Straightening the LCLS
In the Magnet Measurement Facility sits an impressive new measuring tool that will be instrumental in getting the Linac Coherent Light Source online. The Leitz coordinate measuring machine is the last stage of off-site preparation for the 40 LCLS undulators.
"It's the most accurate coordinate measuring device of its size in the U.S.," says Quality Inspection group leader Eric Lundahl. With a base that measures 4.5 meters in length, the new Leitz machine is almost six times as long as and far more sensitive than the next most advanced instrument on campus. Positions can be measured with an accuracy of 4 microns, one tenth the width of a human hair. Its computer-controlled arm holds a ceramic probe capped with a synthetic sapphire, which moves the along the surface of an object much like a fingertip. The location information is sent back to the computer, which produces a three-dimensional map with great accuracy. A pneumatic isolation system under the machine's granite bed blocks out ground vibrations from sources such as passing trucks.
When the 130-meter-long line of undulators is installed, the electron beam will need to run within 50 microns of the ideal beam axis created by the undulator magnets. In order to achieve that level of precision, engineers and physicists in the Metrology department must first determine the beam axis for each undulator. The location of the axis will then be marked using as many as eight tooling balls on the exterior. After that, the Leitz machine will be used to make precision adjustments as the undulator, a quadrupole (for beam focus) and other components are assembled on a girder.
For the first of the 1,850 pound undulators, the measurement process will take as long as two weeks. Lundahl explained that it may seem like a long time, but the machine is quite efficient, nearly four times as fast as the laser trackers that will be used for on-site alignment.
After the undulator girder assemblies are transported to the LCLS site and the electron beam is turned on in 2008, the alignment will be even further perfected using the beam itself. Straightness is paramount to keeping the beam on the right trajectory and maximizing the light output.
The initial undulator tuning process will begin in several weeks. In the meantime, the Leitz machine has already been put to work examining large sections of the Linac. "The new machine is an excellent tool for long, large components," explained Lundahl. "Because it's so big, we can use it to measure complete accelerator sections to figure out how straight they are," he said.
Above image: Eric Lundahl demonstrates how a sapphire-tipped probe will be used to map the inside of the LCLS undulators.