pSASE : concept and implementation at LCLS-II

1. pSASE: concept and implementation at LCLS-II D. Xiang, Y. Ding and Z. Huang, SLAC H. Deng, SINAP 1/30/2013 Thanks to many discussions with S. Reiche, H.-D. Nuhn, …… More information in PRST-AB 16, 010703 (2013)

2. Introduction • SASE: Self-Amplified Spontaneous Emission In FELs, radiation overtakes e-beam by one radiation wavelength  per undulator period • Slippage length: N ≈ 3000 ×0.15 nm ≈ 1.5 fs N N N Radiation fields with distance larger than N evolve independently and therefore are uncorrelated in phase FEL power FEL spectrum Purified SASE (pSASE) FEL

3. SASE: temporal profile and spectrum • Cooperation length: lc =/4πρ (slippage length within one gain length) lb c/2πlc seed c/lb 2πlc FEL power FEL spectrum Purified SASE (pSASE) FEL

4. Improving temporal coherence • Frequency doubler/tripler (SASE HGHG)  = 45 nm  = 9 nm SASE SEHG Feldhaus et al., NIM A, 2004; Xing et al., NIM A, 2011 • iSASE Bandwidth reduction factor Thomson & McNeil, PRL 2008; Dunning, Thomson & McNeil, IPAC10; Pellegrini, Wu et al., 2012 Need many mini-chicanes with moderate B-field • Harmonic lasing Bandwidth reduction factor ~ harmonic number Need many phase shifters to suppress lasing at longer wavelengths McNeil et al, PRL 2006; Schneidmiller & Yurkov, PRST-AB, 2012; Schneidmiller & Yurkov, DESY 12-173, 2012 Purified SASE (pSASE) FEL

5. Harmonic interaction K = 1 K = 3 K = 10 Ratio of power gain length at the n-th harmonic to that at the fundamental: Purified SASE (pSASE) FEL

6. pSASE U13 … U18 U1 U2 U3 … … U7 U8 U9 U10 U11 U12 λ1 =0.6 nm K = 2, λ1 =0.6 nm K=6.3,λ1 =4.2 nm 6 GeV Amplify seed radiation through harmonic interaction Standard SASE SASE with a purified seed length ~ 10 gain length Purified seed further amplified to saturation Increase slippage length by n Provide ~100 MW radiation (seed) while keeping energy spread growth small Enhanced spectral brightness Seed is amplified (~500 MW) and purified Fundamental radiation starts from shot noise; suppression not needed Purified SASE (pSASE) FEL

7. pSASE @ LCLS-II U13 … U18 U1 U2 U3 … … U7 U8 U9 U10 U11 U12 λ1 =0.6 nm K = 2, λ1 =0.6 nm K=6.3,λ1 =4.2 nm 6 GeV At the exit of U10 At the exit of U11 At the exit of U12 At the exit of U9 2πlc≈ 4~ 5 fs 2πlc≈1 fs Purified SASE (pSASE) FEL

8. pSASE @ LCLS-II U13 … U18 U1 U2 U3 … … U7 U8 U9 U10 U11 U12 λ1 =0.6 nm K = 2, λ1 =0.6 nm K=6.3,λ1 =4.2 nm 6 GeV At the exit of U18 SASE pSASE SASE pSASE Average over 10 shots Purified SASE (pSASE) FEL

9. pSASE Self-seeding with central wavelength depending on beam energy Beam energy jitter leads to fluctuations in FEL central wavelength FEL power is not sensitive to beam energy jitter iSASE with continuous phase shifter Shift radiation field by (n-1)λ after each undulatorperiod (FEL power grows in the phase shifter too) Purified SASE (pSASE) FEL

10. Summary and outlook • Easy way to reduce FEL bandwidth (PS/mini-chicane not needed) at ANY wavelength • pSASE FEL power not sensitive to beam energy jitter • Suited for FELs (LCLS-II, EXFEL…) that use variable gap undulators *LCLS-II CDR • Use 17th harmonic interaction (K=10) to enhance the LCLS-II FEL spectral brightness by >10 at NO additional cost • Promising alternative to self-seeding for high rep-rate FELs (NGLS) where the heat might damage the grating required in self-seeding Purified SASE (pSASE) FEL