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LIGO Hanford Observatory NewsSouthridge Students Tackle LIGO Tumbleweed Problem
Bright Prospects: Laser Installation Begins at LIGO Hanford Observatory
The HAM Has Landed!
At Southridge High School in Kennewick, Washington--nearby to the Hanford LIGO Observatory--Mr. James Hendricks, instructor, has challenged students in his Engineering Technology class to "design wind deflection systems to allow tumbleweeds to roll past the facility during a windy day."
As Mr. Hendricks describes in his Technology Challenge: LIGO and the Tumbleweeds , "huge amounts of tumbleweeds accumulate on the windward and leeward side of the buildings after a windy day. The tumbleweed accumulations are so great that workers sometimes do not have access to entrances and other building facilities. Tumbleweeds can also be a fire hazard."
His assessment is no exaggeration. Tumbleweeds have piled so high that office windows have been completely obstructed. Four-wheel-drive vehicles attempting to navigate the beam tube access roads have become bogged down in a jungle of impenetrable tumbleweed. And LIGO has had to reinforce all the lightning rods on the corner station roof (which stand more than 40 feet in the air) because the rods were being snapped off by tumbleweeds stampeding over the building. Every month the Hanford Observatory pays thousands of dollars to have removed the tumbleweeds that accumulate on and around the buildings and beam tube enclosures. The company contracted for this service, Yakima Industries, collects the tumbleweeds by hand and compresses them into bales similar to hay bales. On average per month they generate almost five hundred bales. (See Figure 1 at left.)
The students of Southridge High are searching for less expensive ways of dealing with this troublesome tumbleweed issue. In fact they are currently investigating ways to allow LIGO simply to let the tumbleweeds keep rolling through, but without causing costly damage. Their solution was the invention of "tumbleweed deflectors," as shown in Figures 2 and 3 below.
When I visited Mr. Hendricks' class last April to talk with the students about LIGO, I was surprised to find how well-informed and enthusiastic they were. They had a large-size set of engineering drawings for the facilities and here and there were various models the students had constructed of the corner station and beam tube enclosures, outfitted now with their tumbleweed deflectors. The students were close to presenting their design solutions and were testing them by blowing simulated tumbleweeds (cotton balls) across their LIGO models.
The students' knowledge and curiosity about LIGO was truly remarkable. And so was their instructor, Mr. Hendricks himself. He has taken what would have been the traditional wood and metal shop classes and transformed them into Engineering Technology classes that challenge students to consider and grapple with many of the real-life issues in engineering technology--problems of design, research, testing, communication, marketing and the like. A man of restless innovation, he is now considering ways of putting to profitable use the bales of tumbleweed LIGO is generating.
On July 7, LIGO achieved a major project milestone--"Begin Interferometer Installation"--when installation of the Laser and Input Optics tables began at the LIGO Hanford Observatory (LHO).
The enclosures arrived by moving van on schedule July 7. Hugh Radkins and Corey Gray surveyed the table locations and marked the floor in the corner station experimental hall according to a recently released drawing generated by Paul Kabot of Caltech. Lee Cardenas of the Caltech laser team worked with Doug Cook and Rick Savage to position the tables and enclosures by the end of that week. In Figure 1 at left, Doug and Lee can be seen smiling in the photo of the newly installed table.
Once the laser table was situated, Richard McCarthy was able to run power to the enclosures and install electronics racks for the laser control electronics. Meanwhile, the Caltech laser team of Peter King, Rich Abbott, Virginio Sannibale, Joe Suina and Lee Cardenas was putting the laser and its supporting optics and electronics through the final test phase at Caltech. Just after Labor Day, a bon-voyage party was held at the Pasadena campus as the laser was shipped out and the Pre-Stabilized Laser team headed off to Hanford.
The new laser is an all solid-state device specially developed for LIGO by Lightwave Electronics. It provides 10 Watts of output light in the near infrared (1.06 microns) wavelength from a Nd:YAG crystal that is pumped by several AlGaAs diode lasers. The high power output crystal actually works as a power amplifier, boosting the light injected from a smaller, high precision Nd:YAG laser. This configuration is referred to as a MOPA or master oscillator / power amplifier configuration. The Caltech laser team has tamed the raw output of this laser, using a variety of optical and electronics components, to achieve remarkable levels of stability. The end product of this effort is the pre-stabilized laser, or PSL. Over the time scales that are crucial for gravitational-wave detection (of order 1 to 10 milliseconds), the PSL intensity is stable to six significant figures and the PSL frequency is stable to 14 significant figures.
The partially assembled PSL can be seen in Figure 2 at right, taken on September 14. The Lightwave laser is the optics package to the right at the far end of the enclosure. Various PSL components can be seen in the foreground. The principal frequency-defining component--a small Fabry-Perot cavity encapsulated in a thermally stabilized vacuum chamber--was not yet installed in the enclosure. The laser was turned on later in the week that the photo in Figure 2 was taken.
The light from the Pre-Stabilized Laser gets handed off to the Input Optics team from the University of Florida who will inject the beam into the vacuum system where their vibration-isolated optics can further filter and stabilize the light. (For more on this, see the next article below, "The HAM Has Landed!") During all this laser installation activity the Input Optics team was busy in the LHO Vacuum Assembly lab doing final construction of their optical assemblies. But that story is slated for another day.
On Thursday, July 23 I went out into the corner station to admire the completed First Article of the vibration isolation system for the HAM chambers. HAM refers to a Horizontal Access Module--a type of vacuum chamber which holds the input and output optics that inject laser light into the main LIGO interferometer, and also receives output light from the interferometer. As I viewed it, the HAM optical table appeared to float steadily above its isolation system. But a slight jarring of the system revealed that the apparently stiff layers of springs and steel wobbled like a bowl of jello!
LIGO interferometers require laser light that is highly purified compared to the light emitted by a typical laser. We massage the laser intensity, frequency and mode shape using electro-optical elements in the input optics. Some of this massaging can be done while the light is exposed to air. But eventually, the motion of air molecules and vibrations of the optics prevent us from further improving the quality of the light. At this point, we inject the laser beam into the HAMs where high vacuum and vibration isolation systems prevent blemishing of the light due to moving air and shaky optics. In this quiet environment we can finally achieve the laser beam quality needed for LIGO.
The HAM vibration isolation system, shown in Figure 1 at left, consists of a specially designed aluminum optical table with four legs. Each leg has three layers of special springs with stainless steel disks in between. The mass of the optical table (or a stainless disk), combined with the "springiness" of a single layer of springs, acts to reduce the transmission of vibrations much like the suspension does in a car. The HAM system is a "stack" of three layers of mass and three layers of springs--the equivalent of three car suspensions stacked one atop the other. In a car the payload (that is, the car body and contents) vibrates less than the tires as the car drives over bumps and potholes in the road. The isolation is good for minimizing bumps that are sudden compared to the typical response time of the car's suspension. (You can measure this response time by sitting on a car--preferably your own or a friend's, at any rate someone's with a good-natured tolerance for your scientific curiosity--and then suddenly jumping off. The time it takes for the car to bounce up and down once is the response time.) Forces applied more slowly than the response time feed right through the isolation. So you can drive your car uphill or downhill (which would cause a slow change in tire height) but not get bounced around by potholes (causing a sudden change in tire height). Similarly, in the HAM isolation stacks we can adjust the position of the optical table (slow) but faster vibrations due to ground shaking are blocked.
A car's suspension won't work right without shock absorbers. They keep the car from bouncing uncontrollably when there is tire motion at the typical response time of the car's suspension. The HAM stack needs similar shock absorption to maintain stability. For this, special springs were developed for LIGO by HYTEC of Los Alamos, New Mexico, which have shock absorbers hidden away inside. To accomplish this, the spring coils are made by lining a straight tube of spring material with a sheet of rubber-like damping material. A series of metal slugs, held on a rubber core like beads on a necklace, is fitted inside the lined tube. The whole assembly is then coiled and welded with endcaps to seal the rubbery materials inside the spring. As the spring flexes the rubbery sheet is twisted between the slugs and the tube, absorbing energy from the vibrations. Figure 2 at right is a closer view of one of these springs on a HAM.
Eventually we will build 12 HAM stacks for the LIGO interferometers in both Hanford and Livingston. In this "First Article" test we made just enough hardware for a single HAM. The idea was to assemble and test it under realistic conditions so we could make adjustments to parts and assembly fixtures before starting the main production run. This also gave us practice and data on assembly procedures. For instance, full body cleanroom suits and gloves were used for the first time in the experimental hall, and dust monitors were used to assess their effectiveness. Overall the test was a big success. Our performance objectives were generally achieved and a number of improvements to the final hardware were developed. The factories and machine shops have now been given final modifications to the drawings and the green light to proceed.