Current Issues of the IFO design
This page summarizes the current issues of the LCGT IFO design and provides links to pages discussing the details of each problem.
Arm Cavity Parameters
Finesse
1550 is the default value decided by the IFOBW working group.
Issues
- 1550 could be too high. Need more investigations on what could go wrong.
- Mirror loss = 45ppm per reflection. Is it possible ?
- Actually, the current PRM design allows up to 65ppm without significant power loss (by Kentaro). Reference needed.
g-factor (mirror ROC)
g1 = g2 = sqrt(1/3) = 0.57735 is the conventional number to avoid HOM resonances. For L = 3000.0m, it gives ROC = 7098.08m.
There are several factors to determine the g-factor.
* g-factor determines the beam spot size on the test masses. Thermal noise (especially coating thermal noise) is smaller when the spot size is larger. * So called Sidles-Sigg instability of the arm cavities by the radiation pressure is affected by the choice of g-factor. * The parametric instability is also dependent upon g-factor.
Sidles-Sigg instability
With the current design of the arm cavity power (450kW), the two eigen-frequencies of the radiation-pressure-induced angular springs are 1.66Hz and 0.86Hz for the Sapphire mirror of 30kg (moment of inertia = 0.173 [kg*m^2]). For positive g-factor, 1.66Hz becomes unstable whereas it is 0.86 for negative g-factor.