Electrostatic actuators: from the GEO experience to the 2 nd generation requirements Harald Lück Perugia,

Mode Cleaners Output Mode Cleaner GEO600 Length Control Systems Michelson Interferometer Laser 25 MHz 13 MHz 37 MHz 15 MHz 10 MHz

Hierarchical Control Test Mass Actuators <10Hz >10Hz <0.1Hz 3mm 100 µm 4 µm

HV

Reaction Mass Mirror Operating Principle Dielectric in an inhomogeheous E-field: Force along E need bias voltage for bipolar actuation

Segmented ESD

ESD connections HV

ESD connections HV LV

Reaction mass Mirror ~3 mm gap Bare copper wires

GEO600 ESD data ~3 mm gap between reaction mass and mirror 630 V bias, range 0-900V 900 V → 350µN force → about F = U 2 ε 0 ε r d 3/2 A

Sqrt circuits in MI loop ESD: F  U 2 Sqrt circuits are necessary to give full linear force range for acquisition. Drawback: sqrt circuits are noisy (1µV/sqrt(Hz)

ESD: F  U 2 Sqrt circuits are necessary to give full linear force range for acquisition. Drawback: sqrt circuits are noisy (1µV/sqrt(Hz) Bypassing sqrt circuits after lock is acquired. Sqrt circuits in MI loop

Noise in MI loop HVA noise = 100nV/sqrt(Hz) = 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 Whitening right after mixer: zero 3.5 Hz pole 35 Hz Dewhitening for both split paths Passive dewhit- ening done in HV path dewhiten Whiten

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.

U F Bias Reduction

Reaction Pendulum: 3 coil-magnet actuators at intermediate mass, range ~ 100µm Electrostatic actuation on test mass bias 630V, range 0-900V= 3.5µm Hierarchical Control Test Mass Actuators <10Hz >10Hz <0.1Hz