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    User name Armstrong

    Log entry time 21:45:15 on June24,2001

    Entry number 64457

    This entry is a followup to: 64429

    keyword=suggestion read asymmetry feedback


    Looking back through the charge asymmetry results for the past few days
    (43 runs), I notice that the results oscillate around zero (average was -3.3 ppm
    over this range of runs), with an RMS deviation of 152 ppm. This is what I would
    expect based on the sinusoidal oscillation of amplitude about 250 ppm noted before
    (halog entry 64429) and the fact that we feed back based on an integral of that
    oscillation.

    The one exception is at run 924. Here the value of the half-wave plate rotation
    (psub_aa_pos) was 576. The run was short (roughly a 2 minute run) and the apar
    analysis gave 647+-1069 ppm; the automatic feedback used this value to reset
    the plate to 551. The next run (925) was of normal length, and the resulting
    charge asym. was -585. The feedback then used this value to reset the plate to 574
    (i.e. nearly where it started).

    The moral is that the automatic feedback is a little dangerous if a parity run
    is very short, since it can get a value with huge errors and set the plate to a
    wacky angle. It should recover after the subsequent run, however that run may
    have a huge asymmetry. So, shift workers should watch for this, and if a
    large correction is made after a short run, I'd suggest overriding this by resetting
    the rotatable half-wave plate by hand (i.e in the Helicity Correlated Feedback
    controls GUI).

    Also note: after the reboot of adaqcp, the automatic feedback starts in the
    default case of being off, so I turned it on by hand: "feedback on".