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

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

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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

slide-1
SLIDE 1

Advanced Dielectric Wakefield Accelerator Structures

  • G. Andonian

May 10, 2018 FAST/IOTA CollaboraEon MeeEng Fermilab

slide-2
SLIDE 2

Outline

  • DWA background
  • Relevant Issues & research direcEons
  • Advanced structures & applicaEons

– Bragg boundary – Planar geometry – Woodpile – Beam phase space manipulaEons

  • Conclusion
slide-3
SLIDE 3

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

  • Peak field

eEz,dec ≈ −4Nbr

emec 2

a 8π ε −1 εσ z + a ' ( ) * + , Design parameters:

  • 2. ¥10-12
  • 1. ¥10-12

1.¥10-12

  • 2. ¥10-12
  • 0.5

0.5 1.0

On-axis Ez (single mode structure)

!

!" =

! 2! 2!! ! − 1 !!(! − !)

  • Fundamental mode
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SLIDE 4

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)

slide-5
SLIDE 5

Recent High gradient DWA results

  • High field DWA demonstrated (>GV/m) at SLAC FACET

– 3nC, σz=20µm – Cylindrical geometry – In long (>15 cm) structures – Damping effects (reversible) before reaching breakdown due to high field

  • MoEvaEon to explore alternaEve geometry

SiO2 Vacuum Channel Metal Cladding

100µm

1 2 3 4 5 6 7 −0.25 −0.2 −0.15 −0.1 −0.05 0.05 0.1 0.15 0.2 0.25

KK Reconstruction of the 1 CM Data ζ Position [mm] Intensity [a.u.]

Beam Wake O’Shea Nat Comm 7, 12763 (2016)

slide-6
SLIDE 6

Bragg boundary DWA

  • MoEvaEon:

– Metal ablaEon at high fields in first tests – Explore alternate geometry with no metal

  • Concept:

– Bragg arrays – AlternaEng mulElayer stack (high/low ε) – ConstrucEve interference – Modal confinement in channel

  • Test at BNL ATF
  • Bragg DWA

– SiO2 (ε=3.8) matching layer – Bragg layers: SiO2, ZTA (ε=10.6), 12 periods – L = 1cm – Gap = 240 µm Metal ablaEon

e-beam dm d1 d2 2a (a)

dm d1 d2 2a (b)

Photo of Bragg array

c)

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SLIDE 7

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
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SLIDE 8

Bragg-boundary DWA

  • Experiment:

– Characterize structure modes

  • BNL ATF experiment

– 57MeV, 100pC, σt~1ps – CCR spectral analysis – ReconstrucEon algorithm – Energy modulaEon measured – Agreement with theory/ simulaEon (3D Vorpal, CST)

  • Results:

– Bragg reflector performance – Modal purity for THz source apps

5.0x1011 0.0 0.5 1.0 1.0x1012 1.5x1012

Frequency [Hz] Spectral Intensity [arb. units]

  • 0.002

0.000 0.002

  • 0.5

0.0 0.5 1.0

Step Size [m] Signal [arb. units]

10 12 14 16 18 20 z [mm]

  • 1.5
  • 1.0
  • 0.5

0.5 1.0 1.5 Longitudinal Field [MV/m] z [m]

  • 2.5x106

2.5x106 0.0 y [m] 0.0085 0.0117 0.0149 0.0181 0.0213

  • 0.0030
  • 0.0015

0.0000 0.0015 0.0030 Ez [V/m] z [m]

  • 2.5x106

2.5x106 0.0 x [m] 0.0085 0.0117 0.0149 0.0181 0.0213

  • 0.0030
  • 0.0015

0.0000 0.0015 0.0030 Ez [V/m]

  • G. Andonian, et al., PRL 113, 264801 (2014)

210GHz

CCR AutocorrelaEon

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SLIDE 9

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) “Trade-off” curves as funcEon of beam ellipEcity Ez Fy

slide-10
SLIDE 10

DeflecEon modes in cylindrical DWA

  • Experiment to study effects of deflecEon modes

at SLAC FACET

  • HEM modes seen in spectrum + integrated effect
  • n screen (“kick”)

“deflecEon” vs offset

Offset [µm]

0.2 0.4 0.6 0.8 1 1.2 0.2 0.4 0.6 0.8 1

10 cm, 400/600 um, no offset

0.2 0.4 0.6 0.8 1 1.2 0.2 0.4 0.6 0.8 1

Frequency [THz]

10 cm, 400/600 um, 30 um offset

HEM11 TM01 HEM21 TM02

30µm

Observed at low energy @ PSI

“Passive streaker” BeIoni, et al., PRAB 19, 021304 (2016)

TM01

e-

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SLIDE 11

Slab DWA with asymmetric beams

  • Experiment:

– Drive slab geometry with ellipEcal beams – measure effects of deflecEon modes

  • Reproducible results across

different materials (SiO2,ZTA, CVD)

  • Results: Suppression of effects

from transverse wakes for flat beams

e-beam

a b L

SiO2 Beam Off-axis injecEon vs observed “kick”

−100 −80 −60 −40 −20 20 40 60 80 100 −250 −200 −150 −100 −50 50 100 150 200

Offset in slab [ µm ] Deflection Downstream [ µm ] 1:1 x:y 1:2 x:y 1:3 x:y 1:15 x:y

“round” “flat”

FDTD simulaEon

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SLIDE 12

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, vg, 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)

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SLIDE 13

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, εΝ =2 mm-mrad, σz = 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)

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SLIDE 14

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

  • P. Hoang PRL 120, 164801 (2018)

Interferograms FFT round round ellipEcal ellipEcal

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SLIDE 15

Pulse shaping: High Transformer RaEos

  • Efficiency of DWA
  • TR enhancement from ramped

beams

– Triangle distribuEon – 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:

– “Ramped” beam observed – CCR autocorrelaEon – DeflecEng cavity

Symmetric beam R<2 Ramped beam R>>2

R = Ez,acc Ez,dec ≤ 2

  • G. Andonian, et al., PRL 118, 054802 (2017)

100 200 300 400 15 20 25 30

temporal axis (pixel count) current (arb. units) No DWS DWS

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SLIDE 16

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)

  • 500 GHz structure

– Mode confinement

  • J. B. Rosenzweig, G. Andonian, D. Stratakis, X. Wei

Standing wave structure seen in sims awer beam has passed through structure (OOPIC)

Excite mode with 4-pulse train - OOPIC DWA with horn antenna Mode confinement of Ez (HFSS)

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SLIDE 17

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

  • FAST is unique facility for advanced DWA studies

– Designer structures for fundamental physics – Spectra by design + Beam by design – Explore limits and possibiliEes

  • P. Hoang, “Toothed woodpile”

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