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Entry  Mon May 18 02:18:17 2009, rana, Summary, SEI, Using STACIS w/ a good position sensor stacis40.png
    Reply  Tue May 19 16:08:03 2009, rana, Summary, SEI, EUCLID Picture_6.png
Message ID: 1598     Entry time: Mon May 18 02:18:17 2009     Reply to this: 1608
Author: rana 
Type: Summary 
Category: SEI 
Subject: Using STACIS w/ a good position sensor 
WE turned off STACIS a few years ago because we noticed that it was causing noise below a few Hz and making
the overall velocity between the ends higher than with them off. I'm pretty sure they were causing noise
because they use little geophones which are noisy. Below ~0.2 Hz the horizontal geophones are also probably
limited by tilt-horizontal coupling.

Another concept (based on discussion with Brian Lantz and Matt Evans) is to instead put a good position sensor
between the ground and then blue support beam. Since the the STACIS rubber acts like a Q~2 passive resonance at
20 Hz, the whole seismic system (including the blue beams, in-vac tubes, and internal stack) act like a proof
mass of a seismometer.

So, in principle, if we use a very good position sensor and feedback to the STACIS piezo actuators, we can cancel
the ground motion before it enters the stacks. The initial LIGO OSEMs have a noise of 10^-10 m/rHz above 10 Hz
and going up like 1/f below 10 Hz. The AdvLIGO BOSEMs have a noise of ~2x better. Even better, however, are the
UK's EUCLID interferometric OSEMs (developed by Stuart Aston and Clive Speake).

In the attached plot, I show what we can get if we use these EUCLIDs make a ~60 Hz BW feedback loop w/ STACIS.

BLACK   - raw ground motion measured by the Guralp
MAGENTA - motion after passive STACIS (20 Hz harmonic oscillator with a Q~2)
GREEN   - difference between ground and top of STACIS
YELLOW  - EUCLID noise in air
BLUE    - STACIS top motion with loop on (60 Hz UGF, 1/f^2 below 30 Hz)
CYAN    - same as BLUE, w/ 10x lower noise sensor

One of the SURF projects this summer is to put together a couple different sensors like EUCLID to understand the noise.
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