developing photonic technologies for dielectric laser accelerators rosa letizia lancaster...
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Developing photonic technologies for dielectric laser accelerators
Rosa Letizia
Lancaster University/ Cockcroft Institute
Compact Particle Accelerators Workshop
Cockcroft Institute, 18/04/12
Compact Accelerator Workshop, 18/04/12
What is dielectric accelerator?Limits of metallic structures
• EM wave guiding achieved by outside metal walls
• Phase velocity synchronism is enforced by periodic loading
• Tend to be high-Q structure (long low power pulses)
• Gradient is in order of hundreds MV/m for short structures.
Dielectric structures
• Guiding by either metal walls or Bragg reflector
• Synchronism by manipulating effective index
• Tend to be low-Q structures (short high power pulses)
• Gradient > 1GV/m
Iris-loaded structure
Dielectric-lined waveguide
Bragg waveguide
Compact Accelerator Workshop, 18/04/12
MotivationTo attain significant economy in the size and cost of accelerators based on achievable gradients.
• Lasers can produce larger energy densities than a microwave source higher E-fields
• Dielectric materials can hold off material stress >1GV /m for ps-class pulses
• Lasers are a large market technology with rapid R&D driven by industry
• Short wavelength acceleration leads to sub-fs bunches
• Lithography technologies are developing fast
Compact Accelerator Workshop, 18/04/12
Why Photonic crystals (PhC)?
• Electronic crystal – a familiar analogy• a periodic array of atoms forms a lattice• lattice arrangement defines energy bands
• The OPTICAL ANALOGY – Photonic Band Gap (PBG) crystal• a periodic array of optical materials forms a lattice (dielectric atoms)• allowed energy (wavelength) bands arise
1-D
PhC
2-D
PhC
3-D
PhC
Compact Accelerator Workshop, 18/04/12
Photonics: key benefits
• low losses
• high damage threshold
• enhancement of light-matter interactions
• single mode operation in over-moded structures
• flexibility in design
• tunability of semiconductors electrical properties
Photonic bandgap
Periodicity a
Compact Accelerator Workshop, 18/04/12
PhC technologyWaveguides
Compact Accelerator Workshop, 18/04/12
n1 = 1.0 n2 = 3.376
a = 0.650 m r = 0.45 a
res = 1.545 mQ = 779
res = 1.639 m
Q = 1660
res = 1.422 m
Q = 3223
PhC TechnologyMultimode Resonant Cavity
Compact Accelerator Workshop, 18/04/12
PhC for Particle Accelerators
• An initial experimental work has been directed toward the use of the photonic crystal technology in the context of particle acceleration [1]
- Operating at 17 GHz- Gradient: 35 MV/m
[1] E.I. Smirnova et al., "Demonstration of a 17-GHz, High-Gradient Accelerator with a Photonic-Band-Gap Structure", Phys. Rev. Lett., Vol. 95, pp. 074801, Aug. 2005.
• Successful fabrication and use of a PhC structure in a particle accelerator, whose schematic of the experimental setup and of the PhC structure are shown in figure.
• PhC structures are promising candidate for future accelerator applications because of their ability to effectively damp high order modes and thus suppress wake field generation.
Compact Accelerator Workshop, 18/04/12
Dielectric structures for DLA• high-gradient (> 200 MV/m) • compactness (micron-scale)• low cost • (higher breakdown thresholds, 1-5 GV/m)
Si woodpile PhC waveguide Glass hollow core PhC fiber Double grating (quartz)
[B. Cowan, 2006] [R. Noble, 2007] [T. Plettner 2009]
Compact Accelerator Workshop, 18/04/12
DLA concept
Electron gun
Laser
Image credit: Chris MacGuinness (SLAC)
Compact Accelerator Workshop, 18/04/12
New directions in PhC cavities• PhCs offer a unique way to create resonant cavities for a number of very diverse
fields and recently they have been considered also for accelerator applications.
• By engineering a defect in an otherwise perfect lattice of a PhC, it is possible to design a resonant cavity that can sustain resonant modes with field profiles with fixed shapes.
• By strategically choosing the geometrical parameters of the PhC, it is possible to realise devices, and in particular resonant cavities, for virtually any range of frequencies.
Compact Accelerator Workshop, 18/04/12
New directions in PhC cavitiesHowever, PhC cavities can be highly overmoded thus strategies are needed to completely remove (or at least to highly suppress) higher frequency resonant modes
fn = 0.38
Q 1200
fn = 0.27
Q 500
Q 70
Q 400
Compact Accelerator Workshop, 18/04/12
New directions in PhC cavities
A novel combination of PhC structures and Metamaterial can be considered in order to design resonant cavities with only 1 resonant mode and relatively high Q.
fn = 0.392
Q 1000
Compact Accelerator Workshop, 18/04/12
Surface Plasmon accelerators• metals are lossy at IR frequencies and susceptible to breakdown at high field amplitudes
Surface Wave Accelerator based on Silicon Carbide (SiC):
• Acceleration takes place in the vacuum gap between two parallel SiC plates. • Accelerating field is generated by the surface changes at the SiC/vacuum interface. No
need for metal casing.
j
j
T
Lc
22
22
Is negative in the frequency band:(@ λ=10.6µm is compatible with CO2 laser)
(ionic crystals)
LT
* G Shvets, et al., Advanced accelerator concepts: 11th workshop, (2004)
Compact Accelerator Workshop, 18/04/12
Open questions
• coupling photonics modes IN and OUT
• fabrication much more involved
• glass darkening effect, material damaging
• complex simulations
• heat removal
• survival of the radiation environment
Implementation of real accelerator microstructures challenges
Compact Accelerator Workshop, 18/04/12
Future prospects
• Dielectrics offer higher damage resistance than metals and a natural way to provide synchronism
• Photonic crystal technology allows for unwanted HOMs to radiate out of the accelerator
• Compared to plasma wakefield accelerators, dielectric acceleration is linear, the structure is solid state
• High power structures and beam tests need to be carried out for microwave, THz, and optical technologies in order to identify clearly the suitability of each technique