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    User name paschke

    Log entry time 20:15:50 on July29,2003

    Entry number 108029

    This entry is a followup to: 106995

    keyword=Helicity Reversal (RHWP missing)


    The mystery regarding the ineffective RHWP has been solved: the RHWP is
    probably rotating, but it isn't in the beam line, so that isn't really doing anyone
    any good. It appears that during the work done on the laser table July 22, in which
    they swapped to gun3, then back to gun2, the RHWP was moved and
    (inadvertently) not replaced
    (
    POLOG 1161919    ).

    Removal of the Rotating Halfwave Plate flips the helicity
    of the beam!     Of course this is obvious; its just like the insertion and
    extraction of the usual Insertable Halfwave Plate. And of course the moller
    measurements see this sign correctly,
    Compare

    HALOG 107744     with
    HALOG 105935    .
    I just wanted to point this out, because I can easily imagine a harried grad student
    forgetting this complication when compiling final results; one can't rely on only the
    Beam IHWP sign for the helicity sign (and if you compare Moller measurements,
    that will be obvious).

    As for the effect on charge asymmetry: the charge asymmetry has been reasonably
    controlled by the IA cell operating between 65-80% of its dynamic range. If
    something drifts, and the IA system approaches 90% of its dynamic range, then
    we will need to act. Until then, we can wait for a convenient time for the source
    group to fix this problem (insert the RHWP).