Scales Work as Part of Thermonuclear Ignition Target Assembly
- Friday, 01 June 2007
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.