LIGO Document T080003-v1

Thermal Noise Increase due to a Gold Coated Barrel

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Document type:
T - Technical notes
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16 Jul 2008, 00:00
Phil Willems proposed the addition of a gold coating on the barrel of the test masses and the compensation plates in order to reduce the emissivity and reduce the radial temperature gradients due to radiation from the barrel. Since gold is a lossy material it may also help to passively damp higher order acoustic modes which might otherwise be excited into oscillation by higher order optical cavity modes. However, there is concern that the lossy gold coating might lead to an unacceptably large increase in Brownian thermal noise in the optical readout. The purpose of this technical note is to report on a finite element analysis used to calculate the increase in thermal noise (using Levinís approach) due to the addition of a gold coating.

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Thermal Noise
Notes and Changes:
Notes on use of the Comsol model:
1) developed for comsol 3.2, but works on 3.5a

2) one can integrate the strain energy density (Ws_smaxi) over the subdomain to get the total strain energy due to the Levin applied Gaussian pressure on the HR face [Postprocessing, Subdomain integration ...]

3) However, one cannot use the [Postprocessing, Boundary integration ...] in order to integrate the strain energy density at the boundary.

4) Instead export the strain energy along the barrel to be integrated in Matlab, as follows:
a) Plot the strain energy density in the subdomain
b) Draw a line for a cross-section plot which is on the barrel edge (using the auto end-point selection feature). A figure with a line plot of strain energy density versus depth (z) along the barrel will be displayed.
c) Export the plot in an ASCII file. Select the "Write script that creates postprocessing data". The Matlab script file (*.m) created has the date in the following format:
clear pd
pd.p = [...
Z1 SE1 ;...
Z2 SE2 ;...
Zn SEn ;...
pd = rmfield(pd,'p');

d) Edit the end of the file as follows:
pd.x = pd.p(1,:);
pd.y = pd.p(2,:);
pd = rmfield(pd,'p');

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