Laser beams converging on a target inside the target chamber.

The devices used to compress targets in inertial fusion are generally called drivers. Early research considered several types of drivers, including heavy-ion beams and X-rays generated by nuclear explosions. Since the invention of the laser in 1960, high-power pulsed lasers have become the most widely used drivers in inertial-confinement fusion research.

Laser Principle

The acronym “laser” stands for “light amplification by stimulated emission of radiation”. A laser produces highly coherent light within a narrow range of wavelengths. Energy supplied by a pumping source excites atoms or ions in the laser gain medium, such as neodymium-doped glass. Stimulated emission then causes excited ions to release photons with the same frequency, phase and direction as the stimulating light, amplifying the laser beam.

In a laser oscillator, light is amplified as it passes through the gain medium between mirrors. In large inertial-fusion lasers, a very weak initial pulse is subsequently amplified through a series of preamplifiers and large amplifier stages. After amplification, mirrors and other optical components guide, shape and focus the beam onto the target.

High-Power Fusion Lasers

Preamplifiers at the National Ignition Facility (NIF).

Large inertial-fusion laser systems can occupy entire buildings. The National Ignition Facility, for example, is housed in a ten-storey building about the size of three football fields. A very low-energy initial pulse is generated in a master oscillator, divided into many beamlines and amplified through successive stages. At NIF, this process ultimately produces 192 high-energy laser beams that are precisely directed towards the target.

During amplification, the energy of the initial pulse increases by many orders of magnitude. At NIF, the 192 beams together can deliver up to about 2.2 MJ of ultraviolet laser energy to the target with a peak power of up to about 500 TW. The individual beams are carefully balanced and synchronised so that they reach the target with the required energy and timing. Their optical path lengths and pulse shapes are precisely controlled to achieve the desired implosion symmetry. A complex system of mirrors, amplifiers and precision optical components guides and synchronises the beams before they enter the target chamber.

Energetic Electrons

Large inertial-fusion lasers such as NIF use neodymium-doped glass and initially generate infrared light. For target irradiation, this light is converted to ultraviolet because the shorter wavelength couples more favourably to the target plasma and reduces unwanted production of energetic electrons that could preheat the fuel and hinder compression. At NIF, the 1,053 nm infrared light is frequency-tripled to 351 nm ultraviolet light in the final optics assemblies. The final optics assemblies also focus the ultraviolet beams onto the target.

Pulse Duration

The main laser pulses used to compress inertial-fusion targets typically last only a few nanoseconds, although much shorter pulses are used in some specialised experiments. At the National Ignition Facility, the world’s largest and most energetic laser facility, the beams travel about 1,500 metres from their origin to the target. This journey takes only about 5 microseconds.

The largest inertial-fusion laser facilities are extremely complex and expensive; facilities such as NIF have required investments of several billion dollars.