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    User name Armstrong

    Log entry time 13:07:08 on October 05, 2009

    Entry number 293275

    keyword=More on Compton problem and Hall A laser phase

    Diancheng has worked with MCC and they have adjusted the Hall A laser phase, and apparently the photon detector is happy again at 1800 V. Looks like 200 Hz with laser off, 870 Hz with it on. Having talks to MCC, it seems that they had indeed made an adjustment to the Prebuncher GSET at about the time we had the photon detector problems. Apparently they had noticed a tail on the synchrotron light monitor, and an increase in the "beam interception" on the Master Slit. Perhaps this adjustment was related to what caused the high rate in the photon detector. However, it seems to have been the wrong cure, since the cure instead was to adjust the Hall A laser phase (MCC tells me the photon detector rate was somewhat sensitive to the "Prebuncher GSET", but the cure that really worked was the laser phase. Apparently the *only* available diagnostic is the photon detector rate, and the technique is to turn Compton laser off, and empirically "tweak" the Hall A laser phase and have MCC watch the singles rate from the GSO photon detector.

    Moral of the story: when photon background rate goes up (and GSO HV trips with beam on) ask MCC to tune Hall A laser phase.