1. Injector Physics C. Limborg-Deprey, D. Dowell ,Z. Li. ,H. Loos, J
Injector Physics C.Limborg-Deprey, D.Dowell ,Z.Li*,H.Loos, J.Schmerge, L.Xiao*
(*)
RF Gun modifications
Linac Sections modifications
Risk Mitigation Plans
Commissioning
Safety
Summary

2. RF Systems – November Review
Presentations
Recommendations
Report from J.Wang et al.
Initial Gun UCLA/BNL/SLAC  LCLS Gun version

3. Modifications RF gun Courtesy L.Xiao Gun fabricated at SLAC Dual Feed
RF design complete
Mechanical model in progress
120Hz heat calculations under way
Dual Feed
Suppresses the time dependent dipole mode
Matching phase for 2 feeds by holding mechanical tolerances on both arms
Z coupling (instead of -coupling)
Pulsed heating reduced + easier machining
Racetrack shape compensates for stronger quadrupole mode
15 MHz mode separation adopted
Lower cathode voltage for the 0-mode
“Suppresses” two degrees of freedom in parameter space
Larger radius for coupling cell iris
Reduces RF emittance
Easy to accomodate elliptical curvature to reduce surface field
Shaping of RF pulse for reducing average power
4kW -> 1.8 kW ; cooling channels designed for handling 4kW
Reduce reflected power from gun
LCLS-TN-05-3.pdf
Courtesy L.Xiao

4. Reducing Pulsed Heating with RF Pulse Shaping
1.8 kW kW
Advantages
Factor 2 less power dissipated in structure
Reach higher peak fields due to shorter rf pulse length
Less dark current
Courtesy J.Schmerge

5. Study suggested by T.Smith
0 and Pi Mode
0-mode
Accelerating field
-mode
Study suggested by T.Smith

6. 0 and Pi Mode Separation
 between 0 and PI mode for short klystron pulse
Small and nearly independent of 

7. Geometry changes
Modification of cell-to-cell iris for 15MHz mode separation
Reduces the surface field
could help reach more than120MV/m
9.525
14.85
12.025
13.025
19.050
Prototype Gun
LCLS Gun
Courtesy Z.Li

8. Racetrack Shape in Full Gun
2D- no port
no racetrack (3D)
all ports and racetrack shape (3D)

9. Linac: Dual Coupler at entrance cell
rms head-tail trans. kick for 10ps bunch
Head-tail dipole kick from single feed
Generates emittance growth
Operating point
Kick is reduced by more than 4 times in output coupler
nominal 1nC
Ent. L0a
Exit L0a
Ent. L0b
Exit L0b
(/) at 0 single feed in %
1.8
0.4 12
0.6/0.5
With dual feed reduction head-tail kick reduced by 20
(/) at 0 dual feed in %
0.005
0.04
Dual feed at entrance cell BUT NOT at exit
Quadrupole head-tail not a problem at exit cell

10. Linac Final Design
Using standard WR284 waveguide – eliminate all tapers
(flanges closer to body, to accommodate linac solenoid )
Coupler cell lengthened to match height of WR284 waveguide
Racetrack parameters readjusted
Courtesy J.Chan

11. Cathode QE improvement
H-ion Cleaning Experiment
QE at low voltage (No Shottky Enhancement)
LCLS QE Spec.
255nm
After H-beam cleaning 1.2x10-4
Idt for
H-ion beam
%Carbon on surface
initial 30 1H C 11 2H
0.124 12 3H
0.1818 10 4H
0.614 8
Courtesy
D.Dowell, R.Kirby
Surface unaltered by H-ion beam cleaning contrary to effect of laser cleaning

12. Schottky Enhancement of the QE
QE improvement
Schottky Enhancement of the QE
Some Approximations to theory
More tests planned
More samples (process)
Find optimal H-ion current, integration time and temperature
Implement on LCLS gun?
GTF (measured)
LCLS Specifications
LCLS minimum required
Courtesy D.Dowell

13. “Beer Can” vs “3D ellipsoid”
 = 1.02 mm.mrad;  80% = 0.95 mm.mrad
= 0.57 mm.mrad ;  80% = 0.58 mm.mrad
(standard “cathode” = 0.6 mm.mrad / mm radius - similar meshing parameters)
Sensitivity much reduced and Tuning much easier
Margin to 1mm.mrad very large
Feasibility under study

14. Commissioning Preparation
Gun-To-Linac
Cathode emittance
Line density
Orthogonality of knobs: from minimum energy spread
Beam–Based Alignment of Gun-Solenoid-Linac
135 MeV
Straight Ahead Spectrometer Design modified
More simulations on poor quality beam
Commissioning
Emittance measurement data analysis
Comparison of various techniques in collaboration with DESY team
Converge towards “agreed upon protocol”
Software
Import available software from GTF and DUVFEL
List of all additional software to be written

15. Cathode emittance Image of divergence of source
Assumes cathode = 0.6 mm.mrad
Image of divergence of source
At second screen + Vrf reduced
Measure transverse momentum distribution

16. Orthogonality of knobs
Detect rf  Vrf
To Be updated
with better resolution
In region of interest

17. Longitudinal Uniformity at Gun Exit
8% modulation
1nC 
RF 25
Resolves modulation
at YAG1 location

18. Straight Ahead Spectrometer
Modification of design (added pole face rotation)
Uses matching lattice
Leaves possibility of stealing pulses at ~1 Hz or less, in the future
Dx ~ 1m
x ~0.1
Same tuning

19. Longitudinal Phase Space
resolution < 10 keV
3keV -> 40 keV
will be easily measurable
rms

20. Transported In Injector
Safety
Maximum credible beam (MCB) estimate needed for
Laser tube penetration & personnel access shielding design, locations of beam loss
Review previous estimate of MCB to linac (agrees with original value)
Explosive electron emission from cathode is source of MCB
Over-focused drive laser beam generates plasma at cathode surface
Beam power limited by stored energy in gun and beam loading
Large energy spread limits MCB transported to linac
Documented in SLAC Memo April, 2005 & LCLS-TN-01-2
1210W
4MeV
2030W
44.5MeV
4740W
104MeV
1130W
135MeV
Est. Max. Credible Beam
Transported In Injector
Courtesy D.Dowell

21. Conclusion Gun Linac Risk Mitigation Plans for 1nC Safety
RF design completed
Mechanical design under way (thermal analysis on-going)
Linac
Mechanical design in progress
Risk Mitigation Plans for 1nC
Cathode studies: H-ion cleaning for higher QE
Tuning of 0.2nC completed (see P.Emma)
On-going work on laser pulse shaping
Safety
Commissioning Preparation
Laser steering stabilization
Feedback for Laser Energy/ RF gun (P,)
Commissioning Plans (detailed schedule + software)