Overview

The seismic isolation subsystem serves to attenuate ground motion in the observation band (above 10 Hz) and also to reduce the motion in the control band (frequencies less than 10 Hz). It also provides the capability to align and position the load. Significantly improved seismic isolation will be required for Advanced LIGO to realize the benefit from the reduction in thermal noise due to improvements in the suspension system. The isolation system will be completely replaced, and this offers the opportunity to make a coordinated design including both the controls and the isolation aspects of the interferometer.

Functional Requirements

The top-level constraints on the design of the isolation system can be summarized:

• Seismic attenuation: The amplitude of the seismic noise at the test mass must be equal to or less than the thermal noise of the system (10^-19m/ÖHz at 10 Hz) for the lowest frequencies where observation is planned. We have chosen 10 Hz as, at this frequency, the competing noise sources (suspension thermal noise, radiation pressure, Newtonian background) all conspire to establish a presently irreducible sensitivity level roughly a factor of 30 above the limits imposed by the LIGO facilities, and because technical difficulties in suspension design make a lower goal unrealistic.
• The RMS differential motion of the test masses while the interferometer is locked must be held to a small value (less than 10-14 m) for many reasons: to limit light fluctuations at the antisymmetric port and to limit cross coupling from laser noise sources, as examples. Similarly, the RMS velocity of the test mass must be small enough and the test mass control robust enough that the interferometer can acquire lock. This establishes the requirement on the design of the seismic isolation system in the frequency band from 0.1 to 10 Hz.
• The isolation positioning system must have a large enough control range to allow the interferometer to remain locked for extended periods; our working value is 1 week.
• The system must interface with the rest of the LIGO system, including LIGO vacuum equipment, the adopted suspension design, and system demands on optical layout and control.

Concept

The initial LIGO seismic isolation stack will be replaced with an external (to the vacuum) low-frequency pre-isolator stage, and an in-vacuum two-stage active seismic isolation platform (Figure 1 is taken from the design model). The in-vacuum stages are mechanically connected with stiff springs, yielding typical passive resonances in the 2-8 Hz range. Sensing its motion in 6 degrees of freedom and applying forces in feedback loops to reduce the sensed motion attenuates vibration in each of the two-cascaded stages. The outer stage derives its feedback signal by blending three real sensors for each degree of freedom: a long-period broadband seismometer, a short-period geophone, and a relative position sensor. The inertial sensors (seismometers and geophones) measure the platform's motion with respect to their internal suspended test masses. The position sensor measures displacement with respect to the adjacent stage. The resulting "super-sensor" has adequate signal-to-noise and a simple, resonance-free response from DC to several hundred Hz. The inner stage uses the position sensor and high-sensitivity geophone, and some feed-forward from the outer stage seismometer.

Figure 1 Computer rendering of the conceptual design of the two-stage active isolation system for the test-mass (BSC) vacuum chambers. The outside frame supports the first stage from three trapezoidal blade springs. Three plug-in units carry the sensors and actuators for the unit. The inner second stage is likewise suspended from trapezoidal springs, with the sensor/actuators protruding above the upper surface. The optics are suspended below the inner stage (which forms the interface to the suspension and other isolated parts), and hang below the support structure (HPD).

The outer frame of the isolation system is designed to interface to the existing in-vacuum seismic isolation support system, simplifying the effort required to exchange the present system for the new system. The outer stage is hung from the outer frame using trapezoidal leaf springs to obtain the 2-6 Hz resonances. The inner platform stage is built around a 1.5-m diameter optics table (BSC) or a larger polygonal table (HAM). The mechanical structures are carefully studied to bring the first flexible-body modes well above the ~50 Hz unity gain frequencies of the servo systems. For each suspended optic, the suspension and auxiliary optics (baffles, relay mirrors, etc.) are mounted on an optical table with a regular bolt-hole pattern for flexibility.

We will use commercial, off-the-shelf seismometers that are encapsulated in a removable pod. This allows the sensors to be used as delivered, without concerns for vacuum contamination, and allows a simple exchange if difficulties arise. The actuators consist of permanent magnets and coils in a configuration that encloses the flux to reduce stray fields. These components must meet the stringent LIGO contamination requirements. The multiple-input multiple-output servo control system is realized using digital techniques; 16-bit accuracy with ~2 kHz digitization is sufficient.

The external pre-isolator is used to position the in-vacuum assembly, with a dynamic range of 1 mm, and with a bandwidth of 2 Hz or greater in all six degrees of freedom. This allows feedforward correction of low-frequency ground noise and sufficient dynamic range for Earth tides and thermal or seasonal drifts. We target approximately a factor of 10 reduction of the ~0.16 Hz microseismic motion from feedforward correction in this stage. For corrections up to the 1-cm clearance at each vacuum feedthrough bellows, large screw adjustments are included in series with each external actuator.

The performance of the system, and its initial design, is calculated with a model that includes all solid-body degrees of freedom, and measured or published sensitivity curves (noise and bandwidth) for sensors. It meets the Advanced LIGO requirements with some margin, for both the test-mass (BSC) and auxiliary (HAM) chambers.

The passive isolation of the suspension system provides the final filtering. A sketch of the system as applied to the test-mass vacuum chambers (BSC) is shown in Figure 2; a similar system is designed for the auxiliary optics chambers (HAM). Further details can be found in the subsystem Design Requirements and Conceptual Design documents.

Figure 2 Rendering of isolation system installed in the BSC (Test Mass Chambers), with suspension system attached below. The external preisolator provides the interface between the vertical blue piers and the green horizontal support structure (C. Hardham, Stanford).

R&D Status/Development Issues

A first-generation prototype of the in-vacuum isolation system has shown performance at low- and high-frequencies comparable to the requirements. Testing of a preliminary version of the external pre-isolator is nearing completion and will be installed in Livingston in 2003 as a remedial effort addressing excess local seismic noise. Testing started in December 2002 on a second-generation prototype of the in-vacuum isolation at the Stanford Engineering Test Facility.

Several issues must be addressed. The most significant is identifying the character of the internal mechanical resonances of as-built designs and crafting control laws that meet requirements in this environment. Other issues include minimizing the confusion of tilt with horizontal motion for low-frequency control, the distribution of control authority through the hierarchy, and stability of parameters (for feed-forward and loop gain design). In addition, processors, analog interfaces, and software systems that are compatible with the LIGO standard will be integrated into the subsystem.

Materials issues requiring study include the development of contamination-compatible in-vacuum electromagnetic actuators, and creep and yield behavior of structural materials under stress.

Work Plan

The present LIGO Cooperative Agreement and existing NSF grants to LSC member institutions will support research, development, and design on this subsystem through full-scale tests carried out in the MIT LASTI testbed. These involve control and noise-performance tests of complete systems for both the test-mass and the auxiliary optics vacuum chambers, as well as their integration with the suspensions (SUS).

Advanced LIGO construction will commence with a final design review and with placement of production subcontracts for all seismic subsystem components. Fabricated components must begin arriving at the staging buildings at the two sites in early 2006.

Assembly of complete seismic system units in the staging buildings will take place during 2006. Sufficient systems must be completed at both sites to support installation in the interferometer vacuum chambers in mid-2006.

WBS Definition

This element includes all R&D, design, prototype testing, and hardware for the seismic isolation system upgrade. 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.

Design Requirements Document

Conceptual Design

R&D Activities

Detail Estimate Sheets

Baseline Plan