Yujong Kim, K. Floettmann, and T. Limberg

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November 13th-15th, 2004, KEK, Japan
New Bunch Compressors for ILC
Yujong Kim, K. Floettmann, and T. Limberg DESY
Hamburg, Germany Dongchul Son and Y. Kim The
Center for High Energy Physics, Daegu, Korea
TESLA-S2E-2004-47
Yujong.Kim_at_DESY.de, http//www.desy.de/yjkim
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Bunch Compressor in TESLA TDR
BC Parameters Energy 5.0 GeV Charge 3.2
nC Initial energy spread at DR exit
0.13 Initial rms bunch length 6 mm Final rms
bunch length 0.3 mm Compression factor
20 Initial horizontal emittance 8.0
mm.mrad Initial vertical emittance 0.02 mm.mrad
- One stage sensitive to RF jitter - Many
dipoles twelve for 3.23 deg bending
six for 6.46 deg bending -
ISR CSR problem, even microbunching instability
- Short-range wakefields in TESLA modules are
not included in TDR BC part
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New BC Layout for ILC (10NOV04 Version)
Initial parameters E 5.000 GeV ?? 0.13
(small !) ?z 6.0 mm ?nx 8.0 ?m, ?ny 0.02 ?m
1/8.9 1/2.2
?z 6.00 mm 673 ?m
300 ?m
Q3.2 nC e-beam
BC1
BC2
ACC1
ACC2
ACC4
ACC3
ACC39
Damping Ring
E 5.689 GeV ?? 2.4 R56 236 mm ? 5.3 deg
E 6.0 GeV ?? 2.174 R56 17 mm ? 1.4 deg
13.3 MV/m -21.5 deg
23.4 MV/m -45 deg
24.8 MV/m 170.0 deg
Up to main Linac ELEGANT with CSR, ISR, and
geometric short-range wakefields. but without
space charge
Final parameters E 6.0 GeV ?? 2.173 ?z 300
?m ?nx 8.7 ?m, ?ny 0.02 ?m
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New Bunch Compressors Layout for ILC
From various experiences in Start-To-End
simulations and BC designs for TTF2, European
XFEL, SCSS, PAL XFEL, we have chosen two BC stages
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Twiss parameters along BC beamline
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Longitudinal phase space around modules
Head
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Longitudinal phase space around BC1
Linearization by two ACC39 modules (3.9 GHz)
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Longitudinal phase space around BC2
Edges over (or under)compression by the
nonearlity due to wakefieds, T566, RF curvature
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Longitudinal phase space around BC2
Edges over (or under)compression by the
nonearlity due to wakefieds, T566, RF curvature
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New Bunch Compressors Layout for ILC
From various experiences in Start-To-End
simulation and BC designs for TTF2, European
XFEL, SCSS, PAL XFEL, we have chosen two BC
stages.
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New Bunch Compressors Layout for ILC
From various experiences in Start-To-End
simulation and BC designs for TTF2, European
XFEL, SCSS, PAL XFEL, we have chosen two BC
stages.
8.9 times reduction
2.2 times reduction
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New Bunch Compressors Layout for ILC
From various experiences in Start-To-End
simulation and BC designs for TTF2, European
XFEL, SCSS, PAL XFEL, we have chosen two BC
stages.
8.7 mm.mrad
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New Bunch Compressors Layout for ILC
From various experiences in Start-To-End
simulation and BC designs for TTF2, European
XFEL, SCSS, PAL XFEL, we have chosen two BC
stages.
Before BC1
After BC2
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New Bunch Compressors Layout for ILC
From various experiences in Start-To-End
simulation and BC designs for TTF2, European
XFEL, SCSS, PAL XFEL, we have chosen two BC
stages.
Before BC1
After BC2
15
New Bunch Compressors Layout for ILC
From various experiences in Start-To-End
simulation and BC designs for TTF2, European
XFEL, SCSS, PAL XFEL, we have chosen two BC
stages.
Before BC1
After BC2
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Summary
From various experiences in Start-To-End
simulations and BC designs for TTF2, European
XFEL, SCSS, and PAL XFEL projects, we have chosen
two BC stages, we have designed one possible new
BC layout for ILC. Detail design methods will be
discussed during discussion time. Checking of
possibility of 2nd harmonic modules (2.6 GHz)
We should check end effects by simulating from
gun to damping ring. See next pages, which are
related with discussion.
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Bunch Compressor (BC) Working Principle
Bunch Compressor Layout for SCSS Project - Y. Kim
et al, NIMA 528 (2004) 421
dE
dt
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Coherent Synchrotron Radiation (CSR) in BC
In BC where dispersion is nonzero, bunch length
becomes smaller. Short electron bunches in dipole
can radiate coherently (CSR) at wavelength CSR
from tail electrons can overtakes head electrons
after the overtaking length. CSR
generates correlated energy spread along bunch
Electrons are transversely kicked at the
nonzero dispersion region or in BC Hence,
projected emittance is increased in BC due to CSR.
LOT
Tail
Head
CSR from tail
Electron path
Dipole region
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Coherent Synchrotron Radiation (CSR) in BC
Courtesy of M. Dohlus
Without CSR self-interaction
Head with lower energy
DM3
DM4
DM2
DM1
With CSR self-interaction
Head energy gain by CSR
x
Tail energy loss due to CSR
z
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CSR Wakefield for None Uniform Beams
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LCLS CSR Microbunching Instability in BCs
Courtesy of P. Emma and M. Borland
slice emittance and energy spread are changed !
projected emittance growth is simply steering
of bunch head and tail
0.5 ?m
ELEGANT Tracking
slice emittance is not altered
without space charge force
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Action of Longitudinal Space Charge (LSC)
If there is a density modulation, space charge
pushes electrons from higher density region to
lower density region, creating energy modulation
in the process.
Density modulation
Energy modulation
Space charge oscillation frequency (Z. Huang et
al, PRST-AB Volume 7, 074401, 2004)
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Gain of LSC Microbunching Instability 1D
Gain of LSC Microbunching Instability Z is
intergrated LSC impedance. (Schneidmiller et al,
TESLA-FEL-2003-02). Here amplitude damping due to
emittance and 2D model are ignored !
1-D longitudinal space charge impedance (LSC) per
unit length oscillation frequency are
(Z.
Huang et al, PRST-AB Volume 7, 074401, 2004).
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LSC CSR Microbunching Instability
Induced energy modulation
Initial current density modulation
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Microbunching Instability at TTF2
Analytically estimated maximum gain is about 320
at 2.0 ps
Beam energy 1.0 GeV (2006) Peak current 2.5
kA rms bunch length 50 µm 166 fs Bunch
separation 111 ns Number of bunch per train
7200 Repetition rate 10 Hz Slice transverse
normalized rms emittance 2 µm rms beam size
68 µm rms energy spread 1 MeV _at_ 1.0
GeV Undulator period 27.3 mm Undulator gap 12
mm K-parameter 1.17 Undulator length 30
m Peak magnetic field 0.495 T SASE source
wavelength 6.4 nm 120 nm Saturation length
27 m Peak power 2.8 GW

Average power 40 W

(Schneidmiller et al, TESLA-FEL-2003-02) FEL
pulse length (FWHM) ca. 100 fs Peak (average)
spectral brightness 2.41030 (3.51022)
photons/sec/mrad2/mm2/0.1BW
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New S2E Simulations on TTF2 Microbunching
2.0 ps modulation with 1.5 million particles
(Yujong Kim et al, EPAC2004)
Gain of Microbunching Instability at TTF2 is not
so high !!!
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BC Design Concepts against Microbunching
Under 2.0 ps modulation with 20
amplitude, S-type chicane generates much stronger
microbunching instability after BC3
Therefore, we choose the normal 4-bend chicane
instead of S-type chicane !
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BC Design Concepts against Microbunching
Strong compression at BC1 to increase
uncorrelated energy spread at BC2. Large
uncorrelated energy spread at BC2 by putting the
BC2 at a lower energy region, which is useful to
avoid over-compression at BC2 due to long.
wakefields.
BC for Phase-I of SCSS Project
(Before BC)
(After BC)
To keep longitudinal normalized emittance during
compression, uncorrelated energy spread is
automatically increased by the compression
factor. If we consider space charge force in BC,
this is bigger ! (Private communication with M.
Dohlus)
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Concepts against CSR Chromatic Effects

• Choosing a somewhat larger energy spread, we can
  choose a smaller R56
• Choosing a long drift space ?L to reduce bending
  angle for a required R56

European XFEL (10AUG04 Version)
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Concepts against CSR Chromatic Effects

• Linearization of longitudinal phase space with a
  higher harmonic cavity

- means deceleration !
Linearized range 60 degree 29.2 ps 3.7
times of 8 ps bunch
-70 MeV deceleration
Only C-band Linac
C-band Linac X-band Correction Cavity
SCSS BC
Y. Kim et al, NIMA 528 (2004) 421
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Concepts against CSR Chromatic Effects
European XFEL (10AUG04 Version)
BC1 1.76 µm ? 94 µm Compression factor 18.7
BC2 94 µm ? 21.5 µm Compression facto 4.4
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Concepts against CSR Chromatic Effects

• Strong focusing lattice around BCs to reduce CSR
  induced emittance growth

Strong focusing against CSR ?-functions 0
?-functions 3
Before BC1
After BC1
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Concepts against CSR Chromatic Effects

• Shorter linac with a lower frequency between BC1
  and BC2 to control over-
• compression and CSR at BC2 due to the
  short-range longitudinal wake fields.

Courtesy of P. Emma
BC2 for LCLS
Over-compression after BC2 due to longitudinal
wakefields in the long L2 linac
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Concepts against CSR Chromatic Effects

• Small quadrupole length (shortest 5 cm) around
  BCs to reduce chromatic
• effects.

Triplet with 5cm-10cm-5cm length for TTF2 BC2
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BCs for European XFEL (13JAN04 Version)
With TESLA XFEL Injector, ?n 0.9 ?m
?z 1.76 mm 113 ?m 23
?m
Q1.0 nC e-beam
BC1
BC2
ACC1
ACC2
ACC4
ACC5
RF-GUN
ACC39
ACC6
ACC3
E 510 MeV ?? 1.89 R56 87 mm ? 3.95 deg
E 510 MeV ?? 1.88 R56 4.8 mm ? 0.93 deg
20.65 MV/m 0.0 deg
11.5 MV/m 25.0 MV/m -18.3 deg
34.8 MV/m 160.6 deg
20.5 MV/m -29.6 deg
60 MV/m 38 deg
ASTRA with Space Charge
To the end of Linac ELEGANT with CSR

with geometric wakefields

without space charge
0.0 m
12.0444 m
?z 20.5 ?m
FODO MODULES
1567 m
UNDULATOR, 200 m
ACC7
ACC8
ACC118
ACC9
?
All projected parameters !
E 20.0 GeV ?? 0.008 ?x 37.3 ?m, ?y 31.6
?m, ?z 20.5 ?m ?nx 1.15 ?m, ?ny 0.94 ?m
20.65 MV/m 0.0 deg
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Longitudinal Phase Space (13JAN04 Version)
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Slice Parameters at LINAC END (13JAN04)
??u 4.8E-5 before BC2, which is about four
times higher than that of TTF2 Therefore we are
safe from the strongest microbunching with 2.0 ps
period.
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Slice Parameters at LINAC END (13JAN04)
No slice emittance growth in core !
RED before BC1 BLUE LINAC END
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BCs for LCLS
Without any laser heater and SC-wiggler, ??u
3.0E-6 before BC-2
Laser Heater to give ??u 9.0E-5 at the
undulator entrance
SC-wiggler to give ??u 3.0E-5 before BC-2
6 MeV
rf gun
Linac-X L 0.6 m
Linac-3 L 550 m
new
Linac-1 L 9 m
Linac-2 L 330 m
Linac-0 L 6 m
14.1 GeV
undulator L 130 m
25-1a 30-8c
21-1b 21-1d
X
21-3b 24-6d
...existing linac
BC-1 L 6 m
BC-2 L 22 m
DL-1 L 12 m
DL-2 L 66 m
135 MeV
250 MeV
4.54 GeV
Over-compression at BC2 due to strong
longitudinal wakefield at long L2 linac. Strong
CSR at BC2 due to low compression at BC1 and high
compression at BC2 High beam energy at BC2
Strong microbunching instability due to small
uncorrelated energy spread at BC2
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LCLS BC Performance
Courtesy of P. Emma
Compression factor at BC 1 4.4 Compression
factor at BC2 8.6 ? Strong CSR at BC2
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LCLS BC Performance
Even though projected emittance is large, slice
emittance is good enough for SASE source
saturation when laser heater is operated well.
projected emittance along linac without any heater
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BCs for PAL XFEL (05DEC03 Version)
With S-band Photoinjector, ?n 0.9 ?m
?z 829 ?m 114 ?m
26 ?m
Q1.0 nC e-beam
BC1
K3
X1
X2
X3
K2
X2X
BC2
RF-GUN
E 442 MeV ?? 1.84 R56 38.9 mm ? 3.5 deg
E 700 MeV ?? 1.31 R56 6.7 mm ? 1.45 deg
20.0 MV/m 0.0 deg
30.0 MV/m -45.0 deg
18.0 MV/m 30.5 MV/m 5.5/-2.5 deg
56.75 MV/m 180.0 deg
30.0 MV/m -27.5 deg
120 MV/m 32.02 deg
ASTRA with Space Charge
To the end of Linac ELEGANT with CSR

with geometric wakefields

without space charge
0.0 m
8.7 m
?z 26 ?m
227 m
UNDULATOR, 60 m
K4
K5
K12
K6
?
All projected parameters !
E 3.389 GeV ?? 0.033 ?x 68 ?m, ?y 62 ?m,
?z 26 ?m ?nx 1.0 ?m, ?ny 0.9 ?m
20.0 MV/m 0.0 deg
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Parameters along PAL XFEL Linac
No slice emittance growth in core !
Projected emittance
RED before BC1 BLUE LINAC END
1.0 µm
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Slice Parameters at PAL XFEL LINAC END
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1st BC Performance Comparison
ASTRA simulation and measured result give about
4.0 keV intial uncorrelated energy spread.

LCLS TESLA XFEL
PAL XFEL BC type
Two Stage One Stage
Two Stage Beam energy
250 MeV 511 MeV
442 MeV Relative energy
spread 1.78
1.89 1.84 Uncorrelated
energy spread Bending angle
4.62 deg
3.95 deg 3.5 deg Momentum
compaction R56 35.9 mm
87 mm 38.9 mm Total
chicane length 6.56 m
20.0 m 12.2
m Dipole length
0.20 m 0.30 m
0.30 m Drift length ?L
2.6 m 8.9 m
5.0 m Initial rms bunch length
0.83 mm 1.76 mm
0.82 mm Final rms bunch length
190 ?m 113 ?m
114 ?m Compression factor
4.4 15.6
7.2 Initial projected
emittance 0.900 ?m 0.907
?m 0.900 ?m Final projected
emittance 0.995 ?m
1.020 ?m 1.010 ?m Initial
uncorrelated energy spread is rms value which is
estimated at /- 0.1 mm core region
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2nd BC Performance Comparison
Initial uncorrelated energy spread at BC2 is
increased after compression at BC1

LCLS TESLA XFEL
PAL XFEL BC type
Two Stage One Stage
Two Stage Beam energy
4.54 GeV 511 MeV
700 MeV Relative energy
spread 0.76
1.88 1.31 Uncorrelated
energy spread Bending
angle 1.878 deg
0.930 deg 1.450
deg Momentum compaction R56 22.5 mm
4.8 mm 6.7
mm Total chicane length
22.1 m 20.0 m
12.2 m Dipole length
0.40 m 0.30 m
0.30 m Drift length ?L
10.0 m 8.9
m 5.0 m Initial rms
bunch length 195 ?m
113 ?m 114 ?m Final rms
bunch length 22 ?m
20.5 ?m 26 ?m Compression
factor 8.86
5.5
4.38 Initial projected emittance 0.995
?m 1.020 ?m
1.010 ?m Final projected emittance
2.725 ?m (SW on) 1.200 ?m
1.116 ?m All uncorrelated energy spread is
estimated at /-0.1 mm core region and without
any heater !
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Optimized BCs for various XFEL Projects
PAL XFEL (21JUL04 Version) by Yujong Kim
European XFEL (10AUG04 Version) by Yujong Kim
48
Optimized BCs for various XFEL Projects
6 GeV SPring-8 Compact SASE Source (30SEP04
Version) by Yujong Kim