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Multi-user, High Repetition-Rate,Soft X-ray FEL User Facility
(based on a Collinear Dielectric Wakefield Accelerator)
Euclid Techlabs LLC: C.Jing, A.Kanareykin, P.SchoessowArgonne National Laboratory, HEP: W.Gai, G.Ha, C.Li, J.G.PowerArgonne National Laboratory, APS: R.Lindberg, A.ZholentsNorthern Illinois University: P.Piot
John Power, Argonne
Assessment of Opportunities
High Brightness Beams Workshop, San Juan, Puerto Rico, March 25, 2013
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Multi-user, High Rep Rate, Soft X-ray FEL User Facility
expe
rimen
tal e
nd s
tatio
ns
1 MHz ACCELERATOR(BLACK BOX)
2 GeV50 MeV
Low-emittance injector:• 1 MHz bunch rep. rate
Flexible x-ray beamlines• Tunable pulse length• Seeded• 2 color seeded• SASE
Lasers linked with a fiber-optics time
distribution network
Beam spreader• 100 kHz bunch rep. rate
Capable of serving~2000 scientists/year
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Multi-user soft x-ray FEL facility based on: SRF linac
expe
rimen
tal e
nd s
tatio
ns
Capable of serving~2000 scientists/year
Low-emittance injector:• 1 MHz bunch rep. rate
Flexible x-ray beamlines• Tunable pulse length• Seeded• 2 color seeded• SASE
Lasers linked with a fiber-optics time
distribution network
Beam spreader• 100 kHz bunch rep. rate
2 GeV50 MeV
~50 m~100 m
~ 300 m
~ 250 m
~ 50 m
750m
CW superconducting linac~1MHz bunch rep. rate~2 GeV beam energy~1 kA peak current
Multi-user soft x-ray FEL facility based on: DWFA linac
~50 m
~25 m
CompactBeam
Spreader
Facility Footprint350m x 250m
~50 m
4
~50 m
350m750m
expe
rimen
tal e
nd s
tatio
ns
~30 m
Compact DWFA linac~1MHz bunch rep. rate~2 GeV beam energy~1 kA peak current
~100 m ~50 m
2 GeV
BeamShaper
Dielectric Wakefield Acceleration (DWFA) linac
200 MeV
ConfigurableFEL Array
1 keVX-rays
End S
tations
1.2 GeV100 pC
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Flexible x-ray
beamlines
Flexible accelerator beamlines
……
……
ConfigurableDWFA Accelerator
0.5 keVX-rays
2.4 GeV50 pC
Ultra-flexible facility
Dielectric Wakefield Acceleration (DWFA) linac
Motivation for DWFA for the High Rep Facility Low energy spreader Accelerating gradient > 100 MV/m Room temperature quartz fibers Tunable electron beam energy of a few GeV Tunable peak current > 1KA Bunch rep. rate of the order of 1MHz
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Compact
Inexpensive
Flexible
Many hurdles to overcome as you will see…
Is it possible to replace some of the SRF linac with a DWFA linac??
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CollinearDielectric Wakefield Acceleration
FUNDAMENTALS:
Simple geometry Capable of high gradients Easy dipole mode damping Tunable Inexpensive
Recent results (obtained for Linear Collider development):− 1000MV/m level in the THz domain (UCLA/SLAC group)− 100 MV/m level in the MHz domain (AWA/ANL group)
Cylindrical Dielectric Wakefield Accelerator
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2
2
21( ) exp cos( )
2z
Zn
QW z kz
a
2b 2a
eQ
Cu
-300
-200
-100
0
100
200
300
-0. 25 0. 25 0. 75 1. 25 1. 75 2. 25 2. 75
Di stance (mm)
Wz(M
V/m/
1nC)
Wakefield Amplitude Dependence onAperture or 1/f
1
10
100
1000
10000
100000
0.01 0.1 1 10
Inner Radius a (mm)
Ez(
MV
/m/1
0n
C)
a=240 um; Q=1 nC; bunch length=0.5 ps (FWHM), f=650 GHz
Wake field in dielectric tube induced by a short Gaussian beam
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The Wakefield Theorem and the Transformer Ratio
(Maximum energy gain behind the drive bunch)(Maximum energy loss inside the drive bunch) < 2R = W+
W-=
W-
W+
The R< 2 limit has kept interest in
collinear wakefield accelerators to a
minimum.1.5 0.25 1 2.25 3.5 4.75 6
0.2
0.1
0
0.1
0.2
Wak
efie
ld (
MV
/m/n
C)
CollinearDielectric Wakefield
Acceleration
DRIVE WITNESS
Methods to increase R>2 in a collinear wakefield accelerator
Ramped Bunch
Ramped Bunch Train(demonstrated at ANL)
Reference: Schutt et. al., Nor Ambred, Armenia, (1989)
Reference: Bane et. al., IEEE Trans. Nucl. Sci. NS-32, 3524 (1985)
c
(z)W+
W-z
zd d
W -
W+
d
r (z)
Road map to a high energy gain acceleration
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A case study of an x-ray FEL user facility based on a 2.4 GeV DWFA
EXAMPLE:
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High rep. rate, X-ray FEL user facilitybased on a 2.4 GeV DWFA
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FEL10
FEL2
FEL1
P=320 kW, 1 MHz
~30m
Quartz
DWFA
1.6 nC
TR = 16.5
ID=400 umfreq = 850 GHz
ID, OD, Length 400 m, 464.7 m, 10 cm
, tan 3.75, 0.6x10-4
Freq. of TM01, TM02, TM03 850 GHz, 3092 GHz, 5749 GHz
Q of TM01, TM02, TM03 1260, 3173,4401
r/Q of TM01, TM02, TM03 94.1 k/m, 3.2 k/m, 0.5 k/m
ng of TM01, TM02, TM03 0.592c, 0.794c, 0.813c
Key technology:DWFA RF structure design
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Quartz DWFA ID=400 um
RF pulsed heating • DT ~ 20 ºC
Average thermal heating • Average power load 50 W/cm2
@100 kHz rep rate
How can a small DWFA can handle High Rep Rate????
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RF packet ~333 ps
e
Collinear DWFA• Ultra-short RF pulse (~333 ps)• Heating is much less severe than
microwave accelerator
Quartz DWFA ID=400 um--cooling--
Triangular bunch
Double triangular bunch
TR~10
TR~17
Key technology: drive bunch shaping enhances transformer ratio
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dzzzWE 10 MeV in 10 cm
Key technology: witness bunch generation
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Drive and Witness from the same source bunch minimal timing jitter
QFQD QD QF
Emittance exchangeT
QDQF
B
QDQF
B
B
B
-I -I
QD QF
QDQF
B
QDQF
B
B
B
-I -I
QD QF
TM110 TM010TM010
Deflecting cavity
Emittance exchangeFODO
Double EEX technique: a convenient tool for drive and witness bunch shaping
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Before mask After mask At EEX exitmask
(c)
2 1 0 -1 -2time (ps)
witn
ess
120010008006004002000
curr
ent (
A)
z →x emit. exch. x →z emit. exch.
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Accelerated currentWakefield
Key technology: How to handle beam loading:
Eacc=115 MV/m
~DE=30 MV/m
Gaussian Electron bunch• Large energy spread• Strongly chirped in energy
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Key Technology: Undulator
BAD: Accelerated beam is strongly chirped (little FEL gain) BAD: Using the chirp to compress the beam does not seem to be useful for radiation GOOD: For short beams (<10 mm rms) the energy chirp is approximately linear in time
Longitudinal GradientTapering the undulator strength or period can counteract large energy chirp and maintain gain
Transverse GradientVarying the undulator strength transversely can counteract large energy chirp and maintain gain
𝑡
Δ𝛾
Δ𝛾𝛾 0
𝑥
N
S
Smaller undulator strength K
Larger undulator strength K
Strongly chirped beams for FEL applications
Strongly chirped beams for FEL applications: preliminary results
Linear gain
Nonlinear regime
Tapering the undulator strength K
Power evolution of DWFA beam + undulator taper Power profile near
saturation z/LG = 20Chirped SASE spectrum near saturation z/LG = 20
Some applications favor wide bandwidth21
Example: Longitudinal Gradient witness beam chirp
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Can we reduce energy spread due to beam loading?
Gaussian witness bunch 15 10 5 0 5 10 15 z (um)
110
100
90
80
70
Ener
gy (M
eV)
∙Q=50 pC∙Edec=13.6 MV/m∙Eacc=81.7 MV/m∙sigmaE=5.3%∙R=6
Gaussianbunch
Key idea: Match the curvature of the self-wake to the drive wake
Witness self-wake
Drive-wake
~20x reduction in energy spread
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Reversetriangularbunch ∙Q=50 pC∙Edec=6.3 MV/m∙Eacc=86.3 MV/m
20 10 0 10 20 z (um)
110
100
90
80
70
Ener
gy (M
eV)
Reverse triangular witness bunch
d=0.3%
R=14
Beam pipe OD, 2b 1.14 mm
Dielectric tube OD, 2a 1.24 mm
Waveguide cutoff 298 GHz
Charge of the drive bunch 5 nC
Length of the drive bunch 2.127 ps
Charge of the witness bunch 250 pC
Length of the witness bunch 75 fs
Time between the bunches 9.4 ps
Transformer ratio 3.16
ΔG/G 1.5*10-5
By additionally customizing the shape of the main bunch we designed the configuration which minimizes the wakefield-induced energy spread in the main bunch. The energy spread may be made as low as 0.001%.
Minimization of the energy spread in a witness bunchCourtesy of E. Simakov, LANL
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General (nonlinear) shapes are possible
leaf
Multi-leaf collimator:• Used in medical linacs to shape the x-rays • Each vertical leaf moves independently
Multi-leaf collimator
Varian's 120-leaf multileaf collimator
Varian's high-definition multileaf collimator
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Feedback on desired witness and drive shape
http://varian.mediaroom.com/index.php?s=31899&mode=gallery&cat=2473
QD QF QDQF
B
QDQF
B
B
B
-I -I
QD QF
Emittance exchange
20 10 0 10 20 z (um)
110
100
90
80
70
Ener
gy (M
eV)
Multi-leaf mask Measured Spectrum
FEEDBACK
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Demonstrate EEX based bunch shaping at the Argonne Wakefield Accelerator
BEGINNING EXPERIMENTAL STUDIES 1:
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Demonstrate bunch shaping using a double-dog leg EEX beamline
RF Photocathode Gun
Linac Quads
Mask
20 deg
14 MeV
B1 B2TDC
8 MeV
B1B2 B3 B4
at the AWA Facility
Demonstrate bunch shaping and compare measured shape to 1st order theory Measure EEX transfer matrix Study 2nd order effects in beamline Study space charge effects in beamline
Initial experimental goals:
TheArgonne Wakefield Accelerator Facility Low Energy (14 MeV) beamline
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Demonstrate bunch shaping using a double-dog leg EEX beamline
chirp
RF Photocathode Gun
Linac Quads
multiple masks on motorized actuator
20 deg
14 MeV
B1 B2TDC
8 MeV
B1B2 B3 B4
at the AWA Facility
Key tunable parameters
x’ slopex, y beam size
TheArgonne Wakefield Accelerator Facility Low Energy (14 MeV) beamline
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Demonstrate bunch shaping using a double-dog leg EEX beamline
Example: Experiment I - Shaping capability
Multiple masks will be used to study the bunch shaping capability of the double dog-leg EEX beamline
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Propagation of drive beam through a 10 meter DWFA linac at APS
BEGINNING EXPERIMENTAL STUDIES 2:
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Drive bunch through a ID=400 mm fiber !!! ID=400 um
Goal: Propagate drive bunch through meter scale DWFA• With no focusing
• Beam size will triple in one meter! • External focusing channel around dielectric
• ~10-20 cm focal length• Control SBBU with BNS damping
Drive bunch:• Charge = 1.6 nC• Normalized emittance = 2 mm• Beam energy = 50 MeV (close to the accelerator end)• Beam size = 50 mm (Beta function ≈ 10 cm)
10 m long structure test in APS LEUTL tunnel
1. APS will install LCLS type e-gun in 2013 • 0.5 nC, 500 fs, 1 mm bunches • Beam into the LEUTL tunnel in 2014
2. Propagate beam through 10 m long DWFA at APS• Single Bunch Beam Break Up (SBBU)• Vacuum pumping• Cooling design• etc.
LEUTL tunnel is ~ 40 m long and is ready to accept the beam
Some equipment exists, new equipment and diagnostics will be needed
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Summary The concept: High Repetition-Rate, Soft X-ray FEL User Facility
– 10 DWFAs linacs driven by a single SRF linac – 10 FEL lines @ 100 kHz rep. rate.– Compact, Inexpensive, and Flexible
A working group has started feasibility studies– Parameter studies of the overall concept– Bunch shaping studies at the AWA facility– Beam propagation through a 10m DWFA linac at APS– Modeling of the large energy spread in the FEL – Many more:
• Drive and witness jitter• Dielectric breakdown limitation testing• Etc.
We welcome collaborators and new ideas!