LIGO Document G1100332-v1
- Calibration and actuation forces applied to the test masses of interferometric gravitational wave detectors produce both, local3,4 and bulk5,6 elastic deformations that can significantly alter the motion sensed by the interferometer, especially when the frequencies of the periodic forces is above 1kHz. Our study show an increasing discrepancy between the actual sensed motion and the nominal free - mass motion to 1% at about 4.3kHz for an optimum two - beam Photon Calibrator configuration (2PCal). The discrepancy is weakly dependent on the position of the interferometer (ifo.) beam, but strongly dependent on the PCal location. For a ifo. and PCal deviation of 3mm and 1mm, respectively, we found a 1% discrepancy at 3.5kHz, and as much of 6% at 5kHz. If a three - beam (3PCal) or four - beam PCal (4PCal) configuration were used, the discrepancy could be reduced to 0.2% for frequencies up to 5kHz. This is a consequence of how a particular PCal configuration can effectively excite a normal mode. Nevertheless, a 1mm deviation for 3PCal or 4PCal results in a discrepancy of 1% at 3.5kHz, as in 2PCal case, emphasizing the extra care that one has to have positioning the PCal beams.
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