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Entry  Sun Mar 28 20:05:33 2010, rana, Summary, PSL, FSS Work from Sunday: AOM/VCO level set wrong Untitled.png
    Reply  Sun Mar 28 20:51:31 2010, rana, Summary, PSL, FSS Work from Sunday: Cavity Suspension is Ridiculously Undamped! Untitled.png
       Reply  Sun Mar 28 23:17:46 2010, rana, Summary, PSL, FSS Work from Sunday: noise spec in2.png
          Reply  Sun Mar 28 23:47:47 2010, rana, Summary, PSL, FSS Work from Sunday: Open Loop Gain fssloop.pngsweep2.png
             Reply  Mon Mar 29 01:11:33 2010, rana, Summary, PSL, FSS Work from Sunday: RF Out Spectrum rfout.pngTEK00000.PNGTEK00001.PNGTEK00002.PNG
             Reply  Mon Mar 29 02:07:50 2010, Koji, Summary, PSL, FSS Work from Sunday: Open Loop Gain 
Message ID: 2721     Entry time: Sun Mar 28 20:51:31 2010     In reply to: 2720     Reply to this: 2722
Author: rana 
Type: Summary 
Category: PSL 
Subject: FSS Work from Sunday: Cavity Suspension is Ridiculously Undamped! 

What is the Transfer Function of the suspension of the reference cavity? What were the design requirements? What is the Q and how well does the eddy current damping work? What did Wolfowitz know about the WMD and when? Who cooked the RTV in there and why didn't we use Viton??

To get to the bottom of these questions, today I shook the cavity and measured the response. To read out the pitch and yaw modes separately, I aligned the input beam to be misaligned to the cavity. If the beam is mis-aligned in yaw, for example, the transmitted light power becomes first order sensitive to the yaw motion of the cavity.

In the attached image (10 minute second-trend), you can see the second trends for the transmitted and relfected power. The first ringdown comes from the pitch or vertical mode. The second (shorter) one comes from the yaw misalignment and the yaw shake.

To achieve the up/down shake, I leaned onto the table and pumped it at its eigenfrequency. For the yaw shake, I put two fingers on the RC can's sweater and pushed with several pounds of force at the yaw eigenfrequency (2.6 Hz). For the vertical, I jumped up and down at half the vertical eigenfrequency (4 Hz).

I also made sure that the .SCAN field on these EPICS records were set to 9 so that there is no serious effect from a beating between the eigenfrequency and the EPICS sample rate.

Punchline:

f_vert   = 4 Hz

tau_vert = 90 seconds

Q_vert   = 1000            (yes, that number over there has 3 zeros)

 

f_hor    = 2.6 Hz

tau_hor  = 30 seconds

Q_hor    = 250

 

This is an absurd and probably makes us very sensitive to seismic noise - let's make sure to open up the can and put some real rubber in there to damp it. My guess is that these high Q modes

are just the modes of the last-stage steel spring / pendulum.

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