LIGO Document G0900594-v1
- A major aspect of the upgrade from Initial LIGO to Enhanced LIGO (eLIGO) is a four-fold increase in input laser power. As a result of the subsequent increase in radiation pressure on the test masses, any angular beam misalignments will produce a correspondingly larger torque on the test mass than in Initial LIGO. At full operating power, this radiation pressure torque is large enough that for misalignments which displace the cavity axis, radiation pressure torque overcomes the natural pendulum (suspension) restoring torque, and the uncontrolled arm cavity becomes unstable. This Sigg-Sidles instability requires an upgrade to the Angular Sensing and Control (ASC) subsystem. I will present the design of the eLIGO ASC and its performance on the Livingston and Hanford detectors.
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