Ushiba, Miyo, Akutsu: Cont'd from 17407.
Abstract
Using an easy-made iris at EXA, we aligned the ETMX so that it could have reflected the GreenX to the POP table. Still we could not find the beam.
Details
Further more details on how the ETMX suspension was aligned will be reported by Ushiba-kun, so my report is a kind of overall report and on the optics.
Firstly, without detaching the PD there (PDA100A), we checked and confirmed that the beam shap was not so strange; it has a sharp edge due to the GV windows, as usual.
By looking at the beam at EXA, we thought PR3 should be very slightly tweaked so that the GV window edge shadow should be equally found around the green beam at EXA, and did so (Fig. 1). Then we made a small slit (~ 3 mm dia hole, actually) on the aluminum cover on the EXA chamber flange facing the EXC. Using the slit, we aligned the ETMX so that the reflected beam from ETMX could again go through the slit.
Then we peeled the aluminum cover off, and swept the ETMX in pitch an yaw, but no repsonse was found at the POP GrRFPD DC. So Sad. Then I moved back to PR2 and looked for the reflected beam from ETMX (asked Ushiba-kun to shut the beam at EXA, for example), but no such ones.
Look back...
As 17240 and 17266, let's consider thing quantititatively. Assume the input green is 10mW. Accoring to the wiki, ITMX has 0.06 of Green transmission. So roughly 0.5mW reached at EXA in theory. But just judging from the PDA count, the PDA felt only 10 uW, and extrapolating this number, the actual total power at EXA would be ~ 0.1mW. So 10mW-> 0.1mW is the thing. Then, assume the same happen when going back to the POP table, this 0.1mW would be 1/100 times, meaning 1uW. Assuming 0.5 A/W of the GrRFPD's efficiency, and it seems only 100 ohm at DC for the transimpedance, it seems ~50uV. 1 digital count would be 610uV or so (40V/2^16), so the 50 uV thing could not be identified with this setup.
We + YamaT discussed somehow, and will try some options later:
- Try to set a whitening filter for the signal line from GrRFPD DC to increase the DC gain.
- Try to put a PDA100A with a sufficient gain.
But at the same time, if the power is 1uW, I may be able to see it with my eyeballs... was it spacially too close to the forward beam?? Actually there were several "look-like-so" slowing shaking light pieces found in the chambers... but they did not disappear when Ushiba-kun hided the beam at EXA. I'm wondering where are these shaking light come from; for example, the beam from ITMX seems not such shaken... and the shaking beam spot was small, that would mean they were backward beams and reflected at PR3... but I do not think PR3 was such shakey (if it would be the case, the beam could not stably reach the Xend).
