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    User name Armstrong

    Log entry time 06:58:12 on June22,2004

    Entry number 126629

    keyword=increase current

        We ask for increased current on our "empty" target (to
    see how the septum heating scales). With the laser attenuator
    set at max. setting (600) we only get 37 µA! The 
    QE from this spot on the photocathode must be plummeting like 
    a rock! Oh well, we settle for this. Continue with run 2366
    at this current. Septa temperatures go up by only 0.05 K 
    or so. 
      By the way - the fact that the He causes the septum heating
    and the fact that the dummy target vs. full He cell vs.
    empty He cell heating ratios don't scale with relative radiation 
    lengths suggests to me two (possibly overlapping) conclusions:
      - the septum heating is dominated by stuff getting into septum acceptance and slamming into the coils; the reduced (by how much,
    I'm unsure) acceptance for the windows (at +- 10 cm) for the 
    dummy target and real target windows relative to the rest of the
    cell (where the helium is, natch) explains the result
        - the different properties of scattered particles (mix of 
    hard/soft photons, gammas) from the two materials (He and Al)
    somehow affects the heating; I don't have a good model for
    this, but I can't rule it out,
      6:50 - end run, ask for move to dummy target while we 
        close JT valve to park the He target. Plan: take the
        last hour of beam as asymmetry on the dummy target, to
        get crude upper limit on asymmetry from quasielastic Al