Aspects of materials research for advanced and future generations of gravitational wave detectors

Gravitational waves were predicted by Einstein, in his General Theory of Relativity in 1916. These waves can be thought of as ripples in the curvature of space-time. They have not yet been directly detected but strong indirect evidence of their existence was provided by Hulse and Taylor when they measured the rate of decay of the inspiral motion of a binary system. Research towards direct detection of gravitational radiation from astrophysical sources has been carried out for many years through the design, construction and initial operation of a network of gravitational wave detectors. Direct detection of gravitational waves will provide insights into the astrophysical sources which produce them and will provide a new method of observing events in the Universe. Gravitational waves are quadrupole in nature and produce strains in spacetime, which have extremely small amplitudes. The largest most violent events in the Universe are expected to cause strains of approximately 10^-22 at the Earth in the frequency band of a few Hz to a few kHz. Long baseline interferometry between suspended test masses is currently used to search for the induced strains in space-time, and thus the direct effects of gravitational waves of astrophysical origin. There is currently a network of interferometer detectors running worldwide. A 600 m long detector, GEO600, was built near Hannover in Germany by a collaboration between the Institute for Gravitational Research at the University of Glasgow, the Albert-Einstein-Intitut in Hannover and Golm, the University of Hannover and Cardiff University. There are three detectors in the United States of America forming the LIGO project. Two detectors, one of 4 km arm length and one of 2 km arm length were constructed on a site near Hanford, Washington State, and one detector of 4 km arm length was constructed near Livingston Louisiana. A 3 km detector, Virgo, was built near Cascina, Italy, by a European collaboration involving France, Italy and more recently the Netherlands. Six data collecting science runs have taken place to date with different combinations of these detectors in operation during the various runs; no detections have yet been made. An important noise source in the current operating frequency band of ground-based detectors is the thermal noise of the test mass mirrors in the interferometers, and the mirror suspension elements. The research presented in this thesis focusses on studies of techniques for quantifying and reducing the mechanical loss associated with the suspended mirrors and thus reducing the associated thermal noise thereby increasing detector sensitivity. In particular, experiments were carried out to study the loss of fused silica and investigate aspects of the hydroxy-catalysis bonding process used to joint elements of the test mass suspensions. In addition, silicon was investigated as a potential candidate for use as a mirror substrate material for use in future gravitational wave detectors. In Chapter 1 the nature of gravitational waves is explained and some of the sources which are expected to produce the largest amount of gravitational wave radiation are described. The development of resonant bar detectors and interferometers is given along with the current status of detectors and that of planned future projects. Noise sources which cause limitations to the detector sensitivity are discussed and an important noise source, thermal noise, is described in Chapter 2. Thermal noise is an important noise source in the current frequency band of gravitational wave detectors. Reduction of thermal noise is a major challenge but is possible through careful design of the mirrors and their suspension systems. One technique aimed at minimising the thermal noise of a suspension system involves the creation of a quasi-monolithic suspension system by the use of hydroxy-catalysis bonding. This is a high precision, high strength method of adjoining suspension elements. In Chapter 3 investigations were made of the strengths of hydroxy-catalysis bonds and on the effect on strength of various parameters associated with the most commonly used version of the bonding procedure, and of putting bonds through different treatments. It is shown that the average strength of a hydroxy-catalysis bond between silica substrates is ~ 15 MPa and that somewhat elevated temperature treatment (similar to an airbake) can improve on this strength, but that thermal shock conditions can decrease the strength. These investigations provide information on processes which can be used in the suspension construction to produce the lowest loss, highest strength suspension system. Chapter 4 details mechanical loss measurements of bulk silica at room temperature. Different types of fused silica are studied and techniques to reduce their mechanical loss are discussed along with the effect which time and heat treatments can have on the mechanical loss of a hydroxy-catalysis bond. It is shown that Suprasil 3001 is an acceptable choice of material for the mirrors in gravitational wave detectors and that the mechanical loss of silica can be reduced through heat treatment. In Chapter 5 the mechanical loss of bulk silicon is studied, where silicon forms a potential candidate for future generation gravitational wave detectors. Silicon samples having two different crystal orientations, and , were studied. Both orientations were manufactured and polished by the same vendor and have equivalent doping levels. At room temperature it is seen that the crystal orientation material yielded mechanical loss values which were slightly lower than the material. It is shown that it is possible to further reduce the loss of the material through heat treatment. An upper limit of the mechanical loss of a hydroxy-catalysis bond between silicon substrates is determined and found to be within the range of 0.27 to 0.52. The results presented in this thesis indicate that the mechanical losses of silica suspensions in gravitational wave detectors can be reduced through methods such as heat treatments and, potentially, chemical etching. Silicon is seen to be an interesting candidate for the suspension material in future generation detectors run at cryogenic temperatures

Aspects of materials research for advanced and future generations of gravitational wave detectors

Gravitational waves were predicted by Einstein, in his General Theory of Relativity in 1916. These waves can be thought of as ripples in the curvature of space-time. They have not yet been directly detected but strong indirect evidence of their existence was provided by Hulse and Taylor when they measured the rate of decay of the inspiral motion of a binary system. Research towards direct detection of gravitational radiation from astrophysical sources has been carried out for many years through the design, construction and initial operation of a network of gravitational wave detectors. Direct detection of gravitational waves will provide insights into the astrophysical sources which produce them and will provide a new method of observing events in the Universe. Gravitational waves are quadrupole in nature and produce strains in spacetime, which have extremely small amplitudes. The largest most violent events in the Universe are expected to cause strains of approximately 10^-22 at the Earth in the frequency band of a few Hz to a few kHz. Long baseline interferometry between suspended test masses is currently used to search for the induced strains in space-time, and thus the direct effects of gravitational waves of astrophysical origin. There is currently a network of interferometer detectors running worldwide. A 600 m long detector, GEO600, was built near Hannover in Germany by a collaboration between the Institute for Gravitational Research at the University of Glasgow, the Albert-Einstein-Intitut in Hannover and Golm, the University of Hannover and Cardiff University. There are three detectors in the United States of America forming the LIGO project. Two detectors, one of 4 km arm length and one of 2 km arm length were constructed on a site near Hanford, Washington State, and one detector of 4 km arm length was constructed near Livingston Louisiana. A 3 km detector, Virgo, was built near Cascina, Italy, by a European collaboration involving France, Italy and more recently the Netherlands. Six data collecting science runs have taken place to date with different combinations of these detectors in operation during the various runs; no detections have yet been made. An important noise source in the current operating frequency band of ground-based detectors is the thermal noise of the test mass mirrors in the interferometers, and the mirror suspension elements. The research presented in this thesis focusses on studies of techniques for quantifying and reducing the mechanical loss associated with the suspended mirrors and thus reducing the associated thermal noise thereby increasing detector sensitivity. In particular, experiments were carried out to study the loss of fused silica and investigate aspects of the hydroxy-catalysis bonding process used to joint elements of the test mass suspensions. In addition, silicon was investigated as a potential candidate for use as a mirror substrate material for use in future gravitational wave detectors. In Chapter 1 the nature of gravitational waves is explained and some of the sources which are expected to produce the largest amount of gravitational wave radiation are described. The development of resonant bar detectors and interferometers is given along with the current status of detectors and that of planned future projects. Noise sources which cause limitations to the detector sensitivity are discussed and an important noise source, thermal noise, is described in Chapter 2. Thermal noise is an important noise source in the current frequency band of gravitational wave detectors. Reduction of thermal noise is a major challenge but is possible through careful design of the mirrors and their suspension systems. One technique aimed at minimising the thermal noise of a suspension system involves the creation of a quasi-monolithic suspension system by the use of hydroxy-catalysis bonding. This is a high precision, high strength method of adjoining suspension elements. In Chapter 3 investigations were made of the strengths of hydroxy-catalysis bonds and on the effect on strength of various parameters associated with the most commonly used version of the bonding procedure, and of putting bonds through different treatments. It is shown that the average strength of a hydroxy-catalysis bond between silica substrates is ~ 15 MPa and that somewhat elevated temperature treatment (similar to an airbake) can improve on this strength, but that thermal shock conditions can decrease the strength. These investigations provide information on processes which can be used in the suspension construction to produce the lowest loss, highest strength suspension system. Chapter 4 details mechanical loss measurements of bulk silica at room temperature. Different types of fused silica are studied and techniques to reduce their mechanical loss are discussed along with the effect which time and heat treatments can have on the mechanical loss of a hydroxy-catalysis bond. It is shown that Suprasil 3001 is an acceptable choice of material for the mirrors in gravitational wave detectors and that the mechanical loss of silica can be reduced through heat treatment. In Chapter 5 the mechanical loss of bulk silicon is studied, where silicon forms a potential candidate for future generation gravitational wave detectors. Silicon samples having two different crystal orientations, <100> and <111>, were studied. Both orientations were manufactured and polished by the same vendor and have equivalent doping levels. At room temperature it is seen that the crystal orientation <111> material yielded mechanical loss values which were slightly lower than the <100> material. It is shown that it is possible to further reduce the loss of the material through heat treatment. An upper limit of the mechanical loss of a hydroxy-catalysis bond between silicon substrates is determined and found to be within the range of 0.27 to 0.52. The results presented in this thesis indicate that the mechanical losses of silica suspensions in gravitational wave detectors can be reduced through methods such as heat treatments and, potentially, chemical etching. Silicon is seen to be an interesting candidate for the suspension material in future generation detectors run at cryogenic temperatures.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Measurements of the Young’s modulus of hydroxide catalysis bonds, and the effect on thermal noise in ground-based gravitational wave detectors

With the outstanding results from the detection and observation of gravitational waves from coalescing black holes and neutron star inspirals, it is essential that pathways to further improve the sensitivities of the LIGO and VIRGO detectors are explored. There are a number of factors that potentially limit the sensitivities of the detectors. One such factor is thermal noise, a component of which results from the mechanical loss in the bond material between the silica fibre suspensions and the test mass mirrors. To calculate its magnitude, the Young’s modulus of the bond material has to be known with reasonable accuracy. In this paper we present a new combination of ultrasonic technology and Bayesian analysis to measure the Young’s modulus of hydroxide catalysis bonds between fused silica substrates. Using this novel technique, we measure the bond Young’s modulus to be 18.5 ± 2.0 2.3     GPa . We show that by applying this value to thermal noise models of bonded test masses with suitable attachment geometries, a reduction in suspension thermal noise consistent with an overall design sensitivity improvement allows a factor of 5 increase in event rate to be achieved

Strength of hydroxide catalysis bonds between sapphire, silicon, and fused silica as a function of time

Hydroxide catalysis bonds have formed an integral part of ground-based gravitational wave (GW) observatories since the 1990s. By allowing the creation of quasimonolithic fused silica mirror suspensions in detectors such as GEO600 and Advanced LIGO, their use was crucial to the first ever direct detection of gravitational waves. Following these successes, this bonding technique has been included in advanced next generation cryogenic detector designs. Currently, they are used to create quasimonolithic crystalline sapphire suspensions in the KAGRA detector. They are also planned for use in silicon suspensions of future detectors such as the Einstein Telescope. In this paper we report how the strength of hydroxide catalysis bonds evolves over time, and compare the curing rates of bonds as they form between fused silica substrates to those between sapphire to sapphire and silicon to silicon substrates. For bonds between all three types of substrate material we show that newly formed bonds exhibit slightly higher breaking stresses than bonds cured for longer periods of time. We find that the strength stabilizes at ≥ 15     MPa for bonds cured for up to 30 weeks (7 months). This finding is important to future cryogenic GW detector design as it is crucial to ensure the long term integrity of the suspension interfaces. Monitoring the strength of bonds that have been allowed to cure for shorter lengths of time can also shed light on the chemistry of bond formation

Mirror coating solution for the cryogenic Einstein telescope

Planned, cryogenic gravitational-wave detectors will require improved coatings with a strain thermal noise reduced by a factor of 25 compared to Advanced LIGO. In this article, we present investigations of HfO2 doped with SiO2 as a new coating material for future detectors. Our measurements show an extinction coefficient of k=6×10−6 and a mechanical loss of ϕ=3.8×10−4 at 10,K, which is a factor of 2 below that of SiO2, the currently used low refractive-index coating material. These properties make HfO2 doped with SiO2 ideally suited as a low-index partner material for use with a-Si in the lower part of a multimaterial coating. Based on these results we present a multimaterial coating design which, for the first time, can simultaneously meet the strict requirements on optical absorption and thermal noise of the cryogenic Einstein Telescope

Data on vegetation across forest edges from the FERN(Forest Edge Research Network)

Published versionMany studies have focused on vegetation across forest edges to study impacts of edges created by human activities on forest structure and composition, or patterns of vegetation at inherent natural edges. Our objective was to create a database of plant-related variables across different types of edges from various studies (mainly from across Canada, but also in Brazil and Belize) to facilitate edge research. We compiled data on vegetation along more than 300 transects perpendicular to forest edges adjacent to clear-cuts, burned areas, bogs, lakes, barrens, insect disturbances, and riparian areas from 24 studies conducted over the past three decades. Data were compiled for more than 400 plant species and forest structure variables (e.g., trees, logs, canopy cover). All data were collected with a similar sampling design of quadrats along transects perpendicular to forest edges, but with varying numbers of transects and quadrats, and distances from the edge. The purpose for most of the studies was either to determine the distance of edge influence (edge width) or to explore the pattern of vegetation along the edge to interior gradient. We provide data tables for the cover of plant species and functional groups, the species and size of live and dead trees, the density of saplings, maximum height of functional groups and shrub species, and the cover of functional groups at different heights (vertical distribution of vegetation). The Forest Edge Research Network (FERN) database provides extensive data on many variables that can be used for further study including meta-analyses and can assist in answering questions important to conservation efforts (e.g., how is distance of edge influence from created edges affected by different factors?). We plan to expand this database with subsequent studies from the authors and we invite others to contribute to make this a more global database. The data are released under a CC0 license. When using these data, we ask that you cite this data paper and any relevant publications listed in our metadata file. We also encourage you to contact the first author if you are planning to use or contribute to this database

A Cryogenic Silicon Interferometer for Gravitational-wave Detection

The detection of gravitational waves from compact binary mergers by LIGO has opened the era of gravitational wave astronomy, revealing a previously hidden side of the cosmos. To maximize the reach of the existing LIGO observatory facilities, we have designed a new instrument that will have 5 times the range of Advanced LIGO, or greater than 100 times the event rate. Observations with this new instrument will make possible dramatic steps toward understanding the physics of the nearby universe, as well as observing the universe out to cosmological distances by the detection of binary black hole coalescences. This article presents the instrument design and a quantitative analysis of the anticipated noise floor

A Cryogenic Silicon Interferometer for Gravitational-wave Detection

The detection of gravitational waves from compact binary mergers by LIGO has opened the era of gravitational wave astronomy, revealing a previously hidden side of the cosmos. To maximize the reach of the existing LIGO observatory facilities, we have designed a new instrument able to detect gravitational waves at distances 5 times further away than possible with Advanced LIGO, or at greater than 100 times the event rate. Observations with this new instrument will make possible dramatic steps toward understanding the physics of the nearby Universe, as well as observing the Universe out to cosmological distances by the detection of binary black hole coalescences. This article presents the instrument design and a quantitative analysis of the anticipated noise floor