What happens when 192 of the world’s highest-energy lasers converge on a target the size of a peppercorn filled with hydrogen atoms? Answer: the same thing that happens inside the Sun and the stars: fusion! NIF’s laser beams can create nuclear fusion in the laboratory by generating the same temperatures and pressures that exist in the cores of stars and giant planets and inside nuclear weapons.
At the outset of a NIF experiment, a weak laser pulse—about 1 billionth of a joule—is created, split, and carried on optical fibers to 48 preamplifiers that increase the pulse’s energy by a factor of 10 billion, to a few joules. The 48 beams are then split into four beams each for injection into the 192 main laser amplifier beamlines.
Each beam zooms through two systems of large glass amplifiers, first through the power amplifier and then into the main amplifier. In the main amplifier, a special optical switch traps the light, forcing it to travel back and forth four times, while special deformable mirrors and other devices ensure the beams are high quality, uniform, and smooth.
From the main amplifier, the beam makes a final pass through the power amplifier. By now, the beams’ total energy has grown from 1 billionth of a joule to 4 million joules—all in a few millionths of a second.
The 192 beams proceed to two ten-story switchyards on either side of the target chamber where they are split into quads of 2×2 arrays. Just before entering the target chamber, each quad passes through a final optics assembly, where the pulses are converted from infrared to ultraviolet light and focused onto the target. NIF’s 192 laser beams travel about 1,500 meters from their birth to their destination at the center of the spherical target chamber. Yet the journey from start to finish takes only about 5 microseconds.
How NIF Targets WorkIn a NIF ignition experiment, a tiny capsule containing two forms of hydrogen, deuterium (D) and tritium (T), is suspended inside a cylindrical x-ray “oven” called a hohlraum. When the hohlraum is heated by NIF’s powerful laser beams to temperatures of more than three million degrees Celsius, the resulting x rays heat and blow off, or ablate, the surface of the target capsule. This causes a rocket-like implosion that compresses and heats the DT fuel to extreme temperatures and densities until the hydrogen atoms fuse, releasing large amounts of energy. If the implosion is symmetrical, and compression and temperature in the “hot spot” at the center of the capsule are sufficient, the energy from a self-sustaining fusion reaction outstrips the rate at which x-ray radiation losses and electron conduction cool the implosion—a condition known as ignition.