Five air bearing stages working in concert with high-resolution scales provide the extreme accuracies required for assembling small hydrogen targets used in testing thermonuclear ignition.
The air-bearing system includes sliding mechanical arms with a positioning accuracy of 4 millionths of an inch.“Each target is 28 thousandths of an inch in size, smaller than the tip of a ballpoint pen,” says Ken Abbott, owner of AB Tech, the company that manufactures the system that builds the targets.
The machine is at the National Ignition Facility (NIF) at Lawrence Livermore National Laboratory.For each test, a double shell implosion target is placed in a 30-ft-diameter chamber. The laser beams are fired simultaneously to explode it, demonstrating thermonuclear ignition.
The hydrogen targets are made of silica-based inner sphere, and the manufacturing requirements for surface finish and shell concentricity of the targets are essential to successful explosion.Shell halves are assembled on the custom 5-axis air bearing assembly station to achieve acceptable concentricity.
The components of the machine include three linear air bearings and two rotary air bearings, a PMAC motion controller from Delta Tau, a host PC and application software.The system is capable of positioning target shell halves to within 0.1 µm.
“The only way this is possible is with the use of today’s ultra-precise linear scales for use on the linear slides,” explains Abbott, “Because of the strict accuracies in the specifications, our only choice was the extremely high accuracy scales. “
The system employs LIP 481 scales from HEIDENHAIN Corporation that are exposed linear encoders characterized by high accuracy and measuring steps as small as 0.005 µm depending on the model.Their measuring standard is a phase-grating applied to a substrate of glass, and are typically used in high 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 includes three of these ultra-precise scales with one on each of the X, Y and Z linear axes; a high resolution camera, and a 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.
The technical team spent four to six weeks in final system assembly and adjustment, setting up all of the five axes and testing them each individually to achieve the final specification.Much of the work involved aligning the mechanical elements of the system, since each axis has several degrees of alignment.
With the new hydrogen targets, NIF’s 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 nuclear energy, without doing actual nuclear weapons testing.
Abbott says the success of the system is all about maintaining positioning accuracy at very, very slow speeds.A fast speed for the system is 0.5 inches/sec for aligning components.Small parts measure 28 thousands of an inch in diameter and are positioned on the system, moved around slowly, and very accurately positioned to achieve proper alignment.The smallest incremental move on the system allows the operator to push a button and the X axis will move four millionths of an inch.Moving that slowly while holding position stability is one of the main reasons for using the sophisticated scales to achieve the accuracy and high resolution performance required.