Notes from the Workshop on: Realizing the Potential of Seeded FELs in the Soft X-Ray Regime. Kirsten Hacker Nov

Transcription

1 Notes from the Workshop on: Realizing the Potential of Seeded FELs in the Soft X-Ray Regime Kirsten Hacker Nov

2 Highlights -User expectations -New theory and simulations -New experiment designs -Recent experimental results

3 Users want CONTROL Slide from Schoenlein

4 Users want NGLS (FLASH clone with 2.4 GeV, CW upgrade)

5 Users want NGLS

6 NGLS wants to be like FLASH John Byrd (BNL): fs-synchronization THz diagnostics Alan Fry (SLAC): seeding scheme

7 Destination: seeded 1 nm Self-seeding Advantage : No seed laser problems Disadvantage: Less control External seeding Advantage: sensitivity to seed laser properties Disadvantage: sensitivity to seed laser properties

8 Destination: seeded 1 nm? EEHG or HHG

9 Destination: seeded 1 nm EEHG or HHG

10 Best of both worlds Two-color Experiments, anyone? EEHG or HHG 1 st radiator 2 nd radiator HGHG fresh bunch chicane

11 Self-seeding: sounds easier SASE radiator (linear) monochromator seeded radiator (steady-state)

12 Theory and simulations Reversible heater Intra-bunch scattering Seed laser phase errors Seed laser wavefront errors

13 Huang: reversible heater

14 Huang: reversible heater

15 Comment: Since a low slice energy spread is so important for seeding, why are we moving the seeding experiments into FLASH II and FLASH III where the energy spread is worse than in FLASH I?

16 Stupakov: IBS Concern: coulomb collisions and EEHG Theory: derive diffusion coefficient in beam frame and then transform to lab frame Conclusion: becomes a problem around 200 th harmonic of 200 nm (~1nm). Distance between exit of last chicane and entrance to radiator must be short for seeding at 1 nm

17 Comment: I think other effects will cause problems for EEHG loooong before IBS

18 Ratner et al.: seed phase error Concern: seed phase error harmonic number Theory: short laser pulse, longer e-beam Conclusion: hard limit at ~1 nm in best-case scenario Shortens usable portion of seed

19 Comments: It is nice to see an expression for how the usable portion of a chirped seed is shortened at higher harmonics, BUT If one has an excess of seed power, one may not want to fully compress the seed pulse. A chirped pulse leaves more flexibility to find the correct frequency for the radiator (eases tolerances) Note that one loses seed bandwidth everywhere. There are methods to measure and correct these errors. The scarier problem comes from uncorrelated phase errors!

20 Hacker et al.: seed wavefront Concern: wavefront harmonic number Theory: analytic and numerical predictions of reduction of EEHG bunching factor Conclusion: difficulties arise below 40 nm Analytic expression for wavefront tolerance Optics errors -> adaptive mirrors and/or spatial filter B-integral considerations Compressor mis-alignments -> wave-front rotation & pulse-front tilt

21 Hacker et al.: seed wavefront

22 Numerical simulations: 1 nm EEHG schemes presented by Xiang (SLAC), Penn (BNL), and Reiche (PSI) did not investigate the wavefront error tolerance, although Xiang and Penn did recently attempt to understand the tolerance of their schemes to phase error FLASH nm has studied the tolerances of these errors since early 2011

23 Geloni: self-seeding

24 Geloni: self-seeding

25 Geloni: self-seeding

26 Geloni: self-seeding

27 Experimental results HHG: Lambert (LOA) & Giannessi (SPARC) EEHG: Xiang (SLAC-NLCTA) HGHG: Allaria (ELETTRA)

28 Lambert Giannessi

29 Lambert, LOA: HHG

30 Giannessi:

31 Giannessi:

32 Giannessi:

33 Giannessi:

34 Giannessi:

35 Werin,

36 Xiang

37 Xiang:

38 Xiang:

39 Allaria

40 Allaria: ELETTRA HGHG

41 Allaria: Transverse Coherence

42 Allaria: Phase Shifters

43 Allaria: Transverse Coherence

44 Allaria: Theory & Measurements

45 Allaria: HGHG->stability

46 Conclusions Seeding at ~1 nm will be attempted in 2 ways Self-seeding HGHG cascade with saturation in first stage using either EEHG or HHG FLASH is set to lead in EEHG Trieste leads in HGHG (SLAC has a seriously huge laser team)