Bunch Length Measurements and Status of the Slice
Bunch Length Measurements and Status of the Slice Diagnostics System for ELBE / SCRF Gun J. RUDOLPH, HELMHOLTZ-ZENTRUM BERLIN EUCARD SRF ANNUAL REVIEW 2010 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun
§ OVERVIEW: ELBE AND SRF INJECTOR § BUNCH LENGTH MEASUREMENTS: BASICS, SETUP, RESULTS § SLICE DIAGNOSTIC SYSTEM: BASICS OF ZEROPHASING, SETUP, SIMULATIONS AND SIMULATION RESULTS § PRESENT STATUS Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 2
Overview: ELBE and SRF Injector Why SRF-Gun? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 3
SRF Gun operation parameter § § § DESIGN FOR THE SRF-GUN @ ELBE CW + HIGH REP RATE - 500 KHZ, 13 MHZ MEDIUM AVERAGE CURRENT 1 – 2 MA (< 10 MA) § HIGH ENERGY (STAND ALONE) - 3½ CELLS § LOW - HIGH BUNCH CHARGE 80 PC – 1 NC § LOW TRANSVERSE EMITTANCE 1 – 3 MM MRAD parameter present cavity measured ´ 08 final electron energy ELBE new “high gradient cavity” high charge ELBE high charge 2. 1 Me. V 3 Me. V ≤ 9. 5 Me. V peak field 13. 5 MV/m 18 MV/m 50 MV/m laser rep. rate 1 – 125 k. Hz 13 MHz 2 – 250 k. Hz 13 MHz ≤ 500 k. Hz 15 ps 4 ps 15 ps laser spot size 2. 7 mm 5. 2 mm 5 mm bunch charge ≤ 200 p. C 77 p. C 400 p. C 77 p. C 1 n. C max. aver. current 1 µA 1 m. A 100 µA 1 m. A 0. 5 m. A peak current 13 A 20 A 26 A 20 A 67 A 3± 1 mm mrad @ 80 p. C 2 mm mrad 7. 5 mm mrad 1 mm mrad 2. 5 mm mrad laser pulse length (FWHM) transverse norm. emittance (rms) Energy? Bunch length? Emittance? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 4
Bunch length measurements for SRF Gun § § § CHERENKOV-EFFECT USED TO CONVERT LONGITUDINAL ELECTRON DISTRIBUTION TO LIGHT DISTRIBUTION OF IDENTICAL TEMPORAL DISTRIBUTION LIGHT PULSE IS TRANSPORTED TO STREAKCAMERA BY A LOW-DISPERSIVE FULLY REFLECTIVE BEAMLINE LONGITUDINAL DISTRIBUTION OF LIGHT PULSE IS MEASURED Electron bunch Aerogel Čerenkov radiation Beam transport line Streak camera Analysis Light transport to streak camera? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 5
Setup of reflective Cherenkov-beamline OAP 2500 1 m Source 3 m Board in accelerator tunnel OAP 2500 Radiation protection wall OAP 175 5 m OAP 2500 1 m 2 m 3 m OAP 175 Streak camera Board in streak camera hutch Phase range for bunch length measurement? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 6
Faraday-Cup phase scan for bunch length estimation MEASURED PULSE LENGTH OF PHOTOCATHODE LASER: 6. 6 +/- 0. 5 PS RMS F-CUP PHASE SCAN DATA: – Extraction bunch length: 7. 3 +/- 1. 1 ps rms – Zero crossing at 33 deg. L BUNCH LENGTH MEASUREMENTS FOR PHASE RANGE 30 TO 75 DEGL Machine settings for measurement? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 7
Measurement conditions and machine settings BUNCH LENGTH AS FUNCTION OF INJECTION PHASE MEASUREMENTS 30 TO 75 DEGL (ZERO CROSSING AT 33 DEGL) LASER PULSE LENGTH: 6. 6 PS RMS ( 15. 5 PS FWHM) LASER REP RATE: 50 KHZ BUNCH CHARGE: ~ 8 PC BEAM CURRENT: 0. 45 ΜA (DARK CURRENT 0. 05 ΜA ) AVERAGE FIELD ON AXIS: 7. 2 MV/M ENERGY OF ELECTRONS: 3 MEV 4 MEASUREMENT SERIES, EQUAL CONDITIONS: – 3 x lens to focus light on streak camera (phase scan 1 to 3) results? – 1 x OAP to focus light on Measurement streak camera (phase scan 4) Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 8
Measured bunch length as a function of injection phase and ASTRA simulation SAME DEPENDENCY OF PHASE FOR ALL PHASE SCANS BUNCH LENGTH IS BETWEEN 12 AND 32 PS RMS ASTRA SIMULATION SHOWS MUCH SHORTER PULSES THAN MEASUREMENTS FACTOR 5 ENERGY SPREAD MEASUREMENT INDICATES LONGER BUNCHES: ~ 30 PS RMS Fcup current (DETAILS BY J. TEICHERT) Cherenkov ASTRA Conclusion? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 9
Summary of bunch length measurements BUNCH LENGTH DIFFERENCE OF FACTOR 5 BETWEEN MEASUREMENT AND ASTRA ENERGY SPREAD MEASUREMENTS INDICATE LONGER BUNCHES POSSIBLE MEASUREMENT ERROR SOURCES: – Streak-camera adjustment – Misalignment of Cherenkov beamline – Dispersion in Cherenkov beamline (max. 3 ps) POSSIBLE SOURCES FOR BUNCH LENGTHENING: – Machine setup – Photocathode laser pulse length for high power operation TO DO: – Check laser pulse length Slice emittance measurement? – Check Cherenkov beamline with photocathode laser Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 10
The basic principle of zero-phasing for slice diagnostics § § BUNCH PASSES CAVITY AT ZERO-CROSSING OF RF PHASE CORRELATION BETWEEN ENERGY AND LONGITUDINAL BUNCH POSITION CHIRPED BEAM SEND THROUGH SPECTROMETER LONGITUDINAL DISTRIBUTION TRANSFERRED TO TRANSVERSE DISTRIBUTION, POSITION IN X ON SCREEN CORRESPONDS TO POSITION IN T IN LONGITUDINAL DIRECTION LONGITUDINAL SLICES ACCESSIBLE BEAM SIZE IN Y DIRECTION FOR EACH LONGITUDINAL SLICE MEASURED COMBINATION WITH QUADRUPOLE SCAN TECHNIQUE ALLOWS TO RECONSTRUCT THE VERTICAL EMITTANCE FOR DIFFERENT SLICES Spectrometer? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 11
Properties and principle of a Browne-Buechner spectrometer UNIFORM FIELD MAGNET WITH CIRCULAR B O U N D A R Y green: low energy blue: medium energy red: high energy F 2 F 0 r 2 RO F M 2 M 0 R A D I U S r 1 M 1 y Q x R A N D R F 1 R R D Spectrometer position? E F ELBE / SCRF Gun Bunch Length Measurements and Slice Diagnostics System for 12
Possible alternatives for spectrometer installation positions … “ALTERNATIVE 1”: § “Alternative 2”: spectrometer in dispersive chicane section S P E C T spectrometer R O M E T E R I Bunch Length Measurements and Slice Diagnostics System. N for ELBE / SCRF Gun 13
Possible alternatives for spectrometer installation positions. . . continued “ALTERNATIVE 3 ” : S spectrometer P E C “ALTERNATIVE T 4 R ” O : M E S T P Bunch Length Measurements and Slice Diagnostics System. E for ELBE / SCRF Gun 14
Possible alternatives for spectrometer installation positions. . . continued “ALTERNATIVE 4 B”: spectrometer S P E C T R O M E T E “ALTERNATIVES 1”, “ 2”, “ 4 B”: SIMULATION RESULTS SHOWN R ON FOLLOWING SLIDES “ALTERNATIVE 3” + “ 4”: (NO DETAILED A SIMULATIONS) F – Trouble controlling dispersion and beta-functions T E Simulation of measurement? R Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 15
Elegant simulations REALISTIC BUNCH PROPERTIES AND EMITTANCE (1 MM MRAD NORMALIZED) QUADRUPOLE SCAN WITH 10 QUAD SETTINGS BEAM OPTICS AND PHASE SPACE PLOTS SHOW SIMULATION RESULTS FOR REPRESENTATIVE QUADRUPOLE SETTINGS: – “scan 1” (quad start value, black) – “scan 5” (red) and – “scan 10” (quad end value, blue) – Results of different scans somewhere in these limits M. Borland. elegant: A Flexible SDDS – Compliant Code for Accelerator Simulation. Advanced Photon Source, 2000. LS-287. Beam optics? Bunch Length. PLANE Measurements. OF and Slice Diagnostics System for ELBE / SCRF REMARK: FOCUS SPECTROMETER ISGun. PLANE 16
Comparison of alternative setups: dispersion and beta functions. . . § § “ALTERNATIVE 1”: DISPERSION RISES AT SPECTROMETER AND REACHES 0. 4 M @ SPECTROMETER SCREEN “ALTERNATIVE 2”: DISPERSION RISES AT CHICANE DIPOLE AND IS LOWERED BY SPECTROMETER (DEPENDS ON SLOPE OF DISPERSION) DISPERSION NONZERO @ SPECTROMETER SLIT scanning quad spectrometer “Alternative 1” scanning quad spectrometer “Alternative 2” scanning quad spectrometer scanning quad Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 17
Comparison of alternative setups: dispersion and beta functions. . . continued § “ALTERNATIVE 4 B”: DISPERSION CLOSED AFTER ARC FOR ZEROPHASING, RISING AGAIN AT SPECTROMETER TO VALUE OF 0. 4 M @ SPECTROMETER SCREEN spectrometer “Alternative 4 b” scanning quad spectrometer scanning quad Phase space? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 18
p-x correlation in spectrometer slit plane. . . § § “ALTERNATIVE 1”: NO P-X CORRELATION, ALL ENERGIES EQUALLY CUT BY SPECTROMETER SLIT 80 % OF PARTICLES LEFT “ALTERNATIVE 2”: LINEAR P-X CORRELATION CAUSED BY DISPERSION OF CHICANE DIPOLE, CORRELATION CUT BY SPECTROMETER SLIT, ENERGY CHIRP GETS LOST 4 % OF Spec slit closed Spec slit open PARTICLES LEFT “Alternative 1” “Alternative 2” Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 19
p-x correlation in spectrometer slit plane. . . continued § “ALTERNATIVE 4 B”: NONLINEARITIES IN P-X CORRELATION CAUSED BY 2 ND ORDER DISPERSION, CORRELATION CUT BY SPECTROMETER SLIT HEAD AND TAIL OF BUNCH GET LOST 50 % OF PARTICLES LEFT Spec slit open Spec slit closed “Alternative 4 b” Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 20
p-x correlation in spectrometer focus plane. . . § § “Alternative 1”: linear p-x correlation, spectrometer slit sharpens correlation “Alternative 2”: nonlinear p-x correlation, and loss of energy chirp by slit Spec slit open Spec slit closed “Alternative 1” “Alternative 2” t-x correlation similar to presented p-x correlation! Energy chirp needs to be adjusted to size of spectrometer screen in focal plane! Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 21
p-x correlation in spectrometer focus plane. . . continued § “Alternative 4 b”: nonlinear p-x correlation, correlation cut by slit linear again but head and tail of bunch missing Spec slit open Spec slit closed “Alternative 4 b” t-x correlation similar to presented p-x correlation! Energy chirp needs to be adjusted to size of spectrometer screen in focal plane! Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 22
Vertical phase space in focus plane. . . § § FOCUS SCANNED THROUGH SPECTROMETER FOCUS PLANE BY QUAD SCAN ALTERNATIVE 2: LARGE EDGE FOCUSING EFFECTS BECAUSE OF LARGE BEAM SIZE IN X AT SPECTROMETER Spec slit open Spec slit closed “Alternative 1” “Alternative 2” Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 23
Vertical phase space in focus plane. . . continued § ALTERNATIVE 4 B: ? Spec slit open Spec slit closed “Alternative 4 b” Conclusions of simulations? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 24
Conclusion of simulations SIMULATION PROVED: METHOD WORKING! “ALTERNATIVE 1”: – good solution for slice diagnostics – Advantages of spectrometer can be fully utilised “ALTERNATIVE 2”: – Main problem: spectrometer in dispersive section, energy-time correlation lost – Advantages of spectrometer cannot be used “ALTERNATIVE 4 B”: – 2 nd order dispersion effects nonlinear p-x correlation – No slice diagnostics of bunch head and tail Present status? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 25
Present status March 2010 … § User demands for immediate realisation of energy and energy spread measurements of high accuracy § “Alternative 2” is the only possibility that could be realised immediately Spectrometer installed as “alternative 2” for energy and energy spread measurements, though not appropriate for slice diagnostics § “Alternative 1”: ideal solution, but: beam pipe in spectrometer of small diameter (8 mm) § “Alternative 4 b”: nonlinearities, no ideal solution Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 26
Present status … continued Summary? Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 27
Summary and outlook BUNCH LENGTH MEASUREMENTS: – – Streak camera measurements: 12 ps. . . 32 ps rms (depending on phase) ASTRA simulations: 2 ps … 12 ps rms (depending on phase) Energy spread measurements indicate: ~ 30 ps rms Need further considerations SLICE-DIAGNOSTICS SYSTEM: – – – Different setups for spectrometer installation positions simulated Simulations proved: method appropriate Spectrometer installed at “alternative 2” Need spectrometer position in straight beamline Plan to reinstall at more appropriate “alternative ? ” OUTLOOK: – First slice measurements foreseen for April / May with “alternative 2” – Summer / autumn 2010 installation at “alternative ? ” – Slice measurement using all spectrometer imaging properties Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 28
THANK YOU FOR YOUR ATTENTION! THANKS TO THE ELBE CREW, THE ACCELERATOR PHYSICS GROUP AT HZB AND ESPECIALLY TO A. ARNOLD, M. ABO-BAKR, T. KAMPS, J. TEICHERT AND R. XIANG. Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 29
BACK-UP Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 30
Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 31
t-x correlation in focus plane … Spec slit open Spec slit closed Alternative 1 Alternative 2 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 32
t-x correlation in focus plane … continued Spec slit open Spec slit closed Alternative 4 b Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 33
t-x correlation in focus plane … Spec slit open Spec slit closed Alternative 1 Alternative 2 Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 34
Elegant Simulations –alternative 1 – quadrupole scan LEFT SLIT OPEN / RIGHT CLOSED SLIT Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 35
Elegant Simulations –alternative 2 – quadrupole scan LEFT SLIT OPEN / RIGHT CLOSED SLIT Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 36
Streak image data analysis for minimizing errors § rms time § § x § time § CONSIDERATION OF SINGLE SLICES OF IMAGE GAUSSIAN FIT OVER X-AXIS 10 SLICES WITHIN RMS WIDTH ANALYSIS OF SINGLE SLICES GAUSSIAN FIT FOR INTENSITY PROFILE OF EACH SLICE WEIGHTING OF RMS VALUES WITH CONTAINED INTENSITY GIVES BUNCH LENGTH UNWEIGHTED STANDARD DEVIATION AND MEAN VALUE OF ERRORS DETERMINED BY FITTING PROCEDURE AS OVERALL ERRORS x Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 37
Bunch length and ASTRA (normalized) MEASURED VALUES NORMALIZED TO MEASURED VALUE AT 0 DEGL (16 PS) ASTRA SIMULATED VALUES NORMALIZED TO SIMULATED VALUE FOR 0 DEGL (2. 2 PS) Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 38
ASTRA Simulation der Strahldynamik Bunchladung q = 8 p. C für 0. 4 μA, Emax = 16. 5 MV/m für E = 3 Me. V. Strahlparameter ohne Raumladung am Faraday Cup (bei ca. 1. 2 m) EXTRAKTION VON LADUNG ZWISCHEN -5 UND 90 DEGL RELATIV ZU ZERO CROSSING DER HF TRANSVERSALE UND LONGITUDINALE EMITTANZ, SPOTSIZE, DIVERGENZ STEIGEN AB 60 DEGL STEIL AN Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 39
ASTRA Simulation der Strahldynamik Bunchladung q = 8 p. C für 0. 4 μA, Emax = 16. 5 MV/m Strahlparameter mit Raumladung am Cherenkov. Monitor (bei ca. 4. 4 m) SIMULATIONSERGE BNIS ZUM VERGLEICH MIT MESSUNG Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 40
Diskussion möglicher Fehlerquellen DISPERSION IN ELEKTRONENSTRAHLBEAMLINE AUSRICHTUNG DER STREAK-KAMERA NICHT OPTIMIERT GENAUIGKEIT DER FÜR SIMULATIONEN ANGENOMMENEN MASCHINENPARAM ETER DISPERSION IN OPTISCHER BEAMLINE – Reflektive Optik, aber Elemente, die Dispersion haben: • Aerogel, Luft, Viewports in Cherenkov. Station (Fused Silica), Viewport zwischen Tunnel und Streak-Raum (BK 7) Bunch Length Measurements and Slice Diagnostics System for ELBE / SCRF Gun 41
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