[Hallc_running] (not your usual) Call for Shifts for A1n/d2n in Hall C
xiaochao at jlab.org
Fri Feb 7 22:40:08 EST 2020
Don't Worry, Be Happy.
Don't Worry, Be Happy.
Ever since the first polarized 3He electron scattering experiment, E142 at SLAC, people have been naming these glass cells. Well, how else would you tell these nearly identical glass cells apart? By their exchange symmetry properties??
Don't Worry and Be Happy are the names of the two first glass cells used in the GDH experiment in Hall A. You don't have to ask how the experiment went: Just look at these names, they **scream** the challenge of installing and running this target at JLab the first time. Don't Worry showed a ~25% in-beam polarization and lasted about 8 days. Be Happy lasted less than 4 days, mid-30% in-beam. How many cells were used by GDH? Even the then Ph.D. thesis student of GDH (the current Hall C Run Coordinator) does not know by heart: "We lost count, they just kept exploding", he said. And one should not forget cell Blue. Blue did not even show up in typical documentation of GDH, not even in PhD theses. Blue was installed in the target chamber without problem, but exploded right after because of unrastered beam and no physics data was taken with it. For reference, these glass cells did not need rastering under the pulsed beam of SLAC.
A lot has been learned on the electron beam effect on these glass cells since then. For example, it was found that each cell in average lasts about one month in beam. After that, radiation effect darkens the glass enough to cause increased laser absorption by the glass, and the cell would have a high chance of rupturing. The common wisdom from 6 GeV time is that the cell needs changed proactively after one month worth of running
(or RadCon would have a busy time removing irradiated glass shapnels). But does our 6 GeV wisdom apply to the higher beam energy and current? Although simulation shows similar radiation from 30uA 12 GeV beam to 15uA 6 GeV beam, thanks to the improved cell design, we were not so sure... until now.
On Monday Feb. 10th, cell Dutch will be officially retired. By then it will have seen beam for 31 days, most of which is 30uA at 10.4 GeV. Amazingly, Dutch polarization has not shown any sign of degradation in the most recent week, still holding at 60%+/50%+ in the transverse/longitudinal direction. We do not know yet why the longitudinal polarization is not as high as that in transverse. Could it be that beam irradiation has caused more darkening of the glass in the longitudinal pumping direction? Or is it something else? After all, with 30uA we are venturing into a new terrain of running these polarized 3He cells. After the target enclosure is opened on Monday, the first thing our target experts plan to do is a visual inspection of Dutch, and maybe we will solve the mystery then.
Following the removal of Dutch, new cell BigBrother will be installed and used for the remainder (fingers crossed) running of A1n. In laboratory settings, BigBrotherhas shown even higher polarization than Dutch, and we are keeping our hope high for its in-beam performance as well. On Tuesday, while BigBrother is being pumped to build up polarization, we will carry out 24 hours worth of reference cell runs that are needed by the experiment.
This is an exciting time, however, we can't do everything without your help. We are in dire need of shift workers for the upcoming week, most urgently Tuesday Swing; Wednesday Owl and Swing, and Thursday Owl and Swing. Please come join us. If you sign up for Tuesday swing, you will possibly witness the removal of Dutch and observe the first spinup of BigBrother (all while taking a disk load of data on the reference cell). If you sign up for Wednesday Owl/Swing or Thursday Owl/swing, you will possibly be the one putting beam on BigBrother the first time.
This will be a historical time, don't miss the chance of making yourself a part of the history. After all, it is only one click away! (Tip: see shift sign-up link below).
(not your usual) spokesperson of A1n
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