Last Tuesday I measured the ALS beam profile by the ITMY baffle PDs.

This was before we replaced the ALS Faraday today.

I tilted the beam by the PIT and YAW offset sliders of the TMS table. Then I used Keita's calibration of the sliders from counts to radians (see aLOG entry 3244) to estimate the actual beam tilt.

I swept the beam over the ITMY PD2 baffle PD. The beam was centered on this PD when the offsets were:

PIT -79500, YAW +49041

Initial values were:

PIT -60691, YAW +56641

Sampling a Gaussian beam in X or Y with a PD of finite radius r0, produces and intensity profile defined by the function attached below: xc is the beam center location, and w the beam radius.

I measured the beam radius by fitting the data from the PD scan with this function. Assuming that the ITMY baffle PDs are Perkin Elmer YAG-444AH (although a document by Mike Smith says that they're YAG-444-4AH QPD), then r0=16.3/2mm and the resulting radii are:

Vertical diameter: 59.4 mm

Horizontal diameter: 45.6 mm

The beam is astigmatic and not well matched to the cavity mode.

We are going to improve the mode matching soon.