E-169: Wakefield Acceleration
in Dielectric Structures
The proposed experiments at FACET
J.B. Rosenzweig
UCLA Dept. of Physics and Astronomy
FACET Review — February 19, 2008
E169 Collaboration
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UCLA
H. Badakov, M. Berry, I. Blumenfeld, A. Cook, F.-J. Decker,
M. Hogan, R. Ischebeck, R. Iverson, A. Kanareykin, N. Kirby,
P. Muggli, J.B. Rosenzweig, R. Siemann, M.C. Thompson,
R. Tikhoplav, G. Travish, D. Walz
Department
of Physics and Astronomy, University of California, Los Angeles
Stanford Linear Accelerator Center
University of Southern California
Lawrence Livermore National Laboratory
Euclid TechLabs, LLC
Collaboration spokespersons
E-169 Motivation
 Take advantage of unique experimental
opportunity at SLAC
 FACET: ultra-short intense beams
 Advanced accelerators for high energy frontier
 Very promising path: dielectric wakefields
 Extend successful T-481 investigations
 Dielectric wakes >10 GV/m
 Complete studies of transformational technique
Colliders and the energy frontier
 Colliders uniquely explore
energy frontier
 Exp’l growth in equivalent
beam energy w/time
 Livingston plot: “Moore’s
Law” for accelerators
 We are now falling off plot!
 Challenge in energy, but
not only…luminosity as well
 How to proceed to linear
colliders?
 Mature present techniques
 Discover new approaches
Meeting the energy challenge
 Avoid gigantism
 Cost above all
 Higher fields implied
 Higher fields give physics
challenges
 Linacs: accelerating fields
 Enter world of high energy
density (HED) physics
 Impacts luminosity
challenge…
HED in future colliders:
ultra-high fields in accelerator
 High fields in violent accelerating
systems
eE z /mcw ~ 1
 High field implies high w
 Relativistic oscillations…
 Limit peak power, stored energy
 Challenges
 Breakdown, dark current
 Pulsed heating
 Where is source < 1 cm?
 Approaches
 Superconducting
 High frequency, normal conducting
 Lasers and/or plasma waves, or…
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Scaling the accelerator in size
 Lasers produce copious power (~J, >TW)
 Scale in size by 4 orders of magnitude
  < 1 m gives challenges in beam dynamics, loading
 Reinvent the structure using dielectric (E163, Neptune)
Resonant dielectric
structure schematic
 To jump to GV/m, only need mm-THz
 Must have new source…
Possible new paradigm for high field
accelerators: wakefields
 Coherent radiation from bunched, v~c e- beam
 Any impedance environment
 Powers next generation or exotic schemes
 Plasma, dielectrics…
 Non-resonant, short pulse operation possible
 High fields without breakdown?
 Intense beams needed by other fields
 X-ray FEL, X-rays from Compton scattering
 THz sources for imaging with chemical signature
CLIC V.O.: High gradients, high frequency,
EM power from wakefields
CLIC drive beam
extraction structure
Power
CLIC 30 GHz,
150 MV/m structures
CLIC wakefield-powered resonant scheme
Simpler approach:
Collinear dielectric wakefield accelerator
 Higher accelerating gradients: GV/m level
 Dielectric based, low loss, short pulse
 Higher gradient than optical? Different breakdown mechanism
 No charged particles in beam path… field configuration simpler
 Wakefield collider schemes
 Modular system
 Afterburner possibility
 Spin-offs
 THz radiation source
 Imaging, acceleration…
"Towards a Plasma Wake-field Acceleration-based Linear Collider",
J.B. Rosenzweig, et al., Nucl. Instrum. Methods A 410 532 (1998)
Dielectric Wakefield Accelerator
Electromagnetic characteristics
 Electron bunch drives Cerenkov wake
*
in cylindrical dielectric structure
Variations on structure features
Multimode excitation
 Wakefields accelerate trailing bunch
 Mode wavelengths

Design Parameters
a,b
z
n 

4 b  a
 1
n
 Peak decelerating field




Transformer ratio

Ez on-axis, OOPIC
eE z,dec
4N b re mec 2

 8

a
 z  a
  1

R
E z,acc
2
E z,dec
Extremely good
beam needed
OOPIC Simulation Studies
 Parametric scans
 Heuristic model benchmarking
 Analyze experiments:
Field values
Beam dynamics
Radiation production



1.5 10
Multi-mode excitation (short bunch)
10
E_dec,max (OOPIC)
E_acc max (OOPIC)
E_dec,theory
10
Single mode excitation (longer bunch)
z
E (V/m)
1 10
5 10
9
0
40
60
80
100
120
140
160
a (m)
Example scan, comparison to heuristic model
Fundamental 
Experimental Background
Argonne / BNL experiments
E vs. witness delay
 Proof-of-principle experiments
(W. Gai, et al.)

ANL AATF
 Mode superposition
(J. Power, et al. and S. Shchelkunov, et al.)

ANL AWA, BNL
 Transformer ratio improvement
(J. Power, et al.)
 Beam shaping
 Tunable permittivity structures

For external feeding
(A. Kanareykin, et al.)
Tunable permittivity
Gradients limited to <50 MV/m by available beam
T-481: Test-beam exploration
of breakdown threshold
 Leverage off E167
 Existing optics, diagnostics, protocols
 Goal: breakdown studies
 Al-clad fused silica fibers
 ID 100/200 m, OD 325 m, L=1 cm
 Multi-photon v. tunneling ionization
 Beam parameters predict ≤12 GV/m
longitudinal wakes
 30 GeV, 3 nC, z ≥ 20 m
 48 hr FFTB run, Aug. 2005
 Follow-on planned, no FFTB time
 PRL on breakdown threshold produced
T-481 “octopus” chamber
T481: Beam Observations
 Multiple tube assemblies
 Alignment to beam path
 Scanning of bunch lengths for
wake amplitude variation
 Excellent flexibility: 0.5-12 GV/m
 Vaporization of Al cladding… use
dielectric, more robust
 Breakdown monitored by light
emission
 Correlations to post-mortem
inspection
View end of dielectric tube; frames sorted
by increasing peak current
Breakdown Threshold
Observation
Breakdown Camera Pixel Sum
2.40 107
08170cs
2.20 107
2.00 107
1.80 107
1.60 107
1.40 107
1.20 107
1.00 107
0
50
100
150
Bunch Length Variable
[rms XRAY]
X-ray data yields bunch length, current
200
T-481: Inspection of Structure Damage
Damage consistent with beam-induced discharge
ultrashort
bunch
Bisected fiber
longer
bunch
Aluminum vaporized from pulsed heating!
Laser transmission test
Striking conclusions
 Observed breakdown threshold (field from simulations)
 Esurf >13 GV
 Eacc>5 GV/m!
 Much higher than laser data (1.1 GV/m for 100 psec)
 Tunneling ionization dominant
 Multi-mode excitation gives effective shorter pulses?
E169 at FACET




Approved by SLAC EPAC 12/06
Research >GV/m acceleration scheme in DWA
Push technique for next generation accelerators
Goals:
 Explore breakdown issues in detail
 Varying tube dimensions
 Change impedance, mode content
 Breakdown dependence on wake pulse length







Determine usable field envelope
Coherent Cerenkov radiation measurements:
Explore alternate materials (diamond, etc)
Observe acceleration
Explore alternate structure designs
Examine deflecting modes, transverse BBU
Push to modular DWA demonstration (1 m section)
E-169 at FACET
High-gradient acceleration research
Goals in 3 Phases
 Phase 1: Complete breakdown study
 explore (a, b, z) parameter space
Alternate cladding
Alternate materials (e.g. diamond)
Explore group velocity effect
T  Ld /c  vg  Ld / c  1
 Coherent Cerenkov (CCR) measurement

 Total energy gives field measure
UC 
eN b E z,dec Ld
2
 Harmonics are sensitive z diagnostic
2 
 

 nN b re mec  z L
n z
d


Un 
exp



2
 2b  a  1  
2ab  a 8  1 z   1a


2


2
2
2

z
≥ 20 m
r
< 10 m
U
25 GeV
Q
3 - 5 nC
E-169 at FACET: Phase 2 & 3
 Phase 2: Observe acceleration, explore new designs
 10 cm tube length
 longer bunch, z ~ 150 m
z
150 m
 moderate gradient, 1 GV/m
r
< 10 m
U
25 GeV
Q
3 - 5 nC
 single mode operation
 Phase 3: Scale to 1 m fibers
 Alignment
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 Group velocity & EM exposure
 Transverse BBU
Before & after momentum distributions (OOPIC)
Ez on-axis
Experimental Issues: THz Detection
 Conical launching horns
 Impedance matching to free space
 Direct radiation forward
 Fabrication, test at UCLA Neptune
 Signal-to-noise ratio
 Background of CTR from tube end
 SNR ~ 3 - 5 for 1 cm tube
 Detectors
 Pyroelectric
 Golay cell
 Helium-cooled bolometer
 Michelson interferometer
for autocorrelation
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Autocorreation of coherent edge radiation at BNL ATF, 120 fsec beam
Experimental Issues: Alternate DWA
design, cladding, materials
A. Kanareykin
 Aluminum cladding used in T-481
 Vaporized at even moderate wake amplitudes
 Low threshold from low pressure, thermal environment
 Dielectric cladding
 Lower refractive index provides internal reflection
 Low power loss, damage resistant
 Alternate dielectric: CVD diamond

CVD deposited diamond
 High breakdown threshold
 Doping gives low SEC
 Available for Phase I (Euclid)
Phase 2
 Bragg fibers
 2D photonic band gap structures?
Bragg fiber
Alternate design: Slab structure
 Slab structure familiar from
resonant laser idea
 Suppresses BBU!
 Ultra-short bunch means ~GV/m
fields still obtainable
Example: Ez~ 700 MV/m
E-169 at FACET: Implementation/Diagnostics

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



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New precision alignment vessel
Upstream/downstream OTR screens
for alignment
X-ray stripe
CTR/CCR for bunch length
Imaging magnetic spectrometer
Beam position monitors and beam
current monitors
Controls…
Heavy SLAC involvement
Much shared with E168
E169 Game Plan and Timeline
Design, initial
construction
Go
Cerenkov
production
FACET beam
commissionin
g
UCLA Neptune
experiments
2008
Slab
structures
Breakdown
studies
2009
Alternate
materials
10 cm module
acceleration
Novel
cylindrical
structures
2010
1 m multi-GeV
design study
1 m multi-GeV
acceleration
experiments
(witness beam)
10 cm module
BBU studies
2011
Path to
staging
2012
Conclusions/directions
 Extremely promising initial run
 Collaboration/approach validated
 Physics tantalizing; new regime for dielectric
acceleration must be explored
 Unique opportunity to explore GV/m dielectric
wakes at FACET
 Flexible, ultra-intense beams
 Only possible at SLAC FACET
 Complementary low gradient experiments at Neptune
 Conceptual, experimental, and personnel synergies
with E168…