A major milestone was reached recently when scientists at Lawrence Livermore National Laboratory in California reported that they had demonstrated that laser pulses shot into a cavity can produce the conditions required to trigger nuclear fusion reactions. The finding was a crucial test of principle for Livermore's National Ignition Facility (NIF), the $3.5 billion complex now under construction and expected to start full operations in 2009.
When completed NIF will be, by far, the world’s largest and most energetic laser and a major international scientific resource. Designed to study the physics of matter at extreme densities, pressures, and temperatures, NIF will use 192 laser beams to compress fusion targets to conditions required for thermonuclear ignition and burn. In the process, more energy will be liberated than is used to initiate the fusion reactions.
NIF will employ a process called inertial confinement fusion, in which either laser or x-ray energy striking the outside of the target fuses atoms of deuterioum (hydrogen with one neutron) and tritium (hydrogen with two neutrons) together, creating helium atoms, extra neutrons, and large amounts of energy. NIF is the first laser facility in which the energy released by the fusion fuel can exceed the laser energy used to produce the fusion reaction. The facility has a 10 meter (33 foot)diameter target chamber that contains a gold cylinder the size of a dime, about 6 mm in diameter and 10-mm long, called a Hohlraum, that will eventually receive the energy from all 192 lasers.
When the Hohlraum receives energy from the lasers it then produces x-rays that compress and heats a 2-3 mm fusion capsule, cooled to near absolute zero, containing the fuel, inside the Hohlraum to temperatures and pressures approaching those in a nuclear explosion or the sun. In a self-sustaining fusion reaction, the capsule is compressed to about 20 times the density of lead and heated to the point of ignition, about 100 million C. The release of fusion energy by the fuel drives the rest of fusion reactions until the fuel is spent. Inertial confinement fusion is limited by the amount of fusion fuel in the capsule, and is therfore totally controllable. The 192 lasers have a total power of 1.8 million joules over a period of a few billionths of a seconds. The total energy is about 500 trillion watts, which is nearly 1000 times the power generated in U.S. at any time.
In an actual Inertial fusion power plant, a few pulses of fusion energy per second would heat low-activation coolants, such as ceramic beads, surrounding the targets. These, in turn, would transfer the fusion heat to steam-turbine generators to produce electricity.
During commissioning of the first four laser beams in 2003, the laser system met design specifications for everything from beam quality to energy output. In July, 2003 laser shots in the infrared wavelength using four beams produced a total of 26.5 kilojoules of energy per beam, not only meeting NIF’s design energy requirement of 20 kilojoules per beam but also exceeding the energy of any other infrared laser beamline. In another campaign, NIF produced over 11.4 kilojoules of energy when the infrared light was converted to green light. In an earlier performance campaign laser light that had been frequency-converted from infrared to ultraviolet really proved NIF’s mettle, over 10.4 kilojoules of ultraviolet energy were produced in about 4 billionths of a second.
There is a large amount of desciptive material available at the NIF website that gives details about the facility, especially the way the laser beams are directed, amplified and transformed before they reach the target.
First Steps Toward Fusion at NIF, American Institute of Physics, Physics News Update number 755 #2, November 23, 2005
National Ignition Facility Project, website
NIF Brocure (pdf)
The National Ignition Facility Comes to Life, Science and Technology, September, 2003