Joe Kovalik, Perugia Group, VIRGO Project
The Meeting on Thermal Noise and Low Frequency Noise Sources in Gravitational Wave Detectors took place in Perugia, Italy, on June 4-6.
There has recently been a significant growth of interest in low frequency noise sources in gravitational wave detectors and in particular, thermal noise. This workshop was organized to bring together various international efforts in this field and to discuss current results openly in a workshop format.
All the major international groups were present including: VIRGO, GEO600, LIGO, LIGO science community affiliated groups, TAMA, ACIGA, AURIGA, and Nautilus. The meeting started with a series of review talks on thermal noise.
The rest of the meeting was organized into three main sessions: modelling of thermal noise and low frequency noise sources, materials and geometries for low noise detectors and finally advanced designs.
An informal proceedings will be put together at the end of August and should be available in the fall. More information is available at http://www.pg.infn.it/virgo/PerugiaWorkshop. The main conclusions of the talks and discussions will be given here.
It is quite clear that most of the groups agree on models, measurements and predictions. Moreover, it is also evident that traditional designs based on fused silica test masses with metallic suspension wires are reaching their theoretical limits in experimental tests. Unfortunately, these numbers do not give excellent results. The best Q's for fused silica test masses are no more than 2e7 while typically they can be 5e5. Sapphire shows some promise of higher Q's with preliminary measurements in Perth giving Q's of a few 1e7, but hopefully going up to 1e8. This coupled with the higher frequency internal resonances of sapphire make it a better material from a thermal noise performance viewpoint. There still remains, however, the problems of the optical properties of sapphire.
For the pendulum mode, it seems that traditional designs cannot give Q's better than about 1e6. Fused silica suspension wires do give better results with some experiments showing Q's better than 1e7. Unfortunately, this still is not good enough. Fused silica does give an immediate design improvement without going to drastic design changes.
Advanced ideas do offer some hope, but with a few precautions. There are proposals for electrostatic and magnetic suspensions. Many of the bar groups, however, have shown that there are diverse electrical and magnetic loss mechanisms and noise sources that must be considered beforehand. They could significantly affect the performance of these alternative designs. Active suspensions are also a solution for reducing seismic noise, but they require more effort in order to produce fully functional prototypes.
The final solution is to cool the suspension system. While it is not obviously clear that the test mass where laser power is dissipated can be cooled, the suspensions could be cooled to a few kelvin. This offers two advantages. The thermal noise is reduced by a factor of the square root of temperature. Also, many materials (but not fused silica) have better Q's at low temperature. There is only some preliminary ideas of what research should be performed. This could be a good meeting point for the interferometer community to learn from the bar community with its many years experience in cryogenic, low noise experiments.