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.

Contents

  1. Current Issues of the IFO design
    1. Arm Cavity Parameters
      1. Finesse
      2. g-factor (mirror ROC)
    2. Mirror Size
    3. Original Slides

Arm Cavity Parameters

Finesse

1550 is the default value decided by the IFOBW working group.

Issues

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 (420kW), the two eigen-frequencies of the radiation-pressure-induced angular springs are 1.7Hz and 0.88Hz for the Sapphire mirror of 30kg (moment of inertia = 0.173 [kg*m^2]). For positive g-factor, 1.7Hz becomes unstable whereas it is 0.88 for negative g-factor.

Mirror Size

Mirror size requirements are first set by the beam spot size on each mirror. The mirror radius should be larger than 2.7*(beam radius of 1/e^2) so that the diffraction loss is less than 1ppm.

For LCGT, we do not want to have too many variations of suspensions. This is another factor to be considered when choosing mirror sizes. Different mirror size require different suspension design. So we don't want to have many different sizes of mirrors.

Mirror size issues are discussed here. Mirror Size.

Original Slides

http://gw.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=148