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Electrostatic actuators: from the GEO experience to the 2nd generation requirements Harald Lück Perugia, 21.9.2005 GEO600 Length Control Systems 25 MHz 13 MHz 37 MHz Michelson Interferometer 15 MHz 10 MHz Laser Mode Cleaners Output Mode Cleaner Hierarchical Control Test Mass Actuators <0.1Hz 3mm <10Hz 100 µm >10Hz 4 µm HV Operating Principle Dielectric in an inhomogeheous E-field: Force along need bias voltage for bipolar actuation Reaction Mass Mirror E Segmented ESD ESD connections HV HV ESD connections HV LV HV LV ~3 mm gap Bare copper wires Reaction mass Mirror GEO600 ESD data • ~3 mm gap between reaction mass and mirror • 630 V bias, range 0-900V 900 V → 350µN force → about 3.5µm @DC • F = U2 ε0 εr d3/2 A Sqrt circuits in MI loop ESD: F U2 Sqrt circuits are necessary to give full linear force range for acquisition. Drawback: sqrt circuits are noisy (1µV/sqrt(Hz) =100µV @HV) Sqrt circuits in MI loop ESD: F U2 Sqrt circuits are necessary to give full linear force range for acquisition. Drawback: sqrt circuits are noisy (1µV/sqrt(Hz) =100µV @HV) Bypassing sqrt circuits after lock is acquired. Noise in MI loop HVA noise = 100nV/sqrt(Hz) = 10µV @ HVA HV-amplifier noise can be reduced by decreasing bias voltage or active noise suppression. Suppressing noise introduced by loop electronics needs whitening Whiten MI loop whitening / dewhitening dewhiten Whitening right after mixer: zero 3.5 Hz pole 35 Hz Dewhitening for both split paths dewhiten dewhiten Whiten Passive dewhitening done in HV path Future needs/options • Two (eight) segments to give different actuation ranges for acquisition- and run mode (in prelim. design doc. for Advanced LIGO) Segments for different actuation ranges Future needs/options • Two (eight) segments to give different actuation ranges for acquisition- and run mode (in prelim. design doc. for Advanced LIGO) • Change actuation range (noise) by changing bias voltage (nonlin. effects) Future needs/options • Two (eight) segments to give different actuation ranges for acquisition- and run mode (in prelim. design doc. for Advanced LIGO) • Change actuation range (noise) by changing bias voltage (nonlin. effects) • Investigate problems associated with charged mirrors. Can be minimized by using AC (8192 Hz in Ad. LIGO?) actuation and using symmetrical voltages. Bias Reduction F U Hierarchical Control Test Mass Actuators <0.1Hz <10Hz >10Hz Reaction Pendulum: 3 coil-magnet actuators at intermediate mass, range ~ 100µm Electrostatic actuation on test mass bias 630V, range 0-900V= 3.5µm