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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).

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


Reflectivity was tested:


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.

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