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Ramped-beam generation

This paper discusses the use of beam shaping techniques to enhance the transformer ratio in beam-driven acceleration. Various shaping profiles are explored, including quadratic takeoff with linear ramp and beam shaping via photoemission. Experimental realization and applicability to FACET-II accelerator are also discussed.

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Ramped-beam generation

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  1. Northern Illinois Center for Accelerator and Detector Development Ramped-beam generation Philippe Piot, and Francois Lemery* Department of Physics and Northern Illinois Center for Accelerator & Detector Development, Northern Illinois University, DeKalb IL 60115 Accelerator Physics Center, Fermi National Accelerator Laboratory, Batavia IL 60510 *Present address: Center for Free-Electron Lasers (CFEL), Universitat Hamburg, D-22607 Hamburg, Germany FACET-II Science Opportunities Workshops 12-16 October, 2015 SLAC National Accelerator Laboratory

  2. Introduction & Motivation • Possible multiple-user FEL facility based on wakefield acceleration • Important parameters for this type of beam-driven acceleration is the transformer ratio [Zholents, FEL14 (2014)] e-bunch P. Piot, FACET-II workshop, SLAC 10/12/2015

  3. Transformer-ratio-enhancing shapes • Various current profiles have been proposed over the years [Bane et al., SLAC-PUB 3662 (1985)], • Profiles often challenging to realize in practice • We seek shapes of the form • The wake voltage iswhere and tail wavelengthof mode Green’s function of the e.m. mode excited P. Piot, FACET-II workshop, SLAC 10/12/2015

  4. Quadratic takeoff with linear ramp • profile with equations • decelerating field • accelerating field (being • Transformer ratiousually take Lemery, PRSTAB 18, 081301 (2015) e-bunch P. Piot, FACET-II workshop, SLAC 10/12/2015

  5. Quadratic takeoff with linear ramp P. Piot, FACET-II workshop, SLAC 10/12/2015

  6. Beam shaping via photoemission • photoemission is a technique of choiceshaping accomplished via temporal tailoring of the drive laser. Simulation of laser shaping (left) and produced e- beam (right) including shaper + phoemission response (CsTe) 1 nC Lemery, PRSTAB 18, 081301 (2015) P. Piot, FACET-II workshop, SLAC 10/12/2015

  7. Numerical example 1 nC • Use LCLS RF gun • Form “optimum” shape ~1-m downstream of gun 20 cm 60 cm 100 cm • simulation • -- parabolic + linear ramp 100 cm fromphotocathode P. Piot, FACET-II workshop, SLAC 10/12/2015

  8. Production of tailored bunch for beam-driven acceleration • laser shape +acceleration &compression • close-to-optimalcurrent distribution UW SCRF gun Lemery, PRSTAB 18, 081301 (2015) 5 nC e-bunch P. Piot, FACET-II workshop, SLAC 10/12/2015

  9. Experimental realization (in another context at Fermi@Elettra) measured UV laser shape • optimum laser shape tuned to pre-compensate wakefield- distortions • shape has an invertedparabolic region Penco, PRL 112 044801 (2014) measured w. optimal laser shape simulated evolution of bunch’s current profile measured nominal 0.5 nC P. Piot, FACET-II workshop, SLAC 10/12/2015

  10. Multi-frequency linac • coordinate downstream RF gun • downstream of 2f linac: • downstream of compressor: P. Piot, FACET-II workshop, SLAC 10/12/2015

  11. Multi-frequency linac (2) • Example of computed profiles HEAD HEAD P. Piot, FACET-II workshop, SLAC 10/12/2015

  12. Standard compression Ramped bunch 0.5 kA Head Head Ramped bunch 200 A Ramped bunch 1 kA Head Head Multi-frequency linac: experiment • Example of computed profiles (700 MeV, 0.5 nC) P. Piot, FACET-II workshop, SLAC 10/12/2015

  13. Applicability to FACET-II • proposed injector + sub-GeVlinac has all necessary ingredients for versatile shaping: • photocathode RF gun (w. Ti:Sp laser), • R56 sections, high-harmonic (X-band) • necessary diagnostics [deflecting structure(s)] P. Piot, FACET-II workshop, SLAC 10/12/2015

  14. Summary + outlook • Practical shapes capable of enhancing transformer ratio have recently been proposed • can be produced by conventional photocathode laser shaping, • can also be produced passively (combining wakefield with R56) [Lemery, AAC14 (2014)] • can also be formed by employing a multi-frequency linac[Piot, PRL 108 (2012)] P. Piot, FACET-II workshop, SLAC 10/12/2015

  15. Open questions from FACET-II CDR Fig. 3.1 • For improved efficiency need to understand how to modify these shapes to achieve high-transformer ratio in presence of heavy beam loading, • Witness-bunch formation is not addressed yet and setup-specific (here shown for FLASH). AFTER BC2 GUN EXIT witness drive drive witness P. Piot, FACET-II workshop, SLAC 10/12/2015

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