RF Gun Design and Status D. H. Dowell, LCLS/SLAC Feb 2006 Lehman Review

1. RF Gun Design and StatusD. H. Dowell, LCLS/SLACFeb 2006Lehman 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

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

7. deformation tuner wakefield taper LCLS Gun Cross-Section 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 Resonance tune, bead drop, balance fields Post-braze rf measurements, balancing and calibration Assemble gun, solenoid, bucking coil, vacuum pumps, supports, etc Begin gun integration 6-15-06 Integration of gun with solenoid, bucking coil and supports Magnetic measurements of gun, solenoid and bucking coil system using dummy cathode Characterize magnetic fields of the gun assembly Begin magnetic meas. 7-1-06 Demonstrate 120Hz, 120MV/m operation Begin hot test and conditioning 8-1-06 Install cathode, tune gun, bake and perform hot test Move gun, solenoid and bucking coil system as a unit, under vacuum to S20 for installation Ready for 9-26-06 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 10-10 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 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 • 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 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