Slab-Symmetric DielectricBased Accelerator
Rodney Yoder
UCLA PBPL / Manhattan College
DoE Program Review
UCLA, May 2004
Review: Why Slab Geometry?
Interested in structures in the mm or FIR regime
But— there are well-known limitations:
Cavity structures:
Slab structure:
• Wakefields ~ l3, leading
to bad transverse dynamics
• Transverse wakefields
strongly suppressed
• Machining tolerances
are tough
• Planar structure may be
easier to build and tune
• Accelerating fields
limited by breakdown
• Dielectric breakdown
limit potentially easier
R. Yoder / DoE Review
Slab-Symmetric DielectricLoaded Accelerator
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, en = 6π mm mrad, DE/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 CO2 laser / MARS amplifier (≤ 100 J/pulse)
• “Cold-testing” with 10-µm design still possible
R. Yoder / DoE Review
Basic physics of the structures
• Set l = l0 (vacuum wavelength of laser)
• Fields independent of x (translational symmetry)
• Dispersion relation:  = c2(kx2 + ky2 + kz2)
Periodic coupling enforces kz = /c  vfz = c
• prevents Fabry-Perot mode
Since kx = 0, we must have ky = 0 in gap
Resonant kz values obtained as function of geometry
using dielectric to match boundary conditions
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
R. Yoder / DoE Review
Transverse Wakefield Suppression
2D Simulations using OOPIC
Short pulse (s = 0.4 ps)
Long pulse (s = 4 ps)
Wz
W
200 pC, sr = 120 µm, er = 3.9, a = 0.58 mm, b = 1.44 mm
R. Yoder / DoE Review
Coupling to the structures
• Periodic slots enforce resonant mode
• slot dimensions determine the Q-factor for the structure
• roughly proportional to l0/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, D/ ~ L/w
• Perturbation vanishes for L = lg/4 (quarter-wave matching)
• gives high Q, slow fill
R. Yoder / DoE Review
2D time-dependent simulation
340 µm wavelength
a = 115 µm, b–a = 30 µm
quarter-wavelength slots
Axial field:
• flat wavefronts (no perturbation)
• large field in slot
Transverse field:
• zero at y=0
• zero at peak acceleration
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
(D / = +1.8%)
Slight deformation near slot
Field in slot comparable to peak
Frequency bandwidth ~ 1%
R. Yoder / DoE Review
Filling
Everything depends on the slots…
Quarter-wavelength slots
t = 325 ps
Emax = 15 E0
6 µm slots
t = 70 ps
Emax = 3.8 E0
R. Yoder / DoE Review
Manufacture
• Can use standard semiconductor techniques
• Choices are monolithic vs. two-part
Monolithic
Two-part
- alignment not an issue
- how to tune/deform?
- must avoid very thin
“membrane” as upper layer
- easy tuning
- how to align?
- need precision positioning
in y, z, and azimuthal angle
- possible but expensive
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
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
R. Yoder / DoE Review