Laser-structure accelerators
B. Cowan, M.-C. Lin, B. Schwartz, Tech-X
Corporation
E. Colby, J. England, C. McGuinness, C. Ng,
R. Noble, J. Spencer, SLAC
R. Byer, Stanford University
Outline
• Motivation
• A tour of structure types
– Macroscopic structures
– Grating-enabled slab structures
– Photonic bandgap structures
• Laser-structure concepts
–
–
–
–
Gradient
Efficiency
Beam dynamics
Microfabrication
• Ongoing work
– Computation
– Beam experiments
– Injectors
Motivation: Laser-driven acceleration using
dielectric structures
• High gradient
– Take advantage of intense laser fields
– High dielectric breakdown thresholds
• Efficiency
– Laser wall-plug to optical efficiency continues to improve
– Optics have low loss
• Operate in stable, linear regime
– Many concepts carry over from RF
• Generate attosecond bunches
Macroscopic structures: Demonstration of
microbunching and acceleration
• Optically bunch the beam in IFEL, follow with accelerating
structure
• First observed by Kimura et al. at ATF with 2 IFELs
• Net acceleration using linear structure
demonstrated at SLAC
• Structure used tilted free-space mode
Observation of microbunching: Sears et al.
PRST-AB 11, 061301 (2008)
Free-space accelerating structure
Net acceleration: Sears et al. PRST-AB 11,
101301 (2008)
What’s next for structures?
• Want to develop scalable structure – accelerate over many
Rayleigh lengths
• Need to generate axial electric field
• Speed-of-light phase velocity for matching to high-energy
beam
• How do we scale down RF structures to optical
wavelengths?
– Ideally, use waveguide: Similar to RF, high efficiency
– But for index-guiding (as in conventional fiber-optics) fields in
vacuum are slow waves: Waveguides get complicated
Courtesy G. Travish
At UCLA, we are designing an optical accelerator
consisting of a diffractive optic coupling structure and a
partial reflector
Courtesy G. Travish
A long term goal is to develop a mm-scale, laserpowered, disposable, relativistic particle source
MAP: Micro Accelerator Platform
More slab/grating structures
• Slab structures tend to use gratings: Gratings induce phase
shifts for matching to a particle beam
cylindrical
lens
vacuum
channel
cylindrical
lens
laser
beam
top view
z
Courtesy T. Plettner
electron
beam
y
x
/2

Interlude: Photonic bandgaps (PBGs)
• A photonic crystal is a structure with periodic dielectric
constant
• Like electronic states in solids, EM modes form bands
• Band gaps can form, in which propagation is prohibited
Benefits of photonic bandgaps
• Provide confinement in “defect” — an interruption in the
lattice
• Can confine a speed-of-light mode in all-dielectric structure
– impossible with index (total internal reflection) guiding
• Only confines modes in bandgap frequency range –
automatic HOM damping
Axial field
PBGs with reduced dimension: Fibers
• PBGs can be made with periodic structure in some
dimensions, uniform in others
• Ex. PBG fibers: Periodic in transverse dimension;
longitudinally uniform
• Certain dispersion points ( , kz) are prohibited for all 2D
propagation vectors
Geometry, mode and gap map of fiber structure from
X. E. Lin, PRST-AB 4, 051301 (2001)
PBGs: They’re not just for optical structures!
• HOM damping motivated PBG structure development in the
RF regime
Geometry and modes
of metallic PBG
structure based on
triangular transverse
lattice. From
Smirnova et al., PRL
95, 074801 (2005)
Dielectric Bragg structure, from
Jing et al, NIM A 594, 132 (2008)
Modeling Photonic Band Gap Fibers and Defect Modes
Goals:
1. Design fibers to confine
vphase = c defect modes
within their bandgaps
2. Understand how to
optimize accelerating
mode properties: ZC, vgroup,
Eacc/Emax ,…
Codes:
1. RSOFT – commercial
photonic fiber code using
Fourier transforms
2. CUDOS – Fourier-Bessel
expansion from Univ of
Sydney
Accelerating Modes in Photonic Band Gap Fibers
• Accelerating modes identified as special type of defect mode called
“surface modes”: dispersion relation crosses the vphase=c line and
significant field intensity at defect edge.
• Tunable by changing details of defect boundary.
Modifying Accel. Mode via Defect Radius:
Increasing the Accel. Field:
Ez of 1.89 µm
accel. mode in
Crystal Fibre
HC-1550-02
HC-1550-02
Band Gaps
Rinner(µm)
λ(µm)
Eacc/Emax
ZC(Ω)
Loss
(db/mm)
5.00
1.8946
0.0493
0.136
0.227
5.10
1.8872
0.0660
0.250
0.035
5.20
1.8767
0.0788
0.371
0.029
Courtesy R. Noble et al.
Modified X.E. Lin
hollow core silica fiber
with improved ratio
Eacc/Ez matrix obtained
by filling the first layer
holes with
εr = 1.5 material
3D “woodpile”-based structure
• Has complete bandgap; requires high index
• Lithographic fabrication can allow incorporation of features,
e.g. coupling elements
• Supports speed-of-light, near-lossless accelerating mode
Axial field
Si (εr = 12.1)
Vacuum
PRST-AB 11, 011301
(2008)
Key structure concept: Sustainable gradient
(Also not just for optical structures!)
• Gradient fundamentally limited by breakdown of material
• Huge unexplored territory: What are best parameters?
– 5 orders of magnitude in frequency (RF to optical)
– Lots of materials
(For THz measurements see Thompson et
al., PRL 100, 214801 (2008))
– Relatively little data
Si
Simanovskii et al., PRL 91, 107601 (2003)
Proc. SPIE 6720, 67201M-1
Stuart et al., PRB 53, 1749 (1996)
• One conclusion: Short pulses are good (at least down to
~1 ps)
Woodpile gradient example
• Based on damage threshold of bulk silicon, sustainable
gradient is 300 MeV/m at  = 1550 nm, 1 ps pulse width
– Could get to 400 MeV/m at longer wavelength; GeV/m challenging in
silicon
– Higher-bandgap materials could allow higher gradient
• Achievable with 500 W peak laser power
– Commercially available in fiber systems
• Low group velocity laser
pulse slips 1 ps relative to
particle beam in 100  m
– Frequent coupling & compact
coupler needed
Optical accelerator efficiency
• Bunch charge and optical-to-beam efficiency limited by
wakefields
• Embed accelerator in optical resonator to recycle energy;
use multiple bunches
• Beam can consist of a single optical bunch or a train of
optical bunches spaced by 
From Y. C. Neil Na et al.,
PRST-AB 8, 031301 (2005)
IFEL + chicane
RF electron
bunch
Opticallybunched beam
Efficiency optimization
• Optimize resonator beamsplitter reflectivity and bunch
charge for optimum efficiency
• Efficiency 37% for single bunch, 76% for 100 bunches
• Bunch charge ~few fC, so rep rate must be high
• Energy spread could be problem
efficiency
reflectivity
charge
Beam dynamics considerations
• Structure has small aperture: 1.55  m × 1.41  m
• Structure is not azimuthally symmetric  has strong
transverse focusing and nonlinear forces for off-crest
particles
• Two problems ⇒ one solution
– Idea: Use the optical structures /2
out of phase as focusing elements
– Adjust waveguide geometry to
suppress quadrupole fields during
acceleration
• Geometry is key
Perturb woodpile structure
by adjusting central bar
Effect of geometry change
• 2 modes available; suppress quadrupole field in
accelerating mode and octupole field in focusing mode
• We can now use thin lenses
Quadrupole field
suppressed
Focusing mode
with octupole
field
suppressed:
~ 831 kT/m
magnet
Into guide
Out of guide
Original mode
Beam confinement
• Use accelerating and focusing structures to create thin-lens
F0D0 lattice
• Resulting design has high dynamic aperture, low emittance
growth
Dynamic aperture, on-crest particles
Results for full
6D tracking
simulation over
3m
Emittance
requirement:
 x  9.2 10 10 m,
 y  1.09 10 9 m
87% energy gain
Computational issues
• Computing properties of photonic crystal structures is hard
– High-order mode
– Large computational area
• For n “cladding” layers:
– Computational cell size ~ n2
– Mode number ~ n2
PBG lattice
defect
• Computations can be orders of
magnitude more intensive than for
metal-bounded structures for similar resolution
• High-performance computing is beginning to be brought to
bear
– Advanced dielectric algorithms
– Frequency extraction techniques from time-domain simulation
P. Hommelhoff et al, Kasevich group, Stanford University
laser
beam
field emitter
tip
Field emission tip properties
1. laser-assisted tunneling of electrons
from the atom to free space
2. Highly nonlinear
3. Potential for timed sub-optical
cycle electron emission
e
metal
 tip ~ 10
10
vacuum
m  rad
P. Hommelhoff, Y. Sortais, A. Aghajani-Talesh, M. A. Kasevich, “Field Emission Tip as a Nanometer
Source of Free Electron Femtosecond Pulses”, PRL 96, 077401 (2006)
Summary
• Optical structures hold great promise for laser-driven
acceleration
• Groundwork in place further exploration
– Linear acceleration in vacuum demonstrated
– Several structure designs simulated
– Efficiency and beam focusing concepts described
• Fabrication and experimentation underway
• Much work remains to be done and many exciting ideas to
explore
– Many concepts carry over to other frequency ranges
Acknowledgments
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Collaborators at SLAC/Stanford
J. Rosenzweig, G. Travish (UCLA)
A. Chao, A. Wachsmann (SLAC)
S. Fan, D. Simanovskii (Stanford)
M. Tang (SNF)
Work supported by Department of Energy contracts DEAC02-76SF00515 (SLAC), DE-FG06-97ER41276 (LEAP),
and DE-SC0000839 (SBIR), and by Tech-X Corporation.
• Bob
Diamond structure
• Simulate woodpile structure based on diamond: n = 2.395 at
λ = 1.55 μm
• First, optimize the lattice: Adjust rod width w for largest
bandgap; optimum at w = 0.37a
w
a
Omnidirectional bandgap:
5.4% width-to-center ratio
Step 2: Compute an accelerating mode
Mode parameters (with
Si structure parameters
for comparison):
Si
Diamond
Normalized
frequency a/λ
0.367076
0.426313
Loss
< 0.48
dB/cm
35.3
dB/cm
Damage
impedance
6.10 
5.56 
Characteristic
impedance
460 
241 
Group velocity
0.253c
0.108c
For diamond, electronic bandgap is 5.5
eV, requiring 7 absorption for
ionization at λ = 1.55 μm
Frequency near bandgap edge; loss
might be reduced by altering waveguide
to bring frequency into the gap