The tests performed today were aimed to better understand the contribution of the 56 MHz side band to the HOM content of the B1s2 and B1 beams.
While the ITF was fully locked, we performed the following scans of the 56 MHz modulation depth:
- First we reduced the 56 MHz modulation depth by steps of 1 dB from its nominal value (12 dB) down to 7 dB (we tried to lowered down to 6 dB but the interferometer unlocked). This scan is shown in Fig.1. During this scan the gains and integration times of the B1s2 and B1 cameras were increased in order to better visualize the modes of very high orders. Fig.2 shows the images provided by these cameras when the modulation depth of the 56 MHz is set to 12 dB, and Fig.3 shows the same images for a modulation depth set to 7 dB. Fig.4 and Fig.5 shows the same thing with a log color scale. The modes of very high order do not seem to be impacted by the reduction of the 56 MHz. Instead the central part of the beam seems to be affected, probably because the power in the TEM00 decreases with less 56 MHz side band, but maybe also because lower order modes are affected.
- The second scan of the 56 MHz side band was performed from 12dB to 8 dB (and from 8 dB to 12 dB), with lower gains set on the cameras in order to have images less focused on the very high order modes. The scan is shown in Fig.6. Fig.7 and Fig.8 show the images of the beams for a modulation depth set to 12dB and 8 dB respectively, in linear scale. Fig. 9 and Fig.10 shows the same images in log scale. The shape of the beam (mostly B1s2) seems to be affected, but the conclusion is not clear.
More analysis in progress.
