LIGO Document G0900586-v1

Flame welding tests for monolithic suspensions of the mirrors of future GW interferometers.

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The mass of the mirrors of Gravitational Wave interferometers such as Advanced LIGO will be 40 kg. The suspension of this mirror will be done on four circular-cross section fibres produced from high-purity fused silica. Breaking stresses of pristine silica fibres of up to 6 Gpa have been demonstrated. The probability density function of the breaking stresses is described by a function with two peaks, described in the literature as a bimodal Weibull distribution. To select appropriate fibres it reasonable to pre-test and exclude the weakest fibres in the lower peak of the distribution.
The fibres will be welded to the ‘ears’ attached to the silica mirrors (test masses) since this method of suspension provides the smallest additional losses. However the distribution of the breaking stresses of fibres welded to ears shows a smaller average value. Furthermore a relatively narrow peak is formed associated with the weaker fibres. This effect may be caused by deposition of silica vapour on the surface of the fibre during welding. To examine this assumption, extraction of welding vapour was carried out simultaneously with welding. The distribution of breaking stress was found not to have been modified in that case.
Experience which has been gained during preliminary flame welding experiments has allowed us to demonstrate the suspension of 40 kg on 4 silica fibres. The fibres were pulled using a CO2 laser and welded to the fused silica ears using an oxy-hydrogen flame. The fibre diameter was approximately 400µm and profiles of the fibre shapes correspond approximately to those appropriate for Advanced LIGO. The test mass for this suspension was fabricated from steel plates. This model suspension can be used for further studies of techniques associated with laser and flame welding. Some experimental simulations of the behaviour of test mass suspensions are anticipated as a subject for future work.
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