Idea
The idea of implementing a magnetic suspension for gravitational-wave detectors was first proposed by R. Drever in early 80’s [1]. The initial proposal was to directly levitate the entire test mass without any mechanical contact. This can eliminate the “violin mode” resonances of suspension wires, and the associated suspension thermal noise. In addition, if one can achieve a very low suspension resonance frequency, the seismic noise, which dominates at low frequencies as shown in the sensitivity curve below, can also be well isolated.
Previous Work by Drever
In the early attempts, Drever et al. attached non-conducting ceramic permanent magnets onto the test mass, and levitated the test mass using the attracting force induced by another fixed permanent magnet. With an additional feedback control, they realized a stable magnetic suspension. In addition, a low resonant frequency and long suspension relaxation time were both observed [2]. The main issue was that the attachment of ceramic magnets significantly increases the thermal noise. The noise performance was not as good as expected.
Later, Augst and Drever tried paramagnetic materials (including Gadolinium Gallium Garnet (GGG) and Terbium Gallium Garnet (TGG)) to replace the commonly used material for the test mass [3]. In this case, they were able to directly levitate the test mass without attaching any magnet, and they achieved relatively high quality factors (106 - 107) for the internal modes. The experimental setup is shown in the following figure:
Others
After the pioneering work of Drever, many others followed up. In a recent investigation by Varvella et al. [4], they explored the feasibility of a magnetic suspension for next-generation advanced gravitational-wave detector. They numerically modeled a single suspended magnet scheme and further experimentally confirmed their numerical results. They mainly focused on the DC magnetic levitating force that balances the gravitational force. Instead of attaching a permanent magnet onto the test mass, they proposed to incorporate the magnetic suspension as a part of the multistage suspension system. Their configuration is shown in the figure below:
References:
[1] R. W. P. Drever, Techniques for extending interferometer performance using magnetic levitation and other methods, Proceedings of the International Conference on Gravitational Waves (Source and Detectors), Eds I.Ciufolini -F.Fidecaro World Scientific (March 1996).
[2] Drever, R. W. P. Some New Concepts for Laser Interferometer Gravitational Wave Detectors, Proceedings of the Moriond Workshop on Dark Matter & Cosmology, Quantum Measurements and Experimental Gravitation, Les Arcs, January 1996, ed. J. Tran Thanh Van (Editions Frontieres, 1996).
[3] S. J. Augst and R. W. P. Drever Measurements of Mechanical Q in Levitated Paramagnetic Crystals, CP523, Gravitational Waves: Third Edoardo Amaldi Conference, edited by S. Meshkov.
[4] M. Varvella, E. Callonia, L. Di Fiorea, L. Milanoa,and Nicolas Arnaudb, Feasibility of a magnetic suspension for second generation gravitational wave interferometers, Astroparticle Physics Volume 21, Issue 3, Pages 325-335 (June 2004).