[Keita, Jason, Masayuki]

Summary

The beam alignment inside HAM1 was carried out using JM2 and JM3 after the EOM installation. Alignment was optimized by observing higher-order modes (HOM) and maximizing TRANS peak power. After verifying the presence of a PDH error signal at IOT2, the demodulation phase was adjusted since the new EOM will change the modulation phase due to the location difference. The IMC was then successfully locked using the common mode servo, following the Guardian sequence manually. The tuned IMC-REFL_A_PHASE was 20.87°

Detailed Procedure 

• Beam alignment inside HAM1 was performed using JM2 (scratched) after EOM installation. The beam was aligned to an iris placed between JM2 and JM3 using JM2’s mirror position and angle. The beam position matched the iris within approximately ±1 mm.

• Further alignment between JM2 and JM3 was done using the iris after the periscope. A beam walking procedure was used: JM2 aligned the first iris and JM3 aligned the second.

• After this, the Septum window cover was removed. The beam reached the IOT2 table, but there was a yaw offset. JM2 was further adjusted to correct it (note: yaw in HAM1 corresponds directly to yaw on the IOT2 table).

• A small sweep of JM2 in pitch and yaw revealed a very weak resonance peak. To excite the cavity, a 0.1 Hz, 300 cnts signal was sent to the TEST filter of MC2 M1.

• The TRANS camera showed the presence of a higher-order mode (HOM) with a pitch misalignment. Beam walking was repeated: small adjustments to JM3, followed by peak maximization with JM2. After several iterations, yaw HOM became dominant, so the axis was adjusted accordingly. A final pitch correction increased the TRANS power to approximately 6 cnts, concluding the alignment.

• Returning to the control room, the IOT2 shutter was opened. A PDH error signal was observed, confirming that modulation was present. Since the signal appeared primarily in the Q-phase, the demodulation phase was rotated by 45° using a delay line circuit.

• The common mode servo gain was set to 14 dB. When the input switch was engaged, the system began to catch resonance. By enabling the Lock filter on MC2 M3, the cavity successfully locked. The Lock filter for MC2 M2 was then engaged with gain 0.1, and FM3 was turned on, allowing stable long-term lock. These steps followed the Guardian sequence.

• A 50 Hz, 400 cnts excitation signal was injected into the IMC_L filter, and the delay line phase was tuned to minimize the RF25_Q_ERR peak. The optimal demodulation phase (IMC-REFL_A_PHASE) was determined to be 20.87°.