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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. - PowerPoint PPT Presentation
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A single-shot method for measuring fs bunches in linac-based FELs
Z. Huang, K. Bane, Y. Ding, P. Emma
Growing interests in a few fs and sub-fs x-ray pulses
Introduction
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)
Measurement of 60-mm FEL microbunching at Stanford, 2000
Initially proposed by E. Crosson et al., 1995
BC2 4.3 GeVBC2 4.3 GeV
add a diagnostic chicane R56’add a diagnostic chicane R56’
Run L3 at zero crossing (-90 deg) h3Run L3 at zero crossing (-90 deg) h3L2 (2)
To high-resolutionenergy spectrometerSlightly adjust BC2 R56
=0
Final energy spread/profile corresponds to short bunch length/profileWakefield of long linac must be taken into account
d
z
sz
Over-compression
sdZero-crossing
Apply this method to measure fs bunches
Diagnostic chicane can be part of BC2
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)
LCLS low charge example
After nominal BC2
After adjusted BC2 and L3
Needs to measure ~1e-4 energy spread with a high-resolution spectrometer
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
Linac Wakefield
d
z
sz
Over-compression
Zero-phasing
With wake
d
z
sz
More over-compression
Zero-crossingwith wake
Wakefield un-corrected Wakefield corrected
This simple wake-correction scheme works for almost arbitrary (short) bunch length we want to measure!
N: # of e- L: L3 lengtha: iris radius
Linac-3 wake can be corrected by a bit more over-compressionUsing 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
Wakefield compensation
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)
Wakefield compensation by changing R56
R56’ = -8.08 mm
• Real bunch length• E-spread/chirp
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’=-1/ h3= -7.18 mm
Wakefield un-corrected
Increase BC2 R56 by R56’+R56 = -8.08 mm
Wakefield corrected
A-line as a high-resolution spectrometer
Spectrometer screen (PR18)x = -6.4 mx = 100 mEnergy resolution ~1×10-5
Elegant simulation (20 pC, L2 at -31.5 deg)
L3ENDBC2 END
A-line PR18
~ 2 mm
RMS bunch length (Elegant simulations)
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
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
Backup slides
Phase shift agrees with theory
Wake effect can be corrected empirically by identifying full compression phase through CSR bunch length monitor
• Real bunch length
• E-spread/chirp• E-spread/chirp (shift 2 by 1°)
Wakefield compensation by shifting L2 phase
J. Frisch
R56’ = -7.18 mm