From: "Lucatorto, Tom" To: "Gary Sanders (E-mail)" Cc: "Berger, Beverly K." Subject: Some questions about the readiness for AdvLIGO from the MREFC Pan el Date: Wed, 23 Jul 2003 11:06:03 -0400 X-Mailer: Internet Mail Service (5.5.2656.59) X-RAVMilter-Version: 8.4.1(snapshot 20020919) (acrux) Hi Gary, The MREFC Panel in considering the proposal for AdvLIGO has developed questions about the construction readiness of the seismic isolation stage and the test mass suspension system. In particular, the MREFC Panel focused on two parts of the AdvLIGO Review Panel Report. The first part, connected with the evaluation of the seismic isolation stage in the Report, states: "The dynamical tests of the ETF (Engineering Technology Demonstration at Stanford) demonstrated good rigidity of the system, and good reduction of the tilt-horizontal coupling was achieved. The control of the two-stage platform with 12 degrees of freedom with the required gain in the 1 - 10 Hz region is a challenging task. The initial results of the simulation appear encouraging. The ETF will be invaluable for testing the control strategy and to demonstrate compatibility with the Advanced LIGO requirements. The work will certainly benefit from the JILA seismic vibration isolation experience and from the Stanford's group's experience in control engineering. Recommendations The sensor blending and coupling between different degrees of freedom may cause unexpected excess noise at low frequencies, spoiling the expected reduction of the rms residual motion of the test masses. Extensive control tests will be needed to validate the simulation predictions and to demonstrate the attenuation performance and the robustness of the control scheme." The panel has correctly observed that "The control of the two-stage platform with 12 degrees of freedom with the required gain in the 1 - 10 Hz region is a challenging task". The experiments to date have shown success in operating all 12 degrees of freedom simultaneously, with a suppression of noise by a factor of 300 at 10 Hz, which meets the requirement at that frequency. This has been demonstrated on the pre-prototype, and we look forward to confirming this performance on the Technology Demonstrator at Stanford which is being commissioned now. The astrophysical reach of the system is quite robust against compromises in performance; an increase in seismic noise by a factor of 10 over the design value would still put it below the Newtonian background. The performance of the system at lower frequencies is also challenging. The two techniques in question (combining different sensors to form a wide-bandwidth low-noise system, and using a ground sensor to anticipate the motion at the payload and correcting it in advance) have each been demonstrated and show agreement with separate models. The combination of the two will be tested in the Technology Demonstrator at Stanford, and compared with the existing model for performance, as suggested by the panel. Based on the piece-wise successful comparison of experiment and models of the servo techniques, there is confidence that the complete system will also meet requirements. The second part, connected with the suspension system states: "Recommendation Accelerate the R&D program towards the construction of a full scale prototype allowing the test of the SUS together with the active seismic isolator. In order to facilitate a full test at MIT (LASTI) it is recommended that a full simulation model be developed that includes the suspension, the active seismic isolator, and their associated control systems. In view of the final project, it is advisable to integrate such a model in the end-to-end simulation as soon as possible." The quotes are the complete text from the Review Panel Report. The sections that are in bold italics are the sections quoted in the preamble to the MREFC Panel's overall "lingering" question which is: "Whether the project is really ready to transition from R&D to construction of the final design." Further, in connection with this lingering concern, the MREFC Panel asks: "How important are the results from simulations called for by the review panel? Is it the simulations that provide confidence that the technical challenges will be met? What if they pointed to the need for a different design in a critical part of the system? Could this lead to a different total cost of the project?" My sense is that the MREFC Panel perhaps reacted overly strongly to the word "simulation" and considered its use a sign that the LIGO team was still in the initial stages of trying to decide amongst several different design choices. Whatever the reason for the Panel's concerns however, we would very much appreciate your help in providing answers to the questions by cob (EDT) today. John Hunt, the AD for the MPS Directorate, will be presenting our response to the MREFC Panel tomorrow morning. Thanks in advance, Tom Gary H. Sanders LIGO Laboratory California Institute of Technology 18-34 Pasadena, CA 91125 626-395-2997 626-304-9834 fax