After lunch we attempted to connect the actuator arms to the iris arm used to change the iris diameter. We need two iris diameters: fully open for experiment measurements and a 5mm diameter for beam focusing

To do this the original design uses a cuboid with two rods, one rod is connected to the actuator arm, and the other rod is connected to the iris arm with a hook - see attached. The rods were first cut down to fit into the chamber. This would allow us to change the iris diameter whilst under vacuum.

We decided to first check we could connect the hook to the iris in the central section of the target chamber, because this has the most amount of access space. We mounted the iris onto the bottom of the central lid using it's two legs.

After several attempts we could not find a suitable position for the hook and box combination, for several reasons:

1. The cuboid used to hold the metal hook will hit the iris legs at the limits of the actuator arms movement. This means the iris cannot be completely opened or closed. We could close the iris until a diameter of 13mm is left, however the active region of the S2 detectors starts at ~11.5mm diameter so this will not work when focusing the beam.

2. Also if we mount the hook facing upwards (above the cuboid) it is too high to reach the iris arm

3. If we rotate the hook such that it faces downwards (below the cuboid) it now reaches the iris arm. However, the hooks height needs to be adjustable because the iris arm moves vertically as the iris is opened and closed. This is currently not possible as the hook cannot be loosed without falling into the chamber

Note: when the iris is fully opened or closed its arm is at its furthest vertical distance from the chamber lid. When the iris is halfway between open (or closed) its arm is at its closest vertical distance from the chamber lid.

We noticed the cuboid used to hold the two rods is assymetrical and so attempted switching around the two rods, this did not solve the problem.

We then attempted a simpler solution: We replaced the cuboid and two rods with two long screws placed directly in the actuator arms threaded holes, see second attached photo.

The idea being that we can simply move the iris arm with these two screws - there will be some backlash to this technique because the distance between the two screws is larger than the diameter of the iris arm. This should not be a problem because we will only need two positions.

However we found the screws still collide with the iris legs close to the limits of the iris arm movement.

Most importantly: We realised that either the hook or screw technique will both hit the ribbon cable connecting the detector to the vacuum feedthrough, see attached sketch

The central collar of the chamber does have a second position for the actuator arm, however this will hit the Si diode we intend to use to measure the backscattered beam. So this will not work.

At 18:30 we decided to leave the lab and sleep on this problem.

The only solution seen so far is to focus the beam with no detectors mounted. Then after focusing is complete: vent the chamber, mount the detectors, repump the chamber down to vacuum, and remount the preamplifiers on top of the chamber. This would not be optimal but it may be a temporary fix until a better solution is found.