The LCLS Injector C. Limborg-Deprey Injector-Linac & Spectrometers Nov 20th, 2006

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

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 ) P = P0 eN = 1.2 mm eN = 2.0 mm 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

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

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

injector spectrometer 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 20 sector 21

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

0.5 nC requires only QE of 10-5 with laser energy of 250 J 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 LCLS Min QE Courtesy S.Gilevich Courtesy E.Jongewaard Courtesy D.Dowell

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

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

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

Commissioning 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)

6 MeV transverse measurements YAG02 YAG01 FC01 Courtesy J.Schmerge 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)

Imaging cathode Point-to-point imaging of cathode Virtual cathode Direct determination Uniformity of emission Ellipticity Transv. rise/fall slopes e image : hot spot electrons image Tuning depends E gun Solenoid calibration (best with short laser pulse) Getting initial conditions. Use complementary laser image for init distribution. Find image point of ebeam. Measure emitted distribution. Use in simulations. Courtesy W.Graves Cathode Image at DUVFEL

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 Assumes cathode = 0.6 mm.mrad Image of divergence of source At YAG02 , with Vrf reduced

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

6 MeV Longitudinal measurements Spectrometer 85  Bend YAG01 p(GeV) = 0.3 B(T)(m) YAGG1 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

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

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

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

Longitudinal Phase Space at TCAV 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) DL1 Longitudinal Phase Space at TCAV 135 MeV Spectrometer Spectrometer Screen

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

Risk: Solenoid Alignment Tools 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 Gun BPM2 BPM3 L0a BPM5

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

Single Particle tracker Dark current studies for gun Solenoid Gun YAG01

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

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)

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

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 …)

BACK-UP

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

Emittance compensation Phase space evolution x X’ Drift x X’ x X’ Focusing kick (ex Solenoid RF entrance cell) Defocusing kick (ex: RF kick at exit gun , space charge ) x X’ x X’ 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

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

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 Gun BPM2 BPM3 L0a BPM5

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

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

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 (n) gaussians p 80 <10,90> (2) TP_G02 1.41 1.23 0.97 (3) TP_G03 1.24 1.11 0.91 “Square” 1.08 0.8

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 ) Q [nC] Laser pulse Gun Field 80 mm-mrad proj.mm-mrad z [mm] 0.2 6.5 ps 120 MV/m 0.35 0.44 0.657 110 MV/m 0.37 0.704 1 10 0.91 1.08 0.948 0.95 1.05 0.97 12 0.92 1.02 14 0.88 1.157

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

(while being accelerated) Emittance compensation 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 x X’ e x x m m e e X’ x X’ x X’ Drift + Self-defocusing Drift (while being accelerated) m x e Slices realigned at best Dis. frozen at high energy

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

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

Emittance compensation Gun Solenoid Linac

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

LCLS Injector RF Gun L0a RF section L0b RF section gun spectrometer 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 20 sector 21

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

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

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

Courtesy J.Schmerge, GTF QE from Schottky Scan Direct determination of QE RF for a given Vrf Courtesy J.Schmerge, GTF

More Profile measurement Standard “Beer Can” “3D-Ellipsoid”

Cathode Imaging Point-to-point imaging of cathode Cathode Image at DUVFEL Virtual cathode Virtual cathode Direct determination Uniformity of emission Ellipticity Transv. rise/fall slopes e image : hot spot electrons image Result Correlated to E gun Solenoid calibration (from mask image rotation ) Getting initial conditions. Use complementary laser image for init distribution. Find image point of ebeam. Measure emitted distribution. Use in simulations. Courtesy W.Graves

Cathode emittance Direct determination cathode (with appropriate set of Vrf, rf, Bsolenoid) Momentum distribution  Fundamental for initial model + cathode quality Correlated quantities E gun Solenoid calibration Assumes cathode = 0.6 mm.mrad Image of divergence of source At YAG2 , with Vrf reduced

Imaging source divergence what type of momentum distribution? Momentum at cathode Imaging source divergence what type of momentum distribution?

More Profile measurement Standard “Beer Can” “3D-Ellipsoid”

Solenoid = 98 A Data Parmela Solenoid = 104 A Solenoid = 108 A DUVFEL EXPERIMENT Good match of Slice Emittance and Twiss Parameters Parameters: 200 pC Solenoid = 104 A Solenoid = 108 A

Difficulties of Calibrations  beam at YAG1 varies with Vrf , rf , Gun field balance, charge, Solenoid calibration calibrate Vrf , rf (see slide 18-20) then can possibly detect field unbalanced Fit of DUVFEL measurements

Diagnostics Current Monitors Straight Ahead Spectrometer Wire scanners Cerenkov Radiator OTRs YAGs Gun Spectrometer EO monitor

Including Magnets Treaty Point

1 2 3 4 Linac tunnel ‘Laser Heater’ Straight Ahead Spectrometer 3 screen emittance measurement ‘RF Deflecting cavity’ TCAV1 Emission thermal Uniformity QE 2 3 4 Gun Spectrometer

3 screen emittance measurement ‘RF Deflecting cavity’ TCAV1 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’ Gun S1 S2 L0-1 19.8MV/m L0-2 24 MV/m DL1 Spectrometer 3 screen emittance measurement ‘RF Deflecting cavity’ TCAV1 Spectrometer UV Laser 200 J,  = 255 nm, 5-20 ps, r = 0-1.5 mm

Diagnostics Current Monitors EO monitor Cerenkov Radiator YAGs OTRs Faraday Cup EO monitor Cerenkov Radiator YAGs OTRs Wire scanners Gun Spectrometer Straight Ahead Spectrometer

Injector Layout RF Gun L0a RF section L0b RF section gun spectrometer 6 MeV L0b RF section 62 MeV 135 MeV gun spectrometer Transverse RF deflector injector spectrometer main SLAC Linac sector 20 sector 21

Risk: Solenoid Alignment 2) Error of solenoid position F(Xlaser, rf, Vrf, Bsol, Xsol) = Xf Large systematic errors on Xf (reference 0 cannot be determined) F(Xlaser, rf, Vrf, Bsol, Xsol) = Xf - Xf0 Determining Xsol is a very non-linear problem Xsol = (xsol, xp,sol, ysol, yp,sol) Xf reference (Xf0, Xfp0, Yf0,Yfp0)  8 unknowns instead of 4

Risk: Solenoid Alignment 2) Error of solenoid position 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) Algorithm to determine, in minimum steps, mis-positioning of the solenoid F(X, rf, Vrf, Bsol, Xsol) = Xf Issue : 4 unknowns Xsol = (x,x’,y,y’) Center BPM/Screen cannot be determined with beam BPMs too close together for good accuracy on angle Angle resolution from BPM2-BPM3 > 100 rad But Indirect evaluation of angle using BPM5 Only solution: model based analysis for complicated non-linear problem Solenoid SC0 SC1 SC2 Gun BPM2 BPM3 L0a BPM5

100MV/m, 2 Gaussians, 0.5 nC < {10,90} > ~ 0.6 mm-mrad With 0.6 mm radius beam 80 ~ 0.78 mm-mrad < {10,90} > ~ 0.6 mm-mrad Large margin for emittance increase from errors

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