Following on from last week's Round 2, at Keita's request I took a series of beam profiles between ALS_M11 and ALS_M12 (ALS_M12 is the bottom periscope mirror, so this is the green beam as it goes into WBSC10 (as close as I can get to it, at least)). Using the ALS_M11 mirror as the reference point, I took 4 profiles along the path; the profiler sensor was at 0° for these measurements, and the distances are from ALS_M11 to the profiler's sensor:
| Distance from M11 (mm) | Horizontal Beam Diameter (mm) | Vertical Beam Diameter (mm) |
| 96.5 | 5.05 | 4.26 |
| 121.9 | 5.09 | 4.28 |
| 147.3 | 5.03 | 4.28 |
| 169.5 | 5.05 | 4.26 |
I've attached a beam profile of the beam at the farthest extent from ALS_M11 that I could get the profiler without bumping ALS_M12 (this is the final data point in the above table). All of the profiles taken had this shape to it. To my eye it looks like the top half of the profile is compressed versus the bottom half.
In addition, I rotated the profiler sensor by +45° at each data point to check for any gross off-axis ellipticity. I've attached an example of this, taken at the same location as the first attachment. The beam gets more round when the sensor is rotated, showing no gross off-axis ellipticity; all sensor-rotated profiles showed this shape.
Performed a fit using 2 different programs: manual fit in gnuplot and an automatic fit included with JamMt. Results are attached (0 is the front face of ALS_M11). JamMt and gnuplot generally agree very well, but as can be seen there is absolutely no agreement here. As a back check, I asked Peter to perform a fit using a script he's written (also in gnuplot), see the final attachment. Once again, no agreement. That's 3 different fits, 3 different results. The only conclusion I can come to here is there aren't enough data points to get an accurate fit (remember, only 4 were taken due to space constraints between ALS_M11 and ALS_M12, the requested area for the measurement). We can get some more data if ALS_M10 is used as the reference, but there is an additional problem: the Rayleigh range of the green beam after ALS_L7 is very large (desired spot size of 2.2mm with a 532nm wavelength gives a zr ~= 28.5m). This means that over the ~24 inches we have available to take a beam propagation measurement after ALS_L7 (can't fit the profiler between ALS_L7 and ALS_M10, so we have to go after M10) the spot size is not going to change very much. This is a potential issue for any fit to a beam propagation measurement performed here, and something to be considered should we move forward with additional ALSy green beam propagation measurements.
Somehow I forgot to post this, but I looked at the ISCTEX to see if the lexan plate is on the viewport, and there was none (good).
Then I let the X arm freely swing after the green WFS converged, and look at the arm transmission peaks. Attached is the screen shot when the arm was moving with a reasonably constant velocity, together with the video camera image of each of the modes.
There's a very clean mode mismatch signature with 2nd, 4th and 6th order modes present, and the total power in 00 mode seems to be only about 70% of the total.
Jason's measurement shows that the beam size on the table is OK, and even though it might be clipped on the table it cannot explain the mismatch of this magnitude.
We might have to measure the return beam profile from ETMX.
