[Hallb] Minutes of meeting

[burkert]

Dear Collaborators,

Please find the minutes of today's Hall B meeting at this link: 
http://clasweb.jlab.org/group_meeting_minutes/Physics/

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 
edge energy.

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.


Best regards,
Volker Burkert