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A single-shot method for measuring fs bunches in linac-based FELs

A single-shot method for measuring fs bunches in linac-based FELs. Z. Huang, K. Bane, Y. Ding, P. Emma. Introduction. Growing interests in a few fs and sub-fs x-ray pulses. We (and LCLS users) would like to know the compressed bunch length of the LCLS low charge (20 pC ) beam.

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A single-shot method for measuring fs bunches in linac-based FELs

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  1. A single-shot method for measuring fs bunches in linac-based FELs Z. Huang, K. Bane, Y. Ding, P. Emma

  2. Introduction • Growing interests in a few fs and sub-fs x-ray pulses • We (and LCLS users) would like to know the compressed bunch length of the LCLS low charge (20 pC) beam • LCLS S-band transverse cavity resolution is limit at 10~20 fs • (X-band TCAV resolution ~ 4x smaller) • Needs techniques with1-fs resolution (or even lower) • Traditional RF zero-phasing is insufficient in measuring very short bunches because of its sensitive to the initial energy spread • A longitudinal mapping technique developed by T. Smith’s group overcomes this limitation of RF zero-phasing • We propose to use this technique to measure fs bunches in LCLS • (taking into account wakefield of a long linac, SLAC-PUB-14104, 2010)

  3. Initially proposed by E. Crosson et al., 1995 Measurement of 60-mm FEL microbunching at Stanford, 2000

  4. Apply this method to measure fs bunches To high-resolution energy spectrometer Slightly adjust BC2 R56 add a diagnostic chicane R56’ BC2 4.3 GeV Run L3 at zero crossing (-90 deg) h3 L2 (2) Over-compression Zero-crossing sd d z =0 sz • Diagnostic chicane can be part of BC2 • Final energy spread/profile corresponds to short bunch length/profile • Wakefield of long linac must be taken into account

  5. LCLS low charge example • Run LiTrack with 20 pC setup (L2 phase at -31 deg, under-compression) • Run L3 at -90 deg (10 GeV over 4.3 GeV leads to h3= 139 m-1) • Increase BC2 R56 by R56’ = -1/ h3= -7.18 mm • Turn off Linac-3 wake (discussed in next slides) • Needs to measure ~1e-4 energy spread with a high-resolution spectrometer After adjusted BC2 and L3 After nominal BC2

  6. Linac Wakefield • L3 wake introduces an additional energy spread to the measurement • For very short bunches (<10 mm), wake-induced energy spread (primarily a linear chirp) is independent of bunch length • N: # of e- • L: L3 length • a: iris radius Over-compression More over-compression d d Zero-phasing Zero-crossing with wake z z With wake Wakefield un-corrected Wakefield corrected sz sz • This simple wake-correction scheme works for almost arbitrary (short) bunch length we want to measure!

  7. Wakefield compensation • Linac-3 wake can be corrected by a bit more over-compression • Using stronger chirp in Linac-2 • Or using stronger R56 in BC2 • I2 is peak current in L2 (same for all BC2 compression settings) • IA=17 kA, • h3 is L3 chirp by RF zero-phasing • Preferred wake-correction method is by shifting R56 of BC2, which needs to be increased by ~8.08 mm • R56’ (= -7.18 mm = -1/ h3) and • R56 (≈ -0.9 mm for wake compensation)

  8. Wakefield compensation by changing R56 • Run LiTrack with 20 pC (L2 phase at -31 deg, under-compression) • Run L3 at -90 deg (10 GeV over 553 m leads to h3 = 139 m-1) • Turn on Linac-3 wake Increase BC2 R56 by R56’+R56= -8.08 mm Wakefield corrected • Real bunch length • E-spread/chirp R56’ = -8.08 mm Increase BC2 R56 by R56’=-1/ h3= -7.18 mm Wakefield un-corrected

  9. A-line as a high-resolution spectrometer Spectrometer screen (PR18) x= -6.4 m x = 100 m Energy resolution ~1×10-5

  10. Elegant simulation(20 pC, L2 at -31.5 deg) BC2 END L3END A-line PR18 ~ 2 mm

  11. RMS bunch length (Elegantsimulations) • Temporal resolution = Energy resolution (~1×10-5) divides by h3 ~ 100 m-1 • = 0.1 um or 0.3 fs • Wakefield/CSR/LSC add a systematic error ~0.5 fs

  12. Summary • A single-shot method for measuring fs bunches is studied • An experimental test at the LCLS using the A-line spectrometer is planned • The method requires no extra hardware (besides a high-resolution spectrometer) and may be applicable to other XFEL facilities Thanks R. Iverson, J. Frisch, H. Loos et al. for reviving the A-line spectrometer and for many useful discussions

  13. Backup slides

  14. Wakefield compensation by shifting L2 phase • Real bunch length • E-spread/chirp • E-spread/chirp • (shift 2 by 1°) R56’ = -7.18 mm • Phase shift agrees with theory • Wake effect can be corrected empirically by identifying full compression phase through CSR bunch length monitor J. Frisch

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