These are all multi-billion dollar projects, and what happens when you spend that kind of money is you tend to 'stay the course,' even if it would be wiser to take a few detours along the way.
There are those that say that fusion is an impossible dream, and it's easy to see where they get that from. It's been a theory since the early 1900s and, as yet, nothing has proven it feasible... except in bombs.
Hydrogen bombs have proven several things about the theory, beyond that there is something to it after all. They've also proven just how difficult the physics is to get right.
In a conventional fission bomb, like an enriched uranium bomb, all you have to do is put enough pure uranium in close enough proximity of each other and a runaway reaction will ensue. If you don't use enough uranium, or it's impure, the runaway reaction will just cause a lot of heat and radiation, much like a reactor meltdown. But, if there is enough uranium and it's pure... boommmm!
The more difficult to build and get right is the plutonium fission bomb. The physics are more difficult, but the materials are easier to get, thanks to breeder reactors that literally make the stuff. These, like the fat man dropped on Japan, are generally ten to a hundred times more powerful than uranium style, but require a near perfect squeezing of a ball of plutonium by high explosives to spark the runaway reaction.
With a uranium bomb, you could set it off just by putting too much in the same room, where a plutonium bomb wouldn't go off if you dropped a ton of dynamite on it. This is, in part, why most nuclear reactors use uranium instead of plutonium to generate power. It's much easier.
Going back to the bomb comparison, they add a pocket of fusion fuel to the center of a plutonium bomb to turn it into an H (for hydrogen, the fusion fuel) bomb. The explosion from the plutonium fission bomb is needed to compress the hydrogen into fusion. Nothing else will do. You can use high explosives to squeeze plutonium hard enough to go off, but not hydrogen. It takes the explosive power of a fission bomb to do that.
And that energy requirement translates into civilian reactor designs as well. It takes very little power to control a fission reaction. But it takes the power of a nuclear reactor to power a fusion reactor.
There are ways to do this. LLNL are using lasers to pinch a pea-sized pod of fuel. Sounds easy, but it takes 192 of them, each insanely powerful, and the laser array is bigger than the biggest Walmart you've every seen.
Others use what's called Z-pinch, where they pump a lot of power down ordinary wires, vaporizing them into plasma and creating a massive magnetic vice that implodes on the tiny capsule of fuel. This process has shown remarkable results, but like with the lasers has always fallen short.
The Russians are stuck in their soviet empire mindset of a tokamak, a giant magnetic doughnut that if filled with plasma instead of jelly. It's never worked, like all the rest.
In my scifi I've gone about it a few ways.
In Personal Space I follow the work of LLNL and use laser confinement. I add the twist of pre-compressing the hydrogen into a metallic state inside a diamond anvil capsule, but otherwise I'm fairly traditional. It was, after all, my first book to include fusion.
It comes up again in Waffen where it was revealed that the HB-4 (her plane) fired diamond-anvil pellets filled with fuel at the enemy, and could use its laser array to detonate them, in theory. Before the ridicule begins, keep in mind that being pre-compressed to metallic hydrogen lowers the threshold to fusion (less additional energy needed) and that a typical jumbo jet, if its turbines were used to make power instead of flight, could achieve the power needed at LLNL to achieve fusion in their tests. So the math is as close as I can get it. These would have been very low yield explosions, except for the life-killing neutron component.
But there's also another design in the mix, a kind of diesel-fusion with diamond anvils for pistons and Z-Pinch ignition. It's the most theoretical of them, but scientist believe hydrogen becomes very electrically conductive as it approaches its metal state, a state that can almost be achieved in traditional diamond anvils today. My design would etch a pattern into the anvil heads that would guide the metallic hydrogen into a Z-Pinch geometry (this lets you use the hydrogen itself instead of vaporizing miles of wire, and also adds the heat energy that is lost in a traditional Z-pinch).
You get some heating by the compression stroke, some heating by the Z-pinch, and if you need a little more, the diamond is perfectly able to transmit some external laser light if needed. The resulting explosion would drive the piston down, and much like in a traditional diesel, that energy would be channeled into compressing another cylinder, where the Z-pinch would occur again.
Obviously, the physical size of this engine would be tiny, and I wouldn't expect the man-made diamond pistons to last indefinitely.
And then, even later in the book, it occurred to me that if you used low power X-Ray lasers to watch hydrogen atoms floating around in a chamber (something we are just beginning to be able to do) you can, eventually, predict when they will position themselves into just the right geometry to be 'pinched' by more powerful lasers. This is based on the hunch that most of the power lost in the traditional approaches stems from the disorganized chaos of dealing with gasses of the smallest atoms in the world, in other words, they're trying to sink all the ball on a billiards table with the break, instead of just picking them off one or two at a time. This drastically reduces the energy inputs, assuming solid-state x-ray lasers come to fruition
A few days ago I read that the people at MIT (I think) are adding lasers to their Z-Pinch experiments in an attempt to reach a breakthrough.
Good luck to them.
The math is compelling, and I do believe that it will, ultimately, be a combination of technologies that unlocks this power source for the world. But I also think they're thinking too big, with massive gigawatt plants, when a few kilowatts and tiny diamond anvils may do (if you could find a way to mass produce them).