Friday, February 5, 2016

He3 For Starship Power

One form of power for a starship has been discussed before in this blog, antimatter batteries. This is a nice concept, but it is dependent on unknown physical parameters, namely, the energy necessary to ignite cold antimatter with matter. If it is large compared to kinetic energy at normal temperatures, antimatter can be stored easily. If it is small, confinement would be doable only with some sort of magnetic isolation.

Fusion is much better understood. For a large, really large, starship, fusion might be a possible choice. But what kind? There are a number of possibilities. The easiest fuel to get in large quantities, pure deuterium, is not the easiest to fuse. Deuterium is ubiquitous, and can be extracted easily from water or methane or anything else an alien civilization happens to have on hand. Extraction of this isotope of hydrogen is child’s play compared to isolating isotopes of heavier elements. But the cheapest fuel is not necessarily the best.

It is possible to talk about the cross-sections for different nuclear reactions, but it is much easier to talk about two more derivative quantities. One is the temperature at which the reactions take place best, and the other is the power density that is involved. Temperature is really a measure of the speed of the nuclei in the plasma that is fusing, and power density is really a measure of how many reactions are occurring per second. On Earth, experiments are universally done with a different fuel than pure deuterium, notably, half deuterium and half tritium. It is not a question of temperature. These two reactions, deuterium with another deuterium and deuterium with a tritium, both occur at about the same temperature. However, the reaction rate, i.e., the power density, is a hundred times higher with tritium, at the outset of the reactions.

Deuterium and deuterium together produce not helium, as one might guess, but a mixture of tritium and a proton and helium 3 and a neutron. Then the tritium quickly burns up with the deuterium, but the helium 3 not very much. Helium three prefers a somewhat higher temperature to burn. As the deuterium burns, it produces tritium which then quickly burns up, producing some more energy. So the power density of the deuterium mixture climbs a bit as tritium is produced, but still stays far, far below that of a mixture of deuterium and tritium.

Temperature has implications on the size of the reactor. If we assume nothing but magnetic fields can be used to confine a plasma, and there is a limit to the magnetic field strength that can be created with any arrangement, having a higher temperature means the nuclei will need a larger turning circle to be directed back into the center of the plasma. So, having the same temperature means having the same magnetic field requirements, and about the same size reactor. DD and DT need about the same size thing-a-ma-bob to make it fuse, but the DD power density is much less, so you need more of them.

There isn’t much tritium around, as you have to make it and use it up quick as it decays almost as fast as a ripe papaya. A bit longer, fourteen years or so, but still pretty quick. You make it out of lithium, and for the time lithium can be used up as a fuel source, you can make tritium and burn it with deuterium. So, for planets and alien civilizations blessed with available lithium, the fusion big picture is you refine hydrogen to get deuterium, get some tritium seed to start but soon start using lithium as a receiver of neutrons from the reaction of deuterium and tritium so more tritium is available, and with some luck and good karma, you have a fusion reactor making helium 4 and protons and neutrons.
You are also making helium 3, which will burn up eventually, but if you flow the plasma out every once in a while, you can take out the helium 3. You can also take out the helium 4, which does you no good to sit in the plasma. It is possible to use the helium 4 for balloons, and very good freezers, and you can store the helium 3 somewhere in a big tank.

Why would you want to collect helium 3? It just so happens that a helium 3 reactor produces almost no neutrons. Two helium 3 nuclei fuse into helium 4 and protons. If you are on a starship, and have a choice of protons or neutrons, you should choose protons. Protons are turned by a magnetic field, and they don’t do much transmutation of elements into nasty radioactive ones. Protons do embrittlement, and that has to be dealt with, by annealing or some treatment, maybe as part of a recycling process, but neutrons ignore magnetic fields and just go looking for nuclei to hit and change into something else.

When you have a large quantity of radioactive elements, you have to have something around to shield biological things from the radiation produced by decay. That means some shield mass needs to be added to the starship. And you need some separation processes to take care of the reactor components, all of whom receive the neutron dose. While this discussion is at a very elementary level, it just seems like a good idea to use helium three fusion reactors in a star ship, and leave the DT and DD reactors on the ground, producing helium 3 fuel for the starship.

Perhaps there will be countervailing considerations, like having to run the helium 3 reaction hotter, meaning larger volume, but in space, volume does not necessarily mean weight one-for-one. So, clever people might come up with designs for starship fusion reactors, equipped with shadow shields and radioactivity removal processes for a DT reactor, or without nearly as much for a helium 3 reactor, and see which one has the best power to weight ratio. A few hundred years might be enough to get a definitive answer, or maybe a week for a good guess from a really clever person.

If helium 3 reactors are viable for starship propulsive and hotel power, then this has implications. It means that a lot of ground reactors are necessary to produce and collect enough helium 3 fuel for a single starship. This limits the number of starships an alien civilization could produce or rather stock. If resources provides a limit on the number of ground reactors that can exist in the integrated history of the alien civilization, then it would also provide some valuable information on how many starships could be sent out by such a civilization. There could be other resources that limit the number of starships more than helium 3 collection, but this is a start at figuring out how to get to such a number.

1 comment:

  1. Post gets you thinking about the possibilities. Would love to travel on a starship. Great post!