[Hallb-engineering] Minutes of meeting
burkert at jlab.org
Mon Apr 9 15:11:06 EDT 2012
Please find the minutes of today's Hall B meeting at this link:
Note that we have finally started production data taking with the
linearly polarized photon beam on longitudinally polarized D (and H)
target. We had excellent running over the weekend at 2.2GeV coherent
You also find a discussion by Alexandre Deur on the results of the
electron beam test on HD.
I want to make a few comments on these results, as the data might be
confusing to some.
We have had 2 test runs. The 2nd electron test run had much better
controlled beam properties, and the interpretation is based on these
data. They are, however, also consistent with the 1st test run results.
The main results are:
1) Both H and D in "HD" lose polarizations during beam exposure, and
much more quickly than expected
2) The H in "HD" retains a very long decay constant even after
relatively long exposure to the electron beam
3) The D in "HD" exhibits a strongly reduced decay constant even after
only a relatively small beam exposure.
Conclusions 2) and 3) are based on the graphs showing the polarization
recovery at the end of the run towards the thermal equilibrium when the
polarization had been erased to 0 for both H and D. While the D
polarization recovers quickly (hours), the H polarization remains near
zero with a recovery time of >50 days. The latter number is in fact the
good news, it indicates that little damage has been done to teh H
component due to beam exposure.
The remaining question is how to explain 1), i.e. what caused the
polarization loss during the beam exposure in the first place, in either
case (D and H)? Since both parts are affected in a similar way, they may
have the same origin. A tentative explanation (by the experts) is that
local beam heating during the slow beam raster causes depolarization as
the heat is not carried away quickly enough from the local HD material
to avoid local overheating and polarization loss. If local heating is
the limitation then an improvement of the heat extraction from the HD
material and a much faster beam raster will have to be implemented.
Several ways of accomplishing this are being evaluated.
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