A bunch compressor design
and several X-band FELs
Yipeng Sun, ARD/SLAC
2011-04-13, LCLS-II meeting
Design of two bunch compressors
Presentation Title
Page 2
Magnetic bunch compression
Bunch L phase space
e- source
3-Dip. Chicane
RF
Energy modulation (correlation):
RF structure, laser, wake field etc.
Bunch compression
Page 3
Dispersive region:
chicane, wiggler
arc, dogleg etc.
Different bunch compressors
3(4) dipole chicane, R56 <0, T566 >0
achromatic to any order
Wiggler, R56 <0, T566 >0
achromatic to any order?
Arc, R56 >0, T566 >0
2-Dip. Dogleg w/ quad+sextupole,
R56 >0, T566 tunable
Chicane w/ quadrupole+sextupole,
R56 tunable, T566 tunable
NLCTA
chicane
shape
Dispersion relations
Bunch compression
Page 5
Bunch compressor with dipoles and drifts
Bunch compression
Page 6
General chicane (1)
Bunch compression
Page 7
General chicane (1)
Bunch compression
Page 8
General chicane (2)
Bunch compression
Page 9
General chicane (2)
Bunch compression
Page 10
An FEL with
LCLS injector (S-band+X-band
harmonic)
Plus X-band Linac2 and Linac3
Presentation Title
Page 11
Scaling
250-10pC
Total length of accelerator
Assume 70% RF in linac
Final bunch length versus bunch
charge
Presentation Title
Page 12
Longitudinal wake potential
'long' range
Presentation Title
Page 13
Linac3 length needed for de-chirp after BC2
Presentation Title
Page 14
Accelerator shape (LCLS injector + X-band)
Presentation Title
Page 15
LiTrack, LCLS, 250pC, 3kA
Presentation Title
Page 16
LiTrack, LCLS injector+X-band, 250pC, 3kA
Presentation Title
Page 17
Optics
LCLS
LCLS-Injector + X-band
Presentation Title
Page 18
Elegant simulation, 250 pC, 3 kA (w/ and w/o CSR)
LCLS
LCLS w/o CSR
Presentation Title
Page 19
Elegant simulation, 250 pC, 3 kA
LCLS
LCLS-Injector + X-band (½ R56 in BC2, 0.7 bending angle
Presentation Title
Page 20
Elegant simulation, 250 pC, 5 kA
LCLS (L3, 30degree)
LCLS-Injector + X-band (½ R56 in BC2, 0.7 bending angle)
Presentation Title
Page 21
Elegant simulation, 250 pC, 5 kA, Projected emittance
LCLS (L3, 30degree)
LCLS-Injector + X-band (½ R56 in BC2, 0.7 bending angle)
Presentation Title
Page 22
Elegant simulation, 250 pC, 5 kA, Trajectory
LCLS (L3, 30degree)
LCLS-Injector + X-band (½ R56 in BC2, 0.7 bending angle)
Presentation Title
Page 23
LCLS-Injector + X-band (0.5 R56 in BC2, 0.7 bending angle), 250 pC, 5 kA
BC1 end
BC2 entrance
BC2 end
Presentation Title
Page 24
Linac3 end
Potential X-band advantage over S-band
•Maintain a flat energy profile when pushing for shorter bunch length and higher
peak current (i.e. 6kA at 250pC), due to stronger X-band longitudinal wake in
Linac3, to remove energy correlation (chirp); plus possible cancellation of
nonlinear chirp between RF, wake and CSR effects.
•Similar or smaller CSR emittance growth in BC2, benefiting from a weaker dipole
and a larger energy correlation generated in Linac2 (previous argument)
•Compact (300m vs 1000m, at 14GeV)
•For LCLS, increasing current from 3kA to 6kA requires a smaller L1 phase to generate a
longer bunch in ~400m Linac2, so that the L wake chirp is much smaller, and the bunch is
compressed more in BC2 with same L2 phase; if keeping similar L1 phase and increasing
L2 phase (i.e. from 36d to 37.5), the final energy profile will be very nonlinear.
Presentation Title
Page 25
Elegant simulation, 250 pC, 5 kA
LCLS (L1, 19degree; L2, 36degree; L3, 30degree)
LCLS (L1, 22degree; L2, 37.5degree; L3, 0degree)
Presentation Title
Page 26
An X-band RF based FEL with
optics linearization
250 pC
Presentation Title
Page 27
Bunch length after compression
Final coordinate (square)
Minimum length
Neglect small initial un-correlated energy spread
1st order optimal compression:
2nd order optimal compression:
3rd order optimal compression:
Bunch compression
Page 28
Full compression using optics linearization
1st order dispersion
2nd order dispersion
3rd order dispersion
Bunch compression
Page 29
Minimize CSR (1) short interaction time
Bunch compression
Page 30
New design BC1 (1) first order
B1
0.2m
7 degree
B2
0.2m
3 degree
QF
QD
R56 = 17 mm
B3
0.2m
-3 degree
B4
0.2m
-7 degree
New design BC1 (2) second order
SF1&2
SD1&2
T166 = T266 = 0;
symmetric
K3(SF1) = -K3(SD2)
K3(SF2) = -K3(SD1)
T566 = 170 mm
Minimize CSR (2) phase space matching
general
X’
X’
CSR
CSR
X’
specific
CSR
Large β
x
Small β
x
Optimal
β and α
x
Optimized to minimize CSR
impact on emittance
Bunch compression
Page 33
X-band based 2 stage FEL (1) 250pc, 300micron
Bunch compression
Page 34
Final profile at 7GeV (collimation in middle of BC1)
Presentation Title
Page 35
Slice emittance evolution, 250 pC, 6 kA
BC1 entrance
BC1 end
BC2 entrance
Presentation Title
Page 36
Linac3 end
An X-band RF based FEL with
normal chicane BC
10 pC
Presentation Title
Page 37
Max bunch length w/o harmonic RF
Bunch compression
Page 38
Bunch compressor and linac design
BC1
BC2
Bunch compression
Page 39
Linac cell
X-band based 2 stage FEL (3) 10pc, 40micron
54 MeV (C. Limborg)
6 GeV
FEL simulation Setup
• FEL at 2 keV , 6 Å (FEL at 8 keV, 1.5 Å)
• Electron Charge 10 pC, Centroid Energy 6
GeV, peak current 3 kA with profile as
shown in previous slides
– S2E file down to undulator entrance
• LCLS Undulator with larger gap lw = 3 cm
(1.5 cm); beta-function ~ 15 m
Juhao Wu
FEL performance
1.5 angstrom
6 angstrom
Juhao Wu
Presentation Title
Page 42
BC parameters summary
Bunch compression
Page 43
Possible test at NLCTA
Presentation Title
Page 44
Motivation and simulation condition
Motivation
oDemonstrate effective bunch compression (5 to 10 times) with x-band RF
Scheme 1: use normal chicane + positive RF chirp (current NLCTA)
Scheme 2: use optics w/ higher order dispersion + positive/negative RF
chirp (need to install 4/6 sextupoles in the big chicane)
oInvestigate tolerances on timing jitter, misalignment etc.; emittance growth
Simulation condition:
In Elegant, including transverse and longitudinal wake, coherent synchrotron
radiation (CSR), longitudinal space charge (LSC) and velocity bunching
0.5 million macro-particles
For scheme 1, current operating optics
For scheme 2, new optics
20 pC beam at 5MeV, 0.5ps RMS bunch length, 5e-3 RMS energy spread, 1
m.mrad transverse emittance
Beam energy: 60 MeV at BC1, 120 MeV at BC2
Bunch compression
Page 45
NLCTA optics (current operation)
R56 =-73mm
T566 = 111mm
R56 =-10mm
T566 = 15mm
Bunch compression
Page 46
Scheme 1 (1) L phase, current and bunch length
Initial
Linac1
BC1
Linac2
Bunch compression
Page 47
BC2
Scheme 1 (2) no compression, on crest
Initial
Linac1
BC1
Linac2
Bunch compression
Page 48
BC2
Scheme 1 (3) 2 stage compress 20 times, end
Bunch compression
Page 49
Scheme 1 (4) effect of timing jitter, near full
compression
Timing jitter between laser and RF (assumed same for two RF sections)
On phase
+ 115 fs (0.5 degree)
Bunch compression
Page 50
- 115 fs
Scheme 1 (5) effect of timing jitter, under
compression
Timing jitter between laser and RF (assumed same for two RF sections)
On phase
+ 115 fs (0.5 degree)
Bunch compression
Page 51
- 115 fs
Scheme 2 (1) optics
Install 4/6 sextupoles in the big chicane
6 meters long
Chicane w/ quadrupole+sextupole,
R56 tunable, T566 tunable
Bunch compression
Page 52
Scheme 2 (2) L phase and current
Bunch compression
Page 53
Scheme 2 (3) 1 stage compress 10 times, end
Bunch compression
Page 54
Scheme 2 (4) Sensitivity to timing jitter
Deviation between analytical formulae and simulation due to:
Small difference of beam(RF) parameters being employed
Collective effects in simulation
Bunch compression
Page 55
Thank you for your patience!
I would like to thank the following people for their great help and useful
discussions:
C. Adolphsen , K. Bane, A. Chao, Y. Cai, Y. Ding, J. England, P. Emma, Z. Huang,
C. Limborg, Y. Jiao, Y. Nosochkov, T. Raubenheimer, M. Woodley, W. Wan, J. Wu
Bunch compression
Page 56
Current less sensitive to RF phase jitter
20pC, 80 micron
Bunch compression
Page 57
LCLS
150 MeV
z  0.83 mm
  0.10 %
6 MeV
z  0.83 mm
  0.1 %
rf
gun
Lh
L =0.6 m
rf=-160
250 MeV
z  0.19 mm
  1.8 %
...existing linac
L1
DL-1
L =12 m
R56 0
13.6 GeV
z  0.022 mm
  0.01 %
Paul Emma
L =9 m
rf = -25°
L =6 m
4.3 GeV
z  0.022 mm
  0.76 %
X
L =330 m
rf = -41°
L =550 m
rf = -10°
L2
L3
BC-1
L =6 m
R56= -36 mm
BC-2
L =22 m
R56= -25 mm
undulator
L =120 m
DL-2
L =66 m
R56 = 0
TESLA-XFEL
6 MeV
z  2.0 mm
  0.1 %
120 MeV
z  0.5 mm
Lh   2.0 %
L =1.4 m
rf= -191
L=8m
rf  -22°
rf
gun
L0
C
L = 16 m
rf = -40°
L1
375 MeV
z  0.1 mm
  1.4 %
1.64 GeV
z  0.020 mm
  0.5 %
20.5 GeV
z  0.020 mm
  0.01 %
L = 72 m
rf = -40°
L  850 m
rf = 0°
L2
L3
BC-1
BC-2
L4m
L  14 m
R56= -76 mm R56= -36 mm
undulator
L =? m
BC-3
L 18 m
R56= -11 mm
(2003 parameters)
Energy change + optics (dispersion) (2)
Emittance & trajectory (slice)
For sufficiently large slice number, one can assume same energy change
in one slice
Change slice trajectory
Other terms
Change slice emittance
CSR energy change + phase rotation (smear)
Emittance & trajectory (slice)
For over-compress, CSR-process can be treated as an
integral process, with continuing bunch compression
(lengthening).
Change slice trajectory
& emittance
Negligible