The Swiss Cheese Baffle, MCA1, has been identified as the worst scattering coupling site at LHO (https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=35735 ). The baffle’s Q was so high that even minor damping was expected to significantly reduce its velocity. In addition, the baffle was accessible through ports in the reduction flange between HAM2 and the input mode cleaner tube, so the vent plan was modified at a late date to include damping of this baffle.
Before damping, I mapped out the motion of the baffle using the laser vibrometer. I was able to shine the beam through the camera ports beside the baffle at 3:00 and 9:00 and measure the motion of the suspension ring and the edge of the baffles (either side of the proposed dampers). I found that, at these locations, the baffle edge was, at 12 Hz, moving more than ten times as much as its support ring and about 1/3 as much as the center of the baffle. Measurements at multiple baffle locations suggested to me that the 12 Hz mode was a combination of a torsional mode around the shorter standoffs, a mode where the whole baffle moves back and forth in the beam direction within its suspension ring, and a bullet-like mode where the baffle moves somewhat more at the center. These measurements increased confidence that the selected locations were likely to work.
After doors had been put back on the open chambers, increasing purge air pressure, we pulled blanks covering ports at 4:00 and 8:00 (as viewed from the HAM2 side), one at a time. I reached in through the port at 4:00 and inserted a Viton HAM1 table spring (cork) between the baffle and the baffle support ring at about 3:00, viewable through the camera port (see Figure 1). At the port at 8:00, the baffle is closer than the support ring, so I reached up and through the hole in the baffle at 9:00 and inserted the cork from the HAM3 side. I put a third cork as far away as I could reach from the 8:00 port in the 6:00 direction, reaching about 7:00. Since the baffle and support ring are too far apart at this location for the cork, I inserted the cork between the baffle and the wall of the beam tube. We monitored the effect of each insertion with the vibrometer. The total time during which either port was open was under 45 minutes.
Figure 2 shows before and after spectra from the vibrometer and an accelerometer on the beam tube. Both sets were taken in similar conditions at air pressure. After damping, the 12 Hz peak was reduced to the degree that it no longer appears in the spectra for ambient vibration levels. To observe the peak I had to bang on the beam tube (lower plot). Other regions of the spectrum did not show new peaks. Of course the final evaluation will require time and DARM.
A simple damping scheme like this could be used on other off-table baffles: at least one of the Eye baffles is moving in the micron range.
Robert S., Calum T., Stephen A., Eddie S., Vinny R., Gerardo M.
