1. LIGO Scientific Collaboration
Test Mass Down Select Meeting
LIGO Scientific Collaboration

2. LIGO Scientific Collaboration
Down Select Meeting
Decision Date: June 30, 2004
Process
More meetings (roughly monthly frequency)
Recommendation to the LIGO Lab Directorate
With a report to the LSC
Today
What do we know now?
What do want to know (and can learn) by June 30?
LIGO Scientific Collaboration

3. LIGO Scientific Collaboration
Sapphire Absorption
Lots of work on small pieces
30-50 ppm/cm
3 large pieces measured
~ 35 – 70 ppm/cm spread
Factors of 2 inhomogeneity in sapphire problematic
Annealing looks promising
Need work on larger pieces
Outstanding issues?
Action items?
LIGO Scientific Collaboration

4. LIGO Scientific Collaboration
Thermal Compensation
Necessary for both sapphire and fused silica
Sapphire
If absolute absorption < ppm
Inhomogeneities  spot heating needed
fused silica
More needed for absorption
As noted before, coatings may require some compensation
Prototyped and tested on initial LIGO and GEO
Outstanding issues?
Action items?
LIGO Scientific Collaboration

5. Other Sapphire Parameters – OK or Close?
LIGO Scientific Collaboration

6. LIGO Scientific Collaboration
Fused Silica
Bulk absorption good enough?
Absolute value of < 1 ppm/cm
Inhomogeneities up to few ppm/cm?
Thermal Noise
One large piece > 108
HWS program on annealing getting going
Results by June?
Outstanding issues?
Action Items?
LIGO Scientific Collaboration

7. Fused Silica Parameters – OK or Close?
LIGO Scientific Collaboration

8. LIGO Scientific Collaboration
Coatings I
Thermal Noise: Choice of substrate dictates choice of coating
Thermal and thermo-elastic noise impacts AdL strain
However, titanium (bad actor) common to both sapphire and fused silica coatings
Coating program to improve titanium, look at other coating materials
TNI results
Can get to sapphire?
Outstanding Issues?
Action items?
LIGO Scientific Collaboration

9. LIGO Scientific Collaboration
Coatings II
Can’t forget about coating absorption
AdL arm cavities suffer from changes in surface curvature due to high finesse + coating absorption
Coating absorption inhomogeneities
Large inhomogeneities force HR inhomogeneous thermal compensation
Quiet CO2 laser  RIN < 10-6 rHz
Limits
Sapphire: 1 ppm on 1 ppm
Fused silica: not nailed down (~2 ppm on 0.5 ppm?)
Outstanding Issues?
Action items?
LIGO Scientific Collaboration

10. LIGO Scientific Collaboration
Coatings III
Measurements on surface scatter of LIGO I core optics
Large point scatterers
Imbedded in coatings (?)
AdL Arm cavity loss budget tight  < 75 ppm
Allocation to surface scatter  < 20 ppm
Not met by current LIGO optics!
Not clear how these issues differentiate between sapphire and fused silica
Outstanding Issues?
Action items?
LIGO Scientific Collaboration

11. LIGO Scientific Collaboration
Other Issues
Future of sapphire vendors?
We are a small part of Crystal Systems’ market
GaN laser diode substrates primary market
But, new GaN substrates demonstrated which may impact the sapphire market
Impact on Crystal Systems business?
And other sapphire vendors?
Cost and Delivery impact
Need updated information
Attachment issues? Penultimate Mass considerations?
Clear advantage for FS or sapphire?
LIGO Scientific Collaboration

12. Charging and Patch Fields on TMs
Charging identified as potential problem in AdL
Not much known yet
Putting Li into surface improves conductivity
Effect on Q??
Equally possible for sapphire and fused silica?
Outstanding issues?
Action Items?
LIGO Scientific Collaboration

13. What are the priorities in the next 3 months?
Let’s decide here…
LIGO Scientific Collaboration