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

Entry number 126629

    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

  and/or 
 
    - 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
    background.