Parametric instabilities and their control in advanced interferometer GW detectors

TL;DR

This paper simulates parametric instabilities in Advanced LIGO test masses, analyzing their excitation of acoustic modes and exploring control methods, including thermal tuning, to mitigate these instabilities in different test mass materials.

Contribution

It provides a detailed simulation of parametric instabilities in advanced interferometers and proposes thermal tuning as an effective control method, especially for sapphire test masses.

Findings

Parametric instabilities excite specific acoustic modes in test masses.

Sapphire test masses can be tuned to eliminate instabilities.

Fused silica test masses can reduce but not fully eliminate instabilities.

Abstract

A detailed simulation of Advanced LIGO test mass optical cavities shows that parametric instabilities will excite acoustic modes in the test masses in the frequency range 28-35 kHz and 64-72 kHz. Using nominal Advanced LIGO optical cavity parameters with fused silica test masses, parametric instability excites 7 acoustic modes in each test mass, with parametric gain R up to 7. For the alternative sapphire test masses only 1 acoustic mode is excited in each test mass with R ~ 2. Fine tuning of the test mass radii of curvature cause the instabilities to sweep through various modes with R as high as ~2000. Sapphire test mass cavities can be tuned to completely eliminate instabilities using thermal g-factor tuning with negligible degradation of the noise performance. In the case of fused silica test mass, instabilities can be minimized but not eliminated.