Advanced Dielectric Wakefield Accelerator Structures G. Andonian - PowerPoint PPT Presentation

Advanced Dielectric Wakefield Accelerator Structures G. Andonian May 10, 2018 FAST/IOTA CollaboraEon MeeEng Fermilab

Outline • DWA background • Relevant Issues & research direcEons • Advanced structures & applicaEons – Bragg boundary – Planar geometry – Woodpile – Beam phase space manipulaEons • Conclusion

Dielectric Wakefield Accelerator Candidate for next-gen adv. • Accelerator (GV/m field) Simple geometry • RelaEvisEc beam drives wake in • material Dependent on structure • geometry Present day beams naturally • scale to sub-mm (THz) structures a , b , Q , σ z , ε Design parameters: 2 − 4 N b r e m e c eE z , dec ≈ 1.0 ' * • 8 π Peak field a εσ z + a ) , 0.5 ε − 1 ( + - 2. ¥ 10 - 12 - 1. ¥ 10 - 12 1. ¥ 10 - 12 2. ¥ 10 - 12 • ! 2 ! ! Fundamental mode ! !" = ! − 1 ! ! ( ! − ! ) - 0.5 2 ! On-axis Ez (single mode structure)

DWA ApplicaEons & Research • High gradient applicaEons – HEP: future machine (GV/m fields) • Thompson PRL 100, 214801 (2008) • O’Shea Nat Comm 7, 12763 (2016) – Light Source • A. Zholents Proc FEL14, 993 (2014) – Phase Space manipulaEon – RelaEvisEc e-beam diagnosEcs – THz source • Relevant Research Issues – PracEcally achievable field gradients • Breakdown & High field damping • Joule heaEng at high rep rate – Beam break up – transverse modes – Efficiency, TR – Materials/cladding composiEon – Alternate geometries (slab, woodpile)

Recent High gradient DWA results • High field DWA demonstrated (>GV/m) at SLAC FACET – 3nC, σ z=20µm – Cylindrical geometry – In long (>15 cm) structures – Damping effects (reversible) before reaching breakdown due to high field • MoEvaEon to explore alternaEve geometry KK Reconstruction of the 1 CM Data 0.25 Metal 0.2 Wake Cladding 0.15 SiO 2 Intensity [a.u.] 0.1 0.05 0 Vacuum − 0.05 Channel − 0.1 100µm − 0.15 − 0.2 Beam − 0.25 O’Shea Nat Comm 7, 12763 (2016) 0 1 2 3 4 5 6 7 ζ Position [mm]

Bragg boundary DWA c) MoEvaEon: • – Metal ablaEon at high fields in first tests – Explore alternate geometry with no metal Concept: • Metal ablaEon – Bragg arrays – AlternaEng mulElayer stack (high/low ε ) (a) – ConstrucEve interference d 2 – Modal confinement in channel d 1 d m e-beam Test at BNL ATF • 2a Bragg DWA • – SiO 2 ( ε =3.8) matching layer – Bragg layers: SiO 2 , ZTA ( ε =10.6), 12 periods (b) – L = 1cm – Gap = 240 µm d 2 d 1 d m 2a Photo of Bragg array

BNL ATF experimental layout • CTR interferometer for bunch length/profile reconstrucEon • CCR interferometer for spectral characterizaEon • Out-coupling antenna • Dipole spectrometer for energy modulaEon • Similar setup to FACET experiments and techniques can be used at FAST

Bragg-boundary DWA • Experiment: 1.0 – Characterize structure modes Signal [arb. units] 1.0 Spectral Intensity [arb. units] • BNL ATF experiment 0.5 – 57MeV, 100pC, σ t~1ps 0.0 – CCR spectral analysis 210GHz -0.5 0.5 – ReconstrucEon algorithm -0.002 0.000 0.002 Step Size [m] – Energy modulaEon measured CCR AutocorrelaEon – Agreement with theory/ simulaEon (3D Vorpal, CST) • Results: 0.0 0 5.0x10 11 1.0x10 12 1.5x10 12 Frequency [Hz] – Bragg reflector performance G. Andonian, et al., PRL 113, 264801 (2014) – Modal purity for THz source apps -2.5x106 2.5x106 -2.5x106 2.5x106 0.0 Ez [V/m] 0.0 Ez [V/m] Longitudinal Field [MV/m] 0.0030 0.0030 1.5 1.0 0.0015 0.0015 0.5 y [m] x [m] 0.0000 0.0000 z [mm] 10 12 14 16 18 20 -0.5 -0.0015 -0.0015 -1.0 -0.0030 -0.0030 -1.5 0.0085 0.0117 0.0149 0.0181 0.0213 0.0085 0.0117 0.0149 0.0181 0.0213 z [m] z [m]

Beam Break up DWA can sustain GV/m for future machine, but may be limited by BBU • BBU stems from growth of transverse modes • Suggested to use external FODO channel • – C. Li et al. , PRSTAB 17, 091302 (2014) Suggested to use flat beams with planar structures to miEgate the effect • – A. Tremaine et al. , PRE 56, 7204 (1997) – D. Mihalcea et al. , PRSTAB 15, 081304 (2012) – S. Baturin in prep (2018) E z “Trade-off” curves as funcEon of beam ellipEcity F y

DeflecEon modes in cylindrical DWA • Experiment to study effects of deflecEon modes e - at SLAC FACET 30µm • HEM modes seen in spectrum + integrated effect on screen (“kick”) 1 TM 01 10 cm, 400/600 um, no offset 0.8 “deflecEon” vs offset 0.6 0.4 0.2 0 0 0.2 0.4 0.6 0.8 1 1.2 1 10 cm, 400/600 um, 30 um offset TM 01 0.8 Offset [µm] 0.6 HEM 21 HEM 11 0.4 TM 02 Observed at low energy @ PSI 0.2 0 0 0.2 0.4 0.6 0.8 1 1.2 “Passive streaker” Frequency [THz] BeIoni, et al., PRAB 19, 021304 (2016)

Slab DWA with asymmetric beams • Experiment: L – Drive slab geometry with ellipEcal beams b – measure effects of deflecEon a e-beam modes • Reproducible results across Beam Off-axis injecEon different materials (SiO 2 ,ZTA, CVD) vs observed “kick” 200 • Results: Suppression of effects 1:1 x:y “round” 1:2 x:y from transverse wakes for flat 150 1:3 x:y Deflection Downstream [ µ m ] beams 1:15 x:y 100 50 0 FDTD simulaEon “flat” − 50 − 100 − 150 − 200 SiO2 − 250 − 100 − 80 − 60 − 40 − 20 0 20 40 60 80 100 Offset in slab [ µ m ]

Advanced DWA Structure: woodpile Build off Bragg and slab results • – Advanced DWA structures – No metals (excessive dissipaEon into heat) Tailor spectrum for reduced coupling to • transverse modes (enhance longitudinal) Familiar from DLA • – Extend to DWA Engineer spectral content • – 3D-periodicity gives more control – Modes, v g , raEos – Excited modes in bandgap are confined Woodpile assembled at UCLA • – For experiment at BNL ATF – 125µm Sapphire rods x 2cm – by hand (P. Hoang)

Woodpile simulaEons Woodpile parameters • 125µm x 2cm sapphire rods – 375µm periodicity in x, and z – 250µm gap – Single period structure to understand dynamics – BNL ATF Beam parameters • 57 Mev, ε Ν =2 mm-mrad, σ z = 250µm – “round beam”: 50:50 µm, 150 pC – “ellipEcal beam” 50: 500 µm, 235 pC – Many modes in spectra for round beam • Boundary condiEons require computaEon – Flat beam shows only fundamental – Cross secEon (beam perspecEve) Spectrum of woodpile (simulaEons)

DWA Woodpile experiment Experiment at BNL ATF • – CCR spectral characterizaEon methods – Round beam vs ellipEcal – Shows suppression of spectra – agreement with simulaEons Results important • – Design spectrum – Use bunch length to couple to desired longitudinal modes – Use beam shape to reduce coupling to transverse modes Interferograms FFT round round ellipEcal ellipEcal P. Hoang PRL 120, 164801 (2018)

Pulse shaping: High Transformer RaEos Ramped beam R>>2 Symmetric beam R<2 • Efficiency of DWA • TR enhancement from ramped beams R = E z , acc ≤ 2 – Triangle distribuEon E z , dec – Novel: doorstep, double triangles Techniques: • – EEX, laser shaping, mask in dispersive secEon Shaping with self wakes • – Analogous to bunch train with DWA AnEpov PRL 111, 134801 (2013) • Shaping capabiliEes essenEal for TR • studies Experiment at BNL ATF: • G. Andonian, et al., PRL 118, 054802 (2017) – “Ramped” beam observed – CCR autocorrelaEon 30 No DWS – DeflecEng cavity DWS current (arb. units) 25 20 15 100 200 300 400 temporal axis (pixel count)

Pulse trains + Longitudinally periodic structures • MoEvaEon: – Confine energy of mode inside structure – Near zero group velocity – Longitudinal periodicity - ε (z) OOPIC and HFSS SimulaEons • – a = 50 µm, b = 126 µm – Periodicity = 300 µm – Used both sinusoidal variance of ε and step – Base materials SiO2, diamond ( ε =3.8, 10.6) Excite mode with 4-pulse train - OOPIC 500 GHz structure • – Mode confinement DWA with horn antenna Standing wave structure seen in sims awer beam has passed through structure (OOPIC) J. B. Rosenzweig, G. Andonian, D. Stratakis, X. Wei Mode confinement of Ez (HFSS)

Summary & Future Work • DWA useful tool for accelerator applicaEons – Advanced accelerator, THz source – Phase space manipulaEon, beam diagnosEc • FAST allows opportunity to study in new regime – High average current : Charging/ HeaEng quesEons – High quality bunches : Small/long structures – RTFB transform: Flat beam driven planar DWA P. Hoang, “Toothed woodpile” • FAST is unique facility for advanced DWA studies – Designer structures for fundamental physics – Spectra by design + Beam by design – Explore limits and possibiliEes Acknowledgements: UCLA: S. Barber (LBNL), A. Fukusawa, P. Hoang, B. Naranjo, N. Sudar, O. Williams, J. Rosenzweig, et al. RadiaBeam: F. O’Shea, M. Harrison, A. Murokh, et al., FACET: B. O’Shea, C. Clarke, M. Hogan, V. Yakimenko, et al. U. Chic: S. Baturin, ATF: M. Fedurin, C. Swinson, et al., Work Supported by US DOE HEP