Developing photonic technologies for dielectric laser accelerators

1. Developing photonic technologies for dielectric laser accelerators Rosa Letizia Lancaster University/ Cockcroft Institute r.letizia@lancaster.ac.uk 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-fsbunches • Lithography technologies are developing fast

4. 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

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

6. PhC technology Waveguides

7. PhC Technology Multimode Resonant Cavity res = 1.545 m Q = 779 res = 1.639 m Q = 1660 n1 = 1.0 n2 = 3.376 a = 0.650 m r = 0.45 a res = 1.422 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. 074801, Aug. 2005.

9. Dielectric structures for DLA • high-gradient (> 200 MV/m) • compactness (micron-scale) • low cost • (higher breakdown thresholds, 1-5 GV/m) Double grating (quartz) Si woodpile PhC waveguide Glass hollow core PhCfiber [B. Cowan, 2006] [R. Noble, 2007] [T. Plettner 2009]

10. DLA concept Laser Electron gun 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 fn = 0.27 Q  500 Q  400

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

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