PeterK, RickS

We cleaned an number of optics in the main beam path.

We replaced some optics that seemed to be scattering more light than they should, even after cleaning - the two steering mirrors in the pico-motor-actuated mounts just upstream of the PMC, and the two modematching lenses between the 70-W amplifier and the PMC.

We spent quite a bit of time adjusting the positions of the two modematching lenses between the 70-W amplifier and the PMC.  There seemed to be a broad maximum (fairly weak dependence on lens position), but the modematching was not very good.  Using the power meter in reflection and the PD monitoring the transmitted light, we had about 52 W in transmission and a little over 15 W reflected - about 78% visibility.

We then looked at the beam transmitted through the turning mirror just downstream of the 70-W amplifier.  The shape looked pretty good, but there was a definite pedestal.  We decided to try apertures installed just upstream of the PMC to see if scraping off the pedestal (halo) would improve visibility.  It did.

Here are the numbers:

3 mm diameter aperture, 53.7 W transmitted, 9.4 W reflected. 85% visibility

2.5 mm diameter aperture, 54.4 transmitted, 8.4 W reflected 87% visibility

2 mm diameter aperture, 49.9 W transmited, 6.9 W reflected 88% visibility

These numbers are significantly better than what we had without the apertures.  So we will likely go back to working on the beam quality out of the 70 W amplifier.  Maybe we will spend some time to get the Diagnostic Breadboard working.  It was deigned just for this kind of work.