[Prex] [New Logentry] RC status update 8/31

[jonesdc at jlab.org]

Logentry Text:
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Summary of events
Beam restoration began to the hall on swing Friday. Unfortunately, the optics setup would not allow us to run except at extremely low current without exceeding the set limits for the Compton background detectors. Background rates sometimes even exceeded 7kHz in the Compton region with no beam to Hall A just from Hall C bleedthrough https://logbooks.jlab.org/entry/3836703. No data were during Saturday owl as ops tried various solutions.

There was a meeting with the PD at 9AM on Saturday where Javier reminded us that it was imperative to emphasize beam delivery to C and B and not to prioritize A at their expense especially given the pass change scheduled for Monday. We were given the option of no beam or pull out of the Compton. We elected to pull out of the Compton and informed the shift crew only to be told a few minutes later by MCC that they had found a solution by resteering in the injector around the 0R08 region to keep Hall C's tail from tripping the BLM. This resteer then allowed them to adjust the gang phase on 0L04 to the point where Compton could still run. We got beam back @150muA during Saturday morning. Shortly after returning the PITA feedback did not converge and went to large PITA values ~12000. Hall C's shifters also noted this issue with Aq and inquired about it. They stopped taking data until the issue was resolved using the flipper script. This affected about 20 minutes of data and the a!
 ffected runs were removed from the production data set.

Around noon Saturday the shift crew noted that the dpp had shifted significantly from our usual -1e-4 to -2e-4. I requested that ops fix this and they changed the HallA arc Bdl to give a closer dpp reading. The shift in dpp was actually the result of two things: an actual upward energy shift of ~1.4e-4 when we came back from the CHL crash and a change by ops of the Hall A arc Bdl. There was a lot happening related to energy over Saturday and Sunday so the for clarity here is the sequence:
1. We came back from the CHL trip on Friday with a 1.8e-4 increase in momentum (2175.7 to 2176.1). This should have changed our dpp from -1e-4 to +0.8e-4
2. Instead, as we came back ops also changed the HallA arc BDL relative to which dpp is calibrated. This caused an upward shift in arc BDL of 2.8e-4 which took the dpp reading from +0.8e-4 to -2e-4 which is where we complained thinking the energy was too low. 
3. They then shifted the BDL so that the same energy read -1e-4. We were unaware but Yves caught this error and told ops to revert it which they did bringing dpp back to -2e-4
4. Ops next accommodated my request by shifting the energy of the machine higher by 1e-4 to bring us back to dpp=-1e-4. At this point our dpp looked good but we were actually 2.8e-4 higher in energy than we were before the CHL crash. We ran this way overnight realizing the detector slopes and yields indicated we were higher in energy.
5. Devi noted on Sunday that we were accepting a lot of the inelastic peak and took a position spectrum to verify this (https://logbooks.jlab.org/entry/3837712). He concluded we had 3.5 mm more acceptance than we had on the 25th when Qsquare was taken and 5 mm more than on Aug 16. Since Hall C could not decide whether or not to lower the energy, we just decided to move the detectors. Devi moved them 3.5 mm in the dispersive direction away from the inelastic peak on Sunday around 5PM. We ran this way overnight.

During Owl Monday, beam was rather unstable. Ops slowly reduced our current throughout the night until they finally settled on giving us 135muA. During the 7:45 meeting this morning, Hall C said they needed 3-4 hours down for work in the Hall. Jay said that only 1 hour was required for a pass change and suggested that A and B should continue to take beam until Hall C confirmed they were 1 hour from being ready. This was expected to give us an extra two hours of beam. However, Jessie had a lot scheduled for the down time and had a large contingent of people to go into the hall today at 10 AM and it would have been a major disruption to reschedule. Radcon was also scheduled to sweep the hall at 8:30. The extra couple of hours of beam was foregone to keep these efforts on schedule. 

All weekend energy has been unstable with minute timescale spikes at 5e-4 level and longer timescale with shifts at the 1e-4 level. Hall C is more sensitive to this than we are since these shifts can significantly rotate their polarization. Discussions are ongoing on how to resolve the energy instability. One question is, should we take the time to try to get FFB working again.

At 8:30 this morning the beam was taken away and the shift crew transitioned the Hall to Restricted access after the sweep. All magnets have been ramped down. The shift crew was released. The Hall will remain in Restricted access until at least Day shift on Wednesday. Please coordinate hall work with Jessie and keep the RC informed. 
Note: all shifts are cancelled from now up to and including Thursday Owl. At this time I am asking the Day shift crew to report on Thursday but the details will have to be provided by the next RC.

Thank you to those who filled in the schedule this weekend and made data taking possible!

We will have our usual RC meeting at 3:30 PM.
https://bluejeans.com/256205934


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This is a plain text email for clients that cannot display HTML.  The full logentry can be found online at https://logbooks.jlab.org/entry/3838447
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