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    User name paschke

    Log entry time 12:30:12 on April19,2004

    Entry number 118576

    This entry is a followup to: 118430

    keyword=further notes on septum heating


    Not to paint an unnecessarily grim picture with earlier notes
    regarding septum heating, I list a few factors which may
    allow higher currents for HAPPEX than I had earlier suggested:

    - It is well known that the cooling of the coils is sensitive to the
    setpoint of magnet current, due to improved thermal contact between
    the coils and the (cold) yoke. The coils will cool much more
    effectively when the septa are tuned to the elastic peak for HAPPEX.

    - Thermal conductivity should be roughly linear with temperature in
    this region, and the yoke cooling may be dominated by the thermal
    conductivity of the iron itself. In that case, one would expect a
    significant improvement in cooling as the temperature creeps higher,
    which would presumably cap the temperature rise below the 2K change
    predicted.

    - It has previously been expected that most heating is caused by
    relatively high energy electrons, such as those from elastic
    scattering. The shielding now in place should remove a large
    fraction of the forward-peaked elastic distribution, but the septum
    acceptance itself is, of course, not shielded. Tuning the magnets
    to pass elastic-scattered electrons (as will be done for HAPPEX)
    should cut the rate of electrons striking the bore tube of the magnet.

    - There is limited experience with the Hall A optics currently in use,
    both due to the 8m dispersion and the significant entry angle on
    target made necessary by septum steering. There exists a possibility
    that some significant halo due to these optics is responsible for
    atypical heating in the septum.

    I believe there are enough complications to make a precise estimate
    of the HAPPEX current limit due to septum heating impossible. A more
    comprehensive test of heating would be useful, in which the magnets
    are tuned as for HAPPEX, the longer target is used, and the Hall A
    beam optics are more standard.