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> home > proposal > subsystems: suspension Suspension Subsystem Overview The test-mass suspension subsystem must preserve the low intrinsic mechanical losses (and thus the low thermal noise) in the fused silica suspension fibers and sapphire test mass material. It must provide actuators for length and angular alignment, and attenuate seismic noise. The Advanced LIGO reference design suspension is similar in design to the GEO 600 multiple pendulum suspensions, with requirements to achieve a seismic wall of ~10 Hz. A variety of suspension designs are needed for the main interferometer and input conditioning optics. The suspension forms the interface between the seismic isolation and the suspended optics. It provides seismic isolation and the means to control the orientation and position of the optic. These functions are served while minimally compromising the thermal noise contribution from the test mass mirrors and only introducing a negligible amount of thermal noise from the suspension elements. The optic (which in the case of the main arm cavity mirror serves also as the test mass) is attached to the suspension fiber during the suspension assembly process and becomes part of the suspension assembly. Features on the test mass will be required for attachment and potentially for actuation. The test mass suspension system is mounted (via bolts and/or clamps) to the seismic isolation system by attachment to the SEI optics table. Local signals are generated and fed to actuators to damp solid body motions of the suspension components; in addition, control signals generated by the interferometer sensing/control (ISC) are received and turned into forces on the test mass to obtain and maintain the operational lengths and angular orientation. There are two variants of the test mass suspension: one for the End Test Mass (ETM) which carries potentially non-transmissive actuators behind the optic, and one for the Input Test Mass (ITM) which must leave the input beam free to couple into the Fabry-Perot arm cavity. There are also variants for the beamsplitter, folding mirror, and recycling mirrors; and for the mode cleaner, input matching telescope, and suspended steering mirrors. A multiple-pendulum is the basis. This has two benefits:
The latter feature will allow the elimination of the magnets attached to the test mass in initial LIGO (which are the largest source of excess dissipation on the test mass), and should allow the test mass to reach a mechanical loss (and thus thermal noise) limited principally by the substrate material. Furthermore, eliminating the magnets reduces a potential source of correlation between the interferometers due to correlated environmental magnetic fields. Thus both technical noise and fundamental thermal noise should be substantially reduced in such a suspension. Multiple simple pendulum stages also improve the seismic isolation of the test mass for horizontal excitation of the pendulum support point; this is a valuable feature, but requires augmentation with vertical isolation to be effective. Vertical seismic noise can enter into the noise budget through a variety of cross-coupling mechanisms, most directly due to the curvature of the earth over the baseline of the interferometer. Simple pendulums have high natural frequencies for vertical motion. Thus, another key feature of the suspension is the presence of additional vertical compliance in the upper stages of the suspension to provide lower natural frequencies and consequently better isolation. Further detail can be found in the Design Requirements Document. Key parameters of the test-mass suspension design are listed in Table 1; other suspensions have requirements relaxed from these values. Table 1 Test mass suspension parameters
Concept/Options The testmass mirror is suspended as the lowest mass of a quadruple pendulum as shown in Figure 1; the four stages are in series. Sapphire is the reference design mirror substrate material. However, the basic suspension design is compatible with fused silica masses and a "fall-back" to this alternate may be made shortly before final design. Both materials are amenable to low-loss bonding of the fiber to the test mass. The mass above the mirror- the intermediate mass- is made of a moderately low-mechanical-loss glassy or crystalline material such as fused silica, high-density glass, or low-grade sapphire. The masses at the top are suspended from two cantilever-mounted, approximately trapezoidal, pre-curved, blade springs (inspired by and similar to the VIRGO blade springs), and two steel wires. The blade springs are stressed to about half of the elastic limit. The penultimate mass is suspended from 4 cantilever springs and 2 steel wire loops. Fused silica pieces form the break-off points at the intermediate mass. These are attached to the penultimate and final mass using hydroxy catalysis bonding, which is demonstrated to contribute negligible mechanical loss to the system. The upper support stages suspension wires are not vertical and this gives some control over mode frequencies and coupling factors. Tolerable noise levels at the intermediate mass are within the range of experience on prototype interferometers (10-17m/Hz1/2) and many aspects of the technology have been tested. There are, however, no meaningful test results at less than ~150 Hz. At the top-mass, the main concern is to avoid acoustic emission or creep (vibration due to slipping or deforming parts). Sensing (for damping) of the solid-body modes of the suspension requires an improved local sensor (required performance ~10-12 m/Hz1/2 at 10 Hz) or an alternative servo configuration to meet the subsystem noise performance requirements. Actuation is applied to all masses in a hierarchy of lower force and higher frequency as the test mass is approached. Coils and magnets are used on upper stages, with electrostatics (for locking) and photon pressure (for operation) used on the test mass itself. Other suspended optics will have noise requirements that are less demanding than those for the test masses, but still stricter than the initial LIGO requirements, especially in the 10-50 Hz range. Their suspensions will employ simpler suspensions than those for the test masses, such as the triple suspension design for the mode cleaner mirrors shown in Figure 1. More design detail can be found in additional subsystem documentation.
Figure 1 Test mass suspension design elevation view sketches
Figure 2 Test mass suspension rendering R&D Status/Development Issues The primary role of the suspension is to realize the potential for low thermal noise, and much of the research into suspension development explores the understanding of the materials and defines processes to realize this mission. In addition, design efforts ensure that the seismic attenuation and the control properties of the suspension are optimized, and prototyping efforts ensure that the real performance is understood. The GEO-600 suspensions utilizing the basic multiple-pendulum construction, fused-silica fibers, and hydroxy-catalysis attachments, have been in service since 2001. The systems have been reliable and the controls function as modeled. The noise performance will be demonstrated in 2003. Significant design and modeling of the mode-cleaner triple suspensions has taken place, and successful careful comparison of the quadruple test-mass model with the MIT/GEO prototype has been made. Test mass thermal noise is one of the basic noise limits to performance of the
Advanced LIGO design. To realize the reference design performance, the following
lines of research are being pursued:
Development of the Advanced LIGO version of the suspension starts with the multiple
pendulum scheme based on the GEO 600 suspension, and GEO is leading the trade
studies. Within that framework, there are a number of specific questions to address,
including:
Tests for attenuation, parasitic resonances, and other defects in isolation properties (along with consequent modifications of these pendulums) are a focus of the development effort. GEO will characterize their system with Advanced LIGO requirements in mind. Full-scale controls and noise test prototypes are in development and will be used to test performance against requirements in laboratory-scale experiments. Work Plan The R&D program will include work on this subsystem through full-scale tests of all principal variants of the suspensions in the MIT LASTI testbed. By the completion of that test, the design will have been carried through the design requirements, preliminary design, and substantially through the final design review. A final LASTI test will serve to verify form, fit and conformance to functional requirements. Advanced LIGO construction will commence with the final design review and with placement of production subcontracts for all suspension subsystem components. Fabricated components must begin arriving at the optics/vacuum preparation facilities at the two sites in early 2007. A consortium of the University of Glasgow, University of Birmingham, and Rutherford Appleton Laboratory has proposed to UK funding sources (PPARC) to supply the test-mass suspensions for Advanced LIGO. The GEO group at the University of Glasgow is the originator of the design, and is very well positioned to carry through with this effort. Assembly of complete suspension subsystem units in the site facilities will start in 2006. Suspension of the optics in the completed suspension units will be done at the time of final installation. This will require readiness of optics processing and suspension fiber processing systems at each site. Sufficient systems must be completed at both sites to support installation in the interferometer vacuum chambers early in 2007. WBS Definition This element includes all R&D, design, prototype testing, and hardware for the suspension subsystem upgrade, including suspension fibers and attachment to the core optics. It includes the intermediate masses. This element provides small suspensions mechanical hardware for other subsystems. It includes all physical hardware for sensing and control (including the electrostatic actuator, but not the photon actuator) of suspended masses. It includes all components of active elements including programmable controls items, and software specific to local control of this subsystem. It does not include general controls for the interferometer, nor shared controls infrastructure. It does not include controls hardware and software specific to other subsystems for which the mechanical suspensions are supplied by this element. Quadruple Suspension Design for Advanced LIGO Design Estimate Sheets Baseline Plan |
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updated 05.21.2003 | web