FUSION

To release energy from the fusion of two hydrogen nuclei, one must achieve not only high temperatures, but also a certain "quality of confinement" density times duration for which that density is achieved. The Princeton experiment, using a large torus to confine the hydrogen plasma by a magnetic field, showed that no instabilities arise (as some physicists had feared), and made the researchers confident of reaching 100 million degreesC in the next machine, to be operational in 1981 or 1982. "Breakeven," in which enough energy is generated to power the fusion machine, is still some years away, and when it is achieved (as it surely will be), fusion will be roughly in the state where fission was with the Fermi pile in 1942: It took another 25 years for nuclear fission to penetrate the commercial power sector.

How about the other routes to fusion? What happened to Dr Maglich's project of turning an ion beam back onto itself and letting the hydrogen nuclei smash into each other in a precise path rather than by random thermal motion?

Frankly, we don't know. Maglich's group is still working in Princeton Junction, amidst rumors that we cannot evaluate.

On the other hand, significant progress has been made in laser induced fusion, in which a small pellet containing hydrogen is hit from all sides by gigantic laser light pulses, causing it to implode. If it can be compressed to sufficiently small volumes under these blows, fusion will take place. Los Alamos and Lawrence Livermore Labs are both working this route. LLL has been building Shiva, the world's most powerful laser. Last May 18, Shiva was ready and delivered its first power blow via 20 beams which hit a sandgrain sized hydrogen pellet from 20 sides with a combined wallop of 20 trillion watts, packed into about one ten billionth of a second. Thus rudely hit, the squashed pellet produced 7.5 billion individual fusion reactions, which increased the previous record by a factor of 3, but was still a long way off breakeven, in which the pellet will produce enough energy to power the lasers. LLL is now developing an even mightier laser, the Nova, in order to move closer to that goal.

And there is yet another newcomer to the race - inertial confinement by charged particles. The hydrogen pellet is not hit by light, but by electron or ion beams. The method has apparently been successful in the USSR, and it is now also being vigorously pursued at Sandia Labs in New Mexico.

The basic idea is the same as for laser fusion, but there are some differences. While light will be reflected by the surface of the pellet and will push the content inwards (similar to a boat recoiling when somebody jumps into it and back out again), charged particles will penetrate the outer layers of the pellet and heat them with such vehemence that they explode, and it is this explosion that implodes the interior of the pellet into itself; from here on the story is much like that of laser fusion. To maximize the explosion implosion effect, the spherical pellets are made with two outside layers - an "ablator" on top of a "pusher."

The numbers involved are stunning. To achieve fusion, the pellet must collapse inward with a velocity of 125 miles per second (not a leisurely 125 miles per hour!) until its density has increased 600fold; the energy delivered in a short pulse (10 nanoseconds) must go in at a power level of 100 billion watts. For those 10 nanoseconds, the pellet (about 1/16th inch across) will absorb power at roughly 100 times the power consumption of all of the US!

Some reasons why this new horse is so hopeful:

The method is more efficient than laser fusion, in which electrical energy is turned into light first; particle beams can be produced with smaller losses.

One of the problems with high powered light is "Mirror, mirror on the wall, can you handle wallops of a trillion watts at all?" But particle beams can now be conducted through air in plasma channels (much like Nature does it lightning travels through ionized channels of air, the so called "leaders," which are set up just before the main stroke) and are probably easier to handle in the long run.

To hit a tiny pellet with equal power from each of 20 directions at the same instant is difficult, yet the symmetry requirements for ablation by laser light are very stringent. On the other hand, the explosion implosion mechanism, as scientists at Sandia Labs have found, is to some extent self correcting for deficiencies in symmetry, so that the symmetry requirements can be significantly relaxed.