The physics of fusion are well-understood. Nuclear fusion was achieved before nuclear fission. The most common nuclear fusion reaction--the deuterium-tritium re

The physics of fusion are well-understood. Nuclear fusion was achieved before nuclear fission. The most common nuclear fusion reaction--the deuterium-tritium reaction--can be readily performed by research groups across the world. As others have mentioned, funding is the main reason why energy production from nuclear fusion hasn't taken off. The better question is, what isn't being funded? Ask yourself: how do you hold the sun in a bottle? The answer is that it's very hard, but it's doable. You just need to cool the walls fast enough and make sure the sun doesn't melt the bottle. Based off this rough problem description, the main issues are the following: - Materials: what do you make the bottle out of? Do you know of any materials which can withstand millions of degrees of heat? Probably not, so you should look into using magnets to suspend the sun in the middle of the bottle. - Confinement: how do you hold the sun? The sun (that is, plasma) reacts to magnetic fields. Classical designs such as the tokamak drove the plasma around a donut to keep it confined. Problem there is that you need two magnetic fields: one to drive and one to steer. Some decades ago, people got smart and figured out you could design the road in such a way that the car will always drive straight, so great, you don't need to steer anymore. (This is the main difference between ITER and Wendelstein 7-X; personally, I think ITER will turn out poorly, but they'll learn a lot from building the thing...) - Reactions: guess what? Your sun emits high-energy neutrons, making the bottle radioactive. You could use a different type of sun, but those are expensive and weird or would require you to go to the Moon, which is also expensive. You figure out you can line the walls of the reactor with lithium and breed fuel for your sun, which works but isn't perfect. You know other suns are out there, but no one knows how to build a bottle to hold that sun. - Self-sustainment: you put some ingredients in your bottle, shake it up, and presto, you have a sun. For about ten milliseconds. Problem is, your bottle got too hot and your reaction creates exhaust you have to scrape out. Also, the magnets used to keep the sun in the middle of the bottle draw more energy than your sun makes! That won't do. Self-sustaining fusion reactors are very hard problems to crack because there's so many moving parts. The fusion research community has suffered from underfunding for decades because people wrongfully associate the principles of Chernobyl with ITER. People don't want their politicians funding more Chernobyls, and Bob's your uncle. Because the funding has been so low, the problems which should be solved by now, well, haven't been.