Parametric instabilities in the LCGT arm cavity

TL;DR

This study compares parametric instabilities in LCGT and Advanced LIGO, showing LCGT's reduced instability modes and easier mirror curvature requirements due to different thermal noise mitigation strategies, highlighting cryogenic solutions.

Contribution

It provides a detailed analysis of parametric instabilities in LCGT, demonstrating their reduced severity and the impact of thermal noise reduction methods compared to Advanced LIGO.

Findings

LCGT has 10 times fewer unstable modes than Advanced LIGO.

LCGT's instability strength depends less on mirror curvature accuracy.

Cryogenic mirrors and elastic Q reduction effectively suppress instabilities.

Abstract

We evaluated the parametric instabilities of LCGT (Japanese interferometric gravitational wave detector project) arm cavity. The number of unstable modes of LCGT is 10-times smaller than that of Advanced LIGO (U.S.A.). Since the strength of the instabilities of LCGT depends on the mirror curvature more weakly than that of Advanced LIGO, the requirement of the mirror curvature accuracy is easier to be achieved. The difference in the parametric instabilities between LCGT and Advanced LIGO is because of the thermal noise reduction methods (LCGT, cooling sapphire mirrors; Advanced LIGO, fused silica mirrors with larger laser beams), which are the main strategies of the projects. Elastic Q reduction by the barrel surface (0.2 mm thickness Ta$_2$O$_5$) coating is effective to suppress instabilities in the LCGT arm cavity. Therefore, the cryogenic interferometer is a smart solution for the parametric instabilities in addition to thermal noise and thermal lensing.