SLAC Advanced Accelerator
Research
accelerators beyond 100 MV/m
Eric R. Colby*
Acting Head of Accelerator Research Division
December 14, 2011
* ecolby@slac.stanford.edu
Outline
• Advances in RF concepts
• Novel Acceleration Concepts
• Plasma Wakefield Acceleration
•Recent Progress
•Towards a PWFA linear collider at 1 TeV
• Laser-Driven Dielectric Accelerators
•Recent Progress
•Towards a DLA linear collider at 10 TeV
• Thoughts on the Future
BreakdownProbability 1 pulsemeter
RF Accelerators: Prospects for Further Progress
10
a
a
a
0
10
1
10
2
10
3
10
4
10
5
10
6
10
7
0
100
200
0.105, t 2.0mm, SLAC
0.143, t 2.6mm, SLAC
0.215, t 4.6mm, KEK
300
400
Peak Electric Field MV m
Dual mode accelerating structure
1
1
4
500
• Surface gradients >300 MV/m
have been reliably produced in
single-cell cavities
•Material science to identify the
best materials
•Investigation of roles E and H
play in breakdown process
•Geometry optimization
• Using high gradient RF requires
more efficient, lower cost power
sources
•Novel sources using
•Photoemission sources
•Magnet-free transport
•Lower voltages
Engineering: Gordon Bowden
Solid model: Bob Reed
For details see A.D. Yeremian talk Monday afternoon
Sami Tantawi, SLAC
Plasma Wakefield Acceleration
Progress Demonstrating Key Aspects of Plasma
Wakefield Acceleration
• SLAC/UCLA/USC FFTB experiments 1998 - 2006
– Plasma Wakefield Acceleration of electrons over meter scales
• 50GeV/m accelerating gradient
• Total energy gain of 43GeV
– First plasma acceleration of positrons
– Systematic studies of integrated & time dependent focusing
• electrons (extended propagation, emittance preservation @ 10-4m)
• positrons (halo formation, emittance growth)
– Refraction of electron beam at plasma boundary
– Betatron radiation from strong plasma focusing
• x-rays @ 1014 e-/cc (kT/m)
• gammas (e+ production) @ 1017 e-/cc (MT/m)
– Dielectric Wakefield Acceleration
• Proof of principle studies of material breakdown threshold
– 14GeV/m induced catastrophic breakdown in 1cm long, 100µm diameter
fused Si tubes (we turned the dielectrics into plasmas!)
• 2008 DOE Recognized the need for an ‘Advanced
Plasma Acceleration Facility’
5
Mark Hogan, SLAC
FACET: Facility for Advanced aCcelerator
Experimental Tests
New Installation @ 2km point of
SLAC linac:
•Chicane for bunch compression
•Final Focus for small spots at the
IP
•Experimental Area (25m)
Experiments
here
FACET Beam Parameters
A Unique Facility
for Accelerator Science
Energy
23 GeV
Charge
3 nC
sr
10 µm
sz
20 µm
Peak Current
22 kA
Species
e- & e+
Mark Hogan, SLAC
E200: Plasma Wakefield Acceleration
5 year targeted R&D on single stage plasma accelerator physics
•
•
•
•
Accelerating gradients > 10GeV/m, Focusing fields > 1 MT/m
Meter scale, high density field ionized plasmas (Li, Cs, Rb)
Demonstrate single stage plasma accelerator: meter scale, high gradient, preserved
emittance, low energy spread
First experiments (2012) will quantify head erosion with single high-current bunches
Follow on experiments (2012-2013) will use notch collimator to produce independent
drive & witness bunch
Next phase will use pre-ionized plasmas and tailored profiles to maximize single
stage performance: total energy gain, efficiency
Betatron & Synchrotron radiation, instability studies
Mark Hogan, SLAC
•
•
•
7
Several paths are being investigated
• Non-linear regime with electrons and positrons
– Majority of activity at SLAC by SLAC/UCLA/MPI collaboration; new program
taking shape at DESY
• Large wake amplitude, high efficiency, ion column for extended propagation
and emittance preservation
• More difficult for positrons
• FACET program at SLAC targeted to address many of the issues associated
with applying PWFA to single stage of a PWFA based linear collider
• Linear or quasi-linear regime with electrons
– Active experiments at BNL using tailored electron bunch trains to resonantly
drive plasma wave
• Potential for larger amplitude and higher transformer ratio
• Reduced benefits of ion column: extended propagation, emittance preservation
• Proton Driven Plasmas (PD-PWFA)
– Collaboration led by MPI/CERN delivering full proposal to begin experimental
program at CERN staring in 2015. FNAL beginning to evaluate concepts using
portions of the Tevatron.
• Potential to reach ~TeV e- energy in single long (~400m) plasma stage
• Takes advantage of large stored energy in High Energy proton bunches
• Reach the energy but difficult to get the beam power needed for luminosity
Mark Hogan, SLAC
Proton Wake Expts
Mark Hogan, SLAC
Key features of a PWFA-LC
• Electron drive beam for both electrons and positrons
• High current low gradient efficient 25GeV drive linac
– Similar to linac of CERN CTF3, demonstrated performance
• Multiple plasma cells
– 20 cells, meter long, 25GeV/cell, 35% energy transfer efficiency
• Main / drive bunches
– 2.9E10 / 1E10 PWFA-LC concept will continue to evolve with
further study and simulation
– Bunch charge; Non-gaussian bunch profiles; Flat vs round
beams; SC vs NC pulse format; ...
• FACET facility will investigate and iterate these ideas
through experiments
Mark Hogan, SLAC
IP Parameter Optimization
• Conventional <1TeV LC is typically in low quantum
beamstrahlung regime (when Y=2/3 ωcћ/E <1)
– The luminosity then scales as L ~ δB1/2 Pbeam / εny1/2 (where δB is
beamstrahlung induced energy spread)
• High beamstrahlung regime is typical for CLIC at 3TeV
• PWFA-LC at 1TeV is in high beamstrahlung regime
• Scaling and advantage of high beamstrahlung regime
– L~δB3/2 Pbeam /[σz βy εny]1/2 and δB~σz1/2
– short bunches allow maximizing the luminosity and minimizing
the relative energy loss due to beamstrahlung
• Optimization of PWFA-IP parameters performed with
high quantum beamstrahlung regime formulae and
verified with beam-beam simulations
Mark Hogan, SLAC
Primary Issues for any Plasma-based LC
• Need to understand acceleration of electrons & positrons
• Luminosity drives many issues:
– High beam power (20 MW)
efficient ac-to-beam conversion
– Well defined cms energy
small energy spread
– Small IP spot sizes
small energy spread and small Δε
• These translate into requirements on the plasma acceleration and
the experimental programs
–
–
–
–
High beam loading of e+ and e- (for efficiency)
Acceleration with small energy spread
Preservation of small transverse emittances – maybe flat beams
Bunch repetition rates of 10’s of kHz
• Multiple stages allow better beam control and use of drive-beam
– FACET aims to demonstrate single stage before full system test
Mark Hogan, SLAC
A Self-consistent Design of a 1 TeV e-e+ Linear Collider
Based on PWFA
• TeV collider design has multiple acceleration stages, each
adding ~25 GeV/stage
• FACET PWFA program aims to demonstrate a single stage
with needed Q, E E , efficiency, emittance preservation
• Results will inform designs for future applications (HEP,
Photon Science)
see A. Seryi et al., PAC09 Proceedings
Mark Hogan, SLAC
PWFA-LC Efficiency & Power Estimate
170MW
43MW
Other Systems
127MW
Modulators 83%
Klystrons 65%
Distribution 93%
.83x.65x.93=50%
63.5MW
90%
• Current design uses
Gaussian beams, 35%
efficiency
• Tailored current profiles have
achieved 90% efficiency in
simulation
– Better overall efficiency
– Improves heat management
problem in plasma cell
Plasma Wave
35%
Drive Beam
57MW
Mark Hogan, SLAC
Main Beam
20MW
1 TeV PWFA LC Parameters
Ecm = 1 TeV, L = 1034 cm2s-1
Laser-Driven Dielectric Accelerators
Motivation
P/l 2
Dielectric Damage Threshold
1053nm
2 J/cm2 @ 1 ps
>2 GV/m
30cm
3m
3cm
300nm
3mm
Source Wavelength
*28.5 GeV, 1e10 ppp, 1m x 1m x 600m (20m for SPPS) beam
300m
30m
l
B. C. Stuart, et al, Phys. Rev. Lett., 74,
p.2248ff, (1995).
**350 MeV, 1e10 ppp, 1m x 1m x 1 mm beam
Fused silica, THz range,
~psec exposure
Cockcroft
Inst, U.K.
U. Lancaster,
U.K.
LLNL
LBNL
Stanford
MIT
Q-Peak Inc.
Purdue
Minotech
Engineering
SLAC
UCLA
Radiabeam
Technologies
Incom Inc.
U. Colorado,
Technion,
Israel
NTHU,
Taiwan
Manhattanville
College
Tech-X Corp.
U. Maryland
DOE
• 51 Participants = 46 US+5 International
• 10 Universities
• 4 National Labs / Institutes
• 5 Companies
• Accelerator
• Laser Science & Technology
• Photonics
• Novel Materials
• Simulations
There is a wealth of concepts being developed…
cylindrical
lens
vacuum
channel
cylindrical
lens
laser
beam
top view
z
electron
beam
y
x
l/2
l
Dielectric Laser Acceleration
Primary challenge for laser acceleration: mode is transverse electromagnetic—must develop
longitudinal electric fields to accelerate
Structure Candidates for High-Gradient Accelerators
Projected maximum gradients based on measured material damage threshold data
Photonic Crystal Fiber
Silica, l=1890 nm,
Ez=400 MV/m
Transmission Grating Structure
Photonic Crystal “Woodpile”
Silicon, l=2200nm,
Silica, l=800nm,
Ez=400 MV/m
Waveguiding Structure
Waveguiding Structure
Ez=830 MV/m
Phase Mask Structure
Much tighter (and all solid-state) coupling means
• MW-class not PW-class lasers are needed (Microjoule-class
pulse energies, not 10-100J class laser)
• Shot-to-shot reproducibility is better
Laser Coupling to Structures
Transverse Coupling to Woodpile Structure
~90% power coupling
[courtesy B. Cowan, Z. Wu]
Coupling from transverse waveguide to accelerating mode of the woodpile
structure.
>90% coupling from silicon waveguide input couplers to accelerating channel;
fabrication R&D of test couplers underway [Z. Wu, D. Xu]
Joel England, SLAC
Benchtop Testing of Structures
Phase Stability of PBG Structures
Breakdown Strength of Dielectric Materials
2 J/cm2 ~ Epk=5 GV/m
Al2O3
1 d
N

 9 ppm /˚C
 dT neff .L.T / l
[633 nm HC fiber]
 3˚ phase / ˚C for L = 1000l
S
HfO2
Si3N4
SiO2
S
i
R. Laouar
ZrO2/Y2O3
K. Soong
Joel England, SLAC
DLA: The Laser Acceleration Facility at the NLCTA
(Commissioned March 2007)
E
S
B
Counting Room
(b. 225)
Ti:Sapphire Laser
System
Cl. 10,000 Clean Room
E-163
RF PhotoInjector
Optical Microbuncher
Gun Spectrometer
Next Linear ColliderNext
Test Accelerator
Linear Collider Test Accelerator
The E163 program has advanced rapidly due to three factors:
Experimental Hall
•
A decade of experience conducting this type of experiment at Stanford
•
Extensive NLCTA infrastructure required modest extension to make a functioning facility
•
Experienced help from the Test Facilities staff at every step
E-Beam Microbunching
Net laser acceleration of 1.2 keV
demonstrated for 400 attosec
microbunches using inverse transition
radiation (ITR) at a metal foil.
C.M.S. Sears, et al. “Production and
characterization of attosecond electron bunch
trains,” PRST-AB 11, 061301 (2008)].
C.M.S. Sears, et al. “Phase stable net
acceleration of electrons from a two-stage optical
accelerator.” PRST-AB 11, 101301 (2008).
Joel England, SLAC
First Microaccelerator Demonstration: January 2012
•Will demonstrate gradient
LASER
E. Peralta
1 mm
alignment channels
8
x1mm2
gratings
20x50µm spot
Edgar Peralta, Stanford
Multi-Stage Layout Concept
5 modules per wafer
1 laser per module
(200W per laser)
Loop
period=beam
repetition rate
Phase
control
40 structures per module
1 module = 10 cm long
1 stage = 750 µm long
... x 16,000/TeV
6'' wafer
Main Linac (one half)
5 TeV  12.5 km  400,000 lasers
Thulium fiber laser l2 µm
Joel England, SLAC
E-Beam Pulse Format
30 attosec
N~3e4
 2 mm
159 micropulses per train, 1pC/train
Train length= 1ps300 microns
T_sep=200 nsec
10 TeV DLA LC Parameters
Parameter
CM Energy
Loaded Gradient
Bunch Charge
Bunches per train
Train Rep Rate
Microbunch Length
Design Wavelength
Normalized Emittance
IP Spot Size
Units
GeV
MeV/m
e
#
MHz
micron
micron
micron
nm
DLA
10000
400
3.8E+04
159
5.00
2.6E-03
1.89
1.0E-04
0.06
SCRF
10000
30
3.0E+10
2820
5e-06
320.29
230609.58
10.00
158.00
Disruption Parameter
#
1.28E+02
2.06E-01
Beamstrahlung Parameter
Beam Power
Active Linac Length
#
MW
km
1002.6
24.2
12.5
5.6
339
166.7
Beam Coupling Efficiency
#
0.3
0.3
MW
85.7
1016.5
#
0.50
0.60
Total Wall Plug Power
MW
171.3
1694.1
Beamstrahlung E-loss
%
5.7
91.0
Enhanced Luminosity
/cm^2/s
4.09E+36
1.23E+36
Total Laser/RF Power
Wall-plug to Light Efficiency
Joel England, SLAC
Thoughts for the Future
• Electric fields well beyond 100 MV/m have been sustained
reliably in short rf-driven metal accelerators
– This remains the nearest-term high gradient technology for a greenfield
TeV-class machine
• Fields beyond 50 GV/m have been sustained in dense plasma
wakefield accelerators
– Extraordinary gradient has already been demonstrated with respectable
shot-to-shot stability; demonstrations of high quality and high efficiency
acceleration are imminent
• Fields well beyond 1 GV/m have been sustained in dielectric
accelerator structures in both quasi-CW and broadband
– The very small bunch charge, high repetition rate operation of a DLA
offers the only path to multi-TeV accelerators with acceptable
beamstrahlung
Power and Efficiency
200W avg power
2µm wavelength
Per Module:
Accelerator absorption loss: 5 mW
Cherenkov absorption: 10 mW
Waveguide absorption loss: 577 mW
(assumes SiO2 substrate, 1e-3 dB/m)
assumes near 100% efficiency
of waveguide couplers, splitters
5% coupling loss
e-beam
beam dump
30% coupling to e-beam
(61%)
absorption at vacuum box wall
Joel England, SLAC
200W in a fiber? How about 20kW?
http://www.ipgphotonics.com/documents/documents/HP_Broshure.pdf
PWFA-LC Efficiency & Power Estimate
Mark Hogan, SLAC
Long-Term Timeline
NLCTA-like test facility
for DLA technology
Gain in performance,
Progress towards realization,
New scientific knowledge
Operational
Improvements
Major Project
Engineering Begins
compact multi
stage device
Engineering
Tests Underway
RESEARCH
DEVELOPMENT
Physics Largely
Understood
Present
Concept
Concept implemented
as a working machine
Proof-ofCommunity
Principle
Develops
Experiments
Critical Mass of Experimental
Effort Achieved
(people+facilities)
5 years
10 years
20 years
Adapted from AARD SPC report 2011, E. Colby
Joel England, SLAC
Other Subsystems of PWFA-LC
• Design of injectors, damping rings, bunch compressors,
are similar to designs considered for LC and the
expertise can be reused
– Extensive tests of damping ring concepts at KEK ATF Prototype
Damping Ring and the CESR-TA Test Facility
• Final focus system similar to conventional LC designs
– Tested at Final Focus Test Beam and soon at the ATF2 at KEK
• Present design of Final focus and collimation has full
energy acceptance of slightly greater than 2%
– May need to deal with a factor of two larger energy spread for
PWFA-LC
– Further optimization of Final Focus and Collimation will need to
be performed
Mark Hogan, SLAC
Plasma cells & flexibility of the concept
• Plasma cell is the heart of the PWFA-LC concept
• FACET will be able to produce a wide variety of beams to study the
beam loading and acceleration with different plasmas
– The initial tests will be done with lithium plasma
• In the PWFA-LC, the plasma in each cell needs to be renewed
between bunches and stability of the plasma parameters is crucial
– A high speed hydrogen jet is a possible candidate
• PWFA-LC concept is flexible wrt beam pulse format:
– Bunch spacing presently assumed to be typical of NC drive linac: ~4ns
• It can be doubled with the addition of RF separators and
delay beamlines that can run along the drive linac
• Increasing the bunch spacing by orders of magnitude could
be done with a stretching ring, where the entire drive train
would be stored and then bunches would be extracted oneby-one with fast rise kickers
– Alternately could use a SC drive linac for very long spacing
Mark Hogan, SLAC
Wakefield Simulations
Field monitor
Optimized PBG
Accelerator
Commercial Fiber
Used in Experiment
blue box
= bandgap
blue box
= bandgap
beam
[courtesy Cho Ng]
ACE3P simulation of HC-800-1 fiber with
axial current excitation. Time domain.
current pulse RMS duration = 0.2  /c
[scaled to actual design wavelength]
one mode excited
many modes
excited
Joel England, SLAC
Single-bunch BBU-driven
emittance growth is manageable
Misalignments
N=234
•Xq=Xa=50 nm
•X’o=0 rad, Xo=0
nm
•Grouping=10
Beam
•go=1000
•s=5/360l
•Q=varies
Accelerators
•Ls=1000l
•R=5l
•er=2.31
•G=500 MV/m
Quad Lattice
•Leff=2.5 mm
•y=90o
•Lq=5 m
•Kq=600 T/m
Fit: de=3.0x10-10 q1.28