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A Resonant, THz Slab-Symmetric Dielectric-Based Accelerator. R. B. Yoder and J. B. Rosenzweig Neptune Lab, UCLA. ICFA Advanced Accelerator Workshop Sardinia, July 2002. Introduction: sketch of the idea Basic theory and features Motivation for experiment Wakefield simulations - PowerPoint PPT Presentation
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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
R. Yoder / ICFA Sardinia
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
R. Yoder / ICFA Sardinia
Slab-Symmetric Dielectric-Loaded Accelerator
Slab-Symmetric Dielectric-Loaded Accelerator
xzy
R. Yoder / ICFA Sardinia
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!
R. Yoder / ICFA Sardinia
yzab
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
R. Yoder / ICFA Sardinia
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)
R. Yoder / ICFA Sardinia
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
R. Yoder / ICFA Sardinia
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
R. Yoder / ICFA Sardinia
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
R. Yoder / ICFA Sardinia
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
R. Yoder / ICFA Sardinia
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
R. Yoder / ICFA Sardinia
Resonant fields in GdfidL, time-domainResonant fields in GdfidL, time-domain
R. Yoder / ICFA Sardinia
Time-domain simulation:structure fills
Time-domain simulation:structure fills
R. Yoder / ICFA Sardinia
Time-domain simulation:structure fills
Time-domain simulation:structure fills
QuickTime™ and aGIF decompressor
are needed to see this picture.
R. Yoder / ICFA Sardinia
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
R. Yoder / ICFA Sardinia
• 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:
R. Yoder / ICFA Sardinia
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