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A six-ton block of granite in Linac Coherent Light Source (LCLS) Project
:: 24 October, 2007
The Linac Coherent Light Source (LCLS) project is a collaboration that brings together components and equipment from dozens of suppliers. Last month, Craigslist.org joined that supply chain. Ed Akerstrom, a member of the LCLS undulator assembly team, scored a huge find—a six-ton block of granite—on the popular website and saved SLAC thousands of dollars in the process.
Akerstrom, a machinist who regularly trolls Craigslist.org for deals on tools, found the 15-foot-long granite block by accident. Coincidentally, the seller was associated with a previous SLAC-approved supplier, making acquisition of the stone a lot easier.
"He told me about his find on the phone," said Ben Poling, the undulator assembly team leader. "When he said they only wanted $300 for it, I figured he must have the dimensions wrong."
Granite blocks of this size provide solid, ultra-stable platforms for mounting sensitive equipment. The team, now working in the Collider Hall, had been searching for such a block to use as part of a production line for pre-setting the beam pipe vacuum chambers before they are mounted to the undulator assemblies. Since its arrival, the block has been milled to surface precision that varies less than half of one thousandth of an inch, achieving what Poling terms "AA grade." A stone of this size and quality normally would cost more than $15,000, but thanks to Akerstrom's online acumen, the group spent a fraction of that.
"It cost more to have the stone trucked over to SLAC," said Poling. Once it was finished, the total cost of shipping, mounting and resurfacing was less than $3000, according to Poling.
With the new granite block in place, the assembly team can soon start piecing together the associated structures that will hold the undulator magnets. These massive assemblies will comprise 33 undulators divided among 11 individual cells made up of heavy support pedestals, quadrupole focusing magnets, beam position monitors and related components.
The LCLS is designed such that many of its key systems, including the undulator magnets, can be fine-tuned as necessary without having to take down the machine. Recently, another key component in this fine-tuning system began arriving at SLAC. Called undulator support movers, these devices contain high-precision motors that can make minute adjustments on the fly to keep the undulators aligned to within a few microns.
With a stream of such key components arriving by the week, the undulator assembly team will soon be ready to begin piecing together the undulator assemblies in advance of installation, scheduled to begin next year.
Above image: Ben Poling (left) and Ed Akerstrom with the six-ton granite block Akerstrom acquired on Craigslist.
LCLS Emittance Goals Achieved at Full Bunch Charge
The transverse beam emittance measured in the LCLS injector has reached its design goal of 1.2 microns with a 1-nC bunch charge. Many measurements have been made using both OTR screens and wire-scanners, which confirm this normalized rms emittance level in both transverse planes at an electron energy of 135 MeV. The various measurements at 1 nC extend from 1 micron and on occasion up to 1.5 microns, with an undetermined source of variation. The bunch length is also at its design level of 10 ps FWHM, which produces a peak current of 100 Amperes.
This rms emittance value is measured over the full bunch length (time-projected), but the rms analysis includes only 95% of the core particles. Measurements which use the full transverse distribution are much more sensitive to baseline noise and weak charge tails, and typically report emittance levels closer to 1.5 microns.
The 'time-sliced' emittance has also been measured using an S-band transverse RF deflecting cavity. This method commonly reveals a 0.9-micron horizontal emittance in the 1-picosecond long slice at the center of the bunch. The vertical 'time-sliced' emittance is rarely measured.
The LCLS electron beam has been transported through the full SLAC linac with an energy of up to 16 GeV and has also been time-compressed by a factor of 4-5 in the first stage compressor. The next challenge will be to preserve this low emittance beam through the 1-km linac, both bunch compressors, and into the LCLS undulator, eventually producing 1.5-Angstrom FEL x-ray radiation. Some of these challenges will be the focus of the next phases of LCLS commissioning starting again in December 2007.
Release link: http://www-ssrl.slac.stanford.edu/lcls/
Tags: Linac Coherent Light Source (LCLS) , collaboration , Ed Akerstrom , Ben Poling , Collider Hall , AA grade , granite. ,
