Tuesday, September 1, 2015

Fusion Power and Alien Civilizations

Energy is the lifeblood of a civilization. Any alien civilization that reaches the point where they can do star traveling must have very robust energy sources. They also need robust ways to store and transport energy, and then they need robust ways of using it. Choices seem very limited.

Energy is, at the top level, the domain of physics, a subject we have learned something about in the last couple of centuries. Energy in the universe is governed by the rules of quantum mechanics and its extension to particles. There are only a few types: among particles, there is the strong force and the weak force. These are involved with nuclear binding energy, which is released in fission and fusion processes. Since the universe consists mostly of hydrogen, protons to be exact, fusion is one of the two dominant sources of energy. Next comes electromagnetic force, and this is involved in chemical energy. Lastly comes gravitational force. That’s all there are that we know of, and we have seen no failures of energy conservation that would indicate there is any other, with the possible exception of what is called ‘dark energy’.

Life on earth have been making use of solar energy, which is fusion, since the predecessors of chlorophyll evolved. Fusion releases energy in the sun’s core, heating the outer shell, which radiates photons at the planets. Gravity maintains the pressure in the sun, allowing the internal temperature and pressure to rise past the threshold where fusion starts. So our life is made possible by a partnership between strong force and gravitational force, mediated by heat and then electromagnetic radiation. Everything working together for our benefit.

Some of the radiation is used to form chemical energy, which is transferred around, resulting in edible food, fossil fuels, and combustible materials. Some of it is used to differentially heat the oceans and atmosphere, which, being fluids, like to flow around. This can be tapped by us with windmills, and sailing vessels, including those which use the major currents in the oceans. Gravity has not proved to be very much use locally, with just a little tidal power being used, plus some geothermal energy. We use it a little in storage, with dams on the long term, and aircraft on the short term. After all, we don’t have to fly down under power.

This is enough to get a civilization going, and well on its way to developing technology to a high degree. But then technology is needed to grasp the more potent forms of energy locally, not just from the remote source of the sun. We on Earth have witnessed the birth of fission power, but have just scratched the surface of what can be done with it. As for fusion power, ideas and experiments are underway, progress has been made, but no conclusive victories yet. This is our point in time.

To bring the focus around to the main point of this blog, where are the aliens, we need to ask if there are any visible Great Filters arising in the energy arena. They could arise early on, if there were no trees to provide combustibles at an early stage in civilization’s development, or a bit later, if there were no or very little fossil fuels. If the planet had been around for too long, and its U235 had decayed too far, this would be a third possible Great Filter. They have already been discussed.

Past fission is fusion. This is the power source where fuel would be either very scarce or unlimited. Fusion can be done with a mixture of deuterium (D) and tritium (T), which is the easiest mixture to fuse, as tritium has a high cross-section for fusion with deuterium, compared to other reactions. Tritium has a half-life of about 14 years, which means it disappears shortly after you find some. It has to be made industrially, and fission reactors have been our only source. A fusion reactor is supposed to generate its own from lithium, which reacts with the extra neutron spewed out of the D-T reaction. A clever design might produce more than was consumed.

The status of current research on Earth has been that predicted successful results have been elusive. There are three ideas for doing D-T fusion. Two involve quick pulses of heating in which the D-T mixture fuses and produces energy, to be repeated in rapid succession. One uses magnetic fields to compress the D-T mixture, Z-pinch, and the other uses lasers, the Inertial Confinement Fusion way. The third way involves longer holding time, using large toroidal magnetic fields to hold the hot, fusing plasma, the tokomak plan. All three are being funded and are making gradual progress.

If you step back from them and observe the processes that are being followed, the rate of progress is quite understandable. Some observers, pessimists all, note that the early predictions of when the first nuclear burn would be achieved have not been met. This is simply an indication that the theory of confinement and fusion is complicated, and requires much experimental work to clarify it. There is no reason to think that fusion power will not be achieved, but picking deadlines is not a promising approach.

When we write about alien civilizations, it is about ones which are a millennium further along in science than Earth is, at the very least, and likely many millennia or even millions of years further. Technology is over in these civilizations, as everything has been done. They would understand fusion, and how to do it, and what can be done. We can only make educated guesses.

As noted in another post on technological determinism, looking at the different possibilities is one way to deal with uncertainty about the future. The future of fusion might be that D-T fusion never reaches energy payback, in other words, no machine that can be made will return more energy that was required to build it and run it and fuel it, and also dispose of it and its waste. It wouldn’t matter if D-T burn was achieved, as it would only be a technological triumph, not an economic one. As the machines are forced to grow more complex and involve more costly parts, this possibility becomes more likely. A second alternative is the exact opposite, that a D-T machine of some sort can be built and can produce energy surpluses for the civilization to enjoy.

The same two alternatives exist for the other two varieties of fusion, D-He3 and D-D. He3 is rare, but does not decay; it is a stable isotope.

With the first two successes, but not the third, there is a looming exhaustion problem, when lithium or He3 run short. This could be, depending on the planet, or rather the solar system as these materials are light and easy to transport, a long time, millions of years, or a shorter time, tens of millennia. This is a migration issue.

With the third success, other factors rather than energy would determine the longevity of an alien civilization on its planet. D is available in great quantity wherever hydrogen is.

All this discussion has been about fusion as a Great Filter for a planetary power source. The second possible Great Filter is about fusion as a spaceship power source, both for hotel load and for propulsion. If D-D fusion is not possible economically on the home planet, could it still be useful as a spaceship power source? If one of the large cost factors in the negative result for economic fusion reactors of the D-D type was the extraction cost of the fuel, then it might be possible. The home planet would bear the cost of providing the fuel. Likewise for the construction costs. But if the problem was that the operational power was greater than the net output power, it would not be.

Another factor to take into account is the minimum size of the reactor. If it is huge and heavy, no small ship could use it. Probes could not, small exploration vessels could not, perhaps even small colonizer ships could not. It might be that only ships so large that they would break the bank of the home planet could use D-D fusion power. This is another alternative for a fusion for space travel Great Filter.

Thus, it would be interesting to see if a non-fusion power source, such as fission, could be used for small probes. Here on Earth we have built something smaller in scale, radioisotope power supplies, for planetary and nearby probes. Possibly a small reactor could be constructed in such a way that a probe can use it.

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