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Of all the early theoretical visions of modern communication, Dewey's had the greatest scope. Like Cooley, he expressed great hope for the potential of new media to reconstitute neighborhood community values in a complex industrial society. Dewey firmly believed that the traditional sense of political and moral obligation might be recovered if organized intelligence and scientific inquiry could be made public. But as he retreated from the thorny political problem of how to transform the physical machinery of transmission and circulation, Dewey increasingly took refuge in a more comfortable identity: a philosopher of communication, absorbed in the metaphysical complexities of the communicative process.
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The reason the fusion reactions that fuel the stars run only at the very high temperatures where plasma prevails is simple; two light nuclei can fuse into a heavier one only if they are brought extremely close together. At ordinary temperatures, nuclei are held far apart by the shell of electrons that surrounds each of them. With the electrons gone, as in a "low-temperature" plasma of a few tens of thousands of degrees Celsius, two nuclei still cannot be made to come together because, being both positively charged, they repel each other with enormous force. Temperatures of tens of millions of degrees and more are needed to give the nuclei the tremendous speed required for them to crash together despite their strong electrical repulsion.
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There are two approaches capable of releasing fusion energy on Earth's surface. One is to heat the fuel so rapidly to fusion temperatures that fusion reactions take place before the hot gas has time to blow itself apart. This is called inertial confinement, because inertia holds the atoms together for the fraction of a second needed to get a substantial amount of fusion energy. An extreme case of inertial confinement is what happens in a hydrogen bomb. It may also be possible to make a fusion reactor this way, using lasers or intense beams of electrons or heavy ions as the initial energy source. (In a hydrogen bomb, a fission bomb serves as the trigger that provides the initial heating.)
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The heart of the matter is that electrically charged particles -- namely the electrons and positively charged ions -- cannot move very far perpendicular to the direction of a strong magnetic field. The stronger the magnetic field, the more tightly the particles are constrained. One can think of magnetic fields, in fact, as exerting a pressure on plasma; and by arranging magnets cleverly it is possible to produce a set of magnetic fields that push in on a plasma from virtually every direction. Such an arrangement is often called a "magnetic bottle." Of course, Newton's laws should remind us that if the magnetic field is pushing on the plasma, the plasma must be pushing back.
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Mirror machines operate with the plasma at somewhat lower densities and would be capable, in theory, of running more or less steadily, rather than in pulses. The name mirror comes from the tendency of the moving charged particles to reverse direction (hence, "reflect") if they move into a region where the strength of the magnetic field increases very sharply. The earliest mirror machines contained basically cylindrical plasmas and looked like a section of a theta pinch but with very strong fields at the ends forming the "mirrors." Those plasmas were unstable, and ways were subsequently found to produce more complicated magnetic-field shapes with the basic desirable properties of mirrors but without the strongest instabilities.
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Tokamaks are toroidal (doughnut-shaped) devices of medium plasma density, in which a strong current is made to flow along the tube of plasma itself. This current heats the plasma and produces a magnetic field that helps to suppress instabilities. Tokamaks surged to a position of dominance in world fusion research after important successes were achieved in the USSR with this approach in 1969. (Magnetic-confinement fusion research was declassified by international agreement in 1958, and the field has been a model of international scientific information-sharing ever since.) Large tokamaks that are expected to approach fusion break-even conditions are scheduled for operation in the late 1970s in several parts of the world.
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In magnetic-confinement fusion, the magnets are like the lasers in laser fusion: they are the cornerstone of the system, very expensive, and vulnerable to damage by overheating from their own circulating energy or from energy deposited by neutrons and X-rays. The problem is especially difficult because most magnetic-confinement schemes appear feasible only with superconducting magnets -- special materials whose resistance to electric current drops to zero when they are cooled to the temperature of liquid helium, a few degrees above absolute zero. These magnets must be very close to the fusion plasma they are containing; if they were far away, the confining field would be too weak.
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Fusion offers the potential for significant environmental and safety advantages, compared to fission, in three main respects. First, the fuels and reaction products of fusion are nonradioactive, except for tritium. Being a fuel, it can be burned up and need not become a waste-management problem. Second, there is no chance of a runaway reaction that could release more energy than intended. The conditions under which fusion flourishes are so extreme and so difficult to attain that any malfunction merely causes a departure from those conditions and thereby quenches the reaction. Third, fusion does not automatically produce materials needed to make nuclear explosives; hence, the problem of nuclear theft need not arise with fusion.
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The second radiological hazard of fusion is the material in the reactor structure that becomes radioactive because of bombardment by neutrons. The size of this inventory of activation products depends on what material is used for the inner structure of the reactor. Niobium, which is a good structural material for fusion reactors in other respects, has activation products whose total relative hazard at reactor shutdown would be only 10 to 100 times less than that of the fission products in a fission reactor of the same size. If vanadium can be used instead of niobium, the initial relative hazard would be about 1000 times less than that of fission products.
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Suitably designed fusion-fission hybrids could also be used mainly to make fission fuel by arranging for the fusion neutrons to cause fertile-to-fissile conversions in the blanket. The fuel thus produced could be used in pure fission reactors elsewhere. This approach has been proposed in case pure fission breeder reactors don't work very well. Furthermore, hybrids could be used to fission the long-half-life heavy elements (actinides) produced in pure fission reactors, thus reducing their half-lives to those of fission products. The energetic fusion-reactor neutrons would be much more effective at fissioning actinides than are fission-reactor neutrons.
