1. 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

2. 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

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

4.  = ½(o-2o+o)
 = 1.16 mm.mrad

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

6. 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

7. 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

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

9. Emittance Compensation

10. 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

11. Movie 3
Movie 1
Movie 2
Movie 4

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

13. Preinjector:
SLAC Main Linac Beamline
SECTOR 20
VAULT

14. 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 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

15. ‘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

16. 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

17. 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 %

18. 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

19. 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

20. 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

21. 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
The self-consistent solution is the space charge oscillation

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

23. 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)

24. 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

25. 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)

26. 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

27. Ellipsoid emission bunch

28. Ellipsoid emission bunch

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

30. ‘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

31. 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

32. 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

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

34. R&D Status: GTF Measurements
GTF measurements 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

35. Commissioning Diagnostics1
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

36. 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

37. 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

38. 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

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

40. +/- 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

41. 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)

42. 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

43. 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 = 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

44. 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

45. Injector Schedule

46. 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

47. 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

48. Slice-Emittance Measurement Simulation4
Slice-Emittance Measurement Simulation
sy  bunch length
RF-deflector at 1 MV
slice OTR 10 times
quad scanned

49. 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

50. 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

51. 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

52. 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

53. Single Particle Dynamics
Gun
Solenoid
focusing
defocusing
defocusing
Solenoid focusing

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

55. 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 …

56. (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)
(1.0)
1.3 m (1.3)
104 A
2.1 um (2.8)
(-3.6)
9.8 m (6.8)
108 A
2.7 um (2.7)
(-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