Presentation on theme: "Quadruple Suspension Design for Advanced LIGO"— Presentation transcript:
1 Quadruple Suspension Design for Advanced LIGO Norna A RobertsonLIGO-Caltech & University of Glasgowfor the Advanced LIGO Suspensions TeamaLIGO/aVirgo WorkshopPisa, Italy23rd-24th February 2012
2 Outline of Talk Requirements for test mass suspensions in aLIGO Quadruple suspension designMechanical designLocal and global controlAuxiliary featuresPrototype workCurrent statusConclusions
3 Requirements for Suspension Design for Test Masses Support the optics to minimise the effects ofthermal noise in the suspensionseismic noise acting at the support pointProvide damping of low frequency suspension resonances (local control), and provide means to maintain interferometer arm lengths (global control)while not compromising noise performanceProvide interface with seismic isolation system and core opticsSupport optic so that it is constrained against damage from earthquakesAccommodate a thermal compensation scheme and other systems as needed e.g. vibration absorbersERMEnd Test Masses (ETM)Inner Test Masses (ITM)ERM
4 Suspension Requirements: Test Masses Top-Level Requirements:RequirementValueSuspension Thermal Noise10-19 m/ Hz at 10 Hz (longitudinal)10-16 m/ Hz at 10 Hz (vertical)*#Residual Seismic Noise10-19 m/ Hz at 10 Hz(assumes seismic platform noise2x10-13 m/rt Hz)Pitch and Yaw Noise10-17 rad/ Hz at 10 Hz (assumes beam centering to 1 mm)Technical Noise Sources(e.g. electronic noise, thermal noise from bonds)1/10 of longitudinal thermal noise for each source(since noise terms add in quadrature, each increases total by 0.5%)*assumes 10-3 coupling vert. to long.#except for highest bounce mode peak which can be up to 12 Hz
5 Suspensions and Seismic Isolation – From Initial to Advanced LIGO active isolation platform (2 stages of isolation)hydraulic external pre-isolator (HEPI) (one stage of isolation)4 layer passive isolation stackcoarse & fine actuatorsquadruple pendulum (four stages of isolation) with monolithic silica final stagesingle pendulum on steel wire
6 Thermal Noise Thermally excited vibrations of suspension pendulum modessuspension violin modesmirror substrates + coatingsUse fluctuation-dissipation theorem to estimate magnitude of motionTo minimise:use low loss (high quality factor, Q) materials for mirror and final stage of suspension: use silicause thin, long fibres to reduce effect of losses from bendinguse low loss bonding technique: hydroxide-catalysis bondingreduce thermal noise from upper stages by careful design of break-off/attachment pointslow Q, high noisehigh Q, low noise
7 Initial LIGO used a single stage wire suspension Seismic NoiseSeismic noise limits sensitivity at low frequencies - “seismic wall”Typical seismic noise at quiet site at 10 Hz is ~ few x m/Hzmany orders of magnitude above target noise levelSolution - multiple stages of isolationIsolation required in vertical direction as well as horizontal due to cross-couplingTwo-stage internal isolation platform has target noise level of 2 x m/rt Hz at 10 Hz.require 4 more stages, i.e. quadruple pendulum, to meet target of m/rt HzAdvantage of double over single pendulum, same overall lengthInitial LIGO used a single stage wire suspensionBetter isolationQuadruple pendulum transfer function: predicted longitudinal isolation ~ 3 x 10-7 at 10 Hz
8 Suspension Design for Advanced LIGO: Top Level Features Thermal noise reduction: use monolithic fused silica suspension as final stageDetails in Giles Hammond’s talkSeismic isolation: use quadruple pendulum + 3 stages of maraging steel blades for vertical isolationControl noise minimisation: apply damping at top mass (for 6 degrees of freedom) + use quiet reaction pendulum for global control actuationcoil/magnet actuation at top 3 stageselectrostatic drive at test massDesign is developed from the GEO 600 triple pendulums which have been in operation for more than 10 years.parallel reaction chain for control actuationfour stagessilica fibres40 kg silica test mass8
9 Mechanical Design Starting assumptions Two chains close to identical: reduces design effortApprox. equal masses and lengths, modified as needed – e.g. final stage requirement on vertical bounce freq. <12 Hz, violin mode freq. > 400 Hz drives length.Blades (two at each of three stages) are crossed to reduce footprintTwo wires at top – reduces pitch couplingFour wires at lower stages – to allow damping and alignment from top massAngles, spacing and attachment positions of wires such that all low frequency modes (22 out of 24 total) less than ~5 Hz and visible at top mass.Build carried out firstly with all-metal masses, then monolithic stage and lowest reaction mass swapped in.Surrounding welded support structure stiff as practicableVertical, ZyawpitchrollTransverse, YLongitudinal, X
10 Mechanical Modeling and Results Initial modeling using MATLAB, built up from equations of motion: extension of GEO MATLAB modelSymmetric suspension: longitudinal and pitch coupled, transverse and roll coupled, yaw and vertical uncoupledModel used to optimise design as parameters were varied (masses, moments of inertia, wire lengths, blade parameters, attachment points). Fast turnaround.Values of mode frequencies and observability of modes at top mass in all degrees of freedom checked.Further work using Lagrangian model, firstly in MAPLE and latterly in Mathematica: assumes no symmetry and includes detailed elasticity modelling of wires from beam equation.Mathematica model includes arbitrary frequency-dependent damping on all sources of elasticity –calculate admittance and use fluctuation dissipation theorem for thermal noiseCan export state-space matrices for use in MATLABExample of impulse response in longitudinal with simple (zero-pole) damping controller.First long. mode and third transverse mode
11 Final Design: Some Key Parameters Masses (kg)22, 22, 39.5, 39.5(main chain)22, 22, 53, 26(ETM reaction chain)22, 22, 59, 20(ITM reaction chain)Size of optic340 mm x 200 mmOverall length to CofM1.7 mFinal stage silica fibres600 mm long400 micron diam.Gap between optic and reaction mass5 mm (ETM)20 mm (ITM)Each quad contains approx custom made parts.
12 Final Design: Mode Frequencies Mode frequencies for main chain (Hz)Longitudinal: 0.43, 1.00, 2.01, 3.42Pitch: , 1.24, 1.57, 2.77Vertical: , 2.22, 3.56, 9.27Yaw: 0.60, 1.35, 2.39, 3.05Transverse: 0.46, 1.05, 2.12, 3.31Roll: , 2.63, 5.06, 13.2(Identities are approximate – e.g. pitch and roll are also coupled for some modes due to mass distribution)Highest vertical (9 Hz) and roll (13 Hz) modes are well isolated from external vibrations and undamped by local control.coupledcoupledverticalroll
13 Expected Performance: Residual Seismic Noise Overall seismic noise performance is product of expected noise on final stage of the internal isolation system x transfer function of the quad pendulum.sfsdf
14 Local and Global Control Main (test) ChainReaction ChainLocal control: damping at top mass, separately for each chain, using BOSEMs* (next slide) with 10mm x 10mm NdFe magnetsGlobal control (length and angular) at all four levelsBOSEMs with 10mm x 10mm NdFe magnets at top massBOSEMs with 10mm x 10mm SmCo magnets acting between chains at upper intermediate massAOSEMs** with 2mm x 6mm SmCo magnets acting between chains at penultimate massElectrostatic drive (ESD) using gold pattern on reaction mass/compensator plate at test mass level*BOSEM = Birmingham Optical Sensor/Electromagnetic Actuator**AOSEM = ALIGO OSEM = modified initial LIGO OSEM
15 Local Control BOSEM* sensitivity 7 x m/ Hz at 10 HzRequired longitudinal noise at test mass10-20 m/ Hz at 10 HzRequired damping – decay time to 1/e ~ 10 secs6 BOSEMS on the top mass for 6 degree of freedom (DOF) active damping.In GEO point-to-point feedback to co-located actuators with same control lawIn aLIGO, signals transformed to Euler basis by 6x6 matrix and similar matrix used for the feedback signals: increases design flexibility to achieve required noise performance for each DOF.coilflagIRLEDlensmaskphotodiodemagnet
16 Local Control continued Two approaches investigated::Modal damping: convert 24 DOF MIMO into 24 DOF SISO systems. Each system can be damped independently with a relatively simple filter. Since there are sensors only at the top mass, an estimator is needed to fully reconstruct the dynamics. See below for response in X direction.Multimode damping: extension of the GEO damping scheme. Up to 4 modes per coordinate are damped with a single controller. Gain and phase adjusted for aggressive filtering above the resonances. Method more tolerant of parameter variations.Likely to use modal damping in X for shorter decay time, multimode for other DOFs.Eddy current damping was also considered as supplement: full damping by ECD introduces too much thermal noise.From Strain and Shapiro, submitted to Rev. Sci. Instrum.
17 Example of AOSEM used at penultimate stage Global ControlGlobal control signals can be applied at all four stagesTop three stages have coil/magnet actuatorsFourth stage has electrostatic drive (ESD)Larger forces/lower bandwidth at top, smaller forces/higher bandwidth at bottomTypical forces per actuator1.7 N/A at top and upper intermediate stages0.03 N/A at penultimate stage4 x N/V2 (+/- 400V) at test massMain (test) ChainReaction ChainExample of AOSEM used at penultimate stage
18 Hierarchical Global Control If each stage’s input is the output of the previous stage, any feedback change is automatically registered by the rest of the loop.The stability at each stage is independent from the others.ITMETM
19 Other Features of Quad Design Enhanced gas damping in small gaps (Dolese et al, Phys Rev D 84, 2011)Led to redesign of compensator plate (CP) for ITM, thinner to give larger gapETM to End Reaction Mass (ERM) gap = 5 mm, ITM to CP gap = 20 mmETM/ERM gap kept at 5 mm to allow full ESD force: rely on improving gas pressure at end chambers, and possible future redesign of ERM shapeVibration absorbers added to welded support structure to damp modes2 kg mass sandwiched between viton O-rings within outer casingBroadband damping around first few resonances (~ 100 Hz)Earthquake and bump stops to constrain glass masses have silica tipsReduce charge build-up and transferThermal Compensation Scheme (TCS) featuresRing heaters surrounding ETM and ITMGold-coated barrel on CPETM/ITMCP (purple) ERM (blue)*ERM = end reaction mass
20 Chain Splitting & Installation Arm Ability to split both main and reaction chains below top masses for ease of storage, installation and replace/repair scenarios.++=Installation arm for replace/repair scenarios
21 Prototype Testing at LASTI Several prototypes built and tested at LASTI as design matured over past 10 years.Final “noise” prototype incorporated first full monolithic suspension of 40 kg silica mass, suspended from a BSC-ISI. Investigations includedUnderstanding and developing the quad modelMode frequencies and transfer functionsMulti-mode and modal dampingAspects of hierarchical global control (cavity formed with prototype triple pendulum)Q values of test mass, pendulum modes and and violin modesInvestigation of use of ESD for acoustic mode dampingInvestigation of charge on the optic using the ESD and of discharging using ionized nitrogenInteractions between ISI and quadAssembly, installation and alignment procedures
22 Examples of Transfer Functions from LASTI prototype Example of transfer functions (TF) taken at top mass (drive to response)Top: measured and modelled TF in X direction with no damping appliedMiddle: simulated and measured modal damping of TF in X directionBottom: measurements of yaw TF undamped and dampedyawData from Brett Shapiro
23 Current Status Assembly and installation: 2 full quads with monolithic stages assembled at LHO: one (ITM) installed in chamber, the second (ETM) due for installation early March. These will be used for single arm test.A further 6 assembled and tested in metal form, and sub-assemblies of all the others Total = 12 plus spares.Next installation due at LLO early summer, two ITMs for short Michelson tests.
24 Testing Results: Transfer Functions Driving and Observing at Top Mass
25 Example of bad transfer function due to chains touching (black trace) More Testing ResultsFirst two pitch frequencies are most variable, due to strong dependence on setting of “d” values – vertical separation of wire break-off points from CofM. Typical values ~ 1 mm.Example of bad transfer function due to chains touching (black trace)d1Centre of massd2
26 Some Lessons LearnedTwo reviews carried out on assembly procedures during Changes included:Flag redesign for BOSEMs to improve assembly process and relax tolerance on alignment for minimising cross-couplingAddition of pitch adjustment on upper intermediate mass to help alignment process (masses shown in red)Reduction of particulates:Unplated SmCo magnets (used to reduce Barkhausen noise) found to shed particulates: replaced by Ni-plated versions.Addressed abrasion of bolt heads in high torque locations - use of Nitronic-60 washersRevised tooling for blade adjustments and other alignmentsImproved silica fibre safety after dropped screw broke fibres:more catch trays and shields, addition to fibre guards, use of captured screws, improved tooling, redesign of shims to be more secureAttention paid to routing of cabling up reaction chain to minimise effect on dynamics, especially on pitch
27 ConclusionsThe assembly and installation of the quadruple suspensions for the test masses in Advanced LIGO are well underway.The procedures for assembly, including the monolithic elements, alignment, testing and installation are well understood.The mechanical model of the dynamics of the suspensions is well understood.The measurements of mode frequencies, transfer functions and damping (local control) fit the model well.We look forward to the first operation of two quad suspensions in a long cavity which will start soon at LHO.
28 AcknowledgementsThe Advanced LIGO Suspensions Team consists of more than sixty scientists, mechanical engineers, electronic engineers, designers, technicians and students from eight institutions.USALIGO Caltech: Jay Copti, Philip Croxton, Todd Etzel, Kate Gushwa, Jay Heefner, Alastair Heptonstall, Kristen Holtz, Tim McDonald, Gary McIntyre, Margot Phelps, Norna Robertson, Calum TorrieLIGO Hanford Observatory: Jeff Bartlett, Mark Barton, Betsy Bland, Doug Cook, Jeff Garcia, Jesse Garner, Gerardo Moreno, Gerardo Moreno Jnr., Jason Oberling, Andres Ramirez, Travis Sadecki, David Stone, Vern SandbergLIGO Livingston Observatory: Carl Adams, Anna Aitken, Joe Betzwiezer, Derek Bridges, Virginia Brocato, Anamaria Effler (LSU) , Robert Giglio, Matt Heintze, Ed Merilh, Mike Meyer, Ralph Moffatt, Bobby Moore, Terry Pitre, Janeen Romie, Danny Sellers, Gary Traylor, Gene WintonLIGO MIT: Sam Barnum, Michael Hillard, Jeff Kissel, Rich Mittleman, Brett ShapiroUKUniversity of Glasgow: Angus Bell, Nicola Beveridge, Alan Cumming, Liam Cunningham, Giles Hammond, Jim Hough, Russell Jones, Rahul Kumar, Sheila Rowan, Ken Strain, Marielle Van VeggelRutherford Appleton Laboratory (RAL): Justin Greenhalgh, Tim Hayler, Joe O’Dell, Ian WilmutUniversity of Birmingham: Stuart Aston (now LLO), Ludovico Carbone, Ron Cutler, Alberto VecchioStrathclyde University: Nick Lockerbie, Kirill Tokmakov