We tried measuring the width of aperture of scrapper baffle in front of PR2

Sheila, Mayank

We tried measuring the width of aperture of scrapper baffle in front of PR2.

The plan was to change the beam spot on PR2 mirror while simultaneosly monitoring the circulating power in X (ASC-X_PWR_CIRC_OUT) ( It is a caliberated channel derived from ASC-X_TR_A_SUM_OUT_16 and ASC-X_TR_B_SUM_OUT_16) . When the beam starts hitting the edge of aperture of scrapper baffle the circulating power in X will drop.

In oder to change the PR2 beam spot. We changed the PR3 yaw slider while activating PR2spotmove script (It adjust the slider values of PR2 and PRM and IM4 in such a way the the light going from PR3 to the the Interferometer remaines unchanged while the spot on PR2 moves.)

Steps we followed

1. Lock X-arm.
2. Decrease the PR3 slider untill the Circulating power drops ( due to beam hitting one edge of baffle).
3. Unlock X-arm and Lock PRX. Run A2L on PRCL to estimate beam position on PR2.
4. UnLock PRX and Lock X-arm
5. Increase the PR3 slider untill the Circulating power drops ( due to beam hitting other edge of baffle).
6. Unlock X-arm and Lock PRX. Run A2L on PRCL to estimate beam position on PR2

A2L procedure   

1.     Dither PRM PR2 PR3 at three different frequecy (29Hz , 30 Hz , 31Hz with amplitudes of 30,1000,1000 cts)
2.     Obeserve these peaks in PRCL.
3.     Minimize these peaks by chaning the "A2L gain" on PR2 and PR3 and PRM.  
4.     From "A2L gain" the beam spot on the optics can be estimated.

What we did

1. We started  at  PR3 yaw slider of 96 and TRX value of 0.05 with following a2L gain.

• PR2: -6.25 ( -12.588 mm)
• PRM: 0.52
• PR3: 1.6

       2. Perfomed  A2L on the three mirrors PR2 PRM PR3 to estimate original position.  The gains which minimized the three injection peaks were.

• PR2: -6.     ( -12.084 mm)
• PRM: -1.0
• PR3: 1

       3.The PR3 Yaw slider was decreased to 75.2 the such that the TRX value dropped to 0.047 (6%)
          Perfomed  A2L on the three mirrors PR2 PRM PR3 to estimate one edge of the Baffle.  The gains which minimized the three injection peaks were

• PR2: -5.5  ( -11.077 mm)
• PRM: -1.0
• PR3: 1

       4. The PR3 Yaw slider was increased to 103  such that the TRX value dropped to 0.047 (6%)
           Perfomed  A2L on the three mirrors PR2 PRM PR3 to estimate other edge of the Baffle.  The gains which minimized the three injection peaks were

• PR2: -6.3  ( -12.689 mm)
• PRM: -1.0
• PR3: 1

As expected the beam spot does not move on PR3 and PRM.  

The PR2 beam spot moved by 1.612 mm.

Thoughts: Sheila suggested that this movement of 1.612 mm is too small to see the clipping at the baffle. Most likely the reduction in circulating power was due to something else. To probe this, we tried to go further on the PR3 Yaw sliders (greater than the 103 and less than 75.2) however the X arm was not locking. This was probably because we were not getting the required PDH signal for Xarm locking on the POP port due to misalignment. Sheila suggested that we can 

1. Repeat the same procedure while Pico on POP signal so that we dont loose lock.
2. Alternatively we can choose REFL port signal for Xarm locking and repeat the measuement.