I checked that the makeup air in the PSL enclosure was not limiting us at the 20% nominal setting. However, I found that at 100%, it would limit us. Figure 1 shows the difference in DARM between makeup air on/off  at 100% fan speed. Red is makeup air on, Blue, off.  I am particularly concerned with the regions such as between 15 and 22 Hz where the DARM noise increases but the microphone level does not increase. I don’t think we can make accurate estimates of air current noise using the microphones.

Another source of air currents that may produce jitter are the temperature variations along the beam path. Figure 2 shows a FLIR image of the beam path between the HPO and the DBB. I took this image on Monday. The table surface reading was 23 degrees C while the walls of the HPO were 26 degrees and the walls of the DBB were 26.5. I think that the temperature differences with respect to the table may generate an air current down onto the table, crossing the beam path, and up the sides of the boxes. The space between the Front end and the HPO is a similar channel. Downstream of the jitter-reducing PMC, near the main beam path under the periscope, lies the PI piezo controller (Figure 3). Because of its transformer, it is a warm 32 degrees C, nearly ten degrees hotter than the surrounding environment. The cool air coming in to replace the rising current above the controller may cross the main beam path. This would be very easy to move somewhat further away to test for any change in DARM.