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LIGO Hanford Observatory NewsBeam Tube Bakeout: First Module A Hot Success!
Enthusiasm Abounds As Hanford Hosts Latest NSF Review
Bake lightly, at 300 degrees Fahrenheit, for 30 days. Is this how Aunt Tillie gets that interesting texture in her fruitcake? Not quite. It's how we get a few ounces of water off the walls of a one and a quarter mile-long piece of LIGO Beam Tube. And it was just done for the first time at the Hanford Observatory.
LIGO requires a very good vacuum between its mirrors or else there is a mirage-like effect that makes the mirrors appear to move even when they are standing still. A "very good" vacuum in this case means about a billionth of an atmosphere as LIGO gets started, but eventually we will need a vacuum of about a trillionth of an atmosphere. We decided to go for the final limit up front in preparing our Beam Tube. When we start pumping on the tube, the air actually comes out very nicely in a few days. The problem arises due to so-called condensable molecules, like water vapor and hydrocarbons. These molecules tend to stick lightly onto the stainless steel walls of the Beam Tube. If they would only stick forever, life would be easy. Unfortunately they stick poorly enough that they keep coming off the walls and spoiling the vacuum inside the tube. At the same time, they stick well enough that it takes forever for all these molecules to finally come loose. (Whoever said life was easy!?!) Our strategy is to shake them loose fast enough so that our pumps can gobble them up before they stick to the walls again. A one and a quarter mile-long tube is hard to shake, but we have a trick--call it physicist magic. If we heat the walls up, then the molecules have more energy, and they will shake themselves loose.
This process takes about 30 days of heating the entire length of tube to 300 degrees Fahrenheit. Now, 300 degrees isn't really that hot--many baking recipes use higher temperatures--but what do you use for an oven when your "cake" is longer than a mile?
Basically, we made the tube itself into an oven. We wrapped its four-foot diameter in fiberglass insulation about ten inches thick. (See Figure 1 at left.) Then we ran 1,850 amperes of current down its length. The stainless steel tube effectively acted as its own heating element. Designing and staging this effort took more than a year of hard effort by Bill Althouse, Al Lazzarini, Rai Weiss, Mark Lubinski, Mark Guenther and Kerry Stiff. At the same time, helping staff the field operation round the clock were Sid Bevans, Larry Garelts, Ron Houtrow, Tim Neasham, and Tom Russel. Special portable power sources needed to be acquired. Many were graciously loaned to us by those kindly folks at the Fermi National Accelerator Laboratory in Illinois, and promptly wired up. About one megawatt of electricity was drawn through a series of transformers to provide the large DC currents through the Beam Tube, as well as AC power for the pumps and instrumentation along the tube. A flatbed truck (See Figure 2 below, left) was used as a platform for each 1/2 megawatt power supply, along with its associated transformer and heat exchanger to make the system portable. Special copper cables (Figure 3 below, right), with a total cross-section of 2.4 square inches of copper, carried current back from the tube. Their ends were carefully trimmed in length to balance currents in the various loops.
Next, turbopumps at each end of the tube pumped non-condensable gases, like hydrogen, while eight cryopumps were spaced out along the tube to pump condensable molecules. Roughly 400 thermocouples monitored temperatures along the Beam Tube, and a data acquisition system was set up to monitor the entire bakeout system. Residual gas molecules in the tube were monitored by a mass spectrometer throughout the bake. Metal bellows spaced every 130 feet took up the thermal expansion from the bake and special gauges were used to verify that the mechanical strains on the tube agreed with our structural modeling. In late August we started slowly raising the tube temperature, and we cooled down again in October.
By the end, it had taken a bit more than 30 days to get all the bugs out of the baking process this first time around. When Rai Weiss surveyed the vacuum data and declared that the bakeout goals had been met--and the tube was leak free--we figured this bake was ready for the record books and started moving equipment for the next bake, due to start in mid-November. Now if only we could figure out how Aunt Tillie gets that texture in her fruitcake...
The normally scheduled semi-annual review of the LIGO Construction Project was conducted by a panel of experts on behalf of the National Science Foundation (NSF) on October 27-29, 1998. This was the first time a full review had been held at the Hanford Observatory. It was also the first time some members of the review committee and the NSF had been to Hanford in more than eighteen months, and for other participants it was the first opportunity ever to visit one of the LIGO Observatories.
At left, Head of the Hanford LIGO Observatory, Fred Raab, addresses members of the NSF review panel. At right: Rai Weiss, a man who can truly be called one of the founding fathers of LIGO, and of the laser interferometer gravitational-wave research community in general. Here Rai, acting as spokesman for the entire LIGO Scientific Collaboration, gives his first presentation from the Observatory that he helped to envision.
The project has entered an exciting new phase with facility construction, including the vacuum system, nearing completion. At Hanford, the focus is shifting now to Detector installation. This recent review therefore provided a nice opportunity for the NSF panel members to see the completed facilities and to witness firsthand the various activities in the control room, the optics lab, the laser and vacuum equipment area (LVEA) of the corner station, and along the Y-arm, where equipment used during the recently completed bake of the Y-2 module (see the related article above) was being moved and set up in preparation for baking the Y-1 module. As Don Hartill, the chair of the review panel, remarked, it was an opportunity to "kick the tires." He was also excited, he said, by the enthusiasm that was apparent at Hanford, indicating that this enthusiasm would surely "make it happen."
LIGO is very pleased with the comments provided by the review team during the closeout briefing. The team congratulated LIGO on a successful start of the Detector installation, and commended us on the successful completion of the conventional facilities construction and the bakeout of the first 2-km section of Beam Tube.
The next NSF review, scheduled for April 27-29, 1999, will evaluate the continued progress of the Detector installation, as well as the development of our data analysis and modeling capabilities. This meeting, in another first, will be held at the recently completed LIGO Livingston Observatory.