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
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)
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 BC2 R56
add a diagnostic chicane R56’
L2 (2)
BC2 4.3 GeV
Run L3 at zero crossing (-90 deg) h3
d
Over-compression
Zero-crossing
sd
z
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
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 nominal
BC2
After adjusted
BC2 and L3
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 eL: L3 length
a: iris radius
d
d
Over-compression
More over-compression
Zero-phasing
Zero-crossing
with wake
z
z
With wake
sz
Wakefield un-corrected
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 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)
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
Increase BC2 R56 by R56’=-1/ h3= -7.18 mm
Wakefield un-corrected
R56’ = -8.08 mm
A-line as a high-resolution spectrometer
Spectrometer
screen (PR18)
x = -6.4 m
x = 100 m
Energy resolution
~1×10-5
Elegant simulation (20 pC, L2 at -31.5 deg)
BC2 END
A-line PR18
~ 2 mm
L3END
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 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 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