Everybody contributed but the following people spent a considerable amount of time on this experiment D Edstrom A Halavanau J Hyun P Koback D Mihalcea P Piot J ID: 784116
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Slide1
Outcomes, Failures, and Lessons learnt from the Channeling-experiment effort
Everybody contributed but the following people spent a considerable amount of time on this experiment: D. Edstrom, A. Halavanau, J. Hyun, P. Koback, D. Mihalcea, P. Piot, J. Ruan, W. Rush, T. Sen
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Slide2Overview
Achievements:Lattice studies, on-line matching, Several valuable control tools developedContributed to commissioning of injector pushing the limit (e.g. low charge)Issues + Lesson learn:Dark current and its mitigationsX-ray detector, Shift/personnel to highDiagnostics (e.g. emittance)On-going/Next Steps:Short termLong term
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Slide3Intro
Initial goals1 Observe channeling [failed]2 Demonstrate an incremental capability of combining channeling radiation with a superconducting linac [failed]History about this program Demonstrate high-brightness channeling by combining channeling-radiator with an ultra-low emittance electron source.Sponsored by DARPA AXiS program which was terminated in 2013.
Given the hardware we had acquired we thought we could easily pulled the experiment together with a few local enthusiastic + remote participants
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Slide4Setup cs
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Slide5Setup overview
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goniometer
f
orward X-ray
spectrometer
90-deg X-ray
spectrometer
@43 MeV
Slide6“On-line” matching Developed script to interface
elegant to the control system and be able to do on-line matching from measured CS parametersFast.py toolbox and RunElegantfromCRsettings.pyCustomized save-restore etc…ImageTool.pyAll of these tool could be useful
to others (especially external users who are not interested
in learning ACNET) and will
be documented and made
available (
Github
)
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measured
@X121
Simulated according to
measured initial conditions
Low beta
Nominal beta
1 mm
1 mm
Slide7Note on standard quad settings For most of the running period we had the flat-beam (planned to be skewed in the initial design) turned off
most of the time for BC on and offCould be skewed insupport of 4-D emittance measurementsFlat beam generation 7
Beginning
of run period
Sasha R.
soft optics
Current (A)
Time
Slide8Dark current issues and mitigation
Dark current was a significant issue and mitigation was non trivialLower Gun E-field decrea-ses dark current butLower energy Complication with CC1 stronger over focusingEventually higher emittanceHowever the 90 deg detector should have been “insensitive” to dark current (due to the low Compton cross-section, dark current BC should have been small wrt BS from higher charge photo-emitted bunches
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Slide9Dark current mitigation
No real direct diagnostics for dark current so used the X-ray detector (measuring BS for various conditions):Most of the dark current came from the gun, there seems to be an effect from CC1 in capturing dark current and its gradient was slightly decrease (while increasing the CC2 gradient)Gun collimator is a valuable tool but it was not evident the 12-mm aperture choice is the best one; nevertheless the collimator was inserted and the solenoid adjusted to scrape as much dark current as we could (with Egun set to 41 MV/m). Chicane was used as a final (dispersive) selection.9
Slide10Dark current mitigation in BC
Procedure:Decrease energy spread by looking at X115Increase current while monitoring photoemitted beam transmission and dark current induced X-ray count on detectorIterate until a good compromise is found In this processFindings:It was not possible to scrape only dark current without loosing in photoemitted-beam transmission dark current and main beam have similar energyThere was some drifts in time due to lack of current-regulation on chicane dipolesDark current is scraped on the pipe (but where!!) and this created a BS shower that overwhelmed the 90-deg detector
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Slide11Dark current mitigation
At gun exit dark current and main beam have same energy but separated by 45 deg!11
Dark-current induced BS dominated spectrum on 90-deg detector (this was realized
after
the running period)
Dark current beam-dynamics on-going
Beam energy (MeV)
Laser phase (
deg
)
Dark current
Main beam
Slide12Detector challenges
Amptek detector ended up not well understoodA major challenge was to ensure we operate them in the single-photon regimeConfusion + wrong initial charge choice 50 fC/bunch for the forward detector20 pC/bunch for the 90 deg detector
(this was inconsistent with the expected
reduction from Compton
Xsection
~10
-6
)
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Slide13Other issues
Diagnostics:Emittance measurement is suspicious and several techniques are inconsistent + quad scan too slow for optimizationWould have been good to spend more time (with more people) on understanding the reason for the (most likely) + benchmark with trusted method (e.g. multislit)The video server has reliability issues (however Charlie was very responsive and helpful, did not work at 2 Hz).Would be very helpful to agree on an on-line optics code that all of us could contribute and useELEGANT might be too complicated without an intuitive GUI as used in Sasha R. code.
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Slide14Personnel/scheduleO
nly three people from our team were actually available throughout the full running period: significant burn out, sometime lack of time to think and some thing could have been realized earlierWorried with switches to other experiment as it took very long time to get back to bunch charge conditionMissing a full-time student who own this experiment: the students involved so far were part time and there is a lack of continuity
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Slide15Moving forward Short term:
A lot of data to go through:Emittance data are all going to be re-analyze with various algorithm (during the on-line data we turned off some feature (Aleksei + others)Dark current beam dynamics (Philippe)Spectra are still being analyze – there is a debate on some spectra that appear to show some peaks during the 7/10-7/20 running periodThink thoroughly about what to do nextGoniometer relocation (or not), Xtal replacement, new in-vacuum scattererNew detectors [seeking advice from Arden, ANL/APS detector pool group and other collaborators (co-DNDO grantees)]
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Slide16Moving forward Short term:
much better shielding for the detectorsnot use the chicane for removing DC if added shielding is enoughcheck that spectra in off-axis detector is entirely from Compton scattererdo an immediate background subtraction of spectraAleksei will devote more time on this experiment (trying to find a way to tie up the topic with the main topic of his dissertation)Philippe to attend channeling 2016 and discuss with more experienced people
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Slide17Moving forward Longer term:
Setup is unique and we are trying to setup a larger collaboration with people interestedHZDR stopped any effort (=goniometer is probably ours)This means $$$$ and we are submitting a proposal in September “Compact Bright X-ray Sources beyond Bremsstrahlung” 17