The news of a fusion reactor at the Joint European Torus (JET) in mid-November came after press time, and I only just managed to shoehorn it into the "Echoes and Updates" column. Looking at it closer now, we will find it less of a big deal than the sensation-crazed media dunces thought.
To release nuclear energy from elements at the low end of the chemical table (hydrogen and its isotopes deuterium, tritium are no.1, helium is no. 2), one must fuse two nuclei together rather than split them as at the high end (uranium 235, plutonium 239). This is not easy, because both nuclei have the same positive electric charge and therefore repel each other. To overcome this repulsion and make them crash into each other, they are given a very high velocity, which in particle physics is equivalent to giving them a very high temperature. The well known Lawson Criterion gives the minimum values of pressure (amount of atoms), temperature and duration for which the nuclei must be contained to start spontaneous fusion or "ignition," where part of the released energy is used to keep the temperature and pressure at a point where the fusion will continue on its own without an external energy input. This point is called "scientific breakeven," because its energy budget is balanced
¾no external energy need be supplied after ignition has taken place.But contained in what? There is no known material that will contain particles at velocities corresponding to millions of degrees centigrade; the only way is to ionize the hydrogen (or deuterium with a proton and a neutron in the nucleus, or tritium with two neutrons and a proton) so that its ions will follow a path that can be bent in a magnetic field. Many types of "magnetic bottles" have been devised, but the one that appears the most hopeful is the Tokamak, a Russian acronym for a toroid or donut-like structure with a strong magnetic field around the donut ring and a current of a gas of hydrogen isotopes racing in a circle through the hollow donut.
What happened in England was not scientific breakeven. It was an admirable breakthrough in that the large output energy (20 MW for almost two seconds) exceeded 3% of the input energy, i.e. amounted to more than 3% of breakeven, and it was the greatest amount of power ever liberated from a fusion machine. For this the European scientists deserve great credit.
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Vol. 19, No. 5
Newsletter: Access to Energy Newsletter Archive Volume: Volume 19 Issue/No.: Vol. 19, No. 5 Date: January 01, 1992 09:50 AM Title: Articles of Impeachment
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