Scheduled for completion in 2009, the National Ignition Facility Project (NIF), part of Lawrence Livermore National Laboratory (LLNL, Livermore, CA), is a component of the National Nuclear Security Administration’s (NNSA) Stockpile Stewardship Program, whose mission is to maintain the safety, reliability, and effectiveness of the nation’s nuclear stockpile without underground nuclear testing, banned since 1992. To continue research into thermonuclear ignition, NIF began the Inertial Confinement Fusion (ICF) Program for high-energy density physics. To this end, NIF constructed a complex system of lasers ending in a chamber ten meters in diameter to house tiny fuel capsules called “targets” that are subjected to a high-energy pulse, setting off a small thermonuclear burst. The target assembly machine, part of NIF’s thermonuclear testing system, is custom built by ABTech (Swanzey, NH), using the LIP 481R linear scales from HEIDENHAIN Corp. (Schaumburg, IL).
With an accuracy of up to 4 millionths of an inch and weighing approximately 150 pounds, ABTech’s “5-Axis Assembly Station” mechanism is an air-bearing machine system that includes mechanical arms with the ability to slide, without friction, into position. The overall components of the custom machine include three linear air bearings and two rotary air bearings, a motion controller, host PC and application software. The system is capable of positioning the target shell halves in locations within 0.1 μm. The HEIDENHAIN LIP scales — exposed linear encoders capable of small, highly precise measured steps to 0.005 μm — have a measuring standard that is a phase grating applied to a substrate of glass. The NIF air-bearing system includes three LIP scales, one on each of the X, Y, and Z linear axes. The system is completed with a high-resolution camera and surgical microscope that provide views of the mating components. The system’s bearings produce a thin film of air, providing an environment for precision manufacturing.
Via the ABTech assembly station, films of diamond, plastic, or beryllium are first deposited on spherical silicon substrates called mandrels. The films, ~70 μm thick, are polished, and the target is then hollowed out, filled with cryogenically cooled deuterium-tritium (DT) fuel, and plugged or epoxied together. Diamond-machined, the spherical ABTech targets are two millimeters in diameter, with an inner and outer shell. While currently independent, future designs of the NIF facility include the target assembly as an integrated component of the NIF ignition system in order to better ensure that the cryogenic state of the target is maintained.
When the heating from fusion inside the target exceeds the heating from the driver beams, ignition is reached. Experiments are set for 2010, with high-power lasers or ion particle beams compressing and heating the targets, precipitating a fusion ignition burst (“thermonuclear burn”). The energy pulse that will ignite the target begins with the Injection Laser System, which creates a precisely shaped nanojoule-level laser pulse. This is amplified by a factor of more than 1,000,000 before entering the laser “beampath.” There, two stages of Laser Amplifiers again increase the laser energy, using the world’s largest optical switch to enable multi-pass amplification. Surrounded by vertical arrays of 7,680 flashlamps, the amplifiers, with 16 glass slabs per laser beam (18 if necessary), are arranged in two sections: the main amplifier and the power amplifier. These amplifiers provide 99.9% of NIF’s power and energy.
Next, the Plasma Electrode Pockels Cell (PEPC) uses electrically induced changes in the refractive index of a potassium dihydrogen phosphate (KDP) electro-optic crystal, and contains the laser beams in the main amplifier cavity, causing them to pass four times through the main amplifiers. The PEPC traps the laser between two mirrors as it makes four one-way passes through the main cavity amplifiers before being switched out. The PEPC allows light to pass through or reflect off a combined polarizer. The pulses then travel through two stages of spatial filters, which focus the beams through pinholes to remove high-frequency intensity variations; the precise characteristics of the laser beams are maintained en route to the target chamber.
NIF’s neodymium glass laser generates light at a wavelength of about 1.053 micrometers in the infrared region. Because inertial fusion targets perform more efficiently when driven by ultraviolet radiation, Final Optics Assemblies (FOA) convert the infrared light to ultraviolet, using a system of two nonlinear KDP crystal plates.
The mechanical interface between the FOA and the target is the target chamber, a 1,000,000-pound sphere equipped with stationary shutter-like louvers and a 16"-thick neutron-shielding concrete shell that provides a vacuum environment for the target and mounting and alignment points — 72 for the FOA and 48 for indirect-drive experiments for the 192 laser beams that focus on the target. X-ray spectrometers, microscopes, and cameras are mounted around the equator and at the poles of the target chamber. The targets are positioned, three at a time, in a 1-cm canister, the hohlraum. Laser beams pass in groups of four through the FOA’s frequency- conversion crystals, then through wedge-shaped lenses that focus the ultraviolet beams at the target. Although coming in pulses lasting 3 to 10 nanoseconds, the laser system is equal to 1,000 times the electric generating power of the United States. Delivering 1.8 million joules of ultraviolet laser energy and 500 terawatts of power to the target, the lasers enter the hohlraum from top and bottom, generating x-rays that heat the capsule to Sun-like temperatures, blowing off (“ablating”) the target’s shells. Simultaneously, the x-rays create massive pressures that compress and fuse the atoms of the DT fuel, resulting in a burst of energy. The shortness and small size of the burst is many times smaller than a conventional nuclear explosion.
The new target design has allowed NIF to create thermonuclear ignitions mimicking conditions found in the Sun or an exploding nuclear event. The primary mission of NIF will be to attain fusion ignition in the laboratory, exploring fusion’s potential as a clean, long-term energy source. Such experiments bypass the need for actual nuclear weapons testing while continuing research into the nuclear, fusion, and high-energy density sciences. High-energy density regimes achieved by NIF experiments could also lead to fresh insight into the origin of the universe: though roughly 10% uncompleted, NIF laser beams are already in use to simulate supersonic plasma jets occurring in some galaxies and black holes.