Slab-Symmetric Dielectric-Based Accelerator

Slab-Symmetric Dielectric-Based Accelerator

Rodney YoderUCLA PBPL / Manhattan College

DoE Program ReviewUCLA, May 2004

R. Yoder / DoE Review

Review: Why Slab Geometry?Review: Why Slab Geometry?

Interested in structures in the mm or FIR regimeBut— there are well-known limitations:

Cavity structures:

• Wakefields ~ 3, leadingto 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

R. Yoder / DoE Review

Slab-Symmetric Dielectric-Loaded Accelerator

Slab-Symmetric Dielectric-Loaded Accelerator

R. Yoder / DoE Review

Motivation: experimentMotivation: 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 CO2 laser / MARS amplifier (≤ 100 J/pulse)

• “Cold-testing” with 10-µm design still possible

R. Yoder / DoE Review

Basic physics of the structuresBasic physics of the structures

• Set = 0 (vacuum wavelength of laser)• Fields independent of x (translational symmetry)• Dispersion relation: = c2(kx

2 + ky2 + kz

2)

Periodic coupling enforces kz = /c vz = c • prevents Fabry-Perot mode

Since kx = 0, we must have ky = 0 in gapResonant kz values obtained as function of geometry using dielectric to match boundary conditions

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Ideal accelerating mode, 3D simulationIdeal accelerating mode, 3D simulation

Structure Q ~ 600, r/Q = 25 k/m, so field = 30 MV/m at 50 MW

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Transverse Wakefield SuppressionTransverse 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

Wz

W

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• 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 structuresCoupling to the structures

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2D time-dependent simulation2D 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

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Comparison: Shorter coupling slotsComparison: 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 slotField in slot comparable to peakFrequency bandwidth ~ 1%

R. Yoder / DoE Review

FillingFilling

Quarter-wavelength slots = 325 psEmax = 15 E0

Everything depends on the slots…

6 µm slots = 70 psEmax = 3.8 E0

R. Yoder / DoE Review

ManufactureManufacture

• 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

R. Yoder / DoE Review

Multilayer structure for 1-10µm laser(aka 1-D Photonic Band Gap Accelerator!)

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 substrateEach layer is a quarter wavelength

R. Yoder / DoE Review

ConclusionsConclusions

• 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