1. Slab-Symmetric Dielectric- Based Accelerator Rodney Yoder UCLA PBPL / Manhattan College DoE Program Review UCLA, May 2004

2. R. Yoder / DoE Review Review: Why Slab Geometry?  Interested in structures in the mm or FIR regime  But— there are well-known limitations: Cavity structures: Wakefields ~ 3, leading to bad transverse dynamics Machining tolerances are tough Accelerating fields limited by breakdown Slab structure: Transverse wakefields strongly suppressed Planar structure may be easier to build and tune Dielectric breakdown limit potentially easier

3. R. Yoder / DoE Review Slab-Symmetric Dielectric- Loaded Accelerator

4. R. Yoder / DoE Review Motivation: experiment UCLA project begun mid 1990s, hampered by small device dimensions at 10 µm Scaling to 340 µm gives realistic device dimensions for injection Neptune photoinjector beam a good candidate (E = 11–14 MeV,  n = 6π mm mrad,  E/E = 0.1%, 4 ps bunch length, chicane compressor, can focus to ~ 20-30 µm “slab” beam) Potential for high-power THz generation, using Neptune CO 2 laser / MARS amplifier (≤ 100 J/pulse) “Cold-testing” with 10-µm design still possible

5. R. Yoder / DoE Review Basic physics of the structures Set = 0 (vacuum wavelength of laser) Fields independent of x (translational symmetry) Dispersion relation:   = c 2 (k x 2 + k y 2 + k z 2 ) Periodic coupling enforces k z =  /c  v  z = c prevents Fabry-Perot mode Since k x = 0, we must have k y = 0 in gap  Resonant k z values obtained as function of geometry using dielectric to match boundary conditions

6. R. Yoder / DoE Review Ideal accelerating mode, 3D simulation Structure Q ~ 600, r/Q = 25 k  /m, so field = 30 MV/m at 50 MW

7. R. Yoder / DoE Review Transverse Wakefield Suppression Short pulse (   = 0.4 ps) Long pulse (   = 4 ps) 2D Simulations using OOPIC 200 pC,  r = 120 µm,  r = 3.9, a = 0.58 mm, b = 1.44 mm WzWz WW

8. R. Yoder / DoE Review Periodic slots enforce resonant mode slot dimensions determine the Q-factor for the structure roughly proportional to 0 /w, but filling time depends on depth too Very wide slots are NOT cut off! slots fill with field resonant frequency is perturbed high fields on slot surfaces For small slots,  /  ~ L/w Perturbation vanishes for L = g /4 (quarter-wave matching) gives high Q, slow fill Coupling to the structures

9. R. Yoder / DoE Review 2D time-dependent simulation Axial field: flat wavefronts (no perturbation) large field in slot Transverse field: zero at y=0 zero at peak acceleration 340 µm wavelength a = 115 µm, b–a = 30 µm quarter-wavelength slots

10. R. Yoder / DoE Review Comparison: Shorter coupling slots a = 118 µm, b–a = 16.9 µm silicon (n = 3.41) slots 6 µm long, 5 µm wide Resonant at 334 µm (  /  = +1.8%) Slight deformation near slot Field in slot comparable to peak Frequency bandwidth ~ 1%

11. R. Yoder / DoE Review Filling Quarter-wavelength slots  = 325 ps E max = 15 E 0 Everything depends on the slots… 6 µm slots  = 70 ps E max = 3.8 E 0

12. R. Yoder / DoE Review Manufacture Can use standard semiconductor techniques Choices are monolithic vs. two-part Monolithic - alignment not an issue - how to tune/deform? - must avoid very thin “membrane” as upper layer Two-part - easy tuning - how to align? - need precision positioning in y, z, and azimuthal angle - possible but expensive

13. R. Yoder / DoE Review Multilayer structure for 1-10µm laser (aka 1-D Photonic Band Gap Accelerator!) Metal boundaries won’t work well at IR Investigate dielectric multilayer approach (Bragg reflector) Simulations underway R = 99.2% 9 layers plus substrate Each layer is a quarter wavelength

14. R. Yoder / DoE Review Conclusions Slab structures are attractive for beam quality and gradient; become practical at (sub-)THz for e.g. Neptune We are completing designs for versions with and without metal (scalability to IR) Simulations look good for acceleration; structure cold-tests will be necessary to build and align Working out fabrication issues Questions: Breakdown limits, wakefields Acceleration gradients potentially worth the effort