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    User name V. Sulkosky

    Log entry time 19:01:18 on November 01, 2008

    Entry number 245995

    This entry is a followup to: 245303

    keyword=Analysis of Bulls eye Scan

    I have concluded my analysis of the Harp scans and CODA runs taken for
    the bulls eye scan on October 29th.

    I have been analyzing the BPM and Harp data starting with the Coulomb Sum
    Rule experiment, which performed a few bulls eye scans as well as some
    data taken this past June. The short summary is that the Harp A X
    position compared to the EPICS BPMA X position is now shifted by about
    1 mm. This can be seen in the first attached figure (top left-hand
    corner). Prior to the recent problem with Harp A, the various data for
    Harp A versus the EPICS readout were very consistent. The blue data
    points are from the recent scan, and the red points are accumulated
    data from CSR and this past June.

    There are also some smaller shifts seen in the other positions, but they
    are within 500 um. For Harp A Y and Harp B X, the difference is at the
    300 um level and probably not significant. I did take into account the
    survey results from October for the harps. Compared to earlier surveys,
    the Harp B X and Y offsets changed by about 200 microns. The offsets
    for Harp A changed well within 100 microns.

    The first figure shows the plots with a fit to the earlier data. The
    second figure shows the plots with a fit to the current data. During
    the Coulomb Sum Rule experiment, the coupler for Harp B was slipping.
    This explains the large outlier points for BPMB B greater than -5 mm.
    This problem was fixed in March and the new data lies reasonably close
    to the data I acquired back in June. Due to the nature of the analysis,
    issues in the absolute X position DO NOT translate into a problem in the
    Y direction. In fact, the Harp A Y position shows very good agreement
    with the earlier data with a small systematic shift (0.28 mm).

    This analysis makes the assumption that the EPICS BPM information is
    stable. From my earlier analysis, this appears to be a safe assumption,
    but we know things change over time.

    At the next major down, I think we should have a complete survey of both
    Harps. I am afraid we might not be able to rely on them for an absolute
    measure of the beam position to better than 0.5 to 1.0 mm at this point
    in time. For the time being, I can recalibrate the BPMs from the ADC
    readout using the Harps, but I will use the EPICS position for the
    BPMA X position instead of the Harp results.

    A copy of this log entry has been emailed to: brads,jiang,moffit,jpchen