a resonant, thz slab-symmetric dielectric-based accelerator

A Resonant, THz Slab-Symmetric Dielectric-Based

Accelerator

A Resonant, THz Slab-Symmetric Dielectric-Based

Accelerator

R. B. Yoder and J. B. RosenzweigNeptune Lab, UCLA

ICFA Advanced Accelerator WorkshopSardinia, July 2002

R. Yoder / ICFA Sardinia

OutlineOutline

• Introduction: sketch of the idea• Basic theory and features• Motivation for experiment• Wakefield simulations• 3D electromagnetic simulation• Experimental prospects


Why Slab Geometry?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 easy to build and tune

• Dielectric breakdown limit potentially easier


Slab-Symmetric Dielectric-Loaded Accelerator

Slab-Symmetric Dielectric-Loaded Accelerator

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HistoryHistory

• Dielectric-loaded slow-wave structure for phase-matching is 20+ years old• Inverse Cerenkov accelerator (BNL, Omega-P, Columbia, Dartmouth, …)

• Dielectric wakefield accelerator (ANL, Yale/Omega-P-- current slab work)

• Planar dielectric waveguide is now under investigation in mm-wave regime at SLAC (M. Hill et al., PRL 87, 2001)

• Laser-driven resonant slab-structure proposed at UCLA, 1995– phase velocity not set by dielectric properties

(Rosenzweig, Murokh, Pellegrini, PRL 74, 1995) • This proposal refined: better accelerating mode quality

(Tremaine, Rosenzweig, Schoessow, PRE 56, 1997; Rosenzweig, AAC1998) • … but optical dimensions still too difficult to operate• Now: new THz power source at UCLA— expt possible!


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Basic physics of the structuresBasic physics of the structures

“Infinitely” wide in x

conducting wall

r

Fields must be independent of x

Set = 0 (vacuumwavelength of laser)

Coupling Q-1 ~ w/

Coupling slit, width w

Dispersion relation: = c2(kx

2 + ky2 + kz

2)

Want: vz = c, i.e. kz = /c

Therefore: since kx = 0,must have ky = 0 in gap

Resonant kz values obtained as a function of a, b, r

dielectric layer


Accelerating ModesAccelerating Modes

In the gap (|y| < a)E ~ eikz [otherwise Fabry-Perot]

ky = 0, so Ez is constant in y

•E = 0 Ey ~ kzy

In the dielectric (a< |y|< b)Ez(y) ~ Asin[ky(b–y)]

Ey(y) ~ A kz/ky cos[ky(b–y)]

A ~ E0/sin[ky(b–a)], ky = (r–1) kz

Ez = 0 at y = b, Ez, Ey continuous at y = a

€

kzaεr

εr −1=cot[kz εr −1(b−a)]

(Simplest case: perfect slab)


A

n

n

Ey(

a)/E

0

Solutions for accelerating modesSolutions for accelerating modes = 340 µm resonator, n = 1, 2, 3

-10

-5

0

5

10

0 50 100 150

Ez/E

0

y (µm)

-10

-5

0

5

10

15

0 50 100 150

y (µm)

Ey/

E0

0

5

10

15

20

25

30

35

40

0 2 4 6 8 10 12

0

10

20

30

40

50

60

0 2 4 6 8 10 12


Transverse Wakefield SuppressionTransverse Wakefield Suppression

Short pulse ( = 0.4 ps) Long pulse ( = 4 ps)

Simulations using OOPIC

200 pC, r = 120 µm, r = 3.9, a = 0.58 mm, b = 1.44 mm

Wz

W


Motivation for an experimentMotivation for an experiment

UCLA Neptune Lab:• Photoinjector beam with good parameters, well

understood(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)

• New THz generation experiment beginning, using Neptune CO2 laser / MARS amplifier (≤ 100 J/pulse)

• Opportunity for realistic device dimensions using FIR drive power, and potential multi-MW source


Nonlinear Difference Frequency GenerationNonlinear Difference Frequency Generation(UCLA Elec Eng — S. Tochitsky, P. Musumeci)

2.38°21.6°

10.6 + 10.3 µm 340 µm

• Non-collinear phase matching in isotropic gallium arsenide crystal• Frequency mixing through choice of face angles

• GaAs transmits well in 100–1000 µm range

• Limited by dispersion in crystal, damage threshold

• CO2 laser a natural source of frequency doublets

• Maximum power: 100’s of MW at 340 µm with Neptune laser

• Other possibilities: use low-power tunable laser for several MW at mm-wave (e.g. 300 GHz)

• First experiments underway


Theory vs. Simulation: accelerating mode

Theory vs. Simulation: accelerating mode

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


Resonant fields in GdfidL, time-domainResonant fields in GdfidL, time-domain


Time-domain simulation:structure fills





QuickTime™ and aGIF decompressor

are needed to see this picture.


Wakefield simulationsWakefield simulationsOOPIC: use resonant structure from GdfidL with ‘real’ beam

Longitudinal wakefield period = 340 µm !

Q = 200 pC, a = 115 µm, b = 145 µm, = 3, y = 25 µm

Bunch length 1.2 mmField mostly washed out

Bunch length 120 µmStill only 20 kV retarding potential


• Wakefield measurements• seeing energy change is impossible; maybe misalignment could disrupt the beam

• with FIR bandpass filter can check resonant frequency

• try adjusting gap and verifying mode analysis

• Structure resonances (“cold test”)• use coupling slots as bandpass filter

• Breakdown fields• need to see if we can break down structure in small high-power spot

• Energy change • Energy gain set by structure size and Q, details of coupling slots, power available,

frequency, and laser spot size.

• Gains of a few MeV are possible

Experiments -- and questions:Experiments -- and questions:


ConclusionsConclusions

• Slab structures are attractive for beam quality and gradient; become practical at (sub-)THz

• We are simulating and planning experiments for Neptune; theory appears to be backed up by simulation

• Wakefield is important but will be hard to measure• Breakdown limit still to be established• Acceleration gradients potentially worth the effort

a resonant, thz slab-symmetric dielectric-based accelerator

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