Introduction to the benchmark problem

Transcription

1 Introduction to the benchmark problem M. Krasilnikov (DESY) Working group 4: Low emittance electron guns 37th ICFA Beam Dynamics Workshop Future Light Sources May 6 DESY, Hamburg, Germany

2 Outline Introduction, Photo Injector Test facility at Zeuthen PITZ1 benchmark: Motivation Measurements Phase scan for bunch charge Reference phase check beam size Longitudinal momentum measurements Emittance measurements using slit-scan technique Cathode laser measurements Stability issues Possible sources of errors and discrepancies Conclusions

3 Generic photo injector design Solenoid magnets to counteract space charge induced emittance growth RF gun Laser Longitudinal ps and transverse flat-top laser pulse ~5 MeV Longitudinal phase space Δp z Booster cavity with proper position and gradient Superconducting TESLA module Δz Flatten longitudinal phase space distribution with 3 rd harmonic (3.9 GHz) cavity 3 rd harmonic cavity bunch compressor ~13 MeV Longitudinal phase space Δp z Δz Proper bunch compression diagnostic section Longitudinal phase space Δp z Δz

4 Optimization of the XFEL photo injector ASTRA simulations mm mm mrad Bunch slice z=15m charge density (flat-top, cases 1), a.u slice emittance (flat-top, Ek=.55eV, case 1) 1.4 Erms (flat-top, Ek=.55eV, case 1) slice projected -.6 ε () ε z-<z>, mm mm mrad Xemit (flat-top, Ek=.55eV) case 1 Xrms (flat-top, Ek=.55eV) case z, m Erms, kev 4

5 Photo injector layout and PITZ setup RF gun Laser Superconducting TESLA module (ACC1) 1 MV/m > MV/m 4-5 MeV 3 rd harmonic cavity bunch compressor MeV diagnostic section PITZ (+booster cavity) Photo-Injector Test Facility at Zeuthen (PITZ) 5

6 Photo Injector Test Facility at Zeuthen (PITZ) test facility for FELs: FLASH, XFEL small transverse emittance (1 mm 1 nc) stable production of short bunches with small energy spread further studies: dark current, QE, BBA, thermal emittance, + detailed comparison with simulations extensive R&D on photo injectors in parallel to FLASH operation test and optimize rf guns for subsequent operation at the FLASH and XFEL test new developments (laser, cathodes, beam diagnostics) 6

7 Diagnostics at PITZ1 z=.76m z=.935m z=1.618m z=3.448m z= l=.679m z=.778m z=.68m component property diagnostics cathode laser transverse profile virtual cathode, CCD longitudinal streak-camera charge FC, ICTs beam size screens 1,,3,4, CCD emittance EMSY (slit masks), screens,3, CCD electron beam momentum dipole, screen 5, CCD longitudinal profile radiators(straight)+streak camera longitudinal phase space radiators(straight+dispersive arm)+streak camera 7

8 PITZ1 benchmark problem Simultaneous simulations of a set of the consistent beam measurements (night shift ): Stable machine run (phase drift within <1 deg, cathode laser profiles, measured emittance) Several measurements (laser+electron beam) have been done: Phase scan ~1nC, ICT1 Phase check (several beam size measurements at screen_pp vs. SP Phase) Momentum measurements for SPPhase=Phi+1 deg, SPPhase=Phi+3 deg (x), SPPhase=Phi+5 deg Emittance measurements for (SPPhase=Phi+1 deg) (Imain=318.5A), (SPPhase=Phi+3 deg, SPPhase=Phi+5 deg) (Imain=3;34;36:38;33;33A) Some stability studies (charge,position) 8

9 PITZ1 benchmark problem Beam measurements to be simulates Bunch charge vs. rf phase Longitudinal momentum Beam size vs. rf phase Beam size vs. solenoid Beam emittance vs. main solenoid current Measurements Auxiliary measurements to be used for input Cathode laser transverse intensity distribution Cathode laser temporal distribution Bunch charge stability Cathode laser position stability 9

10 PITZ1 benchmark problem: Phase scan Bunch charge vs. rf gun launch phase measured with ICT SPV=33 ( <Pz>max=5.MeV/c) Imain=3A Ibuck=4A ICT(z=.935m) 14 space charge Schottky 6 4 SP Phase, deg rf phase = PhaseOffset - SP Phase, PhaseOffset to be found charge, nc?aperture??se? 1

11 PITZ1 benchmark problem: Reference rf phase check Reference phase ~ rf gun launch phase of maximum momentum gain Xrms (screen 3) Yrms (screen 3) SP Phase, deg SPV=33 (5.MeV/c) Imain=3A Ibuck=4A Screen 3 (z=.68m) rms size, mm Phi, deg : : R = X + Y rms rms Reference RF Phase Drift : : : :48 rms : :1 11

12 Reference rf phase simulations mm Measurements+Simulations Rrms,measured Rrms,simulated <Pz>,simulated rf phase, deg ΔΦ <Pz>, MeV/c ΔΦ vs. cath. laser parameters practical parameter region 1

13 PITZ1 benchmark problem: Longitudinal momentum measurements Pmean, MeV/c Pmean, MeV/c Prms, kev/c Phi-SPPhase, deg Prms, kev/c SPPhase=Phi+3deg (1) SPPhase=Phi+3deg () SPPhase=Phi+1deg SPPhase=Phi+5deg SPV=33 (5.MeV/c) Imain=8A Ibuck=1A Screen 5 (disp.arm) Pz, MeV/c intensity, a.u. 13

14 PITZ1 benchmark problem: Emittance measurements using slit scan technique ε ε n x n y = βγ X = βγ Y rms rms X Y ' rms ' rms X rms - whole beam rms size at screen (z = 1.618m) 3 ( X ) w ( Y ) wn rms n n rms n ' 1 n= 1 ' 1 n= 1 X rms =, Yrms =, L = 1. 1m 3 3 L L w w n= 1 n n= 1 Beamlet # Slit position-<x> Xrms Xrms 3 n 14

15 PITZ1 benchmark problem: Emittance measurements. Beam size at screens (EMSY) and 3 mm Xrms (EMSY - screen ) Yrms (EMSY - screen ) Imain, A mm SPV=33 (5.MeV/c) SPPhase=Phi+5deg Ibuck=.74847*Imain Screens: (EMSY), Imain, A Xrms (screen 3) Yrms (screen 3) 15

16 PITZ1 benchmark problem: Emittance measurements for SP Phase=Phi+5deg, Imain=36A ε n x = βγ X X βγ rms ' n ' rms; ε y = Yrms Yrms ε n GAvg = ε ε n x n y Screen (EMSY) 3.5 Screen 3 mm mrad EmX EmY EmGAvr Imain, A X-beamlets: X= -.7 Xrms; ; +.7 Xrms Y-beamlets: Y= -.7 Yrms; ; +.7 Yrms

17 PITZ1 benchmark problem: Cathode laser. Transverse 1.5 mm diaphragm: Xrms=.54mm Yrms=.57mm Virtual cathode (CCD camera) 17

18 PITZ1 benchmark problem: Cathode laser. Temporal Streak-camera in UV 18

19 PITZ1 benchmark: Stability charge, nc :: ::53 :5:46 :8:38 :11:31 :14:4 laser mm StDev~4% Charge time/ hh:mm;ss Q(p-p) 1 3 Time, sec Q(area) Laser position stability at Virtual Cathode (19.8.4) <x> <y> StDev Xrms SPV=33 (5.MeV/c) Imain=3A Ibuck=4A SPPhase=Phi ICT1 <X>=(1.517±.8) mm <Y>=(.861±.3) mm Xrms=(.56±.5) mm Yrms=(.58±.4) mm ( X ) StDev ( Y ) ~ Yrms ~ 5% 5 frames* ~% 19

20 PITZ1 Benchmark Problem: EM Fields Bz, T cathode plane Main solenoid (Imain=3A) Bucking solenoid (Ibuck=4A) Ez (balanced) Bz_peak measured, T Imain, A Ez, MV/m Main Solenoid Calibration Bz_peak(mT) =.5871*Imain(A) Field balance in the rf gun cavity Solenoid calibration MF compensation Ibuck=.74847*Imain -. z, m -5

21 Possible sources of errors and discrepancies RF Phase drift and jitter Reference phase determination Cathode laser power and position jitter Field balance (FB=Ecath/Efullcell) Solenoid calibration Cathode laser measurements Beam size measurements using YAG screens Influence of the beam line components (i.e.vacuum mirror), misalignment of the components (i.e. DDC) 1

22 Conclusions A set of consistent beam measurements has been proposed as a benchmark for the theoretical understanding (PITZ1 benchmark problem): implies simultaneous simulation of measured charge, momentum, beam size at different positions, emittance (beamlet size) effects during emission (Schottky, space charge, etc) seem to be of importance for accurate simulations of beam dynamics in photo injector various measurements of projected values simulated simultaneously should provide more reliability for simulated slice parameters of the electron bunch