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Experience with Bunch Shape Monitors at SNS A. Aleksandrov Spallation Neutron Source, Oak Ridge, USA.

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Presentation on theme: "Experience with Bunch Shape Monitors at SNS A. Aleksandrov Spallation Neutron Source, Oak Ridge, USA."— Presentation transcript:

1 Experience with Bunch Shape Monitors at SNS A. Aleksandrov Spallation Neutron Source, Oak Ridge, USA

2 2Managed by UT-Battelle for the U.S. Department of Energy SNS Accelerator Complex Front-End: Produce a 1-msec long, chopped, H- beam 40mA 60Hz 1 GeV LINAC 2.5 MeV LINAC Front-End RTBT HEBT InjectionExtraction RF Collimators 945 ns 1 ms macropulse Current mini-pulse Chopper system makes gaps Liquid Hg Target 1000 MeV 1 ms macropulse 945 ns #1,2,3,4 Ø30mm aperture #5,6,7 Ø120mm aperture

3 3Managed by UT-Battelle for the U.S. Department of Energy SRF, ß=0.81 (48 cavities, all powered) SRF, ß=0.61 (33) (81 total powered) from CCL 186 MeV 379 MeV RFQ (1) DTL (6) CCL (4) 2.5 MeV 86.8 MeV HEBT 805 MHz, 0.55 MW klystron 805 MHz, 5 MW klystron MHz, 2.5 MW klystron Layout of Linac RF with NC and SRF Modules 12345

4 4Managed by UT-Battelle for the U.S. Department of Energy Beam Shape Monitors at SNS CCL BSM HEBT BSM

5 5Managed by UT-Battelle for the U.S. Department of Energy BSM EPICS GUI Courtesy of R. Dickson Fully independent parallel scans Extensive set of troubleshooting and tuning tools

6 6Managed by UT-Battelle for the U.S. Department of Energy BSM use at SNS Drift Tube Linac tank1 commissioning – Confirmed design beam parameters Warm Linac commissioning – Confirmed beam parameters, feedback tuning, linac tuning verification Drift Tube Linac tank 5 troubleshooting – Discovered abnormally large bunch length in CCL Superconducting Linac beam loss investigation – Suspected longitudinal halo problem – Needed large dynamic range for bunch tails measurement Superconducting Linac beam loss mitigation (ongoing) – Need high accuracy of beam dynamics simulation – Need high accuracy and resolution of bunch core measurement HEBT laser stripping experiment preparation (ongoing) commissioning operation R&D

7 7Managed by UT-Battelle for the U.S. Department of Energy Accelerator troubleshooting examples (I) Cavity field droop Cavity phase droop No cavity field droop No cavity phase droop Phase variation along the pulse due to poor RF feedback operation Phase [deg of 805MHz] Time [us]

8 8Managed by UT-Battelle for the U.S. Department of Energy Accelerator troubleshooting example ( II ) Improper setting of the MEBT buncher phase caused longitudinal tail resulting in losses in the HEBT.

9 9Managed by UT-Battelle for the U.S. Department of Energy Accelerator troubleshooting examples (III) DTL5 problem BSM ° BSM ° BSM °

10 10Managed by UT-Battelle for the U.S. Department of Energy Validation of beam simulation CCL1 phase [deg] Bunch RMS size [deg] CCL1 phase [deg] Bunch RMS size [deg]

11 11Managed by UT-Battelle for the U.S. Department of Energy After transition from commissioning to high power operation beam loss is of major importance High beam loss in SCL was unexpected Longitudinal halo was first suspect – Required to measure bunch profile with high dynamic range Intra-beam stripping turned out to be the main source of beam loss – Beam loss is proportional to square of core density – Precise tuning of 3-d beam envelope is required to minimize beam loss

12 12Managed by UT-Battelle for the U.S. Department of Energy Large Dynamic Range Measurements

13 13Managed by UT-Battelle for the U.S. Department of Energy High measurement resolution and accuracy is required to predict beam envelope Measured longitudinal bunch size vs. model Distance [m] RMS phase width [ 1º.5º

14 14Managed by UT-Battelle for the U.S. Department of Energy Modifications to BSM hardware for improving resolution and accuracy Reduced slit size (CCL410 & CCL111) Added external RF deflector tuners ( test BSM ) Added computer controlled corrector magnet polarity switch (all) Added RF phase shifter calibration output (all)

15 15Managed by UT-Battelle for the U.S. Department of Energy Measured BSM performance BSM 107BSM 109BSM 111BSM 410 σ V [ V ]2.28 old slit, ~1mm 1.94 old slit, ~1mm 1.43 new slit, ~0.3mm 1.51 new slit,~0.3mm dφ/dV [º/V] Cable [ft]???? σ φ [ º ] Can expect σ V of ~ 1.5V Can expect dφ/dV of ~.4 º/V σ φ [ º] of ~.6º should be achievable We assume this correction for BSM impulse response :

16 16Managed by UT-Battelle for the U.S. Department of Energy PARMILA simulation vs. measurements CCL4 phase RMS length CCL4 phase

17 17Managed by UT-Battelle for the U.S. Department of Energy Record short bunches measured by BSM410 RMS σ = 1.4° RMS σ = 2.0° 3.5 mA 35 mA

18 18Managed by UT-Battelle for the U.S. Department of Energy Longitudinal emittance comparison Parameter list vs. PARMILA vs. measurements Parameter ListPARMILA no errors, 35mA BSM 410 measurement, 35 mA BSM 410 measurement, 3.5 mA RFQ exit CCL 4

19 19Managed by UT-Battelle for the U.S. Department of Energy Space charge effect study 4 mA17 mA35 mA Target bias [kV] Gauss fit σ RMS σ

20 20Managed by UT-Battelle for the U.S. Department of Energy Space charge effect study Target bias [kV] RMS bunch length

21 21Managed by UT-Battelle for the U.S. Department of Energy Future Goals and Plans Experimental study of longitudinal emittance evolution in SNS linac – We trust longitudinal emittance measurements at CCL exit (BSM410) – Need to measure with confidence emittance at CCL entrance Repair BSM107, modify BSMs 107,109 – Measure longitudinal emittance in SNS MEBT Laser based diagnostic, not BSM BSM development – Laser based BSM concept development (in collaboration with INR, A. Feschenko, Gavrilov) Not high priority – Short pulse 266nm laser for off-line BSM testing In progress – 2.5MeV Beam Test Stand Facility In progress – New high-resolution BSM design Interested, but do not have resources currently

22 22Managed by UT-Battelle for the U.S. Department of Energy What we would like from new design Better magnetic shielding Wire on the same actuator as slit Higher resolution Different wire material ? Higher voltage on the wire?

23 23Managed by UT-Battelle for the U.S. Department of Energy Thanks for your attention

24 24Managed by UT-Battelle for the U.S. Department of Energy Supporting slides

25 25Managed by UT-Battelle for the U.S. Department of Energy BSM resolution a max -180º 180º 0º, 90º slit width beamlet size deflecting voltage To improve resolution: – Decrease slit width – Decrease beamlet size – Increase RF voltage

26 26Managed by UT-Battelle for the U.S. Department of Energy Measuring the beamlet size Deflecting RF is OFF Scan the beamlet across the slit using DC voltage Beamlet width in volts is proportional to its width in mm σ = 1.43 V

27 27Managed by UT-Battelle for the U.S. Department of Energy Deflecting RF is ON Scan the buch profile -> Change the DC corrector voltage by dV -> Scan the buch profile Resulting phase shift is dφ Measuring the (deflector strength) dV = 10 V dφ = ~ 4deg

28 28Managed by UT-Battelle for the U.S. Department of Energy Bunch shape for different CCL4 RF phases BSM410 measurementPARMILA simulation

29 29Managed by UT-Battelle for the U.S. Department of Energy Longitudinal phase space for different CCL4 RF phases (PARMILA)


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