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Why is Controlled Nuclear Fusion so Difficult?

Short answer 1: There is no chain reaction mechanism for fusion.

Fussionable materials, mostly isotopes of hydrogen release energy after they are mashed ("fused") together. However there are repulsive forces between the fusionable nuclei, so a certain amount of energy must first be introduced to overcome these forces.

The situation is analogous to ordinary chemical reactions where a certain energy barrier(activation energy) must be overcome to start the reaction. For example ,the gasoline in the tank of your car does not spontaneously combine with the oxygen in air; It takes a spark to ignite. That spark supplies the activation energy. But, the spark by itself is not enough; the reason gasoline burns so readily is that certain intermediate products with reduced activation energy are produced. These intermediate products will increase in number as the reaction proceeds. An oxygen molecule, for example may split into two atoms of oxygen. The oxygen atoms are more reactive than the original molecule. This doubling of reactants accounts for the rapid, sometimes explosive progress of the reaction. Nuclear fission also relies on a chain reaction; the neutron is the intermediary with (very)low activation energy and that's why fission is a lot easier than fusion. Actually nuclear fission is almost too easy--most of the technical drawbacks arise from the extreme "easiness" of a fission reaction: preventing runaway nuclear reaction in a power plant is a problem. Even the scarcity and expense of suitable fissionable isotopes can also be attributed to the ease of fission;most of these isotopes have fissioned away billions of years ago.

There is no such reaction accelerating mechanism for fusion. Early researchers thought that the "heat" from a small fusion reaction would propagate and cause a self-sustaining fusion "burn", hence the term "thermonuclear". The "spark" would be provided from a smaller fission bomb and it would "ignite" an unlimited amount of fusionable fuel. But, that proved to be wishful thinking: The fission trigger must heat the entire fusion fuel mass, and must hold it together under extremely high pressures. That limits the amount of fusion fuel and therefore the yield for a given size of fission trigger. The fusion yield is limited to about sixty times the fission trigger. The first H-bomb for example produced 3 megatons of fusion energy triggered from a 50 kiloton "spark". That "spark" is more than three Hiroshima bombs! . (The ratio of sixty to one is roughly the ratio of fusion output to activation energy for Deuterium: 4Mev to 50kev. That is not a confidence ) Other types of fusion fuel will improve the ratio, but there will still remain a fixed ratio of input to output. Note that this is very different from true burning: the same spark that ignites a few drops of fuel in an engine cylinder can just as easily ignite a tanker truck

Short answer 2: Low cross section of fusion reaction sets a lower limit on the size of a nuclear reaction.

Cross section measures probability of a reaction. The higher the cross section the more probable the reaction. A chart of cross section vs reactant energy offers a more detailed description of a reaction than the just the activation energy. One can make up for a low reaction cross section by compressing the reactants to higher density. Mean free path, which is the inverse of cross section x density product is the average distance a particle travels to achieve 37% reaction probability. Summary of the reaction cross sections and corresponding mean free paths:

We briefly mentioned Ivy Mike, the 3 Megaton fusion behemoth that used a 50 kiloton bomb for a trigger. You might ask: why did they make such a big bomb to start with? why not start with say 15 kilotons and get 900 kilotons out of it? Wouldn't it be easier to start small? The answer is no; it is not easy to start a small fusion reaction That's because the low cross section of deuterium fusion forces a large size, otherwise the reactants will escape. As you can see from the table, even at a thousand fold compression the mean free paths for the deuterium fusion are of the order of a meter. So, the tank of liquid deuterium had to be a few meters in size to make sure the deuterium nuclei reacted before they escaped. it's just plain stupid to make a tiny BB size bomb but that what the National Ignition Facility is trying to do. Actually the initial (and most likely final) experiments will be with the Deuterium-Tritium reaction--that one does have some chance of fusing. The fusion cross section is about a hundred times greater than pure Deuterium. But relying on Tritium for fusion spoils the hope of "obtaining energy from water". Tritium is NOT a naturally occurring isotope that is extracted from water;it is a radioactive isotope that is currently bred in nuclear reactors. People who say that NIF will produce energy from water are just plain lying. Now, the pipe dreamers at NIF will say that the neutrons from the fusion reaction can be directed on to a target of Lithium and that will produce all the Tritium they need, but Lithium is not water.

The NIF attempts to miniaturize this to a 1 mm pellet of Deuterium Tritium mix.(solid cryogenic).

Instead of a fission initiator which is an energy producer, they will use an inefficient energy consuming UV laser. Furthermore there will be no way of recovering the neutron energy. Also the fuel won't be the environment friendly Deuterium which is harvested from ordinary water—the fusion cross section for pure Deuterium is way to low. Instead a mix of Deuterium-Tritium will be used. Tritium is a radioactive isotope; presently it can only be practically obtained as a byproduct of fission reactors.

The pellet will contain about half a milligram of fusion fuel. If all of this fuel burned with 100% efficiency it would produce the energy equivalent of about 20 lbs of high explosive—enough to blow up a tank. It's doubtful the thirty foot diameter chamber that is supposed to collect all this energy would last very long. But as we have seen from consideration of the mean free path of the reactions, it is doubtful that even 10% of the fuel will burn. In that case, each pellet will yield about 10 Mega joule of energy. (a pint of gasoline yields about 15 Mega joule).

The whole National Ignition Facility is a boondoggle and the people behind it are basically con artists. And I say that because of their deceptive press releases. Note how often Mega joule appears . Do they ever mention that one Mega Joule amounts to less than one third of a kilowatt-hour? That's less than a nickel's worth of electricity in most places. No, of course not. Likewise they never mention that their system of 192 lasers consumes at least 50 mega joules to produce one lousy mega joule. Also they hide the cost of the beautiful gold-plated hohlraum that adorns their web pages and other PR literature. Each of these jewels will be totally consumed in each laser shot.

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