Fusion power, which lights the sun and stars, requires temperatures of millions of degrees to fuse the particles inside plasma, a soup of charged gas that fuels fusion reactions. Here on Earth, scientists developing fusion as a safe, clean and abundant source of energy must produce temperatures hotter than the core of the sun in doughnut-shaped facilities called tokamaks. Much of the power needed to reach these temperatures comes from high-energy beams that physicists pump into the plasma through devices known as neutral beam injectors.