There are two ways to liberate the energy locked in the atomic nucleus. At the "heavy" end of the table of elements (uranium, plutonium), the "splinters" of a smashed nucleus have less mass than the nucleus had; the difference, or mass defect, is the mass that has been converted to energy. But at the "light" end, when two hydrogen nuclei fuse, the resulting helium nucleus is lighter than the combined mass of the two; the mass excess is converted to fusion energy.
Not any hydrogen nucleus will do: It must contain one or two neutrons along with the regular proton, i.e., the hydrogen must be deuterium or tritium, respectively. Deuterium is abundant in nature (sea water), but tritium must be manufactured in a special reaction.
To liberate fusion energy in quantities greater than the energy invested in provoking the fusion, one must achieve a very high temperature of the hydrogen gas and compress it to a sufficiently high density for sufficiently long times. Several ways of achieving this are now under investigation. The oldest is magnetic confinement. The hydrogen is ionized (electrons are torn off the atoms, resulting in a gas of positive and negative ions, or a "plasma"), so that the particles can be accelerated by electric and magnetic fields. "Temperature," for a particle of given mass, is given by its velocity. By various geometric schemes, the plasma is confined by a magnetic field to certain portions of space where the nuclei will fuse if they crash into each other with sufficiently high velocities (temperatures). Typical of this method is the Tokamak now under construction in Princeton.
Alternatively, a tiny pellet of deuterium is hit by laser light of enormous power from all sides; as the pellet is crushed under the blow to small dimensions, the pressure (density of atoms) and temperature (their velocity) will so increase that some atoms will fuse; at a certain rate (which has not yet been achieved) the fusion will become self-sustaining and the entire pellet is burned up. The next pellet then undergoes the same treatment. This is being tried at Berkeley. Last May the Lawrence Livermore Lab began construction of the Nova laser, which will hit a pinhead sized pellet with a wallop of 300 trillion watts for 1 billionth of a second. The hope is that the pellet will produce enough energy to power the lasers bashing it into fusion. The Nova is the 5th generation laser for this purpose. The first phase of the project is the left half¾the right half is the already existing, but less powerful Shiva laser¾which will be housed in a building 330 feet long, will cost $196 million, and should be completed in late 1982. The second phase, replacement of the Shiva by another Nova, has not yet been authorized.
Fusion by hitting deuterium-tritium pellets by particles rather than laser light is being tried by Sandia Labs in New Mexico.
"Migma" fusion does not rely on random thermal motion of nuclei, but seeks to smash them into each other along controlled paths (AtE May 76); we have received no answer to out enquiries, and must assume the project abandoned¾possibly for lack of funds. Now comes news from Russia of yet another method.
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Vol. 7, No. 2
Newsletter: Access to Energy Newsletter Archive Volume: Volume 7 Issue/No.: Vol. 7, No. 2 Date: October 01, 1979 02:47 PM Title: More fundamental than energy
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