EX Cryobaffle

In January we damped the EX cryobaffle through a viewport (75553) because it had not been available for damping during the O3-O4 break and because the scattering noise from it increased with beam power (70808). The baffle has a 4 Hz resonance that matched a 4 Hz mode of the isolation system of one of the HVAC fans, making it particularly noisy early in the run, until we switched fans.  Figure 1 shows that, after damping, about five times more motion of the beam tube is required to produce about the same level of noise in DARM.

Input Arm

During the break, we doubled the angle of the MC baffles in the input arm, so that there would be no specular reflection paths between optics, we added more damping to the MC baffles, added nozzle baffles to each of the nozzles with blank-off flanges, and added an unplanned baffle for a glint (75726).

Figure 2 shows a frequency sweep, injected by a speaker, and the resulting noise in DARM before and immediately after the break. The noise produced by injections above 15 Hz, is greatly reduced immediately after the break. However, we had also hoped to better reduce the noise produced by injections below 15 Hz. I spent last week looking into glints that I could see in the chamber and other potential sources of the residual coupling below 15 Hz. I found that  I could greatly reduce the residual coupling by changing the yaw of the ITMY compensation plate by 50 microradians. The last pair of spectrograms in Fig. 1 shows that this CP angling further reduced the coupling to levels that we had hoped to achieve. 

Figure 3 shows the elimination of noise for a 13.1 Hz injection after angling the ITMY compensation plate in yaw. The second page shows that there is repeatable structure at about 20 microradians in the coupling, suggesting that there are multiple ghost beams or, more likely, multiple retroreflecting structures.  The third page shows a bright light on the wall of a spool piece that bears further investigation.