Motivation

The beam size downstairs in the tunnel was not matching the size set by the iris upstairs.

The beam seemed to diverge further from the compressor to the main chamber.

There were also some deviations from the design parameters in the probe line.

All indicated that the collimation was not perfect, either through wrong positioning of lenses in the beam transport, wrong specification of some lens pairs or an issue even earlier in the laser chain.

Update Jan 2022: the beam appears to be now collimated upstairs and downstairs after adjusting the telescope.

Setup and methods

In laser room

We measured the beam divergence on a rail setup after the deformable mirror in the laser room.
This will enable us to understand the changes required to collimate the beam.


This involves a camera imaging at a diffuser screen.
The camera is a Manta G895B, pixel size 3.45um, magnification ~11 (38 um/pixel).
The first long rail enables a travel range of ~120 cm.


The imaging system was spatially calibrated using a printed paper grid (10mm spacing) on the diffuser screen and using the IR beam as backlighter.
The lens on the camera was locked to make sure the imaging system remains the same over the course of all measurements.


Fig 1: Photo of the setup. Beam enters from the top right and is incident onto diffuser screen.


Instead of measuring the beam size at two locations, we decided to use a 3D-printed mask.
The mask produces an array of beamlets, similar to a pepper-pot for electrons, and the change in spacing indicates divergence.
A bit like an old-school wavefront sensor (or what is used for X-rays).

For measurements with the 1:4 telescope (~40mm beam), we used a 2" mask with 7.5 mm pitch.
http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/45/08.11&pos=2021-11-08T23:37:07

For measurements without the 1:4 telescope (~13mm beam), we used a 1" mask with 4 mm pitch.
http://physics-elog.slac.stanford.edu/facetelog/show.jsp?dir=/2021/46/16.11&pos=2021-11-16T23:13:20


Fig 2: Photo of 3D-printed mask, 2" diameter with 7.5 mm pitch, 2 mm diameter holes.


Fig 3: Example data of IR beam passing through mask and seen on diffuser screen.


Note: When matching the collimation of the CW and pulsed beam or when trying to recover collimation after changing the beam expander, we looked at the focal plane on far field diagnostics.

This could be the microscope objective or InjFar (when set up). Bringing the spot back to focus on these cameras indicates a similar level of collimation.

Afterwards we then quantify the divergence with the rail setup.

In tunnel

We used a similar setup of camera and diffuser in the tunnel. Here we used a Mako-125, lens and same glass diffuser screen along with the masks.

The camera was again positioned on a rail (here for 'rail camera') and measurements were taken at different distances.

Divergence Measurements


Measurements in March 2022 after revamping the beam expander:


Measurements in Jan 2022 and optimising collimation:


Measurements as of Nov 2021 show that the current setup results in a divergent laser beam that is also experiencing even larger divergences at full energy in the final amplifier:

Some of the divergence originates from some modifications to the beam to avoid burning mirrors.

Appendix: Beam size measurements

Simple measurements to identify issues with the collimation, using different methods.

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