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The LCLS Injector C. Limborg-Deprey Injector-Linac & Spectrometers Nov 20th, 2006
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Beam Characterization
Introduction Beamline Layout Emittance compensation Beam Characterization 6 MeV 135 MeV On-line Computation Tools Single-particle Multi-particle
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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 < at 15 GeV ( ~ 1.5Å) n ~ 1.2 m , I ~ 3.2 kA courtesy S. Reiche
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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
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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
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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
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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
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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
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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
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(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
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Emittance compensation
Key parameters Gun (Vrf, rf ) Solenoid field values Laser beam (volume, uniformity) Emission (QE uniformity) Alignment, Steering rf = 2 Solenoid 0.3%
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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)
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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)
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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
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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
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Imaging source divergence
Momentum at cathode Imaging source divergence Best parameters Vrf, ~ 72 MV/m rf ~ 20 degrees Observation at YAG02
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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
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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
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135 MeV Transverse Measurements
See P.Emma Presentation 3 screen emittance (OTRs, WS) Horizontal Slice emittance (TCAV, Quad Scan)
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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
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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 11m resolution ok for modified optics) DL1 Longitudinal Phase Space at TCAV 135 MeV Spectrometer Spectrometer Screen
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resolution 6 keV for nominal optics
resolution 3 keV for modified optics 3keV -> 40 keV will be measurable rms Direct Measurement Projection
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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
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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
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Single Particle tracker
Dark current studies for gun Solenoid Gun YAG01
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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
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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)
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Tuned to matching of -0.6% case Matching performed for each point
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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 …)
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BACK-UP
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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
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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
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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
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Risk: Solenoid Alignment
Tolerance : 250 m, 250rad 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
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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]
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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
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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
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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
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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
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(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
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Emittance compensation
Gun Solenoid Linac Diverging: Space charge RF kick at exit cell Converging: Solenoid RF kick at entrance cell
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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 ‘
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Emittance compensation
Gun Solenoid Linac
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Emittance 1- Emittance 2- Emittance at emission z = 2.34 mm.mrad
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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
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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) < , 14.3 GeV
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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
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Gun Characterization QE, thermal, Uniformity Emission , Bunch Length
YAG1 YAG2 CR1 YAGG1 CRG1
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Courtesy J.Schmerge, GTF
QE from Schottky Scan Direct determination of QE RF for a given Vrf Courtesy J.Schmerge, GTF
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More Profile measurement
Standard “Beer Can” “3D-Ellipsoid”
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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
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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
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Imaging source divergence what type of momentum distribution?
Momentum at cathode Imaging source divergence what type of momentum distribution?
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More Profile measurement
Standard “Beer Can” “3D-Ellipsoid”
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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
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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
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Diagnostics Current Monitors Straight Ahead Spectrometer Wire scanners
Cerenkov Radiator OTRs YAGs Gun Spectrometer EO monitor
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Including Magnets Treaty Point
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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
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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 = mm
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Diagnostics Current Monitors EO monitor Cerenkov Radiator YAGs OTRs
Faraday Cup EO monitor Cerenkov Radiator YAGs OTRs Wire scanners Gun Spectrometer Straight Ahead Spectrometer
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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
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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
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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
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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
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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
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