We already have nuclear fusion and essentially unlimited energy. It exists today, 93 million miles away and right now (14.58 on 5th June, the UK is getting 27.5% of its electrical power from it. Solar energy.
In fact we are getting 100% of our energy from it in the form of fossilised solar energy from ancient plants and animals that was laid down in the past under the ground and under shallow seas.
We also get all of our own animal energy from it in our food.
There are readily available technologies to extract and store solar energy on a huge scale if we had the gumption to do it. A tiny fraction of the Sahara desert set up with either photovoltaic or concentrated solar thermal power plant would supply the entire planet with more energy than it could feasibly use. We already have mature technologies for doing this, but we lack the will to do it at scale.
A California firm is converting sunlight to heat and storing it in molten salt so it can supply electricity when the wind is calm or the sun isn’t shining
www.scientificamerican.com