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| ==== Issues ==== | ''Issues'' |
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| * Mirror optical loss = 45ppm per reflection. Is it possible ? * With the current PRM (R=80%), the acceptable loss to achieve PRG=8.25 (100W input power is assumed) is 58ppm. |
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There are several factors to determine the g-factor. * g-factor determines the beam spot size on the test masses. Some kind of 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. ==== Thermal noise and beam radius ==== Coating Brownian and thermoelastic noise are the largest noise sources; coating is the biggest at f<500Hz and TE is the biggest at 500Hz<f. Coating TN is inversely proportional to the beam radius while TE noise is almost independent from the beam radius at low temperature. ==== 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. [[LCGT/subgroup/ifo/ISC/MirrorSize|Mirror Size]]. == Original Slides == http://gw.icrr.u-tokyo.ac.jp/cgi-bin/DocDB/ShowDocument?docid=148 |
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
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 optical loss = 45ppm per reflection. Is it possible ?
- With the current PRM (R=80%), the acceptable loss to achieve PRG=8.25 (100W input power is assumed) is 58ppm.
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. Some kind of 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.
Thermal noise and beam radius
Coating Brownian and thermoelastic noise are the largest noise sources; coating is the biggest at f<500Hz and TE is the biggest at 500Hz<f. Coating TN is inversely proportional to the beam radius while TE noise is almost independent from the beam radius at low temperature.
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