1. R&D opportunities for photoinjectors Renkai Li 10/12/2015 FACET-II Science Opportunities Workshops 12-16 October, 2015 SLAC National Accelerator Laboratory - Accelerator Physics of Extreme Beams

2. 2 Outline Introduction Shaping the transverse laser profile Blow-out regime operation Collimation Summary Acknowledgement Many helpful discussions with Y. Ding, P. Emma, Z. Huang, and F. Zhou

3. 3 FACET-II and LCLS-I layout FACET-II Layout Preliminary CDR for FACET-II LCLS-I injector PRSTAB 11, 030703 (2008)

4. 4 Key components/technologies for the injector Adapted from S. Di Mitri and M. Cornacchia, Phys. Rep. 539, 1 (2014). Photocathode RF gun Relatively mature, reliable >100 MV/m acceleration gradient Driving most XFELs Up to a few nC charge per pulse

5. 5 Shaping the transverse laser profile Blow-out or pancake regime Uniformly filled ellipsoidal is ideal – linear SC forces and phase-space L. Serafini, AIP Conf. Proc. 413, 321 (1997). O. J. Luiten et al., PRL 93, 094802 (2004). Gaussian bunch Uniformly filled ellipsoidal bunch Space charge forces: Non-linear Slice-dependent Space charge forces: Linear Slice-independent Thermal-emittance-limited beam! Courtesy of B. van der Geer and O. J. Luiten P. Musumeci et al., PRL 100, 244801 (2008) Useful also for longer UV laser F. Zhou et al.,PRST-AB 15, 090701 (2012) Practical and robust in experiment – transverse shaping of ultrashort laser A. Brachmann et al., FEL09, WEOA03

6. 6 Optimal transverse shape – 250 pC 100% rms emittance Model transverse laser profile: Gaussian of rms size σ, cut by an iris of diameter 2R 100k macro part.ε proj (µm)ε slice (µm) Truncated Gaussian 2R=1.4 mm, σ=0.8R 0.47 (100%) 0.35 (95%) 0.35 (100%) 0.29 (95%) Uniform, 2R=1.3mm 0.73 (100%) 0.51 (95%) 0.74 (100%) 0.55 (95%) Optimized norm. emittance for various R and σ Beam energy 135 MeV UV pulse length 1 ps rms 20k macro part. Typical LCLS UV profile

7. 7 Truncated Gaussian v.s. Uniform profiles Uniform, 2R=1.3 mm T-G, 2R=1.4 mm, σ=0.8R 100k macro part.ε proj (µm)ε slice (µm) Truncated Gaussian 2R=1.4 mm, σ=0.8R 0.47 (100%) 0.35 (95%) 0.35 (100%) 0.29 (95%) Uniform, 2R=1.3mm 0.73 (100%) 0.51 (95%) 0.74 (100%) 0.55 (95%) 95% rms emittance

8. 8 Scale to higher charge - FACET-II 2 nC case Slice emittance 100% rms emittance 20k macro part. 100k macro part. 100% rms emittance 20k macro part.

9. 9 Blowout regime operation Short (<100 fs) UV pulse on the cathode Shaped transverse profile, either ideal (spherical) or truncated Gaussian (σ, R) 100k macro part.ε proj (µm)ε slice (µm) UV 1 ps rms, T-G 2R=1.4 mm, σ=0.8R 0.47 (100%) 0.35 (95%) 0.35 (100%) 0.29 (95%) UV 40 fs rms, T-G 2R=1.6 mm, σ=0.6R 0.44 (100%) 0.40 (95%) 0.43 (100%) 0.40 (95%) UV 40 fs rms, spherical, σ=0.35 mm 0.49 (100%) 0.42 (95%) 0.47 (100%) 0.38 (95%) Larger slice emittance than the long pulse case Smaller difference between projected and slice emittance (slices better aligned) 40 fs UV 1 ps UV t-x at gun exit

10. 10 Longitudinal phase space of the blowout regime Self-field is linear also in the LPS for a 3D ellipsoid beam Blowout regime features linear LPS if image-charge and RF effects are insignificant J. T. Moody et al., PRST-AB 12, 070704 (2009) LPS of the blowout beam (a 3 rd -order polynomial removed) LPS of the 1 ps UV beam (a 3 rd -order polynomial removed)

11. 11 Possibility of collimation to improve the brightness Phase space density not uniform inside the beam Possible to pickup the brightest part using apertures (Y. Ding, talk later on beam shaping / longitudinal collimation) Wakefield effects need to be considered Similar studies by F. Zhou et al. in 2013 Work in progress… 20k macro part.ε proj (µm)ε slice (µm) 250 pC, T-G 2R=1.4 mm, σ=0.8R 0.540.40 500→250 pC*, T-G 2R=1.7 mm, σ=1.25R 0.330.29 *with a 300 um diameter aperture at L0b exit Case study: adding a circular collimator, 500 pC → 250 pC Projected and slice emittance closer after collimation Room for further improvement: longi. UV profile, collimator position, …

12. 12 Thermal emittance 12 Thermal emittance Quantum efficiency Dowell and Schmerge, PRST-AB 12, 074201 (2009) Cu: 0.35 mm-mrad/mm w/ mid-10 -5 QE @ PSI Prat, PRST-AB 18, 063401 (2015) Slice emittance almost dominated by thermal emittance after laser shaping Effects of surface roughness Tuning extraction field and photon energy independently Limitation due to laser damage of the cathode and demand on laser power, especially for high charge Big improvement if 0.9→0.5 um/mm

13. 13 New gun geometry - Higher extraction field cathode 1.6 cell type 1.4 cell type 2 cm R. K. Li and P. Musumeci, PRApplied 2, 024003 (2014)

14. 14 Summary and outlook Aiming at lower emittance, higher brightness Shaping the transverse UV profile. Dynamic optical shaping to help operation (Siqi Li et al. at LCLS) Blowout regime operation. Q: the implication of slice energy spread Whether collimation can improve the brightness. Q: instead of shaping the entire beam, can we shape the core to high phase-space density? Optimum location of the collimator? Thermal emittance. Q: understand the source of 0.9 um/mm thermal emittance. Surface roughness? Reduce ε thermal while keeping QE reasonable. Thank you for your attention!