Aspects of fused silica fibres for use in gravitational waves research

Toland, Karl W. (2020) Aspects of fused silica fibres for use in gravitational waves research. PhD thesis, University of Glasgow.

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A pivotal aspect in increasing the sensitivity of the Advanced LIGO (aLIGO) detectors to enable for the detection of gravitational waves was the installation of the monolithic fused silica suspensions in the detectors. 40kg fused silica test masses were installed and suspended by four 400um fused silica fibres to lower the thermal noise of the detector compared to initial LIGO. Along with various other instrument upgrades to increase the sensitivity of the aLIGO detectors, it allowed the LIGO-Virgo Collaboration to directly observe gravitational waves from coalescing binary black hole and binary neutron star systems. There is always a desire to make these detectors even more sensitive by changing parameters of the instrument, and in the case of this thesis, parameters associated with the monolithic suspensions.

This thesis presents various areas of research related to the use of fused silica fibres for gravitational wave research. Firstly, a procedure was documented to standardise the alignment process of the aLIGO fibre pulling machine. Previously, this alignment process was subjective to the user as there were no documented set of instructions, which increased the probability of manufacturing defects to be introduced into the fibre production. This procedure also highlighted common issues that were related to the misalignment of the pulling machine and how these misalignments could be corrected.

One of the manufacturing defects that could be introduced into the fibres were angular misalignments in the thermoelastic nulling region. This became a prominent issue during the commissioning break between O2 and O3 where there were several instances of suspension fibres failing where these misalignments were observed. Though it was not concluded that these were the cause of the failures, an investigation was carried out to determine how great of an effect these angular misalignments have on the ultimate strength of the fibres. This investigation confirmed that these angular misalignments could be introduced into the fibre due to beam distribution misalignment around the stock material during the pulling process. These angular misalignments however did not show any adverse effect on the ultimate strength of the fibre.

The opportunity to work at LHO arose for a four-month period on a long term attachment (LTA). During this LTA, an investigation into the stress fatigue of fused silica fibres was carried out in the fibre production lab at LHO looking at a stress range between 3-5 GPa. The results of these investigations were then compared to an investigation by Proctor in the 1960s to see if there were any differences observed. It was found that data from both LHO and Glasgow did not align with the data that Proctor had obtained in his investigation, but did align with each other, showing that investigations carried out at LHO or Glasgow can be considered to be consistent with each other. Due to the data not aligning with Proctor's results, a follow-on investigation looking at fibres with comparable diameters were investigated.

To do this, a dedicated fibre profiler had to be developed first that was designed specifically to profile fibres within this diameter range. This included being able to profile fibres that were under 10um in diameter. The thin fibre strength tester was also upgraded to add a magnetic encoder to measure the distance travelled by the motorised stage.

An investigation into the stress fatigue of sub-40um diameter fibres was then carried out to compare results with the previous investigation with aLIGO fibres. It was found that fibres that were above 10um in diameter showed strong performances similar to that of Proctor, regardless of the initial stock material being polished, double polished, or no polish. Fibres under 10um however did not show this strong performance and were weaker performing. Polishing the initial stock material did show an increase in suspension time, however these were still weaker than the >10um diameter fibres. To see if this behaviour was consistent with other fibre characteristics, an investigation into the Young's modulus of the ultra-thin fibres was investigated.

A previous investigation into the Young's modulus of ultra-thin fibres was carried out by the author, however there were several factors in the equipment used at the time that lead to large uncertainties being associated with the Young's modulus values obtained. With the new upgraded strength tester and dedicated fibre profiler, this investigation was revisited to see if results obtained were consistent or different to that previously found. The uncertainties associated with the Young's modulus were successfully decreased from 16%, down to 7%. For fibres that had a minimum diameter greater than 10um, the Young's modulus value tended to agree with the accepted value of 72GPa. The average value of three batches that were tested with diameters greater than 10um were found to be (72.3±2.3)GPa, (71.8±1.8)GPa and (75.9±3.0)GPa. This gives a weighted average of (72.7±1.3)GPa. For the batch that had fibres under 10um in diameter, the average Young's modulus value was (63.3±2.7)GPa and a weighted average of (62.8±1.9)GPa. Non-destructive tests on these fibres showed an average value of (62.9±2.8)GPa. This is consistent with the stress fatigue tests where fibres under 10um showed different characteristics.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: gravitational waves, fused silica, monolithic suspension.
Subjects: Q Science > QC Physics
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Funder's Name: Science and Technology Facilities Council (STFC)
Supervisor's Name: Hammond, Prof. Giles and Rowan, Prof. Sheila
Date of Award: 2020
Depositing User: Dr Karl W. Toland
Unique ID: glathesis:2020-81461
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 18 Jun 2020 06:40
Last Modified: 18 Jun 2020 06:44

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