Participants : Marc, Yuefan, Yuhang

Last Friday we did a scan of the FC for IR.

[ In order to obtain a good beam position, the BS pitch correction is close to saturation (9V). It might be useful to move some picomotors in order to have a better beam position and a lower correction]

The scan was performed by modulating the frequency sent to the AOM.

We used the following parameters : 1 MHz for the half peak-to-peak value over a half-period of 2 mHz (the lowest value permitted).

[The "half" values are used as they are what we provided to the AOM frequency generator]

We obtained the result presented in the first picture (FC_scan.png).

Few points to notice :

there is a symmetry at the second peak (if we zoom in we can distinguish 2 peaks)

The vertical scale was not good enough to have a good resolution of the small peaks -> on Monday we will do another scan with 2 scales : 1 to resolve the fundamental mode and another one for the higher order modes.

Anyway, as a preliminary analysis of this FC scan, we fitted the first 2 peaks with an Airy function (fig. FCscan_fit) which gives us a FSR of 62.57 s.

By doing :  1MHz / 2mHz * FSR[s] = FSR[Hz] we find a FSR = 500.560 kHz which is in good agreement with the logbook entry #668.

Another preliminary analysis was trying to estimate the losses due to coupling with higher order modes.

The fundamental peaks were fitted with a gaussian function while the (poor resolved) small peaks were fitted by triangles.

By comparing the ratio between the areas of fundamental mode / fundamental mode + higher order modes, we found a mode-mismatch of roughly 10%.

This analysis will be more precisely performed on Monday.