1. New suspension study for LCGT
I would like to talk about improving LCGT sensitivity. LCGT employ cryogenic, 7th suspension system, RSE and so on for good
. But there is a room to consider another possibility. Mainly this talk is about depressing suspension mode thermal noise.
Erina Nishida
Ochanomizu University
The Graduate School of Humanities and Sciences The Division of Advanced Sciences/ NAOJ

2. LCGT f2f meeting@Tokyo university, Japan, 5th Aug, 2011 Erina Nishida
LCGT Sensitivity
There is a peak at 117Hz
Those depends on the fiber being thick and tunnel tilting.
bLCGT default parameters
Arm length: 3000[m]
Test mass: 30[kg]
Mass temperature: 20[K]
Suspension length: 0.3[m]
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

3. Annoying peak from data analysis
One of problem for data analysis is vertical mode at 117Hz
LIGO mode frequency is about 300～400Hz.
Sensitivity is worse.( lower S/N→lower detection efficiency)
Power is unstable.
(power fluctuation make detection efficiency worse)
It is difficult to analyze this frequency band.
Frequency band this mode could be widen.
>> Laser amplitude noise, seismic noise, 4 wires…
Before begin discussion, let me explain the LCGT design sensitivity. This figure is that, x axis is frequency y axis is noise level. Puple line is .blue line is. Yellow line is .green line is. Red line is. Those line is realized under those default LCGT parameters. So now we have to face with each noise to haunt separately. I focus on those suspension mode thermal noise forward.
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

4. LCGT f2f meeting@Tokyo university, Japan, 5th Aug, 2011 Erina Nishida
Simulation model
fiber
ribbon
Mathmatica Model parameters
●4 sapphire suspensions
●0.30 m length
● 30kg mass
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

5. Why fiber is not effective??
Mechanical lossΦ
Physics Letters A 270 (2000)
‘Ribbon’ thickness could be thinner while the heat
transportation keep holding.
Fro suspension system, the dissipation dilution is calculated as follow. So now it’s rewritten in the fibers and ribbon, which has width. That means Comparing the same area, ribbon thickness could be thinner than fibers, so that ribbon dissipation could be reduced as the heat transport is kept.
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

6. Moving 120Hz peak by changing the fiber shape
Fro suspension system, the dissipation dilution is calculated as follow. So now it’s rewritten in the fibers and ribbon, which has width. That means Comparing the same area, ribbon thickness could be thinner than fibers, so that ribbon dissipation could be reduced as the heat
transport is kept.
The concern about thinner ribbon is the amount of heat transport might be reduced.
Below the thickness is 2.5mm, it depends on the temperature.
Including cooling effect, those peaks doesn’t move to lower.
Without thermal noise increasing, how to make the peak decrease….is
Ribbon could make cross section larger,
then resonant frequency higher.
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

7. Comparison of different ribbon ratio
Ribbon cross section constant
Ribbon thickness h,　width w
Cooling effect　above 0.8W
Thin ribbon can make violin mode move to higher.
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

8. Comparison of different cross sections
Ribbon width constant
Ribbon thickness h
Cooling effect　above 0.8W
7times area make 1st mode to 300Hz
Those parameters are realistic!!
Vertical mode and violin mode move to higher according to the thickness.
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

9. LCGT f2f meeting@Tokyo university, Japan, 5th Aug, 2011 Erina Nishida
Alternative solution to reduce vertical TN
VSPI for vertical mode
Vertical mode could be suppressed by VSPI.
[Vertical Suspension Point Interferometer]
Feedback control
Resonant frequency can
be changed.
Thermal noise can be
reduced.
Lastly let me introduce another technique for vertical mode haunting.
That is VSPI, that is extended from SPI. SPI is another locking cavity above the test mass to attenuate the seismic noise. VSPI is same principal, but different point is as you could imagine that vertical cavity to depress the vertical seismic motion.
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida

10. LCGT f2f meeting@Tokyo university, Japan, 5th Aug, 2011 Erina Nishida
Options for solutions
To avoid the suspension peak at the117Hz
→Use ribbon which has
wide width and thin thickness
To avoid vertical mode
→vertical SPI
Thank you for your attentions!!
LCGT f2f university, Japan, 5th Aug, 2011 Erina Nishida