Scyllac: Toward Pulsed Fusion

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The thermonuclear fusion reactor of the 21st century, when and if it is successfully built, may be either a steadystate device, in which thermonuclear burning proceeds continuously, or a pulsed device, in which the burning proceeds in spurts. The two types represent different attempts to solve the problems of containment, heating and plasma density that stand in the way of an economical fusion reactor. Steady-state efforts are exemplified by the Tokamaks and multipoles that have shown significant advances in plasma confinement lately (SN: 11/ 8, p. 424). These are toroidal, doughnutshaped, devices in which a plasma of ions and electrons is confined by magnetic fields produced by a combination of electrical coils around the outside of the tube and currents inside the plasma itself. These devices have achieved their gain in plasma confinement by using plasma of very low density. The difficulties of confining any plasma in a magnetic field are formidable, and the denser the plasma, the more severe they become. In a toroidal chamber the problem is even greater because it is impossible to make a magnetic field of torus shape that has the same strength on its inside circumference as on its outside circumference. Those who work with Tokamaks and multipoles have generally chosen to try first to achieve maximum confinement and stability at low densities and temperatures and then to increase the density to a level where a practical amount of power could be obtained. The achievements of the Tokamaks, which were developed by Russian scientists, have so excited the U.S. Atomic Energy Commission that it has instituted a program to build five Tokamaks in the United States as quickly as possible (SN: 4/11, p. 373). But supporters of the pulsed devices, who first heat a high-density plasma and then try to confine it, contend that the enthusiasm for Tokamaks is out of proportion. Even the Russians are not as sanguine about Tokamaks as the USAEC, says Dr. James Tuck of Los Alamos Scientific Laboratory. Tokamak does not extrapolate easily into a fusion reactor, he says, citing technical problems such as where to put heat absorbers. Our Scyllac will extrapolate more easily and is much hotter, 10 times hotter. The high-density devices are pulsed because of the means they use to heat high-density plasma quickly. One such method is by the shock and compression of a magnetic implosion, called a theta pinch. Theta-pinch work has been done extensively at the Los Alamos Scientific Laboratory in a series of experiments called Scyllas. These have begotten a more ambitious project, Scyllac, which is now attempting to put together a toroid 15 meters in circumference for high-density plasmas. A fivemeter curved section will be ready for experiment in December. Because of the added difficulties involved in trying to contain a highdensity plasma in a toroid, much of the theta-pinch work has been done in straight tubes. Electric coils around the tube generate a magnetic field that runs along the length of the tube. When the current in the coils is suddenly increased, the strength of the field also increases. This pinch drives the plasma together in the middle of the tube. It causes both shock and compression, which heat the plasma. Temperatures up to 50 million degrees K. have been achieved in the Scyllas, says Dr. Fred L. Ribe of Los Alamos, who heads the Scyllac project. There are two ways in which the plasma can escape from a straight tube: