Critical mechanical technologies for future gravitational-wave astronomy

1. Critical mechanical technologies for future gravitational-wave astronomy • Joseph Giaime, Louisiana State University “LIGO is a mechanical experiment.” Fred Raab

2. Limits to imagination • It is difficult to judge among ideas for advanced detectors until the technology has been tried. We have guessed wrong before. • How do we anticipate success of materials research? • Can we buy 100 bits of material with particular once-demonstrated properties? When? • How hard do we push with our own limited funds on the materials state of the art? • How do we scale reliability versus complexity, in either manufacture or operation? • It takes years of hard work to develop a procurement/manufacturing pipeline for complex precision mechanical, optical and electronic systems. • It is ‘easy’ to sketch out a scheme with many beams and complex servo-controls, but how many engineers will it take to keep it working as a real observatory? • When will non-linear effects bite? Excessive worry causes paralysis.

3. S. Ballmer

4. Design responsiveness • A major design change or upgrade to a LIGO-like detector takes 2-10 years. e.g., HEPI, after years of work by real engineers, to 2 years to bring to LLO once we decided to push it. • Design decisions accumulate, and are expensive to reverse once prototyping has been done. • Adv LIGO was designed to be a ‘sure thing’ for NS/NS sources, possibly neglecting others. • Imagine a graph of expected NS/NS rate versus year, 1985 - present... • Several-year period lowpass filter applied to new source predictions. • Trade-offs: • complexity-driven high performance versus duty cycle. • consistency of data interpretation versus continual improvements. • Should our 3 detectors be alike, or different (and better)? • $, £, €, ¥ versus everything.

5. What do experimenters want? • Theoretical contribution to data analysis widely discussed this week. • Theory tends to produce physical source models, or simulations of existing models. Usually, this means a paper with graphs of h(f) based on a particular set of initial conditions. • To be useful in analysis, one of the two black boxes (green ovals) would be helpful, for each class of sources under study.

6. Post Adv LIGO changes, low frequency & high frequency Theoretical source strengths (Kip Thorne) • Neutron Star & Black Hole Binaries • inspiral • merger • Spinning NS’s • LMXBs • known pulsars • previously unknown? • NS Birth (SN) • tumbling • convection • Stochastic background • big bang • early universe

7. Suspension thermal noise Internal thermal noise Newtonian background,estimate for LIGO sites Seismic ‘cutoff’ at 10 Hz Unified quantum noise dominates at most frequencies for fullpower, broadband tuning Adv LIGO Detector Performance Zones to improve? 10-22 Initial LIGO 10-23 10-24 10-25 1 Hz 10 Hz 100 Hz 1 kHz D. Shoemaker

8. Push to lower frequencies • Gravity gradient forces from ground motion. • Photon pressure on mirrors. • Control system noise.

9. Earth strain noise, Berger & Levine ’72 ‘stochastic’ Earth strain, without excited modes, storms, quakes, etc.

10. Get away from surface waves

11. Low frequency extension 1, DeSalvo, Cella, etc. • Reduced laser power & finesse. • longer TM suspension • larger, heavier mirror, silica. • flat-top transverse FP mode. • MGAS seismic isolation. • fitted subtraction of gradient noise? • underground hollow around TM, to partially cancel GG?

12. Low frequency extension 2, ‘LCGT’ • Kuroda’s Aspen talk... • Underground to avoid surface noise. • Cryogenic suspension, 4-20 K Sapphire TM & fibers. • GAS/Inv pendulum seismic isolation. • Extra vertical active stage above penultimate mass, using vert. FP. • Susp-point interf. • RSE to minimize heat on mirrors

13. Low frequency extension 3, ‘BLITS’ • From W. Johnson’s Aspen talk. Ideas credited to many, including Rowan, Hough, Whitcomb, etc. • Extension right down to gravity gradient. • 1.5 µm laser, 400 W • Suspension/isolation notional design only. • Silicon test mass, 720 kg, transparent at laser wavelength. • Tests mass cooled to 20 K, the lower of the two temp’s when thermal expansion coef goes to zero. • NS/NS range whimsically goes to 900 Mpc. • 130 K ‘sweet spot’ also being looked at.

14. What may be needed • To lower kT noise: • materials research: • substrates, flexures, fibers, bonding • physical properties at cryo temperatures, in interesting geometries. • coating techniques for various substrates and wavelengths. • Cryogenic techniques. • Charge and heat flow and control. • To lower gravity gradient and transmitted seismic noise: • Work to develop seismic models that take ground measurements and correlate with newtonian coupling to test mass. • Lower-frequency seismic isolation platforms • Interferometric displacement sensor development for advanced suspension damping loops. • To lower control system noise (and add robustness): • Hierarchical interferometer alignment control schemes, including • independent sensors (like HEPI) • control reallocation • DSP/Data systems development, with emphases on ease of use during commissioning, and robustness. • To lower interferometer quantum noise: • pay attention to next talk.