Starting from Tuesday morning the 6th of March, we proceed to the reshuffling of some of the optical elements on SDB2.
The main goal of this reshuffling is to have a better balance of the weight on the bench in order to be able to remove some counterweight located on the bench.
Figure 1 shows the optical layout before and figure 2 shows the layout after.
To do this work it was needed to block the bench, it was done in a position close to the he one with the bench suspended (with offsets less than 100 urad and 100 um on the angular and position d.o.f.).
In the absence of a beam coming from the ITF, we used a fibered laser to inject the beam on the different beam path where it was needed.
First we started with B5, we aligned a beam on the camera on photodiode, then we translated the block of photodiodes (B5_PD1 & B5_PD2) 125 mm to the west and 125 mm to the north so that the beam path stays the same. The photodiodes were aligned with the beam previously put in place. It was necessary to move these 2 photodiodes in order to give some room to the B1p_QD2 quadrants, which is moving from the south-east quarter of the bench to the north-east quarter of the bench.
Then we did the same on B1s1 as the 2 mirrors B1s1_M2 and B1s1_M3 have to be translated by about 175 mm.
A beam was aligned to use a reference in order to realigne the 2 mirror we moved.
Finally, we aligned the beam on the B1p path, the beam was aligned on the camera, the 2 photodiodes B1p_PD1 & B1p_PD2 as well as on the B1p quadrants.
A motorized translation stage has been added on B1p_L2, this translation stage has a range of 12.7 mm in total. Thanks to the electronic teams at LAPP and Cascina we were able to connect the new translation stage to a pico-motor driver.
Some mirrors were moved and everything was realigned according to the new layout using the reference beam. It was then possible to close the galvo loops on the 2 quadrants. It has to be noted that the new layout is using one mirror less on the B1p_QD2 path.
A beam dump has also been added in front of the B1p quadrant to dump its reflection.
On the B1s2 path, we replaced the glass beam dump that sees 92% of B1s2 with a AR-coated steel beam dump, this glass beam dump was damaged because it sees lot of powers when the ITF unlocks, see picture 3.
On the B1 path, we put back in place the two originals mirrors (replace temporarily to work on the OMC birefringence tuning, see logbook entry 40631). To do this we also injected a beam on this path and aligned it on the PDs and camera to be used as a reference. Finally the two B1 photodiodes have been replaced with 20 mW photodiodes (B1_PD1 : photodiode air box 08, B1_PD2 : photodiode air box 18). We checked that those 2 photodiodes are working well and are still aligned. However, the DC/audio blending parameters have not yet been updated.
Here is a list of things (along with weight) that have been added (+) and removed on the bench (-) :
+ 448 g : motorized translation stage on B1p_L2.
- 194 g : B1p_L2 column and clamp.
- 616 g : B1p_M1c mirror.
+280 g : Beam dump of the B1p quadrant reflection
- 65 g : feedthrough cable on the bench.
- 448 g : black glass beam dump.
+ 223 g : AR-coated steel beam dump.
On the optics side 372 g were removed from the surface of the bench.
Additional actions, that required the opening of the electronic air tank, were performed to lighten the bench. The motorized counterweight located in the air tank was replaced with a lighter one (2.676 kg vs 0.935 kg for the new one).
The aluminum slab that holds the picomotor drivers in the air-tank was hollow out by the mechanical shop to gain an other 1.105 kg. This reduced the thermal dissipation of the picomotors drivers, but usually they are used only for short times.
Once this work was completed, a first rebalancing of the bench was then made on Thursday evening.
As a general ending comment, everything was aligned according to the new layout with the best possible care considering the absence of beams coming from the ITF.
But anyhow it will probably be needed to fine tune the alignment of the different beams on the photodiodes (longitudinal and quadrant) and cameras when the beam will be reaching again the detection system.