laser-structure accelerators
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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 modeObservation of microbunching: Sears et al. PRST-AB 11, 061301 (2008)
Net acceleration: Sears et al. PRST-AB 11, 101301 (2008)
Free-space accelerating structure
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
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, laser-powered, disposable, relativistic particle source
MAP: Micro Accelerator Platform
Courtesy G. Travish
xyz
laserbeam
cylindrical lensvacuum
channel
electron beam
cylindrical lens
top view
/2
xyz
laserbeam
cylindrical lensvacuum
channel
electron beam
cylindrical lens
top view
/2
top view
/2
More slab/grating structures
• Slab structures tend to use gratings: Gratings induce phase shifts for matching to a particle beam
Courtesy T. Plettner
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 (ω, 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)
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:3. RSOFT – commercial
photonic fiber code using Fourier transforms
4. CUDOS – Fourier-Bessel expansion from Univ of Sydney
Modeling Photonic Band Gap Fibers and Defect Modes
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.
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
Ez of 1.89 µm accel. mode
in Crystal Fibre
HC-1550-02
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
Modifying Accel. Mode via Defect Radius:Increasing the Accel. Field:
HC-1550-02Band Gaps
Courtesy R. Noble et al.
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
Si (εr = 12.1)
Vacuum
Axial field
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– Relatively little data
• One conclusion: Short pulses are good (at least down to~1 ps)
Simanovskii et al., PRL 91, 107601 (2003)
Proc. SPIE 6720, 67201M-1 Stuart et al., PRB 53, 1749 (1996)
(For THz measurements see Thompson et al., PRL 100, 214801 (2008))
Si
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 laserpulse slips 1 ps relative toparticle beam in 100 μm– 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
Optically-bunched
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 μm × 1.41 μm• Structure is not azimuthally symmetric has strong
transverse focusing and nonlinear forces for off-crest particles
Perturb woodpile structure by adjusting central bar
• Two problems ⇒ one solution– Idea: Use the optical
structures π/2 out of phase as focusing elements
– Adjust waveguide geometry to suppress quadrupole fields during acceleration
• Geometry is key
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
Out of guideInto guide Original mode
Quadrupole field
suppressedFocusing mode with octupole field suppressed:~ 831 kT/m magnet
Beam confinement
• Use accelerating and focusing structures to create thin-lens F0D0 lattice
• Resulting design has high dynamic aperture, low emittance growth
Results for full 6D tracking simulation over 3 m
m1009.1
m,102.99
10
y
x
Emittance requirement:
87% energy gain
Dynamic aperture, on-crest particles
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
• Computations can be orders ofmagnitude more intensive than formetal-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
PBG lattice
defect
field emitter tip
Field emission tip properties1. laser-assisted tunneling of
electrons from the atom to free space
2. Highly nonlinear3. Potential for timed sub-optical
cycle electron emission
metal vacuum
e
P. Hommelhoff et al, Kasevich group, Stanford University
laser beam
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)
radm 10~ 10 tip
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
• 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
DE-AC02-76SF00515 (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