1. Developing photonic technologies for dielectric laser accelerators
Rosa Letizia
Lancaster University/ Cockcroft Institute
Compact Particle Accelerators Workshop
Cockcroft Institute, 18/04/12

2. 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.
Iris-loaded structure
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
Dielectric-lined waveguide
Bragg waveguide

3. Motivation
To 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
Dielectrics can withstand higher E-fields in the high frequency range

4. Why Photonic crystals (PhC)?
1-D
PhC
2-D
3-D
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

5. 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
Dielectric photonic crystal could replace metal in many applications requiring reflection in a narrow frequency range such as in walls of waveguides wanted to support a single mode moreover at frequencies higher than several GHz they are less lossy than metals, at near optical frequencies can withstand higher fields than metals, phcs have more adjustable properties than metal (eg the frequency window in which they operate (reflect) can be shifted or narrowed by acting on the bandgap.

6. PhC technology
Waveguides

7. PhC Technology Multimode Resonant Cavity a = 0.650 m r = 0.45 a
res = m
Q = 779
res = m
Q = 1660
n1 = 1.0 n2 = 3.376
a = m r = 0.45 a
res = m
Q = 3223

8. 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
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.
[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 , Aug

9. 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)
High- energy electrons following a zig-zag path are ideal for generating vacuum ultraviolet radiation x-rays. A compact tabletop laser-based accelerator could be the key to an inexpensive and short lambda source easy to approach for every day uses. A laser based e- acc. Would convert the output from a laser into a beam of coherent, short lambda, ultrafast optical pulses for which e- beam with MeV  GeV are required to make these beams.
The laser acc technology being pursued operates in a fashion similar to that of RF linear acc. The main difference is the choice of lambda.
[B. Cowan, 2006]
[R. Noble, 2007]
[T. Plettner 2009]

10. DLA concept Laser Electron gun
Combine today’s commercial laser technology with micro and nanoscale manufactoring techniques to produce table top PA?
Omniguide fiber
Image credit: Chris MacGuinness (SLAC)

11. 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 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.
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.

12. New directions in PhC cavities
However, 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
Q  70
Q  400
fn = 0.27
Q  500
Example of high degree freedom

13. 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
A way to resonate in hollow core

14. 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.
(ionic crystals)
Is negative in the frequency band:
λ=10.6µm is compatible with CO2 laser)
* G Shvets, et al., Advanced accelerator concepts: 11th workshop, (2004)

15. Open questions
Implementation of real accelerator microstructures challenges
coupling photonics modes IN and OUT
fabrication much more involved
glass darkening effect, material damaging
complex simulations
heat removal
survival of the radiation environment

16. 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
Beta < 1; higher frequency methods will require smaller bunch charges and increased repetition rates, experimental advantages