In consequence of issues raised during yesterday's machine studies
meeting, I obtained ECLOUD results for the L3 RFA setup:
45 electron bunches of 2 mA each in a round beampipe ID 3.5"
with no magnetic field. Of particular interest are

1) the energy spectrum of cloud particles, in particular the fraction
  of high-energy particles

2) the cloud spatial distribution to see if the three RFA's might
  see different cloud densities and/or different spectra,

3) the difference  between an electron beam and a positron beam
  in terms of the effect on the cloud density and on the high-energy
  end of the energy spectrum.

The first two pictures show the transverse cloud profile just after the
passage of the 45th bunch.
One clearly sees the repulsive effect of the electron beam.
Electron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_237_1.jpg
Positron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_239_1.jpg

After the passage of six more bunches, all empty, the center part of the
beampipe fills in for the electron beam.
Electron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_237_2.jpg
Positron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_239_2.jpg

These plots also do not show any particular distinction in the clouds
near the positions of the three RFA's.  Note that the primary source
point is on the right in my plots and was on the left in the plots
Shlomo showed yesterday.

The following two plots show the cloud buildup. The cloud for
the positron beam is about a factor of seven denser than for
the electron beam. Also, the cloud decays much more quickly
between bunch passages for the electron beam.  This difference
in decay time is not seen after the beam goes away, indicating
that the slower decay time may be related to kicks on cloud
particles by the beam. Notice also that the decay time changes
dramatically along the 45-bunch train for the positron beam,
in fact, for early bunches, the density actually grows between
bunch passages.
Electron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_237_3.jpg
Positron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_239_3.jpg

The latter plots hint that secondaries are playing a bigger role
for the positron beam, so let's look at the energy distributions
of the cloud particles hitting the wall:
Electron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_237_4.jpg
Positron beam:
http://www.lepp.cornell.edu/~critten/cesrta/ecloud/9may08/ecloud_239_4.jpg

Sure enough, there is a peak related to secondaries in the spectrum
for the positron beam. I don't yet understand why it appears
near 50 eV.  The lower plots in these pictures show the percentage
of the spectrum integrated over energies greater than the energy on the
abcissa.
One can draw the conclusion that 10% of the cloud has energies greater
than 15 eV (60 eV) for the electron (positron) beam.

-- Jim

========================================================
James Crittenden                   Tel. (607) 255-9424
Wilson Synchrotron Laboratory      Fax  (607) 255-8062
Cornell University
Ithaca, New York 14853-8001
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