The things I've read on Nuclear Fusion as an energy source, and why its less and less like "always 30 years away."
First, the basics. Somehow, when light elements (from hydrogen all the way to iron) fuse (come together), energy released. However, to get this done, you need extremely high temperatures to come together, as you need the positively-charged nuclei to smash together, overcoming their repulsive force (things of the same charge repel each other).
Second - how it works. This page here is a good explanation: https://www.iter.org/mach/Tokamak. In a doughnut-shaped chamber, magnetic fields are applied to the fuel (in this case, isotopes of hydogen - deuterium and/or tritium), and eventually, the gas bcomes a plasma - a cloud of positively charged nuclei and the electrons. The magnetic fields confine the plasma, and "squeeze" it, until it reaches millions of kelvins and enormous pressures. This doughnut shape chamber is known as the tokamaks - for the Russian derivation of the design. ITER - what used to be called the International Thermonuclear Experimental Reactor, but now just "ITER" - is the international organisation comprising several countries that are funding and building the reactor. ITER will be the 'prototype' for an even larger design called DEMO (DEMOnstration Power Station(?)) - where they hope they might demonstrate that a power plant is truly possible. Introduction to the whole concept can be found here: https://www.iter.org/proj/inafewlines
Energy from the fusion process will be absorbed by the "divertor" - a set of instruments at the bottom of the tokamak to convert the heat into useful energy - via steam turbines. https://www.iter.org/mach/divertor
I found the entire process fascinating, and it seems that the engineering is at a level of maturity where people will just chip away to make it all possible. What is also exciting is that ITER is not the only game in town; in recent years (past 10 years), several labs and people have described how they want to take slightly different approaches in fusion energy.
A German tokamak derivative works called a stellerator works: https://www.sciencealert.com/tests-confirm-that-germany-s-massive-nuclear-fusion-machine-really-works
The UK government has a different approach to the tokamak, with a spherical tokamak: https://step.ukaea.uk/
American company General Fusion still exists (from 2009): https://www.technologyreview.com/2009/07/31/30440/a-new-approach-to-fusion/
Another American company called Commonwealth Fusion, spun-off from MIT has a new design: https://www.nytimes.com/2020/09/29/climate/nuclear-fusion-reactor.html
An American company called TAE Technologies also has a design: https://www.theverge.com/2019/1/29/18201220/nuclear-fusion-energy-artificial-intelligence-machine-learning-tae-google - smashing together the plasma fusion fuel rather than using magentic fields to confine the plasma.
China's fusion experiment broke a temperature record: https://www.sciencealert.com/china-s-artificial-sun-has-officially-become-hot-enough-for-nuclear-fusion
South Korea managed to confine their plasma for far longer: https://www.iflscience.com/physics/south-korea-nuclear-fusion-world-record/
I found this raft of developments exciting, in a field that I thought previously had been moving slowly. That said, fusion is really still decades away; by then it might just be a promising technology that needs to find alternative uses if solar/wind traditional nuclear-fission plants find their place in the energy system instead.
There are other impotant enabling technologies that will have to be invented or further refined. High-Temperature superconducitivity, preferably at room temperatures, might be important for the powerful magnets that tokamaks require. (HTS is also useful for other bunch of things, such as long-distance electricity transmission with minimal loss, but that's another thing.) This was achieved, but at super high pressures - at the giga-pascal level https://www.sciencemag.org/news/2020/10/after-decades-room-temperature-superconductivity-achieved; working wires would have to do it at normal atmospheres.
Other technologies would be materials that can withstand high temperatures; yet another one would be materials that can withstand high neutron bombardmentl: https://en.wikipedia.org/wiki/Neutron_embrittlement
For fusion reactors, another problem might be tritium production. Tritium is an isotope of hydrogen with 2 neutrons; it has a half-life of just 12 years. It is also used in hydrogen bombs as a booster. A facility that stores or creates tritium would thus also be a weapons proliferation risk. A working fusion reactor that uses tritium would have to be sited near existing nuclear fission power plants. Which might be a problem.
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