[Halld] Update on the Spring 16 run

[Alexandre Deur]

Dear Collaborators,

Last Friday, March 4th, Hovanes, Paul Mattione and Ken Livingston had a first pass at aligning the first production 20 um diamond (JD70-118). A 1h parallel polarization run and a 1h perpendicular polarization run were taken Friday evening. Today, Sunday March 6th, they improved the alignment and 2h parallel and perpendicular polarization runs were taken (one run each). The runplan ( https://halldweb.jlab.org/wiki/index.php/Run_Coordination_Meetings:_Spring_2016_Run#Runplan_time_charts ) expects the production diamond alignments to be done during the first half of March, so we are nicely on schedule. With the thin diamond milestone being reached and since we are almost at the middle of this spring run, it is an appropriate time to summarize the progress made during this run and what remains to be done. (The following summary only mentions the names of the persons responsible for the tasks. Evidently the hard work of many more should be acknowledged, in particular people from the subsystems groups that are maintaining all the subsystems operational during the run, the offline analysis group, the shift workers and the technical support.)

*We closed the Hall on Feb. 4th. Then the accelerator started the beam restoration shortly before midnight on the 5th with the expected ailments associated with starting the beam after a long shut down. In addition to those, the north linac went down for a shift due to a cold box trip, the Solenoid power supply tripped several times tagger magnet once. On Feb. 9th, one magnet in arc 5 partly burned and needed to be replaced. This was done remarkably quickly given the incident and beam was back in the late morning of Feb. 13th. At that point we were about one week beyond schedule without much done except for several accelerator tasks important to us: 
  *beam restoration with adequate radiation levels,
  *establishing the beam position and angle for optimal photon flux transmission to the Hall (Alex Barnes),
  *important progress in commissioning the nA Beam Position Monitors necessary to run at low current (Trent Allison, Brian Bevins), and 
  *progress in commissioning the fast feedback necessary for running with the small 3.4 mm collimator and thin diamond radiator (Trent Allison, Brian Bevins). 
We could effectively start with our own program on Feb. 13th, 2pm with the usual tests on beam quality by checking the behavior of the radiation monitors in function of beam current and radiator thickness (A. Deur). 
So far, the beam availability has been good given that other Halls are running in parallel (Hall B during the week-ends and Hall A during the full week) and given that we are still in a commissioning period. As of tonight (Sunday March 6th), the beam availability has been about 40% (without counting the 1st week when beam was not available. It is 30% when counting this first week which was nearly beamless as far as physics quality beam is concerned). This 40% is to be compared with the 50% efficiency we will expect during physics running, so it is not bad. The runplan was made assuming 40% efficiency and several days for establishing the initial beam, which explains why we are well on schedule. 

*The nA Beam Position Monitors have been commissioned (Trent Allison, Brian Bevins). As a consequence, we were able to run at low beam current (a few nA) and start commissioning the Total Absorption Counter (TAC). 

*The TAC, a device used to calibrate the pair spectrometer (PS) in order to measure the absolute flux of photons in the Hall, is being commissioned and seems to work well (Hovanes, Alex Somov). Some more systematic studies remain to be done.

*The fast feedback (FFB) seems essentially commissioned (Trent Allison, Brian Bevins). The 60 Hz beam position jitter is clearly suppressed when the FFB is enabled (Hovanes). However, more work is necessary because the current beam optics is not well matched to the hardware configuration of the FFB system. When the FFB is off, the beam position and angle are maintained by slow position locks (Arne Freyberger). Those locks maintain the beam position on the last electron beam BPM (5C11b) and on the Active Collimator. The slow motion of the beam seen earlier in the run has been efficiently suppressed thanks to these slow locks. Note that the lock on 5C11b is currently not working, but it should be re-implemented today (Todd Satogata).

*The detectors are behaving well, similarly to the past runs, with the exception of the drift chambers (DC) that trip more often. This is due in part to the higher luminosity we are running at, but also to higher noise level in the DC (Lubomir). This is not yet understood. We are seeing worst radiation levels in the Tagger vault. They are similar to the levels of the 12 GeV Fall 15 run, but 5 times higher (or more depending on the monitor) than the 5.5 Spring 15 run (A. Deur). While it was first assumed that a lower quality beam tune compared to Spring 15 was responsible for the higher levels, the current hypothesis is that these higher levels are intrinsic to the 12 GeV beam with its larger emittance. In any case, the radiation levels in the collimator cave are similar to the Spring 15 levels. The CDC HV scans were performed by Beni Zihlmann and the TOF ones partly done by Alex Ostrovidov).

*An important milestone was reached with the DAQ (Sergey Furletov). It can now handle the data rate expected during the first part of the GlueX run (low luminosity run). The DAQ has been running at 12.5 kHz with typically 97% livetime, and 40 kHz with 85% livetime as of tonight (Sunday March 6th). 

*Trigger studies are done periodically and are being analyzed by Sacha Somov. In addition to the optimization of the production trigger and the trigger efficiency studies, a trigger for the TAC was implemented.

*The luminosity necessary for the first part of the GlueX experiment was achieved on the morning of Feb 28th with first 80nA and then 100 nA of beam current on the 50 um diamond radiator (4.1 10^-4 RL), with good DAQ performances. The drift chamber trips were minimized by increasing the HV trip currents.

*The polarized beam was quickly (re)established on the 50 um diamond on Feb. 17th (Hovanes, Paul Mationne). 
A large body of data in parallel and perpendicular polarization directions, as well as Al radiator background runs are being taken: As of Friday March 4th in the afternoon, we have accumulated 1.99 billion triggers for parallel polarization, 1.98 billion triggers for perpendicular polarization and 0.55 billion triggers for the unpolarized background runs (numbers are courtesy of Elton). This is close to the goal of taking 40% of parallel data, 40% of perp ones and 20% of unpolarized ones. In addition, about 4h of mode 8 data are taken every week. Analysis from Elton showed that most (maybe 90%) of these triggers are useful events. 
The polarized data are taken with the coherent peak edge nominally at 9 GeV. Analysis of the coherent edge spectrum indicates a 35% photon beam polarization (Justin Stevens). Data were also taken with the coherent peak edge at 8.5, 7 and 6 GeV for systematic studies of the beam polarization. 
Analysis of the coherent peak with the 20 um diamond (JD70-118) show a good polarization (about 30% for today's run, Sunday March 6th) but not as high as for the 50 um diamond (Justin Stevens). Analysis of the polarimeter data is consistent with this (Nathan Sparks). 
In addition to the polarized data, we also took a large amount of Al radiator data before Feb. 17th (i.e. before re-aligning the 50 um diamond) for calorimeter calibration purpose (Elton, Adesh). 

*Several reactions are being analyzed, in particular the rho production (Justin Stevens). The large modulation of produced rho vs the angle between the polarization and decay planes is seen with unprecedented precision. An early analysis from Justin on less than 1% of the data collected so far indicates a 34+/-0.02% photon beam polarization, consistent with the coherent edge fit. 

*The triplet Polarimeter is being commissioned, essentially in parallel with other activities (Nathan Sparks, Michael Dugger). As just mentioned, the polarimeter is already providing important diagnostics on the photon beam polarization with different diamonds. 

*The solenoid is doing well. Apart from an early trip (followed by two other trips, human-induced, and consequences of the first trip) on Sun. Feb 7th, there has been no problem for almost a month of continuous running at 1200A. It is also worth mentioning that other subsystems are operating well too, including the cryogenic target.

*We have taken several empty target runs in two periods. they prove useful for checking our vertex reconstruction (Simon Taylor) and other systematic studies (DAQ rate, noise in DC: Lubomir). Empty target reconstruction still shows a slightly shorter target (29.3 +/-0.0 cm) compared to expectation (29.87 +/-0.08), as seen in the Spring 15 run (other runs in Fall 14 and 15 did not use the cryogenic target).

*The Level-3 trigger commissioning has started (David Lawrence) with twice 4h of beam dedicated to it. More commissioning remains to be done.

*The alignment of the quadrupole in front of the tagger magnet was checked and found to be well aligned, with the electron beam position on the tagger dump changing by about 50 um when the quadrupole is turned on to full current (Mark Dalton). Its effect on the microscope remains to be analyzed.  

*Data to check the microscope alignment were taken (Richard Jones, Alex Barnes). More need to be taken.

*Beam transport optics has been taken to better understand the Hall D line. The data is being analyzed (Todd Satogata).

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What remains to be done:

*Implement a new optics (Todd Satogata) better adapted to the hardware configuration of the FFB system and then establish that the FFB is reliably working.

*Finalize the study of the first 20 um diamond (JD70-118) and study the two other diamonds (JD70-111 and JD70-119) (Hovanes, Paul Mationne, Ken Livinstone). 

*Complete the TOF HV scans (a few more hours are necessary) (Sacha Ostrovidov, Mark Ito).

*Continue Level-3 trigger commissioning, with another 4h of work planned in the coming week (David Lawrence). 

*Continue the trigger commissioning (Sacha Somov). Periodic few hours of study are planned throughout the Spring 16 run.

*Study the TAC with the small 3.4 mm collimator and various combinations of the PS and Triplet Polarimeter converters (Hovanes, Sacha Somov).

*Do several TAC runs throughout the Spring 16 run to study the consistency of the PS calibration (Sacha Somov, Hovanes).

*Production running on one of the 20 um diamonds.

*A few more empty target runs throughout the Spring 16 run.

*Implement the fast Active Collimator calibration procedure using beam rastering (Todd Satogata). This is becoming more important since it was realized that Active Collimator calibrations must be redone for different photon fluxes: We ran for several days before realizing (Alex Barnes, Richard Jones) that the photon beam position on the collimator was not optimal at larger photon flux because the Active Collimator calibration was off at this flux.

*Running with the 3.4 mm collimator.

*Finalize microscope alignment (3 periods of 1h tests. Richard Jones, Alex Barnes).

*Perform PS magnetic field scans (Sacha Somov).

*Check the effect of the quadrupole in front of the Tagger on the microscope (4 periods of 4h tests. Alex Barnes, Richard Jones, Dan Sober).  

*Understand the "satellite peaks" seen on the photon beam profiler (need to perform x-y scans of the electron beam on the radiator).

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So the mid-run assessment is that we are well on schedule with many of our most important commissioning tasks done or well underway, a large amount of polarized data taken with the 50 um test diamond, and the scare of a badly behaving solenoid reducing as weeks go by. We had no significant problems compared to previous runs. Notable glitches are the first solenoid trip and a short temperature rise in the Hall (Feb. 25th, 1am) due to an AC trip. We still have several commissioning tasks to complete, including checking the other thin diamonds and running on the 3.4 mm collimator. It remains to be seen whether the coming weeks will be as efficient as the past few ones, since Hall C will start its commissioning in 10 days from now, and Hall A will go to maximum energy (5 passes) at some point not yet announced. With Hall A at 5 passes, the beam separation may be less reliable than presently.

Best regards,
 
Alexandre Deur, Hall D Run Coordinator.
 
RC cell: 757-383-5542
Office Phone: 757-269-7526