Despite the laser’s enormous size, all the real action takes place inside a tiny metal cylinder called a hohlraum, which is the German word for ‘cavity’. But this particular cavity is specially designed to contain a nuclear fusion reaction. The hohlraums used in NIF are less than a centimetre in length and half a centimetre in diameter; they house a millimetre-scale target pellet made from a mixture of tritium, deuterium and ordinary hydrogen. When the hohlraum is flooded with high-intensity laser light, the target pellet absorbs all that energy and a sudden burst of nuclear fusion ensues.
At first glance this may seem something of an anticlimax. Why go to all the trouble of building the world’s biggest, most powerful laser for such a specialised task? The answer lies in the fact that fusion reactions are critical to the performance of nuclear weapons, which the US relies on as its ultimate line of defence. In the early days, the only way to do fusion research was to carry out test explosions, which were highly damaging to the environment due to the hazardous radiation they produced. Facilities like NIF provide a much safer, cleaner way to carry out the same kind of research.