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  • Special Feature - LIGO II
    frequencies until shot noise takes over at about 100 Hz This buffeting of the masses grows with the laser power again as the square root of the power and so it becomes clear that an optimum laser power exists a power such that the sensing noise at high frequencies is reduced to an acceptable level but one where the low frequency buffeting of the test masses by radiation pressure is not so great as to impact the low frequency performance We call the LIGO II design a quantum limited interferometer due to the fact that at all frequencies the LIGO II sensitivity is limited by the quantum nature of light Since the buffeting is a force it makes sense that this noise source falls as 1 f 2 and that the motion associated with it becomes smaller if the mass is greater This leads us to the second significant change from LIGO I the test masses are to be 30 kg rather than LIGO I s 10 kg to hold down the radiation pressure noise and allow a higher laser power It is probable that we will find ways to circumvent this naive quantum limit in the future through quantum non demolition techniques Stay tuned to the LIGO Newsletter over the next decade for late breaking updates Thermal Noise Over a broad range of frequencies the sensitivity of LIGO I will be limited by the Brownian motion of the test masses The test masses are in thermal equilibrium with the surrounding heat bath at a carefully regulated 20 degrees Celsius and thermodynamics tells us that each mechanical mode of the test masses and their wire loop pendulum suspensions in the case of LIGO I has kT of energy where k is Boltzmann s constant This energy is expressed as a random motion of the test mass where the distribution of the motion as a function of frequency is determined by details of the losses which limit the mechanical Q of the system test mass or suspension To reduce this noise one wants to gather the noise into the peak near the mechanical resonances by choosing materials and processes which maximize the mechanical Q and place the resonances either below the frequencies of interest the pendulum suspension modes around 1 Hz or well above the frequencies of interest the test mass internal modes 10 kHz and higher This introduces two very important changes from LIGO I First we plan to use sapphire instead of fused quartz for the test mass material Sapphire has very low mechanical losses and also a high speed of sound and a high density These are all advantageous for the thermal noise and for the radiation pressure noise However to obtain sapphire in the size required for a LIGO test mass order of 25 cm diameter 10 cm thickness and of an optical quality sufficient for the interferometric sensing requires a significant development effort but will be rewarded with a much reduced thermal noise curve 4 Fig

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9911news/9911two.html (2015-06-02)
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  • LIGO Livingston Observatory News
    enormous increase about one hundred thousand fold in the rate at which water can be removed from the system 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 Important Milestone 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 The Roadless Traveled Contributed by Gerry Stapfer HABAMUS VIA 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

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9911news/9911liv.html (2015-06-02)
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  • LIGO Hanford Observatory News
    at Caltech and MIT Over the past decade there have been typically only about a dozen people in the U S who could be left alone to operate one of these gravitational wave detector test beds The challenge now is to come up with about two dozen people at both of the Hanford and Livingston observatories who can run diagnose and fix interferometers and manage the online data analysis How do you quadruple the knowledgeable population of interferometer experts in a few short years One aspect of a solution has been to design and build equipment with more of a turn key nature than the campus test beds which were designed to be operated by the designers The second is to find people with a range of skills and backgrounds lots of enthusiasm a sense of adventure and then fill in whatever else is needed to make them experts in this new field At Hanford we have been able to attract people with the requisite qualities early and to have them work with the designers to install and commission hardware Now that people have developed a visual memory of the hardware and knowledge of the practical aspects of the instrumentation the time is ripe for a systematic education in interferometer science and engineering The first step in that education is LIGO Basic Training a series of lectures aimed at developing a common set of conceptual tools for understanding the science of LIGO Topics will include Basic Calculus Spectral Analysis Statistics Basic Electronics Vacuum Practice Mechanics Geophysics Thermodynamics Optics Quantum Mechanics Special Relativity General Relativity and Astronomy The emphasis of LIGO Basic Training is on concepts rather than details For instance visualizing a partial derivative counts for more than being adept at calculating it since a computer would likely do the

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9911news/9911han.html (2015-06-02)
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  • LIGO Hanford Observatory News
    actually performed to remediate desert soil disturbed in constructing LIGO Hanford experiences severe winds often powerful enough to scoop out as much as 18 inches of the sandy soil in a single storm But the new landscaping put an end to that The winds still try to battle back in the form of invading tumbleweeds Left unchecked these weeds would drift onto our roads causing further problems The solution we have adopted is to use a hay baling machine to bale the tumbleweeds A few of the tumbleweeds bales can be seen in the foreground of the photo These are then deposited onto naturally eroding sand dunes near the end station Eventually this should help the naturally occurring vegetation to recolonize the sand dunes The next big meeting for our new building will be the LIGO Science Collaboration meeting in March Currently we expect a crowd of about 90 people Goodbye CB I Contributed by Fred Raab On February 10 1998 Brad Shaw turned in his keys to the LIGO facilities in Hanford Washington Brad was the last employee of Chicago Bridge Iron CB I to leave the LIGO Hanford Observatory site Driving off site as killdeer were starting to call their mates for a fresh season of renewal Brad was leaving behind him a monumental structure one of the largest vacuum systems in the world Brad his CB I colleagues and LIGO s engineers and physicists had a product of which to be proud Stretching out through the sagebrush the two orthogonal arms of the LIGO vacuum system each measure two and a half miles long and are easily visible on the commuter flights between Pasco and Seattle The vacuum volume of the tubes is about 9 000 cubic meters comparable to the capacity of 15 000 typical kitchen

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9802news/9802han.html (2015-06-02)
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  • LIGO Livingston Observatory News
    on the X arm CB I fabricates the tube sections at a facility near Denham Springs about 25 miles from the observatory site The fabrication equipment was set up and requalified here after being moved last summer from its previous facility in Pasco Washington Nearly three fourths of the 400 tube sections required to finish the observatory have been fabricated Weld quality continues to be exceptional Of the nearly 700 tube sections spiral welded for both observatory sites all have passed the rigorous helium leak test The photograph above shows a small portion of the Beam Tube installed on the slab partially covered by the concrete enclosure A wrapped pump port valve a fixed tube support and the tube s vacuum stiffening rings are evident Although rain is more common at the Livingston site installation has not been slowed in fact it s at an even higher rate than that achieved at the Hanford site Civil Construction At Livingston Nearly Complete Contributed by Gerry Stapfer Civil construction of the Livingston Observatory is winding its way toward completion LIGO staff has taken joint occupancy of the Operational Support Building OSB and has enthusiastically moved into their new quarters Despite stubborn non

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9802news/9802liv.html (2015-06-02)
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  • LIGO Caltech News
    additional mirror Demonstration of operation in a power recycled mode proceeded through a step by step process of bringing subsystems of increasing complexity within the interferometer into their correct operating configurations or lock At each step comparisons of the observed interferometer behavior were made to the lock acquisition model SMAC developed earlier SMAC the Single Mode Acquisition Code was used to design the interferometer control topology and we utilized its predictions here to lock the subsystems in the correct order For more about SMAC see the articles in newsletter Volume 1 Number 6 and Volume 2 Number 6 Some differences with the model were found For example intuitively we expected a sign flip in the beamsplitter servo when the second arm came into lock this is also what the software model predicts When we removed the sign flip the interferometer leapt into lock and we observed solid lock sections of 10 minutes or more There are two most likely reasons why the sign flip is unnecessary One is that the carrier is undercoupled in the interferometer i e the light coming back into the recycling cavity from the front mirror of the Fabry Perot cavity is mostly from reflection off the back of the mirror and only a little bit of the light is due to leakage of stored light out of the Fabry Perot cavity Alternatively it may be due to poor mode matching into the interferometer i e the intensity profile of the beam stored in the Fabry Perot cavity does not smoothly match the intensity profile of the light within the recycling cavity allowing us to effectively lock the beamsplitter on non mode matched light Both possibilities are now under investigation Figure 1 The configuration of the 40 Meter Laser Interferometer on the Caltech campus The recycling mirror seen at left in the diagram was installed last fall Figure 2 belows shows the power recycled interferometer acquiring and then losing lock There are four traces shown the lower two show the south and east arm transmitted light levels SAT and EAT voltages These are measures of the amount of optical power circulating in each arm a higher level indicates greater circulating power The upper trace labeled APD shows the light level incident on the photodetector of Figure 1 The trace below labeled SPD shows the amount of light returning to the laser from the recycling cavity In the diagram the ground level for each trace is indicated by the arrow beside the trace number Also the APD and SPD have reversed polarities so that increasing light levels are indicated by decreasing voltages At the beginning of the plot the power recycled Michelson is resonant for the sidebands and anti resonant for the carrier The east arm comes into resonance for the carrier after approximately 100 msec Since the power recycled Michelson cavity is essentially anti resonant for the carrier light very little light leaks into the east arm and hence the light build up in this cavity

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9802news/9802cit.html (2015-06-02)
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  • LIGO MIT News
    the separate suspended cavity This is a level which is satisfactory for the final phase of the Phase Noise Interferometer enabling the next phase of research to begin measurements of the phase noise at the LIGO sensitivity on a recycled suspended Michelson interferometer It also shows that the stabilization technology is in place for LIGO The additional optical components the beamsplitter and second Michelson mirror were prepared for in vacuum suspension attachments suspension balancing and vacuum baking with the suspension controllers accurately diagonalized ensuring that both sensors indicate and actuators deliver pure translations A modest data acquisition system was set up Sensors and feedback for additional degrees of alignment freedom were installed An Indium Gallium Arsenide photodetector amplifier was prepared which will be used to sense the light in an early prototype test for the LIGO Length Sensing Control photodetector This final stage of the experiment was commissioned in September and has made rapid progress since that time The complete power recycled Michelson has reached a new sensitivity limit in phase of less than 2x10 10 rad sqrt Hz as shown in Figure 2 Shot noise dominates down to 400 Hz and corresponds to a circulating power of 150 watts This is to be compared to the LIGO requirement of 8x10 11 rad sqrt Hz and a planned circulating power of 300 w The present experiment is limited by thermal focusing in the beamsplitter a lower loss material is planned for LIGO A modification in the geometry of the layout may allow higher sensitivities to be reached but the present sensitivity limit suffices to research problems of photodetection and light scatter at LIGO like phase sensitivities At lower frequencies research has led to significant reductions in the noise level One large group of resonances in the best Argon laser phase

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9802news/9802mit.html (2015-06-02)
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  • LIGO Hanford Observatory News
    the light into and extract it back from the main interferometer There are six HAMs for each interferometer making twelve altogether at Hanford There are many more parts to the vacuum equipment in the experimental halls including numerous pumps manifolds bellows gate valves and liquid nitrogen filled cryopumps Once all the equipment was bolted together pipefitters and electricians were able to make the plumbing and electrical connections that allow the equipment to work together Then the vacuum chambers in each experimental hall can be pumped down The pumps do an effective job of getting air out of the chambers in less than a day but molecules of gas that are lightly stuck to the chamber walls are harder to remove Like Stick On Notes they do eventually come loose but take many years to do so and would degrade the vacuum in the chambers for a long time Our solution is to bake the chambers for several days at about 150 degrees Centigrade or a cookie baking 300 degrees Fahrenheit The extra heat gives the gas molecules enough energy to break free of the walls and make it to our pumps like ripping off the Stick On Note and throwing it in the trash Bake outs have now been completed in the two mid station buildings and in the end station on the Y arm Figure 3 at left shows the Y arm mid station experimental hall being baked The shiny covers on the BSC and other hardware are heating blankets with aluminum foil covers used during the bake The blue boxes are power supplies that feed current to the blankets There are temperature monitors underneath the blankets and a control system in the boxes adjusts the blanket currents to get uniform temperatures everywhere Once the chambers cool the pressure is approximately a trillionth of an atmosphere A mass spectrometer is then used to identify the nature of any residual gas left behind This will be analyzed by scientists to ensure the chambers are clean enough to start the installation of interferometer components this summer Hanford Hosts Second LIGO Scientific Collaboration Conference Contributed by Rainer Weiss The second meeting of the LIGO Scientific Collaboration LSC was held this March at the Hanford Washington site Attended by about 100 people the plenary sessions of the meeting were convened to discuss a draft Publications and Presentations policy This is a very important policy as it describes how the Collaboration will determine who is an author on each scientific publication a key to earning recognition for the work reported In addition a forum was held on the criteria for the detection of gravitational waves Discussion of the Publications policy lead to several suggestions for improvements in the draft version that had been circulated The committee responsible will revise the draft and bring it to a vote of the Collaboration Council at the August meeting in Boulder Colorado When ready the revised draft will be posted on the LSC web site hopefully by May

    Original URL path: http://www.ligo.caltech.edu/LIGO_web/9803news/9803han.html (2015-06-02)
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