LCGT IFO Mirror Size
Contents
Mirror size requirements from the beam spot size
The minimum mirror sizes are calculated requiring less than 1ppm diffraction loss. This translates into the mirror sizes 2.7 times larger than the beam spot size. The situation is a bit more complicated for BS. Hiro Yamamoto calculated the loss of the light power in the PRC by the finite size of the BS using FFT simulation. BSLoss.pdf. Now we have to check what is the acceptable loss level for PRC ? 1000ppm ?
Optics |
Beam Spot Size (1/e^2 radius) |
Minimum Mirror Size (diameter) |
ITM |
3.5 cm |
19 cm |
ETM |
3.5 cm |
19 cm |
BS |
3.5 cm |
3.5*6*sqrt(2)+2*t/sqrt(2*n^2-1), where t is the thickness and n is the refractive index. |
PR3 |
3.5 cm |
19 cm |
SR3 |
3.5 cm |
19 cm |
PR2 |
less than 1 cm |
10 cm is enough |
SR2 |
less than 1 cm |
10 cm is enough |
PRM |
less than 1 cm |
10 cm is enough |
SRM |
less than 1 cm |
10 cm is enough |
Mirror Size Plans Considering Suspension Varieties
Plan A
For the 3rd year (iLCGT) configuration
- All the mirrors except for BS are 25 cm x 15 cm fused silica of 16 kg. They are all suspended by the Type B suspension platform with the IP and filter stages fixed.
- BS is 37 cm x 6.85 cm fused silica to make the weight 16 kg.
For the bLCGT
- The ITMs and ETMs are replaced with 25 cm x 15 cm sapphire mirror of 30 kg. The suspension platforms are also replaced with Type A.
Plan B
This was proposed by D. Tatsumi.
For the iLCGT
- ITMs, ETMs, BS, PR3 and SR3 (the mirrors which handles large beam) are all 38 cm x 12 cm fused silica of 30 kg, suspended by the Type A SAS platform.
- PRM, PR2, SRM, SR2 are 10 cm class TAMA sized mirror. These are suspended from a smaller simple suspensions, probably in common with MC suspensions.Issues
For the bLCGT
- ITMs and ETMs are replaced with 25 cm x 15 cm sapphire mirrors.
Advantages
- No need to replace suspensions between iLCGT and bLCGT.
- PRM, PR2, SRM, SR2 may be easily movable (depending on their suspension design). This is important to adjust the folding telescope in the PRC and SRC.
- Further reduce the number of suspension types by using the same design for MC mirrors and the small IFO mirrors.
Disadvantages
- 38 cm x 12 cm BS will have a larger loss than 37 cm x 6.85 cm one. Need to be checked.
- PR3 and SR3 are unnecessarily large.
To be checked
- What is the seismic isolation requirements for PRC/SRC mirrors ?
Mirror requirement summary from the IFO group
- The minimum mirror sizes are as shown in the table above.
- ITMs and ETMs should be at least 30 kg for bLCGT.
- It is necessary that PRM, SRM, PR2 and SR2 can be moved by about 50 cm after installation to compensate for the fabrication error of the ROCs of those mirrors.
Mirror name convention
