Testing Tiny Targets for Big Results

Manufacturing Engineering,  Apr 2007  

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ABTech Inc. (Swanzey, NH) is a supplier of precision-motion systems with an extensive background in the design and manufacturing of air bearings and oil hydrostatic bearings for geometry measurement systems and ultraprecision diamond machining and grinding equipment.

The development of small hydrogen targets to test thermonuclear ignition is like a science in itself requiring highly accurate targets, explains Kenneth D. Abbott, AB Tech owner. His company has manufactured the custom air-bearing motion system that builds those targets.

The new custom machine from AB Tech that is now in place at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory (LLNL; Livermore, CA) has recently passed inspection to begin its work.

LLNL at the University of California employs 8000 and is federally funded to do research in a variety of areas, including nuclear power, national defense, and the environment. The MF lab is currently on track to house the world's largest laser that will involve 192 individual laser beams that will be repeatedly trained onto high precision tiny hydrogen targets.

With the new hydrogen targets, NIFs experiments promise to produce temperatures and densities like those of the Sun or in an exploding nuclear weapon. These experiments will help scientists sustain confidence in the nuclear energy, without doing actual nuclear weapons testing (stopped in the US during the 1970s), as well as produce additional benefits in basic science and fusion energy research.

With each test, a double shell implosion target will be placed in a 30' (9.1-m) diam chamber with the laser beams fired simultaneously to explode it, demonstrating thermonuclear ignition. Its output will be measured for use in a variety of ways.

The manufactured hydrogen targets themselves use capsules of fusion fuel and are heated to thermonuclear ignition. The targets are made of silica-based inner sphere; the manufacturing requirements for surface finish and shell concentricity of the targets are essential to successful explosion. The shell halves are assembled on ABTech's custom five-axis air-bearing assembly station in order to achieve acceptable concentricity.

Weighing about 150 Ib (68 kg), this air-bearing machine system includes mechanical arms that can slide, without friction, into position, with accuracy of up to 0.000004" (0.0002 mm). "Each target is 0.028" [0.71 mm], smaller than the top of a ballpoint pen," says Abbott.

The overall components of this unique 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 to locations within 0.1 µm. "The only way this is now possible is with the use of today's ultraprecise linear scales for use on the linear slides," explains Abbott, "and because of the strict accuracies required in NIF's specifications, our only choice was the extremely high accuracy LIP 481 scales from Heidenhain Corp. [Schaumburg, IL]. It's really amazing what they can accomplish today."

The Heidenhain LIP scales are exposed linear encoders that are characterized by high accuracy together with small measuring steps as small as 0.005pm, depending on the model. Their measuring standard is a phase grating applied to a substrate of glass, and they are typically used in the highest precision machines such as diamond lathes for optical components, facing lathes for magnetic storage disks, and measuring microscopes and semiconductor equipment machines.

The ABTech air-bearing system at NlF includes three of these ultraprecise Heidenhain scales, one on each of the X, Y, Z-linear axes. The entire system is completed with a high-resolution camera and surgical microscope that provides views of the mating components.

The new system's bearings produce a thin film of air similar to the layer of air that allows a puck to move smoothly across an air hockey table. Precision manufacturing takes place from there.

"This ABTech machine is a significant improvement from what NIF was using to develop early stage targets," says Abbott. "Our project is a complicated device, having taken about eight months to develop. Because of the high accuracies, it was crucial that we received assistance from the Heidenhain representative. His highly technical expertise was instrumental in helping us with the installation alignments. This machine is now truly like no other," Abbott says. Circle 223

Copyright Society of Manufacturing Engineers Apr 2007
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