LCLS Injector and Spectrometers for Emittance Compensation

1. The LCLS Injector C. Limborg-DepreyInjector-Linac & SpectrometersNov 20th, 2006 LCLS

2. Introduction Beamline Layout Emittance compensation Beam Characterization 6 MeV 135 MeV On-line Computation Tools Single-particle Multi-particle

3. eN = 1.2 mm P = P0 LCLS • SASE occurs along undulator from a very dense high energy electron bunch with small divergence and small energy spread • Driver • 1- “Injector” high brightness beam • 2- “Linac” compression, acceleration (+ preservation of emittance ) eN = 2.0mm P = P0/100 P  exp(z/Lg) with Lg  (/I)1/3  < 2.10-4 at 15 GeV ( ~ 1.5Å) n ~ 1.2 m , I ~ 3.2 kA courtesy S. Reiche

4. LCLS Injector • High Brightness beam driver • to generate high charge density in the 6D phase space • Technology RF Photoinjector + “Emittance compensation” Challenges Laser performance, stability ELM fields quality Optimization

5. focusing solenoid dual rf power feed cathode flange LCLS Injector Klystron Gallery Laser Room BC1 compressor DogLeg Gun 2 linacs

6. LCLS Injector Design Parameters RF Gun 6 MeV n~1.6 mm-mrad E, uncorr ~ 3 keV Design n,slice ~1.0 mm-mrad 100 A (1nC, 10 ps) 62 MeV n ~1.0 mm-mrad E, uncorr ~ 3keV gun spectrometer 135 MeV n ~1.0 mm-mrad E, uncorr ~ 40keV “Laser heater “ (2008) L1 RF section (21-1b) main SLAC Linac injector spectrometer sector 21 sector 20

7. LCLS Injector Diagnostics YAG screen RF Gun • trajectory (BPMs) • emittance (+ slice) • energy spread (+ slice) • bunch length (+ dist.) • charge (+ dark current) YAG screen YAG screen YAG screen (YAG screen) gun spectrometer Transverse RF deflector OTR & wire OTR & wire OTR & wire OTR & wire main SLAC Linac injector spectrometer YAG YAG & OTR

8. LCLS Min QE RF Photo-Injector 1- Laser system • Pulse (~ cylindrical shape + uniform) • Energy (@ 255nm) 2- Photocathode • Quantum Efficiency (QE) • Uniformity of emission 3- High gradient RF gun 0.5 nC requires only QE of 10-5 with laser energy of 250 J Courtesy S.Gilevich Courtesy E.Jongewaard Courtesy D.Dowell

9. Emittance x x’ = dx/dz z x  = 2.34 mm.mrad  = 1.16 mm.mrad 1- Slice/projected 2- At emission : cathode emittance  = 0.75 mm.mrad Distribution of transverse momentum Px,Py of photo-electrons extracted from cathode

10. X’ X’ X’ x x x Solenoid Focusing lens Emittance compensation Solenoid Linac Gun m X’ e m e Space charge force Smaller at end of bunch (e) than at middle (m) z e x Drift (while being accelerated) Drift + Space charge X’ X’ m x x m e Slices realigned at best Distribution frozen at high energy e

12. rf = 2  Emittance compensation Key parameters Gun (Vrf, rf) Solenoid field values Laser beam (volume, uniformity) Emission (QE uniformity) Alignment, Steering Solenoid0.3%

13. Commissioning baseline parameters • 2007 baseline parameters • Limits • Q ≤ 500pC • repetition rate ≤ 30Hz • Gun gradient ≤ 120 MV/m • Satisfactory results for 2007 would be •  < 1 mm-mrad for Q > 200 pC (much more forgiving on laser characteristics) • Optimization • For given charge, vary • Laser radius, rf, Vrf, Bsolenoid 1,Bsolenoid 2, VL0a • Steering (laser pointing, steering in L0a)

14. Courtesy J.Schmerge 6 MeV transverse measurements YAG02 YAG01 FC01 Solenoid • Emission characterization • QE Charge vs laser energy (for different rf) (total QE & QE vs x,y position laser on cathode) • Cathode Uniformity • Cathode emittance measurement ( also Px,Py distribution)

15. Imaging cathode Point-to-point imaging of cathode Virtual cathode Virtual cathode Direct determination Uniformity of emission Ellipticity Transv. rise/fall slopes e image : hot spot Tuning depends E gun Solenoid calibration (best with short laser pulse) electrons image Courtesy W.Graves Cathode Image at DUVFEL

16. Assumes cathode = 0.6 mm.mrad Cathode emittance Cathode emittance • Direct determination • cathode (with appropriate set of Vrf, rf, Bsolenoid) • Momentum distribution •  initial model + cathode quality Infinite-to-point imaging of cathode Image of divergence of source At YAG02 , with Vrf reduced

17. Momentum at cathode Imaging source divergence • Best parameters • Vrf, ~ 72 MV/m • rf ~ 20 degrees • Observation at YAG02

18. 6 MeV Longitudinal measurements Spectrometer 85  Bend YAG01 p(GeV) = 0.3 B(T)(m) • Energy • Absolute energy • Calibration Vrf vs Prf (MW) • Correlated Energy Spread • Optimal rf • Slice thermal emittance • Relay imaging system from YAG01 to YAGG1 • Uniformity of line density YAGG1

19. 6 MeV Longitudinal measurements Temporal pulse , … using quadrupoles to project manageable size on screen High Charge operation Head Horizontal Projection Linear Scaling of Energy atYAG01 8% modulation on laser pulse at YAG01 at YAGG1

20. 3 screen emittance (OTRs, WS) Horizontal Slice emittance (TCAV, Quad Scan) 135 MeV Transverse Measurements See P.Emma Presentation

21. DL1 135 MeV Spectrometer 135 MeV Spectrometer 35 degrees Bend 135 MeV Longitudinal measurements • Energy • Correlated Energy Spread • Uncorrelated Energy Spread • Bunch Length • Direct Longitudinal Phase Space • Slice vertical emittance

22. DL1 Longitudinal Phase Space at TCAV 135 MeV Spectrometer Spectrometer Screen 135 MeV Longitudinal measurements • Direct Longitudinal Phase Space measurement • Transverse deflecting cavity  y / time correlation • (with V = 1MV, 0.5mrad over 10ps ) • Spectrometer  x / energy correlation • Resolution requirements : 7 m for nominal optics • (OTRS1 has 11m resolution ok for modified optics)

23. resolution 6 keV for nominal optics • resolution 3 keV for modified optics • 3keV -> 40 keV will be measurable rms Direct Measurement Projection

24. Tools Risk: Solenoid Alignment • Single Particle Tracker in Matlab for on-line modeling • gun to L0a (including misalignment of components + earth magnetic field) • to be extended to DL1 (i.e linacs + quads) & spectrometers • Support tool for : • Steering (SC0 in Solenoid 1) • Alignment • Search for imaging parameters (cathode, divergence, dark current … ) • Energy calibration (Vrf, rf ) • All low charge measurements Solenoid SC0 SC1 SC2 BPM2 BPM3 BPM5 Gun L0a

25. Single Particle Tracker : steering in L0a • After steering • <0.1mm,<0.12mrad Risk: Solenoid Alignment • Using SC0, SC1 • Solenoid misaligned • Bearth • Without steering • 4mm, 4mrad Solenoid Solenoid SC0 SC1 SC2 BPM2 BPM3 BPM5 Gun L0a

26. Single Particle tracker Dark current studies for gun Solenoid Gun YAG01

27. Tools : Multi-particle tracking • Start-to-end simulations • Using Linux cluster (64 to 128 processors) • Methodology • Input Laser distribution • Use model • Track through injector • Compare simulations/data • Feed other codes ( ELEGANT, GENESIS/) for downstream transport • Correct model (from calibration with beam) • Code choices for Injector • PARMELA , IMPACT , ASTRA Useful in control room only if runs do not exceed 15 minutes from cathode to DL1 for 3D with 200k particles Example of simulation: emittance compensation with PARMELA

28. Parameter Scan : Beta matching • Scan parameters : (rf, Vrf , B solenoid) • Large Variation of betatron function while varying Bsolenoid • Rematching necessary for emittance measurements • 3 screen emittance : best resolution for perfect parabola (with 60 degrees phase advance between screens)

29. Tuned to matching of -0.6% case Matching performed for each point Tuned to matching of 1.8% case

30. Commissioning Strategy • 1st Pass : beam to 135 MeV • Start-up equipment to reach 135 MeV • Software checkout • 2nd Pass : first order optics • steering • matching • 3rd Pass : • Characterization: • Transverse emittance (slice & projected) • Longitudinal phase space (slice energy spread) • Optimization : • Fine tuning of gun • Scan of parameters ( scans of solenoid fields, phase, voltage …)

31. BACK-UP

32. LCLS Injector Magnets Gun Solenoid RF Gun L0a Solenoid QA01,QA02 BXG QE01,QE02 QG01,QG02 QE03,QE04 QM01,QM02 BX01 BX02 QB main SLAC Linac BXS QS01,QS02

33. X’ X’ X’ X’ X’ x x x x x Drift Focusing kick (ex Solenoid RF entrance cell) Emittance compensation • Phase space evolution Defocusing kick (ex: RF kick at exit gun , space charge ) • Solenoid kicks are energy dependent • Space charge kicks are density dependent Space charge (defocusing) in drift on a converging beam Space charge (defocusing) in drift on a diverging beam

34. 100MV/m, 2 Gaussians, 0.5 nC < {10,90} > ~ 0.7 mm-mrad Parameters improved by using a 0.6 mm radius beam 80 ~ 0.78 mm-mrad < {10,90} > ~ 0.6 mm-mrad

35. Risk: Solenoid Alignment • Tolerance : 250 m, 250rad w.r.t to gun electrical axis • Requires beam based alignment • Method : • 1) Determine center of cathode • 2) Determine error in position of solenoid with few steps of solenoid motion • F(X, rf, Vrf, Bsol, Xsol) = Xf • 4 unknowns Xsol = (x,x’,y,y’) • Center BPM/Screen cannot be determined with beam • Angle resolution from BPM2-BPM3 > 100 rad • Code • Single Particle tracking in Matlab for on-line modeling ( similar to V-code) • gun to L0a (including misalignment of components + earth magnetic field) • to be extended to DL1 (i.e linacs + quads) Solenoid SC0 SC1 SC2 BPM2 BPM3 BPM5 Gun L0a

36. Solenoid Risk: Solenoid Alignment • 1) Center of cathode • Steer laser centroid on 2D grid • Scan Gun RF phase YAG02 Gun Centroid on cathode Centroid on YAG01, rf [24,36]

37. Risk: Solenoid Alignment • 2) Mispositioning Solenoid • (Position, Angle ) does not vary with strength when the Solenoid aligned Vary Solenoid strength • Assume center cathode known to better than 50 m • Assume axis gun on screen is known within 50 m • Requires at least 8 motions of solenoid

38. Risk : Laser Pulse shape • Difficult to meet specifications • Rise time 1 ps • Uniformity 10% ptp • Pulse stacker • with 2-3 gaussians give satisfactory performances • Even better for compression (flatter at 135 MeV ) NEED to ADD standard pulse

39. Risk : Gun field • if 120MV/m difficult (breakdowns and large dark current) • 110 MV/m gives performances very similar to 120 MV/m • 100 MV/m is also gives acceptable performances (see next slide) • ( for 0.5 nC, 80 < 0.8 mm-mrad )

40. 100MV/m, 2 Gaussians, 0.5 nC < {10,90} > ~ 0.7 mm-mrad Parameters improved by using a 0.6 mm radius beam 80 ~ 0.78 mm-mrad < {10,90} > ~ 0.6 mm-mrad

41. Emittance compensation X’ X’ X’ X’ x x x x e m e Space charge force : Smaller at end of bunch (e) than at middle (m) z m X’ Drift + Self-defocusing X’ Focusing lens e x x m m e e X’ Drift + Self-defocusing Drift (while being accelerated) m x e Slices realigned at best Dis. frozen at high energy

42. Emittance compensation Gun Solenoid Linac Diverging: Space charge RF kick at exit cell • Converging: Solenoid • RF kick at entrance cell

43. Self-field from Relativistic Electron Beams Ez Er  r  ‘  1/r2 r B Er Er = 6MV/m for a 1nC in cylinder of (r=1 mm, L =3 mm)

44. Emittance compensation Gun Linac Solenoid

45. Emittance z  = 2.34 mm.mrad  = 1.16 mm.mrad  = 0.75 mm.mrad 1- Emittance 2- Emittance at emission

46. LCLS Injector RF Gun L0a RF section 6 MeV L0b RF section 62 MeV gun spectrometer Transverse RF deflector 135 MeV L1 RF section (21-1b) main SLAC Linac injector spectrometer sector 21 sector 20

47. 135 MeV 250 MeV 10 ps 4.5 GeV 2.3 ps 15 GeV 230 fs Photoinjector High charge and small emittance Compressors Small bunch length Preserving emittance n,slice ~ 1.2 mm.mrad Ipk = 3.4 kA(230fs, 1nC)  < 5.10-4 , 14.3 GeV

48. LCLS Injector 6 MeV  = 1.6 m ,un. = 3keV 63 MeV  = 1.08 m ,un. = 3keV 135 MeV  = 1.07 m ,un. = 3keV 135 MeV  = 1.07 m ,un. = 40keV Linac tunnel ‘Laser Heater’ DL1 Gun S1 S2 L0-2 24 MV/m L0-1 19.8MV/m Spectrometer 3 screen emittance measurement ‘RF Deflecting cavity’ TCAV1 Spectrometer UV Laser 200 J,  = 255 nm, 10ps, r = 1.2 mm

50. Gun Characterization QE, thermal, Uniformity Emission , Bunch Length YAG1 YAG2 CR1 YAGG1 CRG1