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    User name silwal

    Log entry time 15:02:57 on November 20, 2009

    Entry number 301377

    keyword=FFB woes and dithering

    Recently we have had problems with FFB being paused for much longer than
    dithering requires, which has resulted in beam instabilities.

    The most recent long pause that I could trace (going through the
    archives) was on Wednesday, Nov18th @ 10:55 pm or so. This was right
    after the compton epics was rebooted:


    The red trace on the first attachment is the compton orbit lock pause,
    which is 1 during a dithering cycle and 0 otherwise. The blue trace is
    the hall A FFB pause, which is also 1 during dithering cycle and 0
    otherwise. Before the epics reboot, dithering cycle is normal. After the
    reboot, the connection to the compton server is lost, and the connection
    seems to have been restored (according to the archive) only at about
    12:46 am or so, after about 2 hrs, after which dithering is back to its
    normal cycle. So the compton epics hungup seems to have interrupted the
    dithering cycle is this case (something I will try to adapt the dithering
    script to so that this does not happen again).

    The second attachment shows a slightly longer than usual compton/FFB
    pause. This is due to CODA hungup:


    Looks like the counting house crate (this is where the dithering script
    is run) hungup in the middle of the dithering cycle, before the script
    got to compton/FFB unpause commands.

    Besides these I can't see anything unusual in the dithering cycle. There
    are periods where it has been turned off/on manually, but only spotty
    records of it in halog. It would be nice to have shift crews log every
    time they turn the dithering on/off.

    For now, as long as we make sure that the compton epics channels are up,
    dithering should be fine. For compton tuning or compton epics reboot, we
    should turn the dithering off.

    A copy of this log entry has been emailed to: paschke@jlab.org, xiaochao@jlab.org, rom@jlab.org, riordan@jlab.org

    Figure 1

    Figure 2