Ushiba, Tanaka
## Abstract
As response to klog31861, we measured an amount of a beam miscentering on ITMX in the PIT direction with 7.3 Hz PIT resonance peak. The measured miscentering amount is 9.75 mm, this is close to Komori-san's assumption in klog31861.
## What we did
- As reported in klog31861, we needed to measure a miscentering amount on ITMX PIT.
- First of all, we tried to use a resonance around 44 Hz of ITMX PIT in order to measue the amount of a beam miscenter on ITMX in the PIT direction. So we injected a white-noise bandpassed between 42 Hz t0 45 Hz to identify the resonance frequency. As a result, we found the resonance frequency is 43.4 Hz.
- Then, we excited the 43.4 Hz resonance with a sine wave injected from K1:VIS-ITMX_TM_TEST_P_EXC. However, we could not find the 43.4 Hz resonance in the PIT oplev signal (K1:VIS-ITMX_TM_WIT_P_DQ) (fig.1) even though OMC-TRANS_DC_{A,B}_IN1 values became close to their saturationi limit (>20000 cnts) (fig.2). If the amount of miscentering was 1 cm, the estimated PIT fluctuation was 1e-12/1e-2 = 1e-10 rad = 1e-4 urad, On the other hand, The current oplev noise level seemed to be 1e-3 urad. Therefore, we need to increase the excitation amplitude to more than 2-order to see the signal. So we gave up to use this resonance.
- Next, we tried to use a resonance around 7.5 Hz of ITMX PIT to measure the miscentering. We turned off the ITMX NBDAMP for 7.5 Hz, then the 7.34 Hz peak raised in the oplev signal. So we excited the 7.34 Hz resonance by injecting a sine wave with 0.3 cnt from TEST_P_EXC. According to Pitagoras's theorem, (length displacement [mm])/(beam miscentering [mm]) = tan (ITMX misalignment [rad]) ~ (ITMX misalignment [rad]), therefore, we can obtain (beam miscentering [mm]) ~ (length displacement [mm])/ITMX misalignment [rad]). So we measured the TF from the oplev (K1:VIS-ITMX_TM_WIT_P_DQ) which was calibrated to "rad" to DARM displacement (K1:CAL-CS_PROC_DARM_DISPLACEMENT_DQ) which was calibrated to "mm" at 7.34 Hz.
- The upper right panels in Fig. 3 shows the results. From the value of this TFs at 7.34 Hz, we obtained 9.57 mm as the beam miscentering amount.
- We also confiremd whether the result was changed in the case of the forced oscillation, or in the case of the free oscillation. We turned off the excitation and measured the same TF at 7.34 Hz. Fig. 4 shows the result. we also obtained 9.76 mm. This value is consistent with the one in the case of the forced oscillation.
- This is close to Komori-san's assumption in klog31861.
