A singleshot method for measuring fs bunches in
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 p. C) beam LCLS S-band transverse cavity resolution is limit at 10~20 fs (X-band TCAV resolution ~ 4 x smaller) Needs techniques with 1 -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)
Initially proposed by E. Crosson et al. , 1995 Measurement of 60 -mm FEL microbunching at Stanford, 2000
Apply this method to measure fs bunches To high-resolution energy spectrometer Slightly adjust BC 2 R 56 add a diagnostic chicane R 56’ L 2 ( 2) BC 2 4. 3 Ge. V Run L 3 at zero crossing (-90 deg) h 3 d Overcompression Zero-crossing sd =0 z sz Diagnostic chicane can be part of BC 2 Final energy spread/profile corresponds to short bunch length/profile Wakefield of long linac must be taken into account
LCLS low charge example Run Li. Track with 20 p. C setup (L 2 phase at -31 deg, under-compression) Run L 3 at -90 deg (10 Ge. V over 4. 3 Ge. V leads to h 3 = 139 m-1) Increase BC 2 R 56 by R 56’ = -1/ h 3 = -7. 18 mm Turn off Linac-3 wake (discussed in next slides) Needs to measure ~1 e-4 energy spread with a high-resolution spectrometer After nominal BC 2 After adjusted BC 2 and L 3
Linac Wakefield L 3 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: L 3 length a: iris radius d d Overcompression Zero-phasing More overcompression Zero-crossing with wake z sz Wakefield un-corrected With wake z sz Wakefield corrected This simple wake-correction scheme works for almost arbitrary (short) bunch length we want to measure!
Wakefield compensation Linac-3 wake can be corrected by a bit more over-compression Using stronger chirp in Linac-2 Or using stronger R 56 in BC 2 I 2 is peak current in L 2 (same for all BC 2 compression settings) IA=17 k. A, h 3 is L 3 chirp by RF zero-phasing Preferred wake-correction method is by shifting R 56 of BC 2, which needs to be increased by ~8. 08 mm R 56’ (= -7. 18 mm = -1/ h 3 ) and R 56 (≈ -0. 9 mm for wake compensation)
Wakefield compensation by changing R 56 Run Li. Track with 20 p. C (L 2 phase at -31 deg, under-compression) Run L 3 at -90 deg (10 Ge. V over 553 m leads to h 3 = 139 m-1) Turn on Linac-3 wake Increase BC 2 R 56 by R 56’+ R 56 = -8. 08 mm Wakefield corrected • Real bunch length • E-spread/chirp Increase BC 2 R 56 by R 56’=-1/ h 3= -7. 18 mm Wakefield un-corrected R 56’ = -8. 08 mm
A-line as a high-resolution spectrometer Spectrometer screen (PR 18) x = -6. 4 m x = 100 m Energy resolution ~1× 10 -5
Elegant simulation (20 p. C, L 2 at -31. 5 deg) BC 2 END A-line PR 18 ~ 2 mm L 3 END
RMS bunch length (Elegant simulations) Temporal resolution = Energy resolution (~1× 10 -5) divides by h 3 ~ 100 m-1 = 0. 1 um or 0. 3 fs Wakefield/CSR/LSC add a systematic error ~0. 5 fs
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 highresolution 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
Backup slides
Wakefield compensation by shifting L 2 phase • Real bunch length • E-spread/chirp (shift 2 by 1°) R 56’ = -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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