Kovil, I know little about the operations of the Z Machine. I study nuclear collisions at RHIC as I mentioned in the first post in this thread.
ta152h0 mentions starting a nuclear reaction (well, reaxction
), and I assume he's talking about fission reactions. Fission doesn't require high temperatures, but it does require using nuclei that are metastable to breaking apart (which is generally only true of some isotopes of heavier elements). The experiments in the Z Machine don't use such elements. While we could hypothetically use such elements in nuclear collisions studies, we don't bring anywhere near enough of them together to start a chain reaction.
Fusion reactions are yet a different story. For fusion, nuclei must be forced close enough to each other to overcome their electric charge repulsion (a long-range force) to the point where the attractive strong nuclear force (a very short range force) holds them together. For this, you need extremely high pressures (such as under the immense gravity inside a star). Again, high temperatures are not necessary. In fact, it's preferrable not to have temperatures too high, or nuclei won't even stick together any more (the protons and neutrons will have too much motion for the nuclear force to bind them). If you look up fusion experiments, they are all about generating extremely high pressures.
In both fission and fusion, energy is released, which can result in an increase in temperature. So high temperatures are an effect, not a cause.
In nuclear collisions, we do achieve very high pressures when we bang two nuclei together very hard. In this case, energy that was in the form of directed motion to begin with (each nucleus is moving very fast towards the other), can become heat as the particles begin hitting each other and scattering around (heat is in essence random motion of constituent particles), raising the temperature very high. But we collide things so hard that we're in the regime I said you don't want for fusion: the nuclei break apart! We destroy the nuclei. In fact, at RHIC, we collide them so hard that we destroy individual protons and neutrons, breaking them apart into still smaller particles.
So, to recap, in nuclear collisions:
1) We collide too hard for fusion to occur.
2) We don't have a lot of ready-to-break-apart elements around for fission chain reactions to occur.
3) And, even if we do make microscopic black holes (an uncertainty at this point), they're too small to attract any nearby matter, and too hot to live for more than 10^-23 seconds (remember that collisions like these happen in nature all the time: cosmic rays striking our atmosphere, and no black holes have swallowed the earth).
-Gene
