The LCLS Gun

1. The LCLS Gun David H. Dowell and Friends SLAC National Accelerator Laboratory ICFA Beam Dynamics Workshop on Future Light Sources March 1-5, 2010 • The LCLS Injector • Design features of the LCLS Gun • LCLS gun performance • Cathode contamination • Brightest possible beam • Summary and Conclusions

2. Major Components of the LCLS Injector Drive Laser Located in room above gun Dual Feed S-band Linac S-Band Gun & Solenoid +Diagnostics

3. Design Features of the LCLS Gun & Injector Pulsed heating mitigated with longitudinal coupling and increasing radius of RF aperture • Optimum emittance compensation, Ferrario working point • Increased RF mode separation to minimize RF mode beating • Symmetric RF fields in gun and s-band linacs • Z-coupling to minimize pulsed heating for long gun life • Improved cooling for 120 Hz operation at up to 140 MV/m • Full wakefield mitigation in gun-to-linac beamline • Emittance compensation solenoid field with quadrupole correctors • Stable and reliable diode-pumped drive laser • Cathode surface roughness <40 nm peak-to-peak, low dark current Z-coupling e-beam Dual RF feed and racetrack shape in full cell eliminate dipole and quadrupole RF fields. Cathode assembly on flange • L. Xiao, R.F. Boyce, D.H. Dowell, Z. Li, C. Limborg-Deprey, J. Schmerge, “Dual Feed RF Gun Design for LCLS,” Proceedings of 2005 Particle Accelerator Conference. • C. Limborg et al., “RF Design of the LCLS Gun”, LCLS-TN-05-3, May 2005. • D. H. Dowell et al., “Development of the LCLS RF Gun”, ICFA Dynamics Newsletter, No. 46, August 2008. http://www-bd.fnal.gov/icfabd/Newsletter46.pdf

4. The Emittance Compensation Solenoid Gun Solenoid in SLAC Mag. Meas. Lab Relatively strong effect on the beam emittance, especially at high charge Expected ~setting from mag. meas. Quad field over probe length (gauss) Quad field phase (deg) Max on quad corrector rotating coil bucking coil Distance from solenoid center (m) Quad Correctors: long quads on Gun1 long & short on Gun2 Solenoid requires <0.1% precision long quad wires 0.2% Normalized emittance (microns) 1 nC Solenoid Int. Field kG-m short PC quads

5. Low Charge (20 pC) Measurements and Analysis Space charge emittance*: Low charge slice emittance meas. at 20 pC Emittance vs. laser size at constant charge (20 pC, blue) and constant charge density (red) • j: the current surface density • I A : the Alfven current, 17000 amps • x: space charge form factor • x : rms transverse beam size • L z : bunch length (full width) parameterized space charge form factor * K-J. Kim, NIM A275(1989)2001-218

6. Summary of LCLS Injector Performance Projected Emittance vs. Charge R. Akre et al., PRST-AB (2007) Y. Ding et al., PRL 102, 254801(2009)

7. Cathode Contamination • Three sources of cathode contamination • Residual contaminants left by fabrication, handling and storage • Contamination by the gun vacuum • Ambient vacuum • Operating vacuum • Contamination during operation due to molecular cracking: • By the laser • By the electron beam For LCLS contamination by molecular cracking is the most problematic. Electron beam emission image of the cathode after >1 year of operation. The UV laser beam has left a QE hole at its location. This will be discussed in more detail in the Cathodes Overview talk on Thursday

8. The Brightest Beam Possible - How much can the LCLS gun emittance be lowered? - Assume all linear and non-linear space charge effects can be corrected/compensated for, assume the cathode is perfectly flat and the cathode physics is correct. Then the lower limit on the emittance depends on the thermal emittance for the divergence and the space charge limit for the beam size: 20 pC Meas. in LCLS Gun at 50 MV/m

9. Summary and Conclusions • Design Features of the LCLS gun and injector: • Optimum emittance compensation, Ferrario working point • Increased RF mode separation to minimize RF mode beating • Restores field balance between cells • Reduces correlated energy spread => chromatic aberration in solenoid • Gun RF tuning and field balance is more tolerant to geometry and temperature • Symmetric RF fields in gun and s-band linacs: elimination of dipole and quadrupole field errors • Emittance compensation solenoid field with quadrupole correctors: Mostly effective at high charge • Z-coupling to minimize pulsed heating for long gun life • Improved cooling for 120 Hz operation • Full wakefield mitigation in gun-to-linac beamline • Stable and reliable diode-pumped drive laser • Cathode surface roughness less than 40 nm peak-to-peak, low dark current, low thermal emittance • The ultimate emittance based on thermal emission and the space charge limit • Improving the LCLS gun performance: • “Three issues: cathodes, cathodes, cathodes!!“, G. Neil • QE robustness and uniformity. (see Cathode Overview talk) • Large (apparent) low charge emittance in gun • High thermal emittance? Poor theory? Poor measurement? • Growth due to surface roughness, aberrations etc.? • Understand role of solenoid quadrupole in emittance compensation • Correcting for quadrupole space charge fields? • Correcting spherical aberration of solenoid? • Good diagnostics are essential! Emission from LCLS cathode And did I mention cathodes?

10. Acknowledgements Special Thanks to the LCLS Gun Group who built the gun and the Commissioning Team who allowed me to show their results. • LCLS Injector Commissioning Team: • R. Akre • A. Brachmann • J. Castro • Y. Ding • D. Dowell • P. Emma • J. Frisch • S. Gilevich • G. Hays • Ph. Hering • Z. Huang • R. Iverson • C. Limborg-Deprey • H. Loos • Miahnahri • J. Schmerge • J. Turner • J. Welch • W. White • J. Wu LCLS Gun Group: Erik Jongewaard & Klystron Dept Cecile Limborg-Deprey John Schmerge Bob Kirby C. Rivetta Zenghai Li Liling Xiao Juwen Wang Jim Lewandowski Arnold Vlieks Valery Dolgashev