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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 at around 200 ps pulse duration.
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Instead a reflective attenuator design is employed which is harder to align.
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2022 RP attenuator (currently installed)
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.
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
Fig 23: 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.
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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.