General requirement

RP requires attenuation to 14 mJ, so 1.75% at 800 mJ input.

The beam has a ~10-13 mm diameter (will depend on final collimation) and is uncompressed, so at around 200 ps pulse duration.

Pre-2022 RP attenuator

Currently installed RP attenuator.

Consists of 4 mirrors, a wave plate and thin-film polariser

Requires p-pol as input. A wave plate before the RP attenuator (not shown) changes s- to p-pol for this.

Attenuates to 0.5*0.8*0.5*0.1 = 0.02 (2%)

Maybe a bit more attenuation due to the polariser, as this technically is not enough for a 800 mJ beam.

Issues:

• Appears to introduce astigmatism
• Ghosts when changing polarisation
• Loads of different optics (splitting ratio and polarisation)

Fig 1: Currently installed RP-attenuator. Red arrows indicate the beam path.

2022 RP attenuator

4 mirrors and s-pol as input (one wave plate needs to be removed).

Mirrors are 35%R from MPO, AR coating on back and 2 degree wedge.

Attenuation should be down to 0.35^4 = 0.015 (1.5%) or 800 mJ * 0.35^4 = 12 mJ

Reflectivity tests

• Individual mirrors (Dec 2021): http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/49/07.12&pos=2021-12-07T14:37:00

• Whole test setup (Jan 2022): http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2022/04/26.01&pos=2022-01-26T16:03:26

Wedge orientation

We added a wedge and AR coating to remove potential ghosts.

We determined that the thick side of the wedge needs to alternate sides when looking from the back of the mirror mounts.

That is in order to avoid that wedges compensate each other.

The thick side of the wedge is where the pencil markings are.

Wavefront

Need to check this:

Probably look at focal spot cameras, NF/FF or so with and without these beam splitters.

Identify if there is any change in spot quality. Might need the SAGAs for this.

The collimation will in any case vary if we do this at the current test position before the relay imaging.

Quote for mirrors (arrived first week Dec):