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Name: Unknown
Status: N/A
Age: N/A
Location: N/A
Country: N/A
Date: Around 1993


Question:
World's only operational fusion reactor is the sun. Probably the most practical application of fusion energy is converting photons to electrons using photo cells, or converting infrared into hot water using roof-top collectors. Why have you guys not figured this out yet?



Replies:
Well, That is a good point. However, there are actually 10^24 or so other "operational fusion reactors" out there besides the sun... The universe is pretty big, and maybe our perspectives are all too narrow! The biggest problem with solar power is it is practically impossible to concentrate - it needs to be collected over huge areas, preferably sunny areas. Actually, the easiest use of solar power is hydroelectric (how did the water get up there in the first place?) and as you probably are aware, this already accounts for quite a substantial portion of electrical generation, but huge hydro power projects (the most efficient way of using it) tend to cause huge environmental catastrophes, which people are no longer willing to accept. Would other solar schemes be less environmentally unfriendly? What about all the chemicals used in semiconductor manufacture to make all those solar cells? How long do those things last, anyway? And I had a friend with a house near New York who had one of those roof-top collectors. You want to buy a house that only has hot water in the summer? Fossil fuel use also uses "old" solar power - energy sent by the sun millions of years ago (unless Thomas Gold is right about where oil comes from). The only truly non-solar power sources on the earth are geothermal, tide-based, and nuclear (fission or fusion), and only the nuclear ones are likely to produce a substantial amount of power - anyway, I also think solar energy will eventually be our best source.

A. Smith


The major hurdle in inducing fusion is getting the fuel nuclei sufficiently close to each other so that the nuclear strong force, which binds the protons and neutrons in a nucleus, can kick in. As you know, nuclei have positive charges so they repel each other. A number of strategies have been suggested: (1) Heat the fuel (usually some isotope of hydrogen) to a very high temperature so that the nuclei will be moving very fast; this enhances the probability that the colliding nuclei will get close enough to fuse (that is how the sun works). The problem is confining the heated fuel. The sun does it with its gravity, a feat we cannot accomplish on earth. Nor can we use a material "bottle"; when the hot fuel touches the walls of the container, it cools the fuel. So there have been attempts to contain the fuel with a magnetic "bottle", but this has also had a lot of technical difficulties. (2) Blast solid tritium "pellets" with intense laser light. The heating takes place so rapidly that there is not a need to confine the fuel. This would produce bursts of fusion, instead of a continuous fusion reaction So far, the energy needed to induce the fusion exceeds the energy produced. (3) "Cold fusion". This uses the ability of certain materials such as palladium to absorb hydrogen. The palladium electrons partially shield the hydrogen nuclei from each others' positive charges, and it was believed that the nuclei could be caused to get close enough for fusion to take place. So far, this appears to be a dead end. (4) Perhaps the most exotic idea: induce the replacement of electrons in hydrogen atoms by muons. This is referred to as muon- catalyzed fusion, and sometimes is also called cold fusion. Because the muon is much heavier than the electron, it orbits at a much smaller distance from a hydrogen nucleus, providing the same sort of shielding as in (3). Then the two nuclei in the hydrogen molecule can, in theory, get close enough for fusion to occur. The problem here is that muons are unstable; they decay about 2 microseconds after being produced. So they would have to be produced in large quantities then efficiently directed into the fuel to induce fusion before they decayed.

R.C. Winther



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