System Layout
ST
E. Allaria, D. Bacescu, L. Badano, C. Bontoiu, D. Castronovo, F. Cianciosi, M.
Cornacchia, P. Craievich, M. Danailov, G. D’Auria, G. De Ninno, S. Di Mitri, B.
Diviacco, M. Ferianis, E. Karantzoulis, S. Milton, S. Noe’, G. Penco, A. Rubino,
L. Rumiz, S. Spampinati, S. Tazzari, M. Trovo’, M. Veronese, D. Zangrando
LBNL J. Corlett, W. Fawley, I. Pogorelov, J. Qiang, M. Venturini, R. Wells, A.
Zholents
Outline:
• Schematic of the machine and design parameters
• Global physics considerations
• Area-by-area:
o design considerations (physics related)
o optics
o total length and diagnostic purposes (sketch)
• Status of the activities
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Schematic of the Machine1 and Design Parameters
LASER
GUN HEATER
diagnostics areas
BC1
BC2
LINAC ~ 150 m
TL
TL ~ 50/70 m
This a cartoon. Buildings and undulator systems no longer look like this
“Medium ” bunch
Bunch length
Peak current
Emittance(slice)
Energy spread(slice)
Flatness, d2E/dt2
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700 fs (flat part)
800 A
1.5 micron
<150 keV
<0.8 MeV/ps2
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“Long” bunch
1400 fs (flat part)
500 A
1.5 micron
<150 keV
<0.2 MeV/ps2
1
D. Bacescu, R. Wells et al.
2
Physics phenomena affecting the e-dynamics
Non-linear effects in bunch compression: rf waveform, T566
Longitudinal and transverse wake fields
Space charge effects (mainly longitudinal)
Coherent synchrotron radiation (CSR)
Chromatic and geometric aberrations
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Physics considerations: Geometric Wake fields1
BTWs flatten the longitudinal
phase space
BTWs placed at
the end of the
linac (L3 and L4)
strong for
short bunches
BTWs twist the transverse
profile
tail
strong for
long bunches
BTWs placed at
the end of the
linac (L3 and L4)
head
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P. Craievich et al.
4
Physics considerations: Magnetic Compression
δ2,E2
BC2
PI
• smaller R56
• Landau
damping
• space charge
• CSR
• linear E-chirp
• BBU
• ………
• nonlinear E-chirp
• ………
⇒ 2 BCs configuration found for MLB and LB.
It provides E-flatness and I-uniformity within the FEL specs.
⇒ 1 BC configuration found for MLB.
In addition, it provides a slice E-spread within the FEL specs.
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Physics considerations: Magnetic Compression
5 MeV
95 MeV
230 MeV
600 MeV
c.f. = 3.5/2.5
60A, 10ps
c.f. = 3.0/4.5
5-15 keV
1.2 GeV
800A, 1ps
500A, 1.8ps
⇒ 2 BCs configuration found for MLB and LB.
It provides E-flatness and I-uniformity within the FEL specs.
⇒ 1 BC configuration found for MLB.
In addition, it provides a slice E-spread within the FEL specs.
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Physics considerations: µ-bunching instability
DRIFT
DIPOLE
ρi
Z LSC
Z CSR
DX REGION
∆γ
Dx
Gρi
Longitudinal Landau damping
Transverse Landau damping
λ < λ// ≡ 2πR56Cσ δ ,i
λ < λ⊥ ≡ 2π ε x H
But few keVs σδ ,i from the photoinjector
is too small to suppress low–frequency µBI
But
large H leads to projected
emittance growth by CSR excitation
• We need a laser heater
• Optics in the chicanes is tuneable2.
It can be relaxed in a compromise with
the CSR induced emittance growth
• Single compression1 scheme for MLB has
been studied
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1
A. Zholents et al.
2 A. Zholents, D. Wang
7
µ-bunching instability: workshop overview
List of participants (total 25)
M. Abo-Bakr E. Allaria G. Bassi S. Biedron M. Borland M. Cornacchia P. Craievich G.
Dattoli S. Di Mitri M. Dohlus J. Ellison W. Fawley Z. Huang T. Limberg M. Migliorati S.
Milton G. Penco J. Qiang M. Trovò S. Spampinati S. Tazzari M. Venturini X. J. Wang J. Wu
A. Zholents
Items of discussion
• Analytical theory limitations.
• 1-D vs. 3-D impedance model (CSR, LSC)
• Code benchmarking and results for FERMI
Basic conclusions
If one trusts CSR and LSC impedance modeling, then one must trust µBI predictions
(and take care of an instability gain >100).
There is a satisfactory convergence of predictions (codes, theory) of the microbunch
instability that provides a reasonable basis for the project design (laser heater, FEL
performance).
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µ-bunching instability: results for FERMI
elegant:
filamented
space
for
0.2%
density modulations
phase
initial
IMPACT: laser heater set to 7 keV rms
Laser heater switched off
J. QIANG
M.BORLAND
Vlasov solver: optimize tuning of laser heater
Fermi MLB in 2 stage compression:
min. σδ,f ≈ 180 keV rms
Fermi MLB in 1 stage compression:
min. σδ,f ≈ 120 keV rms
M.VENTURINI
against FEL spec. of 150 keV rms
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Photoinjector
1.65 m
0.70 m
5 MeV
95 MeV
Total Length = 10 m. Diagnostics: εth, E, σδ
Emittance compensation1 in space charge regime sets the physical requirement for
the distance between the cathode and the 1st acc. module.
A compromise has been reached with engineering constraints2 to include
diagnostics and an external mirror for the laser normal incidence on the photocathode.
1 G. Penco, M. Trovo’
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2
F. Cianciosi
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Laser Heater1
LH Chicane (θb=3.5o)
Matching
Total Length = 13 m. Diagnostics: εproj, E, σδ
Four quadrupoles match the transverse phase space of the incoming beam to the
linac lattice.
A small chicane allows the external seeding for beam heating. Some space has
been saved for additional diagnostics.
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B. Diviacco, S. Spampinati et al. 11
Laser Heater1
screen for
e-γ alignment
laser, λ=780 nm
undulator
2.2
0.55
0.2
space for additional
diagnostics
3.6
Four quadrupoles match the transverse
phase space of the incoming beam to
the linac lattice.
A small chicane allows the external
seeding for beam heating. Some space
has
been
saved
for
additional
diagnostics.
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∆µ=60o
1
B. Diviacco, S. Spampinati et al. 12
First compression
X-band
off-crest
Total Length = 32 m
X-band cavity linearizes the longitudinal phase space. It has been put in the
middle of Linac 1 to minimize chromatic aberrations1.
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First compression
<30 cm
Total Length = 32 m
A movable chicane makes diagnostics life easier. Its length is dictated by the
small bending angle (few degrees), long lateral drifts to minimize CSR effects
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BC1 Diagnostic section
3 screens: Operation ≡ Diagnostic 5 screens: Only Diagnostic, resol. improved
Vertical
RF Deflector
∆µ=45o
BC1
Total Length = 20 m. Diagnostics: E, σz, εproj, σδproj, εxsli, σδsli,……
Mostly complete characterization of the e-beam before and after compression
(the chicane can be switched on/off). Absolute bunch length measurement1.
Total length dictated by the relative phase advance screen-to-screen (εproj) and
deflector-to-screen (εslice). Measurement accuracy improved by 5-screen method
with optional strong focusing2 (only diagnostic mode).
1 P. Craievich, M. Veronese et al.
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2
A. Zholents et al.
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Linac 4
Linac 4
0.5 MeV/ps2
local bumps
Local bumps suppress the “banana shape”
400 µm
40 µm
700 fs
Total Length = 35 m
700 fs
Longitudinal wake field cancels the linear E-chirp
Distributed trajectory bumps suppress the single bunch BBU instability
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Transfer line: Diagnostics1, Collimation and Beam Dump
E-Collimator,
Diagnostic, Feedback
G-Collimators
∆µ=45o
undulators &
beam DUMP
Total Length = 33 m (up to the 1st bend). Diagnostics: E, σz, εproj, σδproj, εsli, σδsli,……
Complete characterization of the e-beam before entering into the undulators
Optics has been separately optimized for diagnostic and operation mode2
Dipoles located beyond the present linac tunnel to operate the linac commissioning
while the undulator hall is in construction
1 P. Craievich, M. Veronese et al.
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2
A. Zholents et al.
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Spreader
2.5 o
+
+
-I
2m 1m
-I
-I
-
-
LINAC
FEL-1
+
FEL-2
+
Q
Q
Total Length = 40 m (FEL-1), 20 m (FEL-2).
FEL lines separation dictates the spreader length for FEL-1. Further reduction
foreseen from 2 m to 1 m.
Strong focusing due to: (i) double bend achromat; (ii) adjustable isochronicity; (iii)
CSR induced projected emittance growth is cancelled by –I transport matrices1.
Perturbed isochronicity (∆R56∼1 mm) drives µ-bunching instability at λ ~ 1 µm. A
stricter control of R56 is required2.
⇓
TL OPTICS REFINEMENT
1
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A. Zholents
2 M. Venturini, A. Zholents
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OPTICS
ACCELERATION
Numbers….
LINAC (L=150m)
Units
# Accel. Modules
16
Max. energy gain
47, 47, 120
MV
78.9
MV
Ave. gradient per module
15
MV/m
Accelerator fill factor
56
%
Off-crest acceleration
92 (MLB), 95 (LB)
%
Ave. energy gain per module
LINAC (L=150m)
TL FEL-1 (L=40m)
TL FEL-2 (L=20m)
# Quads/m
0.2
0.7
1.4
<k> [m-2]
1
2.5
2.5
15, 15
22, 20
12, 10
∆βx/<βx>, ∆βy/<βy>
2, 2
3, 3
3, 3
# Steerers/m
0.2
0.5
0.9
# BPMs/m
0.2
0.6
1.1
<βx>, <βy> [m]
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Status of the Activities
Physics design is 95% complete from the photoinjector to the spreader.
Fermi CDR contains the working design for the MLB and the LB option in the
double compression scheme.
LBNL simulations and benchmarking demonstrate the validity of the single
compression scheme to suppress the µ-bunching instability.
Now refining some items so that the technical design, i.e. the engineering
details, can be completed:
TL optics
3D CSR emittance growth
collimation
beam dump transfer line
resistive wall wake fields (drift inter-modules, TL)
feedback systems
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