E-169: Wakefield Acceleration in Dielectric Structures The proposed experiments at FACET
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Transcript E-169: Wakefield Acceleration in Dielectric Structures The proposed experiments at FACET
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
2b a 1
2ab a 8 1 z 1a
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
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…