May 11 2021

Magnets and Fusion

Technology is often interdependent. Electric cars are dependent on battery technology. Tall skyscrapers were not possible without the elevator. Modern rocketry requires computer technology. And the promise of fusion reactors is largely dependent on our ability to make really powerful magnets. Recent progress in powerful magnet technology may be moving us closer to the reality of commercial fusion.

Fusion is the process that powers stars. Stars, like our sun, start out as mostly hydrogen. Their intense gravity will squeeze that hydrogen gas into a dense ball, causing the hydrogen to heat up to millions of degrees – 15 million degrees C. At this temperature the hydrogen is stripped of its electrons, forming a state of matter known as plasma. In fact, most of the normal matter in the universe is in the plasma phase. A hydrogen nucleus is basically a proton, which has a positive charge. Like charges repel, so all those positive protons are trying to push each other apart. This is overcome by the power of gravity. If the ball of hydrogen is massive enough then the core will be compact enough that the hydrogen ions will be fused together into helium. This process releases a tremendous amount of energy and heat, which further pushes the star outward. Stars then reach an equilibrium point where the outward pressure of fusion and magnetic repulsion balances the inward force of gravity. When enough helium builds up in the core, the hydrogen in the outer layers of the core is no longer dense enough to fuse, so the star collapses until the pressure is great enough to fuse the helium together. This keeps happening, depending on the mass of the star (it has to be massive enough to fuse the heavier elements) until the most massive stars get to iron in their core. Iron does not produce energy when it is fused, so it cannot act as fuel to keep the star going. The core will then collapse and result in a supernova.

Scientists are trying to reproduce the fusion of hydrogen into helium on the Earth. We have already done this in one-off explosive events, called hydrogen bombs. But we want to do this in a steady controlled fashion in order to access all that heat energy to drive turbines and generate electricity. This has been a project for decades, and despite steady progress never seems to get closer (like running down a hall in a horror movie with the camera effect that makes it look like you are making no progress). But once again we are being told that this time they really mean it and we are getting close.

The most common fusion reactor designs being worked on today use a powerful magnetic field in order to squeeze the hydrogen plasma into a dense ball to get it to fuse – using magnetic fields to do the work of gravity in the sun (the other major design is inertial confinement using lasers). One common magnetic design is the Tokamak, which looks like a doughnut. The UK is working on a more apple-shaped design. Regardless of these details, what matters most is the power and efficiency of the magnets. The key is that the reactor must generate a powerful enough magnetic field to produce fusion without costing more electricity than the fusion will generate (which would be pointless). This was first accomplished in 2014, with a 150 picosecond fusion reaction done on an experimental (not production) scale. This was a great milestone, but still does not put us in range of a commercial fusion power plant.

A new magnet design, however, may finally get us to this point. As the BBC reports:

Weighing 10 tonnes, the D-shaped magnet is big enough for a person to step through. Around 300km of a very special electromagnetic tape is wound into that D-shape.

The tape itself is a feat of engineering that has taken decades to develop. Thin layers of superconducting rare-earth barium copper oxide (ReBCO) are deposited on a metal tape. When cooled that bundle of tape can conduct electricity extremely efficiently, which is essential as 40,000 amps will pass through it, enough electricity to power a small town.

The tape is superconducting at -253 C, which is warm for a superconductor and is practical to cool. Superconducting makes the magnets energy efficient. With this new design the UK company believes they can have a commercial prototype by the early 2030s. This is similar to efforts in the US and Europe. Even if we are being optimistic, that would mean that fusion plants would be putting energy into the grid probably no earlier than 2050. This is definitely an energy solution, if everything works out, for the second half of this century. It also remains to be seen if fusion reactors will be cost effective – we have to consider how advanced wind and solar will be by 2050. But they may be competing more with nuclear fission and whatever remains of fossil fuel energy production. So perhaps ideally we will build and maintain fission plants to provide baseload energy for the next 30-60 years, displacing all fossil fuel for energy production. As the fission plants we start building now begin to retire perhaps we can replace them with fusion plants, starting around 2050 or so.

Fusion is considered a green energy, as the energy production itself does not produce CO2. There will be some released in the construction of the plant, which is true of all big power plants. Fusion plants will use deuterium, an isotope of hydrogen with one proton and one neutron, that is widely available and can be made from water. They also use tritium, with two neutron, which can be bred in the reactor itself. The end product is helium, a noble gas. Perhaps that helium can be used to fill a new fleet of Zeppelins for low carbon cargo transport. Or, helium-3 can be used as fuel in some forms of fusion reactors. He3 can also potentially be mined on the Moon.

It is, of course, hard to say now if all this is in our future, but it’s possible. Various projects are competing to the finish line of commercial fusion, because the payoff will likely be great.

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