Fission weapons (aka "atomic bombs") did bring an end to World War II, but nuclear scientists did not rest there. The second way to use nuclear physics to release vast quantities of energy is by nuclear fusion. By 1951 the first fusion weapon had been designed, the Teller-Ulam thermonuclear weapon (aka "H-bomb"). Fission was now old-hat, fusion was tapping the same source of power as the freaking sun. It was the energy of the future.
And as year overtakes year, fusion power remains the "energy of the future", it never becomes the energy of today. As with most things it is far more difficult to do something constructive than to do something destructive. Scientists all over the world have been trying to develop fusion power since the 1950's, and they are still far away from the "break-even" point (where they actually get more energy out of the fusion reactor than they put in to kick start it). They keep working on it, though, because the benefits are huge. You get more energy from a gram of fuel, there is no chance of a runaway reaction (it is hard enough just to keep the reaction running), no chance of large-scale releases of radioactivity, little or no atmospheric pollution, the fuel is mostly harmless light elements in small quantities, waste has only short-lived radioactivity, and it does not produce weapons grade plutonium as a by-product.
There are two basic operations possible in the universe, analysis and synthesis. That is, breaking one large object into smaller parts, or assembly smaller parts into one larger object. The ancients called this "solve et coagula." With fission, you take one large unstable atom and break it into fission fragments (aka "split the atom"). With fusion, you take two or more small atoms and fuse them into one larger atom.
In both cases, when you weigh the things you start with and weigh the result, you will find the result weighs less. This is know as the binding energy mass defect. It represents the amount of matter that is turned into energy. Everybody knows that e = mc2, but unless you've had a physics class you may not know that c (the speed of light in a vacuum) is a mind-boggling huge number, and squaring a mind-boggling huge number makes it astronomically huger. Bottom line is that microscopic amounts of matter create titanic amounts of energy.
The conversion is 1 atomic mass unit = 931.494028(±0.000023) MeV.
As a side note, fission and radioactive decay makes atoms become smaller atoms, until the atoms become atoms of lead, where they are stable (i.e., they do not decay or otherwise undergo fission). Fusion, on the other hand, releases energy as you fuse larger and larger atoms, until the atoms grow such that they are atoms of iron. After than, fusing heavier atoms actually consumes energy instead of releasing it.
Golden-aged science fiction authors E.E."Doc" Smith and John W. Campbell jr. noted this and postulated space-opera science that required elements in the middle of the periodic table for direct conversion of all the mass into energy. In Doc Smith's "Skylark" series the element was copper (63 nucleons) and in John Campbell's The Space Beyond the element was iron (56 nucleons). But I digress.
|p||Proton, ionized Hydrogen||1.007276|
(or Neutron Radiation)
|1H||Hydrogen-1, common Hydrogen||1.00794|
|3He||The infamous Helium-3||3.014932|
|4He||Helium-4, common Helium|
(or Alpha Radiation)
|7Li||Lithium-7, common Lithium|
|11B||Boron-11, common Boron||11.00931|
The Particles table gives the symbols of the various fusion fuels. The particle mass is given if you want to amuse yourself by calculating the binding energy mass defect of various reactions.
Jerry Pournelle is pretty sure tramp spacecraft owners will call Deuterium "Dee". For the same reason people call automobiles "cars."
Tritium is annoying since it has a fast half-life of only 12.32 years; e.g., after about twelve years half of your tritium has decayed into Helium-3. Use it or lose it. This is why there are no tritium mines. Most reactor designs that use tritium incorporate a tritium breeder.
The infamous Helium-3 is often touted as an economic motive for space industrialization, unfortunately it is not a very good one. There are no Helium-3 mines on Terra, so it is hard to obtain. Space enthusiasts trumpet the fact that there are helium-3 deposits on the moon that can be mined, but they don't mention that it is in a very low concentration. You have to process over 100 million tons of Lunar regolith to obtain one lousy ton of helium 3.
It is possible to manufacture the stuff, but it takes lots of neutrons. Basically you breed tritium and wait for it to decay. There is lots of helium-3 available in the atmosphere of Saturn and Uranus, if your space infrastructure is up to the task of traveling that far from Terra. Helium-3 concentration is estimated at about 10 parts per million, which beats the heck out of Luna. Jupiter has helium-3 as well, but its steep gravity well makes it uneconomical to harvest.
Helium-4 is also called an alpha particle. It is a charged particle. This means that any fusion reaction that produces alpha particles can be used to generate electricity. The particles are directed by magnetic fields and trappped to extract electrical current. This can be useful if you wish to use that reaction for both propulsion and ship's electricity.
|D + D||⇒||T|
|p + 11B||⇒||3×4He||8.7||69.97||100%|
|D + T||⇒||4He|
|D + 3He||⇒||4He|
+ 2 ve
+ 3 γ
|n + 6Li||⇒||T|
|n + 7Li||⇒||T|
- Reaction: Input fusion fuels ⇒ reaction products (for example the fourth row shows that fusing one nuclei of deuterium with one nuclei of tritium results in one helium-4 nuclei, one neutron, and 17.6 MeV of energy)
- MeV / fusion: mega-electron-volts of energy from each individual fusion event (as per the equation above)
- MeV / particle: mega-electron-volts of energy in each particle
- TJ/kg: Terajoules of energy from one kilogram of fusion fuel (I calculated this, use at your own risk)
- Thermal / Neutrons / Radiation: Breakdown of energy released into thermal energy, neutron energy, and Bremsstrahlung radiation energy
- 1000 MW burn g/s: grams per second of fusion fuel requred to burn at a rate of 1000 megawatts (I calculated this as well, treat this also as suspect)
- L-C: Lawson criterion, how hard it is to start and maintain the reaction
- A-N: Aneutronic, does the reaction produce neutrons? Please note that even if there is no n symbol in the results column neutrons can still be produced by side-reactions
- Exhaust velocity: Exhaust velocity for a pure fusion engine
The Reaction table displays the various fusion reactions that look promising for power plants and spacecraft. Note that the Deuterium + Deuterium reaction has two possible outcomes and thus two rows in the table. Each outcome has about a 50% chance of occurring. The two lithium reactions are not power or rocket reactions, they are the tritium breeding methods mentioned above.
There are many fusion reactions, but only a few are suitable for use as power sources or rocket fuels. There are lots of limitations that you can read about here. Of the candidates, you want to use those with low Lawson criterion, which measures how hard it is to start and maintain the reaction. It is a plus if the reaction only produces charged particles, since these can be turned into elecctricity directly, instead of having to be converted into heat first.
Finally it is a plus if the reaction does not release neutrons, because they are not only dangerous radiation, but they have the nasty habit of weakening engine parts ("Neutron embrittlement"), and transmuting engine parts into radioactive elements ("Neutron activation"). Unless you are using the neutrons to breed tritium. In addition, the fusion reaction energy used to make neutrons is basically wasted since they radiate isotropically. That means they produce zero thrust. Though there is theoretically a way to turn the neutrons into thrust and avoid the need for radiation shields with Nuclear Magnetic Spin Alignment.
The D + 3He reaction is of particular interest for rocket propulsion, since all the products are charged particles. This means the they can be directed by a magnetic field exhaust nozzle. Having said that, while D-3He is aneutronic, if you mix a bunch of deuterium and helium-3, some of the deuterium is going to be wayward and insist upon fusing with other deuterium instead of helium-3 like you want. Sadly D-D fusion reactions do produce neutrons. In theory it is possible to use spin-polarized 3He in the fusion fuel to absorb the neutrons. You will get less energy out of each gram of fusion fuel, but with the advantage of a lot less deadly neutron radiation.
The p + 11B reaction is the celebrated Hydrogen-Boron fusion, sometimes called "thermonuclear fission" as opposed to the more common "thermonuclear fusion". It too is aneutronic, but it does have two nasty side reactions. One makes a Carbon-12 atom and a gamma ray, the other makes a Nitrogen-14 atom and a neutron. The first side reaction is quite a bit less likely than the desired reaction, but gamma rays are harmful and quite penetrating. The second side reaction occurs with secondary alpha particles before they are thermalized. However the main draw-back is a truely ugly Lawson criterion. D+3He only has a Lawson of 16, Hydrogen-Boron has an overwhelming 500. On the plus side, in theory the Lawson criterion can be lowered by using antiprotons as a catalyst. Recently (2015) one study suggested that using picosecond laser pulses instead of microsecond could make things easier. In 2018 Chirped pulse amplification lasers were suggested to ignite hydrogen-boron fusion, papers here, here, and here. This has been patented by HB11 Energy Pty. Ltd.
The Deuterium-Tritium reaction is easy to ignite (low Lawson criterion), but it uses that pesky decaying tritium. Hydrogen-Boron (a proton is an ionized hydrogen atom) has the advantage of being aneutronic, but is very difficult to ignite, with a whopping Lawson criterion of 500! Helium-3+Helium-3 is also aneutronic, but helium-3 is hard to come by. Which is probably why I could not find any source quoting its Lawson criterion.
The Deuterium-Deuterium reaction looks sort of lackluster, and it is. But only if you stop there. Notice that of the two reaction chains one produces tritium and the second produces helium-3. Both of which will react with deuterium. The idea is to send the reaction products into an afterburner with deuterium to extract even more fusion energy. This is called the "Catalyzed D-D fuel cycle". The other even more attractive option is Catalyzed D-D + D fuel cycle, where you send the neutrons into a deuterium breeder.
Proton-proton fusion is what the Sun uses, and what Bussard Ramjets would like to use. Four protons fuse to create an atom of helium-4 and 26.73 MeV of energy. Trouble is that the Lawson criterion is off the top of the chart. Trying to get four protons to simultaneously fuse is almost impossible, short of using an actual star.
Ultra-dense deuterium (UDD) is an exotic form of metallic hydrogen called Rydberg matter. As you can probably figure out from the name the stuff is dense. Real dense. As in 1028 to 1029 grams per cubic centimeter dense. About a million times denser than frozen deuterium.
For our purposes the interesting point is it is about 150 times as dense as your average pellet of fusion fuel when laser-compressed to peak compression. Yes, this means do you not need metric-assloads of laser energy to crush the fuel pellet, a pellet just sitting on the table is already at 150 times the needed compression. It is pre-compressed. All you need is a miniscule 3 kilojoules worth of laser energy to ignite the stuff. That is pocket-change compared to what 200-odd compression lasers require. In fact it is so little that a single laser can handle the job. This results in a vast savings on laser mass and capacitor mass.
The laser pulse has to be quick, so the power rating is a scary 1 petawatt. But by the same token since the pulse is quick, it only require the aforesaid 3 kilojoules of energy.
Since you do not have to compress the fuel you can avoid all sorts of inconvienient hydrodynamic instabilities and plasma-laser interation problems.
You also have virtually unlimited "fusion gain". Meaning that with a conventional IC fusion engine there is a maximum fuel pellet size due to the hydrodynamic instabilities and the geometric increase in compression laser power. With UDD you can make the fuel pellet as large as you want (well, as large as the engine can handle without blowing up at any rate). With other laser intertial confinement fusion, if you make the pellets larger, you have to make the laser array larger as well. Not so with the UDD drive. The fusion gain depends solely on the size of the pellet, you do not have to make the lasers bigger.
An important safety tip: since UDD has such absurdly low ignition energy, there is a statistical change a large number of UDD atoms would undergo fusion spontaneously. This dangerous instability means the spacecraft will carry ordinary deuterium fuel and only convert it into UDD immediatly before use.
The cherry on top of the sundae is UDD fusion does not produce deadly neutron radiation. The reaction is aneutronic. Instead it produces charged muons, which are not only easier to deal with, but also can be directly converted into electricity. Left alone, the muons quickly decay into ordinary electrons and similar particles.
And since deuterium is plentiful in ordinary seawater, you do not have to go strip mining Lunar regolith or set up atmospheric scoop operations around Jupiter were you to use a fusion reaction requiring Helium-3.
Sounds too good to be true, I hear you say. Well, there are a couple of drawbacks.
The minor drawback is that D-D fusion has a specific impulse (and exhaust velocity) which is about half of what you can get out of D-T fusion or D-He3 fusion. This drastically increases the mass ratio required for a given mission delta-V. Having said that it is still much better than what you'll get out of chemical or fission engines.
But the major drawback is UDD might not even have that magic ultra-density.
You see, the vast majority of the UDD-related papers has been published by a single scientific group at University of Gothenburg, Sweden, led by Dr. L. Holmlid. Currently there are no third-party confirmations about UDD observations and generally very few discussions about it in the scientific community. Until the density figure is confirmed, it might be all a pipe-dream.
For example, the Deuterium-Tritium reaction is rather nice, except for the two aforementioned problems.
Unstable tritium has a half-life of only 12 years. That means in 12 years half of your tritium has decayed into helium-3. 3He is useful, but not when you want tritium.
Neutron radiation help kill strong bodies, three ways. But a big annoyance is they represent wasted fusion energy. We want all the reaction energy becoming thrust. Since neutrons radiate isotropically, they provide a net zero thrust.
A tritium fuel breeder attempts to combine the problems so they solve each other.
What you do is take an engine that ses the D-T reaction, and wrap the reaction chamber in a blanket of liquid lithium-6 (in a tank). The lithium acts like a radiation shield, sucking up most of the neutrons. This mostly solves the radiation problem. Secondly, when a lithium atom absorbs a neutron, it transmutes into a helium-4 atom and a tritium atom. The lithium blanket breeds fresh tritium fuel. This is neutron activation but in a useful form. Breeding tritium partially solves the problem of neutrons wasting reaction energy by recycling some of them into fresh fuel. Example engines using tritium breeders are the Gasdynamic Mirror and the fictional STARFIRE Fusion Afterburner
As a side note, there is an unobtaininum method of forcing the neutrons to produce thrust and preventing them from destroying the engine. This is called Nuclear Magnetic Spin Alignment. It breeds zero new fuel, but rocket designers will gladly trade that for the prospect of neutron thrust and avoiding the need for massive radiation shields.
Justin Ball had an interesting idea involving a deuterium breeder which he explains in his paper Maximizing Specific Energy By Breeding Deuterium. Granted that deuterium is totally stable and is not particularly rare, but hear him out.
The idea is to use a Deuterium-Deuterium fusion reaction wrapped in a blanket of ordinary water. The deadly neutrons will be soaked up by the water. Water molecules are composed of oxygen atoms and hydrogen atoms. Neutrons more or less bounce right off oxygen atoms, but they will be absorbed by hydrogen atoms. Which transmutes the hydrogen into … deuterium fuel.
Dr. Ball calls this "Catalyzed D-D + D" fuel cycle. From the outside it looks like D + D ⇒ 4He + 23.8 MeV, which is a fantastic energy release.