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

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)

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

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

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

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

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

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

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 6-15-06 Integration of gun with solenoid, bucking coil and supports Magnetic measurements of gun, solenoid and bucking coil system using dummy cathode Begin magnetic meas. 7-1-06 Characterize magnetic fields of the gun assembly Begin hot test and conditioning 8-1-06 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 9-26-06 installation at S20

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

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

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

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

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

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

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

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