The goal of many NIF experiments is to create a self-sustaining “burn” of fusion fuel (the hydrogen isotopes deuterium and tritium) in which the energy from self-heating, or “bootstrapping,” outstrips the rate at which x-ray losses and electron conduction cool the implosion—an event called ignition. In moving closer to achieving ignition, NIF researchers are fulfilling the vision of early laser pioneers who conceived of using the x rays generated by a powerful, brief laser pulse to fuse hydrogen isotopes and liberate copious amounts of energy.
Following the completion of NIF construction in March 2009, scientists focused on installing, qualifying, and integrating the facility’s many systems and the required scientific platforms to support a wide variety of experiments. Precision experiments devoted to ignition began in May 2011 and have since produced unprecedented high-energy-density environments.
Ignition experiments began as part of the National Nuclear Security Administration’s National Ignition Campaign. This campaign, which started in 2006 and ended September 30, 2012, had two principal goals: developing a platform for ignition and high-energy-density applications (including target and diagnostic fabrication) and transitioning NIF to routine operations as the world’s preeminent high-energy-density science user facility.
Over the course of the campaign, NIF researchers steadily increased the laser’s energy and power, culminating on July 5, 2012, when the laser system’s 192 beams delivered more than 1.8 megajoules of ultraviolet light (nearly 100 times more energy than any other laser has demonstrated) and more than 500 trillion watts of power to the center of the Target Chamber. Although ignition was not achieved during the campaign, experiments are continuing, and a large body of scientific knowledge and major new experimental, diagnostic, and target manufacturing capabilities continue to be developed and validated.
NIF ignition experiments use “indirect-drive” geometry to heat the interior of a centimeter-scale cylindrical gold can called a hohlraum. Laser beams enter through laser entrance holes on the top and bottom ends of the can. Laser light hitting the inner walls of the hohlraum is converted into x rays that raise the temperature more than 3 million degrees.
The x rays heat the surface of a fuel capsule mounted in the center of the hohlraum, causing the capsule to implode like a spherical rocket at velocities greater than 350 kilometers per second. When the imploding shell collapses, the interior volume is reduced by almost a factor of 100,000, heating and compressing the frozen deuterium-tritium fuel layer contained within the capsule. The resulting hot center, called the hot spot, reaches temperatures of 20 to 40 million degrees with densities of 50 to 100 grams per cubic centimeter. This hot spot is surrounded by a region with a density of 600 to 800 grams per cubic centimeter at a temperature of 1 to 2 million kelvins. This density, greater than that in the center of the sun, is by far the highest density ever achieved in a laboratory.
The goal of current NIF experiments is to increase the density of the hot spot by a factor of three at about the same temperature as already achieved. Under those conditions, the fusion reaction rate would be sufficient to generate ignition. Current experiments routinely produce a density sufficient to “stop,” or absorb the energy from alpha particles (nuclei of helium atoms) produced by the fusion reactions in the hot spot. This process, known as alpha heating, further heats the assembled fuel and enhances the energy yield. This is a critical milestone on the road to ignition.
An early-morning NIF shot on August 13, 2013, released nearly 3 × 1015 neutrons, or about eight kilojoules (8,000 joules) of neutron energy—about three times the previous record neutron yield for cryogenic implosions. An approximately 50-percent yield enhancement was achieved due to alpha heating. A subsequent experiment in January 2014 surpassed that record, producing 9.6 × 1015 neutrons, or 27 kilojoules of fusion energy; more than half of the yield (14.5 kilojoules) was attributed to alpha heating. In these “high-foot” experiments, NIF researchers turned up the power of the NIF lasers during the “picket” that occurs at the beginning of the laser pulse in an effort to make the target capsule’s imploding shell, or ablator, more resistant to breakup; previous experiments had shown that capsule instabilities were reducing target compression and degrading performance. The added power increased the radiation temperature in the “foot” or trough period of the pulse, which improved the ablator’s stability while reducing compression later in the implosion.
The results of these experiments were remarkably close to simulations and provide important information for better understanding and improving NIF’s performance. Next steps in the high-foot campaign include exploring higher implosion speeds to increase performance, including using the high-foot approach with high-density carbon (diamond) and beryllium ablators, thinner plastic ablators, and different hohlraum configurations. Research also is under way to find alternatives to two engineering features—the ultrathin “tent” that suspends the target capsule inside the hohlraum and the tiny fill tube used to inject fusion fuel into the capsule. Both features are believed to contribute to perturbations that limit implosion performance (see “Climbing the Mountain of Fusion Ignition: An Interview with Omar Hurricane”).