The LCLS Injector C.Limborg-Deprey Emittance compensation Minimum emittance thermal emittance Linear emittance compensation for ideal laser beams Nominal and alternate tunings Beamline layout 1nC, 0.2 nC last year modifications Laser heater RF structures How much can we believe PARMELA GTF, DUVFEL PARMELA vs experiment Code comparison What could we be missing? Commissioning measurements Spectrometers Emittance measurement 6D measurements

Emittance Compensation Photocathode RF gun adequate to generate coldest electron beam photoemission produces some transverse momentum px “cathode emittance” ~  x  px also called “intrinsic emittance” or “minimum” emittance We want to preserve at best the beam emittance along the transport line (space charge, wakefield, CSR …) Space charge very strong at low energy  generates large energy spread Appropriate choice and tuning of components allow to compensate for variation in transverse dimension (size, divergence) due to chromatic effects = Compensate for the mismatch between slices Gun Solenoid Linac

Emittance Compensation 10 Courtesy J.Schmerge Theory (perfect surface) ~ 0.3 mm.mrad /mm radius Measured ~ 0.6 mm.mrad /mm radius

 = ½(o-2o+o)  = 1.16 mm.mrad

Single Particle Dynamics Single particle dynamics in gun Gun focusing defocusing defocusing focusing

Single Particle Dynamics RF effects are non linear RF Kicks are time dependent: so vary along the bunch Are not be compensated for Very small contribution to total ~ 0.1 mm.mrad in our S-Band Gun Electric field effects focusing defocusing defocusing Magnetic field effects focusing

Single Particle Dynamics Gun Solenoid Gun Solenoid Linac Solenoid focusing focal length energy dependent Focusing kick at entrance of Linac Time dependent Used in emittance Compensation process

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

Emittance Compensation

Gun S1 S2 Linac Movies 1,2,3 : thermal = 0.72 mm.mrad Movie 4 : thermal = 0 mm.mrad Linac Movie 3 Movie 4 Movie 1 Movie 2 3D Ellipsoid Space Charge linear with r ,  optimal shape for perfect emittance compensation

Movie 3 Movie 1 Movie 2 Movie 4

Movie 1 Movie 3 Movie 4 Movie 2  = 0.75 mm.mrad  = 2.34 mm.mrad

Preinjector: SLAC Main Linac Beamline SECTOR 20 VAULT

Goal parameters P = P0 P = P0/100 eN = 1.2 mm eN = 2.0 mm Parameter Value Peak Current 100 A Charge 1 nC Normalized Transverse Emittance: Projected/Slice < 1.2 / 1.0 micron (rms) Repetition Rate 120 Hz Energy 135 MeV Energy Spread@135 MeV: Projected/Slice 0.1 / 0.01 % (rms) Gun Laser Stability 0.50 ps (rms) Booster Mean Phase Stability 0.1 deg (rms) Charge Stability 2 % (rms) Bunch Length Stability 5 % (rms) P = P0 eN = 1.2 mm P = P0/100 eN = 2.0 mm

‘RF Deflecting cavity’ TCAV1 3 screen emittance measurement 6 MeV Gun S1 S2 L0-1 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

Nominal tuning proj = 0.954 , 80 = 0.89 mm.mrad Rise/fall 0.7 ps projected [mm.mrad] 0.954 1.028 1.141 80% [mm.mrad] 0.894 0.935 0.986 <slice >10..90[mm.mrad] 0.849 0.877 0.901 <slice >1..100[mm.mrad] 0.906 0.953 1.004

Tolerance as a function of single parameter variation Solenoid 1 0.3% Egun 0.5% gun 2.5  Balance~ 3% is ok Linac Field 12 % (EFinal = 150 MeV ) Solenoid 2 20%

Stability requirements Defined after combining errors Param. Nom. Units Stability Requirements Sol1 2.7235 kG  0.02 % Sol2 0.748  1 % Gun Phase 27.25   /0-X  0.1 Gun Field 120 MV/m  0.5% Charge 1 nC  5% L01Field 18  2.5%  Small margin left for laser parameters variation

Tolerances – Alignment and Laser Uniformity Param. Type Tolerance Units Solenoid 1 Transverse Position 500 m Angular Position 1.5 mrad Laser 100 Laser Uniformity Transverse (Slope)  10 % NA Transverse (Cross) Longitudinal 30% ptp Freq.> 1THz (1ps) 20% ptp Freq.< 1THz Linac 1 150 120 rad Linac 2 Same as Linac 1 Solenoid 2 Same as Solenoid 1 (*) combined with uniformity of QE

Requirements on Laser Pulse - Summary Transverse 10 % ptp maximum on emission uniformity Longitudinal  =120 m  =240 m  =480 m 5% ok for emittance But too much for LSC

Longitudinal Space Charge Instability LSC observed at the DUVFEL Courtesy of Timur Shaftan Also observed at TTF Current Density Energy Simulations and theoretical studies Z.Huang et al. PhysRev. SLAC-PUB-10334 J.Wu et al. LCLS Tech Note , SLAC-PUB-10430 G.Geloni. Et al. DESY 04-112 The self-consistent solution is the space charge oscillation

ASTRA/ PARMELA Simulations , Amplitude = +/- 5%,  = 100 mm GUN EXIT 6 MeV ENERGY CURRENT

End L02 135 MeV ENERGY CURRENT Microstructure at the end of the injector Laser Heater provide enough energy spread (40keV) for “Landau damping” preventing -further amplification of the microbunching - the increase an energy spread (as it needs to remain < the FEL parameter)

Alternate tunings for cylindrical bunch 1nC, long pulse th = 0.6 mm.mrad per mm laser spot size reduce rlaser to 0.85 mm BUT to keep charge density same order lengthen bunch Start with th = 0.51 mm.mrad

Alternate tunings for improving  Name Q (nC) Laser pulse (ps) r (mm) th (mm.rad) 80 RF ()  80  5% Nominal 1 10 1.2 0.72 0.9 32 2.5 1 nC, 17.5 ps 17.5 0.85 0.5 0.75 33 1.5 0.2nC,10ps 0.2 0.39 0.234 0.38 37 0.2nC,5ps 5 0.42 0.25 0.37 th = 0.6 mm.mrad per mm laser spot size minimum r best , BUT limit on minimum radius = space charge limit (ignoring Shottky) Esc = Q / ( r2 o) example: for 1nC, r = 1.2mm, Esc = 25 MV/m ( 12) for 1nC, r = 0.85 mm, Esc = 50 MV/m ( 25) for 0.2 nC, r = 0.3mm, Esc = 80 MV/m ( 42) for 0.2 nC, r = 0.42mm, Esc = 40 MV/m ( 20)

without damaging too much the slice emittance A 5ps laser pulse improves dramatically the peak current compared to the 10ps laser pulse case without damaging too much the slice emittance

Ellipsoid emission bunch

Ellipsoid emission bunch

Ellipsoid emission bunch square ellipsoid Longitudinal Phase Space Ek [MeV] vs T [ps] Exit gun Entrance L01 Exit L01 Exit L02

‘RF Deflecting cavity’ TCAV1 3 screen emittance measurement 6 MeV Gun S1 S2 L0-1 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

Can we believe PARMELA? Sensitivity studies fine since relative evolution Meshing : by hand in PARMELA , automated in ASTRA criteria well understood Benchmarks - w.r.t experiences Proved importance of data on initial distribution Fitted the slice parameters such as  , , projected , slice - w.r.t other codes Seems that extraction agree with PIC codes (experiment to be revisited for low accelerating voltage) Still need to compute fields for lossy copper

DUVFEL measurements 200 pC Good Agreement Slice Emittance and Twiss Parameters for the various solenoid fields After including thermal emittance, gun field balance between the two cells, transverse non-uniformity and longitudinal profile Solenoid = 104 A Solenoid = 98 A

DUVFEL Measurements Thermal emittance experiment Confirms the 0.6 mm.mrad per mm radius of laser spot size

R&D Status: GTF Measurements GTF measurements - 1.5 mm.mrad for 130A head tail 300pC slice = 1.5 mm.mrad for 130 A ~ close to LCLS requirements Similar measurements at the DUVFEL facility (Spring 2002) Spectrometer Image of Slice Quad Scan Data -1.5 -1 -0.5 0.5 1 50 100 150 Time (ps) Peak Current (A) Instantaneous Peak Current 5 10 1 2 n (mm mrad) Slice Emittances  longitudinal emittance Slice number

Commissioning Diagnostics 1 Linac tunnel Uniformity + Thermal emittance ‘Laser Heater’ YAG1 YAG2 Straight Ahead Spectrometer 3 screen emittance measurement ‘RF Deflecting cavity’ TCAV1 2 3 4 Gun Spectrometer

Emission uniformity 1 Point-to-point imaging of cathode on YAG1 Laser masking of cathode image at DUVFEL Above: Laser cathode image with mask removed showing smooth profile. Below: Resulting electron beam showing hot spot of emission. Above: Laser cathode image of air force mask in laser room. Below: Resulting electron beam at pop 2. Getting initial conditions. Use complementary laser image for init distribution. Find image point of ebeam. Measure emitted distribution. Use in simulations. Courtesy W.Graves

Thermal Emittance 1 Infinite-to-point imaging what type of momentum distribution? Very good resolution of divergence YAG2 == Image of divergence of source Assumes th = 0.6 mm.mrad

Gun Spectrometer 2 Energy Correlated Energy Spread for all charges YAG01 Spectrometer YAGG1 YAGG2 Quadrupoles Energy Absolute energy Alignment using laser Spectrometer field calibration Correlated Energy Spread for all charges Uncorrelated energy spread for low charges Introducing a time-energy correlation (varying injection phase) Slice thermal emittance Relay imaging system from YAG1 to spectrometer screens Point-to-point imaging in both planes Uniformity of line density

High Charge Operation : 1nC Nominal tuning – no quadrupole on – Very good linearity Longitudinal at YAG1 YAGG1 YAGG1

+/- 8% modulation on laser beam Resolves line density uniformity at high charge +/- 8% modulation on laser beam YAG1 RF + 25 / nominal Quadrupoles on for manageable image size Resolves modulation

135MeV Diagnostics 6D beam measurements Horizontal slice emittance Vertical deflecting cavity + 3screen Vertical slice emittance Quad scan + spectrometer Quad Scan + Dogleg bend Verification of thermal emittance Longitudinal Phase space Vertical deflecting cavity + spectrometer Efficiency of laser heater (spectrometer has 10 keV resolution) Laser Heater Transverse RF Cavity OTR Emittance Screens DL1 Bend Straight Ahead Spectrometer Point-to-point imaging of the 75 m waist (OTR5)

Longitudinal Phase Space at waist Spectrometer + Vertical deflecting cavity  Direct longitudinal Phase Space representation Longitudinal Phase Space at waist Transverse deflecting cavity  y / time correlation (1mrad over 10ps ) Spectrometer  x / energy correlation rms fwhm From PARMELA simulations (assuming 1m emittance), resolution of less than 10 keV

RF Gun – Racetrack in full cell 2d-: no port = benchmark omega3p/sf 3d-cylin: with coupling ports- cell cylindrical 3d-rtrack: with coupling ports- cell racetrack Full : with laser ports + racetrack Full retuned: with laser ports + racetrack+ retuned x = y =0.88  x = 0.96 y =1.01 x = y = 0.90 x = 0.97 , y = 0.99 x = 0.91, y = 0.915 b d From L.Xiao, ACD/SLAC

RF Studies- L01 coupler Dipole moment From Z.Li, L.Xiao, ACD/SLAC Single feed for 10 ps Dual feed Quadrupole moment Dual feed Dual feed +rtrack

Injector Schedule

Conclusion Gained confidence in PARMELA/ASTRA vs experiment vs other codes Injector computations based on large thermal emittance Discrepancy on thermal emittance by a factor of 2 between exp. and simulations remains to be understood If that value is confirmed: probably too large  at 1nC so run 0.2 nC Laser Pulse shaping and uniformity is critical to reach parameter goals

Acknowledgements Many thanks to S.Gierman, J.Schmerge, J.Lewellen, D.Dowell, W.Graves, T.Shaftan, Z.Huang, J.Wu, P.Emma, S.Lydia, J.Qi, M.Ferrarrio, K.Floetmann, L.Serafini, P.Bolton, P.Krejcik, HD Nuhn, S.Reiche, M.Cornacchia, J.Galayda

Slice-Emittance Measurement Simulation 4 Slice-Emittance Measurement Simulation sy  bunch length RF-deflector at 1 MV slice OTR 10 times quad scanned

Slice-Emittance Measurement Simulation (slice-y-emittance also simulated in BC1-center) Injector at 135 MeV with S-band RF-deflector at 1 MV = meas. sim. = calc. = y distribution = actual (same SLAC slice-e code used at BNL/SDL) slice-5

RF Gun – Mode 0 studies Study suggested by T.Smith From Z.Li, ACD/SLAC 120MV/m Non-negligeable effect dF 3.4 MHz 8 MHz 3s, Vcath. in 0 mode 11.77 MV/m 4.96 MV/m 0.82s, Vcath. in 0mode 10 MV/m 5.7 MV/m From Z.Li, ACD/SLAC 3.4MHz mode separation 8MHz mode separation Solution : Klystron Pulse shaping Study of 12 MHz mode separation

GTF 1.6 cell S-band gun Laser Port Photocathode Currently using a single crystal (100) Cu cathode Electron Beam Exit “Half” Cell Full Cell

GTF 1.6 cell S-band RF gun LCLS Modifications: Dual rf feed Waveguide Feed LCLS Modifications: Dual rf feed Cathode plate with brazed cathode plug Load lock 120 Hz cooling Full and ½ cell power monitors and remote tuners Full Cell Power Monitor

Single Particle Dynamics Gun Solenoid focusing defocusing defocusing Solenoid focusing

Search for better tuning for the 2.8 FHWM case With 1ps rise/fall time, assuming r = 0.42 mm & Retuning

Tolerance and stability as a function of single parameter variation Egun 0.5% gun 2.5  Egun 0.5% Solenoid 1 0.3% Gun S1 S2 L0-1 19.8MV/m L0-2 24 MV/m …

(parmela in parentheses) Solenoid = 98 A Data Slice emittance vs solenoid strength. Charge = 200 pC. Parmela Projected Values (parmela in parentheses) Solenoid Eyn Alpha Beta 98 A 3.7 um (3.2) 0.4 (1.0) 1.3 m (1.3) 104 A 2.1 um (2.8) -6.9 (-3.6) 9.8 m (6.8) 108 A 2.7 um (2.7) -9.0 (-9.6) 45 m (36) Solenoid = 108 A Solenoid = 104 A Slice emittance not constant! Nonlinear phase space shapes, higher order moments to phase ellipse are changed by focusing magnets