A Resonant, THz Slab-Symmetric Dielectric-Based Accelerator

1. 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

2. Introduction: sketch of the idea Basic theory and features Motivation for experiment Wakefield simulations 3D electromagnetic simulation Experimental prospects Outline R. Yoder / ICFA Sardinia

3. Why Slab Geometry? • Interested in structures in the mm or FIR regime • But— there are well-known limitations: Cavity structures: • Wakefields ~ l3, 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

4. Slab-Symmetric Dielectric-Loaded Accelerator R. Yoder / ICFA Sardinia

5. 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! History R. Yoder / ICFA Sardinia

6. Basic physics of the structures Set l = l0 (vacuum wavelength of laser) “Infinitely” wide in x conducting wall Fields must be independent of x Dispersion relation:  = c2(kx2 + ky2 + kz2) er dielectric layer Want: vfz = c, i.e. kz = w/c Therefore: since kx = 0, must have ky = 0 in gap Resonant kz values obtained as a function of a, b, er Coupling slit, width w Coupling Q-1 ~ w/l R. Yoder / ICFA Sardinia

7. Accelerating Modes (Simplest case: perfect slab) E ~ eikz [otherwise Fabry-Perot] ky = 0, so Ez is constant in y •E = 0  Ey ~ kzy In the gap (|y| < a) Ez = 0 at y = b, Ez, eEy continuous at y = a Ez(y) ~ Asin[ky(b–y)] Ey(y) ~ Akz/kycos[ky(b–y)] A ~ E0/sin[ky(b–a)], ky = (er–1) kz In the dielectric (a< |y|< b) R. Yoder / ICFA Sardinia

8. Solutions for accelerating modes  = 340 µm resonator, n = 1, 2, 3 A Ez/E0 n y (µm) Ey(a)/E0 Ey/E0 n R. Yoder / ICFA Sardinia y (µm)

9. Transverse Wakefield Suppression Simulations using OOPIC Long pulse (s = 4 ps) Short pulse (s = 0.4 ps) Wz W 200 pC, sr = 120 µm, er = 3.9, a = 0.58 mm, b = 1.44 mm R. Yoder / ICFA Sardinia

10. Motivation for an experiment UCLA Neptune Lab: • Photoinjector beam with good parameters, well understood (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) • 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

11. Nonlinear Difference Frequency Generation (UCLA Elec Eng — S. Tochitsky, P. Musumeci) • 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 10.6 + 10.3 µm 340 µm R. Yoder / ICFA Sardinia

12. 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

13. Resonant fields in GdfidL, time-domain R. Yoder / ICFA Sardinia

14. Time-domain simulation:structure fills R. Yoder / ICFA Sardinia

15. Time-domain simulation:structure fills R. Yoder / ICFA Sardinia

16. Wakefield simulations OOPIC: use resonant structure from GdfidL with ‘real’ beam Longitudinal wakefield period = 340 µm ! Bunch length 1.2 mm Field mostly washed out Bunch length 120 µm Still only 20 kV retarding potential Q = 200 pC, a = 115 µm, b = 145 µm, e = 3, sy = 25 µm R. Yoder / ICFA Sardinia

17. Experiments -- and questions: • 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 R. Yoder / ICFA Sardinia

18. Conclusions • 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 R. Yoder / ICFA Sardinia