General requirement
RP requires attenuation to 14 mJ, so 1.75% at 800 mJ input, 1.56% at 900 mJ and 1.4% at 1000 mJ.
The beam has a ~10-13 mm diameter (will depend on final collimation) and is uncompressed at around 200 ps pulse duration.
The potential fluence on the mirrors can be in excess of 1 J/cm^2.
The attenuator also needs to be failsafe. That means that damage to the optics should result in a lower transmission and make the system more safe as a result.
That means that transmissive attenuators, e.g. two compensating mirrors at 45 degrees, can not be used.
Instead a reflective attenuator design is employed which is harder to align.
2022 RP attenuator (currently installed)
RP attenuator delay:
From shift on 06/16/2022
EOS signal to e-beam with laser on: 1256.0620 ns
From shift on 06/17/2022
EOS signal to e-beam with laser on: 1256.5190 ns
The RP-attenuator adds ~ 457 (+- 2) ps to the laser path.
Description
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 < 14 mJ
Fig 3: Test setup for new RP-attenuator. Was installed just outside MPA.
Fig 4: New RP attenuator installed (Feb 15 2022)
The latest measurements indicate up to 1.4% transmission, so more than 900 mJ input would be supported.
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
- Whole setup (Feb 2022): http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2022/07/15.02&pos=2022-02-15T16:17:08
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.
Update Jan 2022:
To avoid any kind of confusion, we now made sure that the thin part of the wedge is on the top, resulting in all back reflections being pointed downwards.
The thick side of the wedge is where the pencil markings are, although not perfectly centred.
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 2021):
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.
Fig 2: Photo of old RP attenuator before decommissioning. Note that a different hole pair was used by now.