Laser-powered dielectric-structuresfor the production ofhigh-brightness electron and x-ray beams

Gil TravishParticle Beam Physics LaboratoryUCLA Department of Physics & Astronomy

on behalf of the MAP team

Material stolen from... lots of people including Chris Seers, Chris McGuinness, Eric Colby, Joel England Charlie Brau, Jonathan Jarvis, Tomas Plettner

predictionA particle accelerator “on a chip”, capable of

producing intense pulses of relativistic electrons and x-rays will be widely available

in 10 years

no plasmas were harmedin the making of this presentation

A laser-powered dielectric accelerator can provide relativistic electron beams and x-rays in a chip-scale device

+ laser(s)

Our long term goal is to develop a mm-scale, laser-powered, disposable, relativistic particle source

Large Application Space:

Industrial

• Petroleum Exploration

• Non-Destructive Testing (NDT)

X-ray Photolithography

• Medical

• Cardiology

• Veterinary

• Medical Imaging

Defense

• Homeland Security & Military

>10m

<10mm

x1000

Breakdown limits scale favorably with wavelength and dielectric materials support high fields

10-15

10-13

10-11

10-9

10-7

10-5

10-3

10-1

100

102

104

106

108

1010

1012

1014

Pu

lse

Le

ng

th [

s]

Frequency [Hz]

GHz THz IR-VISfs

ps

ns

us

Conventional RF

DWA

L

A

S

E

R

Du (1996)~GV/m

T-481

Breakdown LimitsConventional Structure

Eacc ~ Prf/λ

in metals...

Of available power sources at wavelengths shorter than microwaves, lasers are the most capable

lack of sources, materials and fabrication technology force us to make a leap from Microwave to Optical

Optical-scale dielectric-structures promise GeV/m gradients and naturally short bunches

+ very short pulses+ very high repetition rate+/- low charge- no track record- limited R&D work! The red-headed stepchild of AA

Gradients x10-x100 metalStructural control of fieldsMany possible geometries

Scalable fabrication

Tolerances:PWFA: ~300nmLWFA: ~30nmMAP: ~10nm

The choice of accelerator technology impacts the possible light source configurations...

RF Optical

Gradient

Energy gain per period

Repetition Rate

Charge per Bunch

Bunch Length

10-100 MeV/m 1-10 GeV/m

1 MeV 1 keV

100 Hz 10-100 MHz

0.1 - 1+ nC 0.01-1 pC

1-100 ps 1-100 fs

key: charge and time scale; not gradient

McGuinness

Optical structures naturally have sub-fs time structures and favor high rep. rate operation

3.3 fs charge capture< 1 fs

//

Fill Time ~ 1 ps Fill Time ~ 1-5 ps

Optical Cycles

Laser Pulse

femtosec

picosec

//Emitter Pulse

nanosecEmission Time ~1 ns

100-1000 ns(1-10 MHz)

Macropulse

Micropulse

Laser Cycle

An example of a soft x-ray FEL-based source reveals the need for new undulator approaches

Parameter Value

Wavelength 6 nm

Beam energy 25.5 MeV

Energy spread 10-4

Emittance (norm.) 0.06 µm (doh!)

Charge 1 pC (whew!)

Peak current 750 A

Undulator parameter 1

Undulator period 20 µm

Focusing betafunction ~ 3 mm

Gain length 500 µm

FEL parameter ~3 x 10-3

Saturation length 6 mm (LOL)

x-ray flux per bunch ~5 x 108

Lcoop/σL<1: 1-2 spikes

106 electrons; 108 photons

Pellegrini and Travish

... the undulator technology has at least as much impact on the FEL design.

PM Micro/Pulsed RF Optical

Period

Parameter

Gap

Status

>1 cm 0.1 - 1 mm 0.1-1 cm 1-20µm

1-10 <1 ~1 ~1

5 mm 1 mm 1+ cm 20-100µm?

Mature some SC work stalled paper

!opt " 3# n$4%&Lg

Focusing is an addition issue:

Optical-scale accelerator structures

At SLAC, the E-163 AARD team is producing a set of laser-driven dielectric micro-accelerators

10 µm

HC-1060 Fiber

4 Layer Structure (10/08)

2 Layer Structure (6/08)

PBG

Woodpile

PBG-fiber-based structures afford large apertures and scalability to HEP-length structures

X. E. Lin “Photonic bandgap fiber accelerator,” PRSTAB 4, 051301 (2001)input port!

absorbing boundary!

HFSS: custom dielectric waveguide coupler

Efficient coupling to the accelerating mode of a PBG fiber is complicated by various issues:➡overmoded: coupling to other modes drains away input power➡extra modes are lossy and difficult to simulate➡initial simulation results from overlap with accelerating mode: ~

12%

~2.5 GV/m

Planar structures offer beam dynamics advantages as well as ease of coupling power

Flat beam LS: modes? coherence? undulator?

MAP Logpile Grating

The MAP structure consists of a diffractive optic coupling structure and a partial reflector

cot kz ! "1 b " a( )#

$%& = kza ! "1 ! !

For gap a anddielectric b-a

idealizedresonance:

The design of the relativistic structure is mature and includes realistic material properties.

Ez = E0 cos(! z c) !

laser

Material measurements in the Nanolab are fed back to simulations

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(#%" (#&" (#'" (#)" (#*" (#+"

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Measured refractive index of ZrO2/Y2O3 deposited by

sputtering

Uniform ZrO2 showing grain size of tens of nanometers with

optimized condition

Discovery Denton Sputterer

Gaertner Ellipsometer

Simulations including acceleration and beam dynamics are underway.Resonant Fields (@ t = 7 ps)

Incident laser

y(m)

x(m)

Ex (V/m)

t(s)

t(s)

Ex (V/m)

Ex (V/m)

Input laser source• can correspond to actual Ti:Al2O3 laser

Energy Distributions

Energy Gain

Prototype structures are starting to be produced.

Full scale structure DBR

96.2nm

96.2nm

92.4nm

130.8nm

134.6nm

Structure  Dimension:  300nmX250μmX1000

287.6nm

Integration of a full structure has been developed. Process control improvements of fabrication is ongoing.

We are planning a ß=1 MAP beam de/acceleration experiment at SLAC’s E163

How can we produce a low-beta structure?

at 1 GeV/m, each period only produces 1KeV1000 periods only yields 1 MeV1 TeV requires 1 billion periods

Creating a sub-relativistic MAP is hard: the coupling and periodicity are one and the same

tapered structure

periodicity variation

two-color operation

DTL-like Solutions

periodicity skipping

Thick Glass Substrate

!!/"

!! 2!

laser light

β

z  (cm)

0.3

1

0.65

0 0.5 1

rapid change in velocity

The accelerating field may die off before the

particle fullly dephases

The low beta structure is now the critical technical risk. Multiple approaches are being tried.

Reflective DBR is short enough to let F-P modes leak out

Periodic metal layer lets FP leak out, but reinforces standing wave

800 nm 800 nm

800 nm incident laser

400 nm

800 nm incident laser

400 nm

DBR

matching  acceleration

Acceleration: coupling slot separation of βλ. Causes strong divergent force.⇒ cannot achieve simultaneous transverse focusing and longitudinal stability

Beam dynamics are challenging in optical scale structures due to large transverse forces

FODO scheme proposed for focusing, stability (being studied)

e-

Ultra-short period undulators

RF & Laser based undulators offer advantages but demand excellent uniformity and are undeveloped

δaUaU

<< ρGood:large aperturehigh fieldssmooth bore (wakefields)tunable

Ugly:

Beating can create larger periodsRF waveguide undulators can work

Bad:betatron motionpower loss along waveguidemodes and cutoffs

Issues:Readily available laser technologyEfficient path to longer periodsBetter than OPO/OPA?Ripples ok?

800nm + 1µm = 20µm

A grating based undulator can produce an intermediate-period device

Plettner and Byer, Phys. Rev. ST Accel. Beams 11, 030704 (2008)Barriers:Smith Purcell parasitic radiationAttosecond pulses and synchronizationLow fields?Period limit? (300µm)

Beam powered devices have also been considered: Image charge undulator (Wakefield)

Y. Zhang et al., NIM A 507 (2003) 459–463

Issues:Another beam?Advantage over RF?Energy loss?Acronym challenged (ICU)

A MAP-based undulator structure has been designed

For E=3 GV/m,Beqv=10 Tesla

Undulator Period = Laser Phase Flip

E-field

…………

λu >> λlaser

waveplate

Good mode quality has been found but phase flips are hard laser

an all optical light source

It is possible to have an all-laser-powered x-ray source using optical accelerator structures...

... but compromises must be made

low energy+

optical undulator=

QFEL

high energy+

conventional undulator=

FEL but long

A hard x-ray light source powered entirely by lasers and on a laptop scale will be a Quantum FEL

Parameter Optical Und. Conventional

FEL Wavelength ~0.1 Å (10 keV)~0.1 Å (10 keV)

Beam energy 10s MeV 100s MeV

Emittance (norm.) 0.06 µm0.06 µm

Current 2000 A2000 A

Charge 1 fC (whew! ~104 e-)1 fC (whew! ~104 e-)

FEL Parameter (ρ) 10-5 10-3

Undulator parameter 10-3 ~1

Undulator period 1-20 µm 1 cm

Saturation length ~10 cm ~1 m

!! / E " 6 #10$4becauseone photon emitted recoils > FEL bandwidth, ρ

We have the opportunity to develop a suite of on-chip particle beam tools

all using laser-driven dielectric structure

guns

monolithic structures

undulators

coherent THz/x-ray sourcesIFEL accelerator

sub-relativistic structures

muons, protons, ions

deflecting cavities

ultra-fast sources

focusing

ICS Gamma-Ray Source

Funding:NNSADTRAUCLADOE

Acknowledgments

Team:Rodney YoderJianyun Zhou (Postdoc - Fabrication)Josh McNeur (Grad - Simulations)Hristo Badakov (Engineer)Several past and present students...