1. RF Gun Design and Status D. H. Dowell, LCLS/SLAC Feb 2006 Lehman Review
Overview of RF Gun Specifications and Technical Issues
Physics Requirements
Technical Issues
Review Process
Gun Design Approach
Differences between LCLS gun and BNL/SLAC/UCLA gun
LCLS gun mechanical design details
Fabrication and Testing Plans
Fabrication and RF cold testing
Integration and magnetic measurements
The hot test
Photocathode Developments
Status and Summary

2. RF Gun Requirements Parameter Value Peak Current 100 A Charge 1 nC
Normalized Transverse Emittance: Projected/Slice
<1.2 / 1.0 micron (rms)
Uncorrelated Energy Spread
3keV (rms)
Repetition Rate
120 Hz
Peak Cathode Field
120MV/m
Gun Laser Stability
<0.50 ps (rms)
Gun RF Phase Stability
<0.1 deg (rms)
Quantum Efficiency
3x10-5
Charge Stability
<2 % (rms)
Bunch Length Stability
<5 % (rms)

3. Technical Reviews of Gun
LCLS Injector Technical Review, Nov 2003
Review of overall injector concept
Chicane incorporated into laser heater design
Injector RF Technical Review, Nov 2004
Settled several technical issues and specifications
Recommended fabricating at SLAC
RF Gun Mechanical Design Review, Aug 2005
Approved final mechanical design of gun
Injector Water System Review, Dec 2005
PDR of water system for gun

4. RG Gun Technical Issues
Stable operation at 120 Hz and 120 MV/m
Gun designed with thermal margin
Designed for 4kW corresponding to 140MV/m
RF pulse shaping reduces average power to 1.2kW
Pulse heating of coupling ports
Using z-coupling instead of theta-coupling
RF field symmetry
Dual feed to eliminate dipole field
Full cell has racetrack shape to eliminate quadrupole field
High field operation
Cavity shaped so highest field is on cathode
Mode-beating of p- and 0-modes
Increased mode separation from 3.2 to 15MHz
Cathode
Developing improved cathode processes
Incorporated processes into design

5. Summary of Design Details
Dual rf feed to eliminate phase asymmetry due to power flow
Racetrack shape in full cell cavities to correct for quadrupole field
Increased mode separation to 15 MHz to reduce mode beating
Reshaped the iris to reduce field 10% below cathode field
Z-coupling to reduce pulse heating
Deformation tuning to eliminate rf tuners
Cooling designed for 120Hz, 120MV/m operation
New cathode mounting allows adjustment of rf seal under vacuum
Dipole and quadrupole field corrections to solenoid
Characterize solenoid with hall probe and rotating coil measurements
Bucking coil to cancel solenoid field
Possibility of in-situ H-beam and/or H-plasma cleaning on gun

6. LCLS gun vs. BNL/SLAC/UCLA gun
BNL/SLAC/UCLA; GTF
LCLS Gun 1
cathode field
100MV/m
120MV/m
rf feed
single w/compensation port
dual feed
cavity shape
circular
racetrack
0-p mode separation
3.2MHz
15MHz
repetition rate
10Hz
120Hz
peak quadrupole field
4 mrad/mm
0.1 mrad/mm
rf tuners
plunger/stub
deformation
cathode
copper or Mg
copper
rf coupling
theta (azimuth)
z (longitudinal)
b-coupling
1.3
2.0
laser incidence
grazing or normal
normal

7. LCLS Gun Cross-Section
deformation tuner
wakefield taper
RF seal adjustment
at atmosphere
cathode cooling
August 2005 LCLS RF Gun Mechanical Design Review
Compliments E. Jongewaard

8. Final Gun-Solenoid Assembly
Cut-away view of LCLS Gun
Cathode weld
assembly
August 2005 LCLS RF Gun Mechanical
Design Review
Compliments E. Jongewaard

9. Fabrication and Testing Plans
Gun parts fabricated, some parts brazed
Adjust dimensions
for freq. and coupling
Pre-braze cold rf test of gun and machine to final dimensions
Final braze
Post-braze rf measurements, balancing and calibration
Resonance tune,
bead drop, balance fields
Assemble gun, solenoid, bucking coil, vacuum pumps,
supports, etc
Begin gun
integration
Integration of gun with solenoid, bucking coil and supports
Magnetic measurements of gun, solenoid and bucking coil system using dummy cathode
Begin magnetic meas
Characterize magnetic fields of the gun assembly
Begin hot test and conditioning
Install cathode, tune gun, bake and perform hot test
Demonstrate 120Hz, 120MV/m operation
Move gun, solenoid and bucking coil system as a unit, under vacuum to S20 for installation
Ready for
installation at S20

10. Cold Testing of RF Gun Performed in rf lab (rm. 150) in klystron dept.
Pre-braze testing and machining to final dimensions
Clamping gun parts; design & build fixtures
Measure 0 and p mode frequencies, mode separation, temperature
Coupling of each and both ports
Post-braze testing
Requires water system with temperature control
Configure gun for beam drop, gun axis vertical
Measure:
Coupling
Mode frequencies
Field balance and symmetry (bead drop), adjust using tuners and cathode plate
After gun is balanced:
Calibrate rf probes

11. Magnetic Measurements of Gun-Solenoid Assembly
Gun protected by axial tube running through gun + solenoid + valve assembly
Measure magnetic fields of the complete gun, solenoid, bucking coil assembly (including valve)
Measure fields with three-axis probe, mostly interested in Bz vs. z relative to cathode
Gun solenoid has been characterized separately for field vs. I, multipole correction, standardization, etc.
Determine bucking coil currents needed to cancel field at gun cathode
Determine if magnetic fields are distorted by assembled components

12. Gun Hot Test Cathode installation and re-tuning
Vacuum bake assembly to torr range
Measure:
Thermal distribution vs. rf power
Field balance vs. rf power
Dark current
Vacuum levels and contaminates with RGA
Demonstrate 120MV/m and 120Hz operation
Seal gun after test and move assembly to S20 under vacuum

13. Gun Hot Test Setup Hardware from GTF Thermal management Controls
Camera & Lens
Hardware from GTF
YAG/FC, camera lens, video, computer
Solenoid power supply
Thermal management
Cooling without temperature feedback
Track RF frequency with gun temperature
Controls
No LLRF, “chase frequency”
Parameter logging
Algorithm for automatic processing
Pump
Gun
Valve
YAG/FC
To Klystron
Wave Guide
Solenoid
Pump
rf power splitter & windows

14. Photocathode Developments
Modified Surface Science Lab load lock to accept and
characterize GTF-style cathodes
An existing chamber is being modified for LCLS cathodes
LCLS cathodes could be modified to fit in Surface Mat. Sci. Lab load lock
Processed cathode for ARDB gun
Data show QE of copper is very sensitive to atmospheric exposure
Repeatedly cleaned cathode to theoretical QE
H-ion gun purchased using SSRL funds
Plans for testing on gun
Use on the LCLS cathode chamber (above)
Theory of copper (metal) cathodes improved
Studies with Cu samples
AFM surface measurements
QE, XPS=>H-cleaning=>QE,XPS=> exposure=>QE,XPS=>H-cleaning=>QE,XPS

15. LCLS RF Gun Design/Fab Status
Design work complete of the gun vacuum envelope
Work started on bucking coil and integration to gun
Drafting
All but ~8 PF’s released and all but 3 SA’s released
Working on the top 2 SA’s for release
Cathode assembly release pending bucking coil design
Fabrication
All fab parts on order except:
Cathode assembly (see above)
Double window assembly
Slide compliments E. Jongewaard

16. NC program is first verified Full cell machining in progress
on aluminum part
Full cell machining in progress
Dual feed waveguide

17. Summary
Gun design and plan was verified and improved by the review process
Engineering is nearly complete and fabrication has begun
Planning for testing, integration and installation:
Gun fabrication complete: June 2006
Integration gun assembly and magnetic measurements: July 2006
Hot test: August 2006
Studies of cathodes & processing techniques begun
Gun assembly will be ready for Oct 2006 installation at Sector 20