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LIGO Livingston Observatory News
Status of the Beam Tube Bakeout
The Roadless Traveled
School Visits to the LIGO Livingston Observatory
The first bake out of a beam tube module at the LIGO Livingston Observatory (LLO) is now complete. The half of the X-arm (the west arm) closest to the end station was successfully baked under vacuum at 168 C for 18 days, after which the outgassing rate for water vapor in the beam tube dropped from about 10-10 torr liter sec-1 cm-2 before the bake out, to less than 10-17 afterwards.
The beam tube is heated using two 5000 amp power supplies which are connected across the beam tube. Its electrical resistance (about 140 ohms) causes ohmic heating which warms the stainless steel beam tube and causes water and other light volatiles absorbed on the inner surface of the beam tube to dissociate from the surface so they can be pumped out of the vacuum system. Two turbo pumps and eight cryo pumps along the beam tube continue to pump on the beam tube during bake out. Since the binding energy of water to the beam tube is quite small, so the outgassing rate of water from the beam tube surface doubles about every nine degrees. Thus, heating the beam tube from ambient temperature to as high a temperature as is prudently possible and mechanically safe--which turns out to be around 170 C--produces an enormous increase (about one hundred thousand fold) in the rate at which water can be removed from the system.
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The temperature profile and beam tube total pressure graphs,
shown above in Figures 1 and 2 respectively, were provided by
Bill Althouse, LIGO's Beam Tube Bakeout Manager. These
illustrate how the beam tube was heated and how the internal
pressure decreased with time. The partial pressures for various
amu's are shown in different colors on the plot. The vertical
features are measurements of the instrumental background of the
residual gas analyzer, during which time the RGA was valved off
from the beam tube.
Next, Figure 3 at right (also provided by Bill Althouse) shows partial pressures of the residual gas within the beam tube following bake out. Note the enormous difference between the partial pressure of hydrogen and that of all the other species. Water, which dominated the total pressure of the beam tube prior to the bake, is now insignificant compared to the hydrogen outgassing rate. The removal of water is the biggest single motivation for undertaking the bake out. For comparison, the bake results from each of the LIGO Hanford beam tube modules are also shown.
This is an important milestone in the commissioning of LLO. Preparation for this activity began last year with Kerry Stiff's extended visit to the LIGO Hanford Observatory (LHO) to participate in the bake out activities there. Under Cecil Franklin and Allen Sibley's local direction, in coordination with Bill Althouse's overall supervision, the equipment was shipped from LHO to LLO beginning last May, and then reinstalled here starting around the beginning of July. A crew of operators was hired, an electrical contractor (MMR of Baton Rouge) was brought on-board, and everyone was trained in the techniques required to operate the vacuum equipment, power system, and the data acquisition system. Round-the-clock staffing was required during the time the tube was being heated, and Rai Weiss came from MIT to be present for key measurements.
The bake out team is now moving all the instrumentation as well as the enormous amount of cabling to the other half of the X-arm so that it can be baked out. The turn-on date for the electrical power to heat the beam tube is around November 1 and the bake out should be completed before Thanksgiving.
"Rome was not built in one day," as the saying goes. But perhaps "Road wasn't built in a day" is a more appropriate version if what we're describing is the LIGO Livingston Observatory's access road.
It was eight years ago that the then Governor-Elect E. Edwards promised that the State of Louisiana would provide an all-weather access road to the Livingston Observatory. Five years later, in January of 1997, all of the legalities had been resolved, and construction of the new road began. But after only a few months, it had to be halted again to allow our contractors access for the construction of the Observatory. The road work did not resume again until February of this year. Then, on September 27, 1999--a date that will live in asphalt history--the finished road was turned over to LIGO.
After all these years of driving to work as if going on safari--dodging potholes, squinting through dust clouds and navigating shallow lakes--cruising smoothly down our new access road will be a real pleasure. As the pictures below show, our new all-weather road is quite an improvement from the old one, and our crew here is pretty convivial about the convenience.
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As the state approved access road is now complete,
it is possible for school groups to visit the LIGO
Livingston Observatory
using school buses. We welcome
this opportunity to share LIGO with our community!
In the first three weeks since the access road
opened, the sixth grade at Doyle Elementary School,
Livingston, and the Honors Physics
students at Denham
Springs High School in Denham Springs, have toured our
facility, and more visits are already scheduled.
For those working with a school who think they would
like to pay the Observatory a visit, please call the
main Observatory number
at 225-686-3100 to schedule
a time for your group to come.
At left and center, Doyle Elementary School children tour
the Livingston Observatory. At right, Honors Physics students
from
Denham Springs High School make a visit.
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