Hydrogen Fusion

the elusive holy grail of the energy scientists

Fusion is what powers stars, including our own Sun. Therefore, I am using fusion power right now, and so are you.

Greenpeace seem to regard fusion powerplant development work as a waste of time and money. They point out that fusion research has been going on for fifty years with no commercial product resulting. Perhaps unsurprisingly, they reckon that research should be terminated and the funds channeled into building more wind turbines.

It is, indeed, a point worthy of discussion that although the hydrogen (fusion) bomb was perfected by Los Alamos scientist Edward Teller in 1952, only a few years after nuclear energy itself was discovered, here we are in the 21st Century, still lacking a non-explosive application of fusion power. Why is that?

It's not as if we need a proof of concept. We already have one in the sky, providing most of our heat and light here on Earth. Fusion works. If you still need convincing, take a look at the YouTube video of Dr Teller's application of fusion to produce an almighty bang, a forty-mile high mushroom and a large chunk missing out of a South Pacific island. Yep, it works, and it releases phenomenal amounts of energy. What's more, unlike nuclear fission, it does this using some very commonplace and inexpensive substances as fuel.

From this situation, you might get to thinking that fusion reactions must be extremely hard to demonstrate, even in a well-equipped lab. In fact, fusion is so easy to demonstrate on the lab bench that quite a few of the better-equipped home physics experimenters have done so.

The CERN facility, set up to help us understand the building-blocks of atomic nuclei, has three particle accelerators of progressively increasing power. Even the smallest of the three is over-specified for the job of demonstrating hydrogen fusion.

It is a little-known fact that fusion is quite easy to demonstrate, and has been so for many years. The Farnsworth fusor -developed by the same Farnsworth who invented the TV system which was eventually adopted by the whole world- operates using electrostatic acceleration. It is a simple enough device that many have been built as home science projects. It serves in industry as a source of fast neutrons for materials analysis. Unfortunately it cannot achieve breakeven power generation, owing to its internal losses being greater than fusion energy generated.  It nevertheless serves as an ideal demonstrator to silence the naysayers who claim that fusion is impossible.

Thus, the issue in fact isn't one of demonstrating fusion, but of achieving what researchers variously term breakeven or ignition - that is, a sustained fusion reaction which produces at least enough energy to power the reactor itself, without outside help. In other words, power out being equal to or greater than power in.

To better understand why that happens to be so much harder than simply demonstrating a fusion reaction on the lab bench, it is instructive to look at the known energy-producing fusion reactions: 

Hydrogen to hydrogen

The simplest fusion reaction of all consists of combining two hydrogen nuclei. This is what we probably think of when we talk about 'hydrogen fusion' but in reality it is a rather unusual event. Hydrogen nuclei are simply single protons. The issue here is that there is no atom consisting of just two protons, thus either the product cannot form in the first place, or would have to immediately undergo a radioactive decay into some other more stable element.

In practice, straight hydrogen fusion is what powers the Sun, and this takes place not in one neat fusion step, but by a torturously complex series of reactions, eventually arriving at helium. (Two protons, two neutrons) 

The fact that the solar reaction is relatively complex is what determines that the sun burns slowly and steadily. Which is convenient for us. Because it is so slow, it is really only feasible to generate energy this way with very large amounts of fuel. As, in a star.  For a smaller powerplant such as we might construct, the output would be too low to useful.

 

The reaction which powers the Sun involves fusing ordinary hydrogen (protium) by way of a fairly complex series of interactions, in multiple stages, to eventually form helium. Because this multi-stage reaction is relatively slow, the Sun is able to burn with a constant heat output over a very long time. The key factor in making this particular fusion route possible is one of sheer size.  With so large an amount of fuel available, and the immense gravity of a star to tightly compress the fuel in its core, the reaction is able maintain a temperature of about fifteen million Celsius, at which it is self-sustaining. Unfortunately this particular fusion process doesn't lend itself to smaller-scale implementations, depending as it does on the presence of vast amounts of fuel and strong gravity.

The H-bomb, on the other hand, relies on a much faster and more direct fusion reaction, that between deuterium (heavy hydrogen) nuclei.  It achieves ignition by way of two successive actions, very heavy compression of the deuterium fuel, and then heating to at least a hundred million Celsius, several times hotter than the Sun's core. It is notable that heating alone, even to such astronomic temperatures, would not achieve a satisfactory ignition, where energy out exceeds energy in. For that to occur, compression of the fuel is a prerequisite. The compression and heating are provided in turn by two plutonium chain reactions. The duration of resulting fusion event is extremely short, at about 40 nanoseconds. It stand to reason that this approach can only function once, since it will destroy the equipment which creates the conditions for fusion.

So, I think it should be apparent that our two working examples of breakeven fusion both depend on their own rather specific, and hard-to-replicate conditions, to achieve ignition. A powerstation reactor isn't going to have billions of tons of fuel on-hand, so the Sun's approach to fusion is not so suitable here. Exerting gigabars of pressure on whatever quantity of fuel the reactor uses will not be easy either, although that may not be quite as impossible as it sounds.