Minutes from target meeting held July 5 2007 in 888

These minutes are available also from:

http://wwwcompass.cern.ch/compass/detector/target/Minutes/2007/

a) Jacques: helium recovery line oxygen level went up to 100 ppm on Tuesday 
10:30. The normal level about 10 ppm was recovered 22:40 in the evening. 
Jaakko: reason could be collected impurity in dilution cryostat or in magnet.
For example air backflow from the magnet quench release valve PV162 could
introduce air into the liquid helium vessel. See PT-FAQ->PolTargHePurity.

b) Jacques: on Wednesday morning polarizations +82%, -80% and +73% were 
measured giving relaxation times 5500 h, 3500 h and 2800 h. Still heater
was kept at 8.0 V in frozen spin mode with the dipole field. This gives
a little higher 3He flow rate compared to the previous transverse data taking
period with 6.0 V still heater, but the relaxations times are almost the same. 

c) Jacques: on Wednesday at 12:00 polarization reversal with microwaves was 
started. At 20:30 slow discharge in the solenoid power supply. Probably one
phase missing in the CERN normal network. The solenoid current ramping down
0.05 A/s, the trim heaters permanently on (high consumption of LHe) and 
NMR signals not any more visible. Delicate procedure to recover normal 
operation of the solenoid power supply. At 300 A solenoid current dipole
ramped up to 100 A (0.1 T). At 01:00 Thursday morning solenoid at 100 A with
ramping speed 1 A/min. Reset solenoid PSU and start programme transverse->2.5T.
The nominal 2.5 T field recovered 01:30. The liquid helium level in the buffer
dewar decreased to 40 % due to the trim coil heaters. Restart the polarization 
buildup at 02:15. On Thursday 21:00 polarizations -83.6 %, +80.6 % and -80.2 % 
were reached.

d) Small confusion with the NMR configuration files on Wednesday morning. 
Jaakko updated the operation procedure on Wiki 
http://wwwcompass.cern.ch/twiki/bin/view/PolarizedTarget/PolTargTransMode


Comments from Fabrice:

Dear Jaakko,

I would like to comment your assumption concerning the pollution which
could come from the opening of the magnet quench valve. It looks hardly
possible since the valve did not open during this period. In addition,
when it opens it is done in the big quench vent pipe which is opened to
air after more than 20 meter and several elbows making difficult an air
backflow. The opening of this pipe is directed downwards and probably
already full of helium gas from the previous "quenches" which stays
trapped. In case of "quench" the pressure of the magnet cryostat drops
to the atmospheric pressure and not lower. So contamination by air looks
seriously improbable.

I already cross-checked why the cryo group the history of the pollution
at that time but their trend is not precise in time enough to know in
what side the pollution appeared first (a backflow from the purifier
could also explain the problem). I can just say that regeneration of
purifier has started AFTER (~11:00) that pollution appeared on our
sensor on the platform(~10:40).

Fabrice