RF-Gun beam based alignment at PITZ/FLASH

1. RF-Gun beam based alignment at PITZ/FLASH M.Krasilnikov, DESY Zeuthen LCLS Injector Commissioning Workshop (ICW) October 9-11, 2006

2. Outline • Beam-Based Alignment (BBA) of RF gun: • Fields and geometry • BBA motivation • Cathode laser BBA: • PITZ • FLASH • Limitations of laser BBA • Main solenoid BBA: • Solenoid micromover system • Main problems and possible solutions • Limitations of solenoid BBA • Conclusions M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

3. RF-Gun: fields and geometry (PITZ and FLASH) • sol.mech.axis ≠mag.axis • sol.mech.axis ≠cavity el.axis • solenoid tilt angles • fields overlapping M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

4. RF-Gun Alignment Motivation ASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV) Emittance growth (@z=15m) due to RF -gun misalignment Solenoid tilt effect For Ecath=25MV/m solenoid tilt angle of ~3mrad is equivalent to 1 mm transverse offset 5% emittance growth (lower limit estimations): laser DR<800um or solenoid DR<500um or solenoid Dangle<1.5mrad (0.086deg) No effect in the matching section has been considered! M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

5. Cathode Laser Alignment (FLASH) Laser spot positioning at the photo cathode: + Transverse displacement and angle of the laser beam can be independently changed • - • Iris position has to be adjusted for every mirror movement • No VC and e-beam simultaneously M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

6. Cathode Laser Alignment (PITZ) M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

7. BBA of Laser on Cathode z=0.276m z=0 z=0.935m z=0.778m Basic measurement: Mean position of electron beam at LOW.Screen1 (DoubleDiagCross at z=0.778m) vs. RF gun launch phase (SP Phase) • Conditions: • Main and bucking solenoids off • All steerers off • Dipole (even it is ~0.2m after the screen) degaussed and off • Bunch charge ~10pC, pulse train 10-50 laser pulses • Moderate RF power in the gun: 0.8-1.5MW (exclude dark current)→Pz~2.6-3.3MeV/c • Preliminay (rough) laser alignment: • Using scintillating cathode • Centering in dark current images M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

8. BBA of Laser on Cathode Simulations Measurements&Simulations Beam charge, Transverse rms Size, Mean Momentum Vs. RF Phase Measuredand Simulated beam charge at z=0.78m Vs. RF Phase Beam spot at Diag.Cross screen (z=0.78m) “Normal” phases “Low” phases, rf focused beam MeasuredandSimulatedbeam rms size at z=0.78m Vs. RF Phase Beam Offset at z=0.78m Vs. RF Phase (0.5 mm vertical laser offset on the cathode has been assumed) M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

9. Cathode Laser BBA. Test Movement Measurement: Beam offset vs. rf phase Step 1: Laser test movement Measurement: Beam offset vs. rf phase Step 2: Laser alignment One of results:(X0,Y0) – preliminary coordinates of the center at the screen M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

10. Cathode Laser BBA. Difficulties • Laser intensity non-homogeneity • Laser position jitter • Possible damage of YAG screen homogeneity • Earth magnetic field (x,y,z)~(0.02mT,-0.03mT,-0.009mT) stdev<X>=13um stdev<Y>=11um Extreme example of a damaged screen M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

11. Main Solenoid BBA. Micromover system main solenoid beam axis M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

12. Main Solenoid BBA z=0.276m z=0 Basic measurement: Mean position of electron beam at LOW.Screen1 (DoubleDiagCross at z=0.778m) vs. Main solenoid current z=0.935m z=0.778m • Conditions: • Bucking solenoid off • All steerers off (or consider in simulations) • Dipole (even it is ~0.2m after the screen) degaussed and off • Bunch charge ~10pC, pulse train 10-50 laser pulses – to be tuned for Imain • Moderate RF power in the gun: 0.8-1.5MW (exclude dark current )→Pz~2.6-3.3MeV/c • RF launch phase* • Laser spot size possibly small M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

13. Main Solenoid BBA From the laser BBA:X0=13.0mm Y0=14.8mm Beam position (<X>,<Y>) at LOW.Scr1(z=0.778m) as a function Imain • E-beam displacement with a main solenoid sweep. Possible reasons: • Solenoid transverse offset (Xsol,Ysol) • Solenoid tilt angles (AngleX, AngleY) • Laser (small) offset (Xlas,Ylas) from the center Imain=0A • Other factors to be considered: • RF gun launch phase and gradient • Small offset (from X0,Y0 obtained after laser BBA) • Solenoid calibration Imain=320A M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

14. Main Solenoid BBA. RF-gun launch phase and gradient streak-camera measurement F0=-18deg (21.4MV/m;22deg) M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

15. Simulation Tool Alignment Utility of the V-code - fast tracking code based on the method of moments of particle distribution function M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

16. Main Solenoid BBA. Test movement Simultaneous simulations of beam position before and after test movement (DXsol=0.4mm) • Advantages: • Same RF gun launch phase and gradient • Same offset X0*,Y0* • Same solenoid calibration M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

17. Main Solenoid BBA M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

18. Main Solenoid BBA. Difficulties • Laser BBA difficulties: • Laser intensity non-homogeneity • Laser position jitter • Damaged YAG screen • + • RF phase and gradient jitter • Some uncertainty in the solenoid micromover (especially angles), z-position M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

19. RF-Gun BBA. Conclusions • BBA of Laser on Cathode: • routine procedure at PITZ (FLASH) • based on a measurement of beam position vs. gun launchphase • test movement of the laser beam allows to determine a displacement vector for the laser beam centering • Main solenoid BBA: • multi-parameter task • based on beam position simulation vs. main solenoid current • test movements of the main solenoid and/or cathode laser allows to reduce uncertainty in main solenoid misalignment • Possible improvements: • implement earth magnetic field in BBA procedures • use BPM (LOW.BPM1) for solenoid BBA • more details on RF field profile • solenoid relative displacement online measurement M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

20. M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

21. Reference RF Phase. Beam Size measurements Transverse Beam Size at Screen 3 as a Function of RF Launch Phase for Various Main Solenoid Currents Fmax M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

22. PITZ1 Benchmark Problem: Fields • Field balance in the rf gun cavity • Solenoid calibration • MF compensation • Ibuck=0.074847*Imain M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

23. BBA: Step 0 – Rough Laser Alignment on the Cathode Rough laser alignment using dark current symmetry. TTF Dark current at TTF at screen Gun3 (z = 1.27m) Without beam With beam • Dark current rings originate from the edge of the Cs2Te coating and plug spring region • A laser spot being aligned on the cathode center results in an electron beam centered with dark current rings M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

24. BBA: Step 0 – Rough Laser Alignment on the Cathode Rough laser alignment using dark current symmetry. PITZ Without beam With beam Screen Diag. Cross (z = 0.87m) Screen_PP (z = 2.62m) M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

25. @Diag.Cross, “low phase” y y Pz x x x @Diag.Cross @PP Screen @Dispersive Arm RF Gun Alignment: Satellites observation Vacuum mirror

26. BBA of laser on cathode SP phase = -20 precision of mirror adjustment:better than 20 µm SP phase = -50 SP phase = -130 SP phase = -70 SP phase = -100 SP phase = -125 SP phase = -90 SP phase = -110 SP phase = -106 SP phase = -114 SP phase = -120 SP phase = -16 SP phase = -112 SP phase = -116 SP phase = -30 M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”

27. RF-Gun Alignment Motivation ASTRA Simulations of FLASH (VUV-FEL) Injector (150MeV) trajectory Emittance degradation in the matching section is not included emittance M.Krasilnikov, DESY “RF-Gun BBA at PITZ/FLASH”