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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 , 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.
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 or 137 mm (+- 0.6) to the laser path.
Description:
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4 mirrors and s-pol as input (one wave plate needs to be removed).
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Attenuation should be down to 0.35^4 = 0.015 (1.5%) or 800 mJ * 0.35^4 = 12 mJ < 14 mJ
Henrik's test setup (originally placed just outside the MPA):
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
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- 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.
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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:
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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.