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 optical loss = 45ppm per reflection. Is it possible? (by Hiro Yamamoto)

• With the current PRM (R=80%), the acceptable loss to achieve PRG=8.25 (100 W 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 = 3000.0 m, it gives ROC = 7098.08 m.

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<500 Hz and TE is the biggest at 500 Hz<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 (420 kW), the two eigen-frequencies of the radiation-pressure-induced angular springs are 1.7 Hz and 0.88 Hz for the Sapphire mirror of 30 kg (moment of inertia = 0.173 [kg*m²]). For positive g-factor, 1.7 Hz becomes unstable whereas it is 0.88 for negative g-factor.

• Note: radiation_spring_freq.pdf

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.

Mirror Specs

Detection Ports

MC length

• Currently the MC length is set 13.32 m. The frequency of the RF sidebands that can transmit the MC is an integer multiple of 11.25 MHz. Providing two sideband fields at 11.25 MHz and 45 MHz and using the beats of these frequencies, we do not have much room for one more SB, i.e. non-resonant SB for ASC, at a reasonable frequency. Tatsumi-san suggested that we double the MC length and make a room for a NRSB (e.g. 16.875 MHz).
• Another issue on the SB frequency is that there may be a "bad number" frequency that overlaps with a radio frequency or something.

Original Slides

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

Technical issues on the control scheme

Disscussion is here.