapping such magnetic fields we can make a small tear in an extremely thin area in the surface of the magnetic field container. We may call this a condition in which a soft and flexible material has partially covered the hard and strong container.

What types of roles does this tear perform? Through experiments using the LHD or research through computer simulations, we have learned that the structure of such a tear stops impurities in the vicinity of the container’s surface and that the tear blocks their penetration into the plasma core. Because iron, carbons, and other impurities that emerge from the vacuum vessel cause a lowering of the plasma’s temperature, this effect is important for enhancing the plasma’s performance. Moreover, from recent experiments we have confirmed that a tear in the container’s surface mitigates the heat input into the device into which the plasma that has emerged from the container flows. Such an effect is thought to be necessary also for the future of the fusion reactor, and brings the possibility that a small tear in the surface of the magnetic field container may offer a great contribution to the realization of fusion energy.

The magnetic field container which confines the plasma might be better at times to have a small tear.