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Message ID: 545     Entry time: Tue Mar 22 15:59:49 2011
Author: frank, tara 
Type: DailyProgress 
Category: RefCav 
Subject: Q measurement for RCAV new suspension 

Today we tried to measure Q factor from RCAV's suspension.  By measuring the FWHM of the peak and the ring down signal,

however, the results are no good, we cannot excite the mode to see clear ring down patterns, and the FWHM for the excited peaks are too narrow.

So we present here the motion of the top stack(vertical translation combined with pitch) and the cavity's motion 

 

After we changed RCAV's suspension, beat becomes worse between 10 - 100Hz from many mechanical peaks.

Since we changed only the spring of the suspension, we expect that the peaks should originate from vertical motion.\

 

We used optical lever to observe the front mirror's pitch motion. The PZT is placed under the chamber's flange  to excite the modes.

Only f = 19.88 and 25.69 Hz can be excited, but still very small, other modes show no ring up during the excitation at all.

Then we measured the stack's top motion, by shadow sensing technique. From the setup it will sense both vertical translation and pitch

motions of the cavity. Most of the peaks from the stack match with the cavity's motion, except a broad peak around 65 Hz which we don't

see on the cavity (from shadow sensing and oplev)

seis_v.png

 

We know the calibration factor from oplev, so the motion of the cavity in rad/ rt Hz measured by oplev is plotted.

- calibration on QPD 46.1 V/m  (10mV over 200 micron)

- lever length  1.8 m  ->  46.1 V/m *1.8 m = 83 V/rad

- divide the signal by 83 V/rad to get rad/ rt Hz

pitch.png

 

 

 

 

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