Nuclear fusion always appears just a few decades away from commercial viability. Refining the atomic reaction to generate more energy has foiled scientists for generations. Fusion, which occurs when two positively-charged nuclei merge, takes an immense amount of energy to produce in reactors, but only returns a fraction of that energy to create electricity.

But if they succeed, it promises to deliver a reliable clean energy source at a time when few other options exist. Private sector investors that recognize this promise are now putting their money into fusion energy startups.

But if they succeed, it promises to deliver a reliable clean energy source at a time when few other options exist.

One such company, Helion Energy, announced on Nov. 5 it raised $500 million in its latest fundraising round, said to be the largest-ever single fundraising round for a private fusion firm. The funding includes an additional $1.7 billion tied to certain performance milestones. Like most fusion energy startups, Helion focuses on electricity. It says it has achieved the necessary temperature threshold (100 million degrees Celsius) for its generators to power a clean energy power plant.