How does fusion power ‘work,’ and will it ever be viable?

 

 If we are to continue advancing as a species and consuming more and more power per person, then there are only two possible endpoints for human power production, and they’re both fusion. Either we figure out how to soak up and use a large portion of the energy falling on the Earth from our solar system’s huge, distant fusion reaction (solar power) or we figure out how to create and sustain smaller, more manageable fusion reactions right here on Earth (fusion power). In either case, the energy that could possibly let the Earth’s entire population ascend well beyond a modern first-world lifestyle is contained in the very makeup of the universe itself.
First principle: Matter and energy are interchangeable and, in a certain philosophical sense, are basically the same thing entirely. Einstein was the one who first put this idea into mathematical form: Energy is equal to mass times the speed of light, squared (E=mc²). Remember that c, the speed of light, is a finite number, so c² is a finite number as well — an absolutely enormous one. So, without needing too much mathematical education, we can see one thing right off the bat: If this equation is correct, then just a tiny, tiny bit of matter corresponds with a whole, whole lot of energy.
Take two protons (we can think of these as being nuclei of the element with one proton, hydrogen), and weigh them. Now, fuse those two separate protons together to form a single two-proton atom (a helium nucleus), and weigh this product again. What you’ll find is that the fusion product weighs very, very, very slightly less than the individual protons that went into it. And since we all know that matter cannot be created or destroyed, there’s only one possible explanation: That infinitesimally small amount of lost matter has been converted into an astonishing amount of energy.
Primarily, that energy is released in the form of heat. In principle, we should be able to use this energy the same way we do almost every other type heat source: Boil water to make steam to turn a turbine to make electricity. The problem is overcoming all the practical impediments to actually doing this.

Here’s the tokamak at the JET fusion lab in the UK – a smaller version of the one bound for ITER

The first problem with fusion power is fusion itself: How do we do it? There are a number of ways, but the simplest are not at all useful for power production; a thermonuclear device triggers a fusion bomb by using the explosive force of a small fission bomb, for instance, but nuclear-bombing a pellet of hydrogen fuel just isn’t a sustainable option for power generation. On the other hand, we can already safely and reliably force fusion between single atoms in high-powered particle accelerators, but fusing just two individual atoms into one won’t release the volume of energy we need. Particle accelerator aren’t structured for harvesting heat as power, in any case.

The X-Ray “Z Machine” studies fusion problems for Sandia National Labs.

So, the challenge of creating fusion has led to two major schools of thought: Either we use simple physical force to collapse a sample of hydrogen down so powerfully and rapidly that the atoms at the center begins to fuse (called inertial confinement), or we use high-powered magnets to contain the hydrogen sample as we heat it further and further to create fusion through simple input of energy (called magnetic confinement). Inertial confinement has to create its implosive force with batteries of high-tech lasers, or even huge mechanical hammers, while magnetic confinement requires equally finicky and expensive magnetic tokamak rigs.
In both cases, the challenge is not really creating fusion, but sustaining it. The first fusion reaction, created by us, has to release enough energy and do it in such a way as to cause further fusion reactions in the sample, which in turn cause more fusion reactions, and so on. This is basically the cascading reaction that continues uncontrollably in a thermonuclear bomb, but this time in a form that can be controlled — mostly because it’s occurring in a fuel pellet that weighs only about a millionth of the one we load into a hydrogen bomb.

Right now, fusion power schemes are all held up in basically the same place: getting more power out of a fusion reaction than we need to insert to keep that reaction going. Put differently, the challenge is learning how to create fusion for little enough energy in that the energy we get out can still be used to make some net amount of electricity. All modern research reactors can create fusion, and most can even sustain it to some extent, but they currently all have to spend far more electricity to do so than their fusion reaction could ever be used to generate.
One laser-based (inertial) approach did manage to get more energy out of a fusion reaction than the fusion fuel took in, however the fusion fuel only took in a tiny fraction of the overall amount of laser energy they shot at it — still a big milestone, but only one of the two they’ll need to pass to generate their first joule of net electricity.

This wouldn’t be a problem with fusion…

Should we ever actually get it working, the advantages of fusion power would be enormous. Fusion power uses as its fuel isotopes of hydrogen, which does not need to be mined from the ground. It releases no airborne carbon or other atmospheric contaminants of any kind. A fusion plant would also produce no long-lived toxic byproducts in need of disposal.
Like a fission reactor, fusion reactors would need to be heavily shielded to contain the radiation the reaction produces, but unlike a fission reactor, we wouldn’t need to worry too much about explosions. The heavy hydrogen isotopes used to create fusion aren’t inherently very radioactive when just sitting there, as uranium, plutonium, and thorium are, so we don’t have to be so concerned if they get accidentally strewn about a couple of kilometer radius. Tritium can be a bit hazardous if it enters the body via air, food, or water, but its half-life in the body is very short, and only chronic exposure would likely cause real medical issues.
So, we still hope for fusion breakthroughs. It could be a source of almost infinite abundance for mankind. We don’t yet know how much a final reactor might cost to build, or how low we might be able to bring the costs of fuel production. But only we humans can learn to keep a star as a pet, and to do so as cheaply as possible.