Gold Coatings in Advanced LIGO

1. Gold Coatings in Advanced LIGO Phil Willems LIGO R&D

2. Motivated by Thermal Compensation • Given the proximity of the compensation plates to the input test masses, heating of the ITMs by the CPs is unavoidable • Here is the ITM heating frm the baseline CP design: LIGO R&D

3. Potential Solutions • Just live with it • Possible, but not ideal. TCS power would need to be larger, and the spatial profile of the thermal lens will evolve on very long time scales. • Move the CP away from the ITM • Not very attractive. CPs are best placed between the BS and ITMs, and there is just no space except inside beam tubes. CPs between the BS and RMs can work for TCS but make the dedicated sensors useless, and still space is very tight. New suspensions would be needed. • Heat the CP less • This is the hot point design- build the IFO to want 0.5W central heating and supply it with TCS at lower powers. • Asphericity of self-heating still must be corrected by some other asphericity in the recycling cavities, such as corrective coating on CPs (expensive) of thermoelastic bump on intracavity mirror (Muzammil’s clever idea) • If significant nonuniform absorption needs compensation we may need high TCS power anyway • Accept the heating and immunize ourselves from its effects LIGO R&D

4. Enter the Gold Coating • Heating of the ITM is acceptable so long as it is radially uniform. This can be accomplished by: • Heating its AR face uniformly • Thermally insulating its barrel • A barrel-insulated CP radiates an admirably uniform blackbody from its back face regardless of the heating profile on its front face, so simply putting it close to the ITM AR face guarantees uniform heating. This we can do by making the CP thicker (extra cost for material; added mass welcome to SUS) • A low-emissivity surface between the ITM barrel and outside world provides thermal insulation. This could be a nearby gold-coated shield, or a gold coating on the ITM barrel itself. LIGO R&D

5. New Thermal Solution Remaining ‘thermal lens is dominated by ITM flexure, which is very modest. LIGO R&D

6. Design of the gold coating • A pure gold coating on fused silica is very fragile and not very adhesive. • An underlayer of e.g. nickel or inconel is required for adhesion- perhaps only a few nanometers • An overlayer of dielectric material is required for protection • The thickness of the gold itself would likely be 0.1-1 microns, just enough for high IR reflectivity and no more (maybe less, given Glasgow experience) LIGO R&D

7. Implications for COC • Cost • Thermal noise • Charging • Scatter LIGO R&D

8. Cost • Gold coatings for IR optics are widely used and available from many sources- no basic R&D required. • However, thermal noise issues may require thinner coatings than industry standard. • Also, our optics are very large and must be kept strictly clean. • Helena estimates very roughly that these coatings will add 50% to the coating cost per optic. LIGO R&D

9. Thermal Noise • A very simple model suggests that a 1 micron layer with Q=100 will increase thermal noise of the substrate ~5x. This is clearly unacceptable. • A 0.1 micron layer or Q=1000 will increase the thermal noise of the substrate by ~50%. The HR coating thermal noise dominates the substrate thermal noise so this is probably acceptable. • A 0.1 micron layer and Q=1000 will increase the thermal noise of the substrate by ~5%. This is clearly acceptable. • All the above is being checked by Andri. • Measurements of Q on real coatings is essential. LIGO R&D

10. Noise Unrelated to Coating Q • Metallic layers on the test mass may cause eddy current damping. • Amount of damping depends upon magnetic field environment- estimates of effect are needed • Level of damping may be reducible by depositing coating in many unconnected segments • Addition of conducting loop to test mass causes forces if magnetic field fluctuates- this can be stopped by a single thin gap somewhere on the barrel. LIGO R&D

11. Charging • Having a conductive layer surrounding the test mass could be such an advantage w.r.t. charging that we may want to do this to the ETMs as well. • Charge anywhere on the barrel is instantly and uniformly distributed, leading to smaller surface charge densities and less coupling to nearby ground planes. • If earthquake stops are also conductive then earthquakes would discharge our optics. • We could install a remotely positionable discharge wire to control optic charge at will- no more vacuum burps. LIGO R&D

12. What about the dielectric overlayer? • This might be thin enough to have useful conductivity, or be tweaked in composition to make it so. • Even if not, it is so thin that surface charges will induce image charges in the gold layer that effectively cancel them locally- surface charge is effectively redistributed around the barrel (and could be removed using the discharge wire) LIGO R&D

13. What about the test mass faces? • The HR and AR face of the ITM must remain pristine, so no conductive coating there • The HR face of the ETM must remain pristine, but a very weakly conductive coating could likely be added to the AR face LIGO R&D

14. Scatter • The gold coating will obviously reflect any scattered light incident upon it, but: • The barrel of the test mass is one of the most positionally stable places on earth, so it would inject very little noise into the IFO. LIGO R&D