Fifty Years of
Stellar Laser Research
THE National Ignition Facility (NIF) at Lawrence Livermore is the world’s largest and highest energy laser system. Since completion in March 2009, NIF’s 192 beams have produced more than 1 million joules of ultraviolet light and completed over 170 target experiments. NIF illustrates what the Laboratory does best: bring together multidisciplinary teams to solve big science and engineering challenges. Over the past decade, NIF scientists and engineers have overcome technical obstacles with ingenuity, determination, and hard work to deliver a facility for the ages.
NIF’s principal goals are to achieve ignition of a deuterium–tritium fuel capsule and to explore high-energy-density physics regimes needed for experiments in national security, fusion energy, and frontier scientific discovery. In addition to supporting the National Nuclear Security Administration’s Stockpile Stewardship Program, success in achieving controlled thermonuclear fusion will position NIF as the world’s preeminent facility for the study of inertial fusion energy and the physics of matter under extreme temperature, density, and pressure. In fact, ignition and net energy gain on NIF will be a major step toward developing inertial fusion energy as a baseload energy source.
NIF is the most recent laser in a long and successful laser program at the Laboratory. It seems fitting to celebrate the Laboratory’s laser program in this issue of S&TR because May 16, 2010, marks the 50th anniversary of the demonstration of the laser. With its highly coherent and focusable light, the instrument caught the attention of Livermore scientists. In particular, physicist John Nuckolls immediately began to study the possibility of using powerful, short laser pulses to compress and ignite a small quantity of fusion fuel composed of tritium and deuterium, two isotopes of hydrogen, in a process called inertial confinement fusion.
These original calculations revealed that heating the fusion fuel with only laser energy would not be enough to generate net energy gain, even with lasers producing as much as 1 million joules. To achieve energy gain, the laser would have to compress the fuel to 1,000 times its liquid density. Scaling up existing lasers was a daunting and high-risk task both scientifically and financially.
Over four decades, the Laboratory designed and built a series of ever bigger, more complex, and more powerful lasers. In 1974, Livermore completed the one-beam, 10-joule Janus laser and used it in fusion experiments to demonstrate for the first time a thermonuclear reaction in laser-imploded fuel capsules. Next,
a two-beam Janus laser was used to gain a better understanding of laser–plasma and thermonuclear physics. In 1975, the one-beam Cyclops became the prototype for the future Shiva laser.
The next year, the two-beam Argus came online, which increased knowledge about laser–target interactions and laser propagation limits. Argus was the first laser with spatial filters, enabling the beam to be relayed from one amplifier to another while eliminating intensity fluctuations that led to optical damage.
In 1977, the 20-beam Shiva became the world’s most powerful laser delivering 10.2 kilojoules of energy in less than a billionth of a second in its first full-power firing. In June 1979, Shiva compressed fusion fuel to a density 50 to 100 times greater than its liquid density. Even more important, Shiva experiments showed that infrared laser light had too long a wavelength to reach fusion energy gain, as energetic suprathermal electrons generated by infrared light in the target’s plasma absorb the laser light and inhibit compression.
Novette, which began operation in 1983, was the first laser to be engineered with optical frequency converters made of potassium dihydrogen phosphate (KDP) crystals, which converted the infrared light to shorter wavelengths. Novette was a test bed and interim target facility between Shiva and the 10-beam Nova, the next system in line. Ten times more powerful than Shiva, Nova produced the largest laser fusion yield to date in 1986, a record 11 trillion fusion neutrons. The following year, Nova compressed a fusion fuel target to about one-thirtieth of its original diameter, close to that needed for ignition and fusion gain.
Work on Nova prepared Livermore to begin construction of NIF. Beamlet, the prototype of NIF, was also essential to demonstrating the viability of the new laser system. Operated at the Laboratory between 1994 and 1998, Beamlet showed that the multipass laser architecture conceived for NIF was capable of meeting the fluence (energy per unit area) requirements prescribed by the National Academy of Sciences, a necessary milestone to proceed with NIF.
Ground was broken for the stadium-size NIF in May 1997, and the facility was formally dedicated on May 29, 2009. NIF is designed to deliver a total energy of 1.8 million joules of ultraviolet light to the center of a 10-meter-diameter target chamber. The article A Stellar Performance describes how NIF’s laser system has demonstrated the precision, flexibility, and reliability required for repeated ignition experiments and the ability to become an international user facility.