Laser-powered dielectric-structures for the production of high-brightness electron and x-ray beams
Laser-powered dielectric-structuresfor the production ofhigh-brightness electron and x-ray beamsGil TravishParticle Beam Physics LaboratoryUCLA Department of Physics & Astronomyon behalf of the MAP teamMaterial 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 relativisticelectrons 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 providerelativistic 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 sourceLarge Application Space: Industrial • Petroleum Exploration <10mm • Non-Destructive Testing (NDT) X-ray Photolithography • Medical x1000 • Cardiology • Veterinary • Medical Imaging Defense • Homeland Security & Military >10m
Breakdown limits scale favorably with wavelength and dielectric materials support high ﬁelds GHz THz IR-VIS 10-15 fs 10-13 T-481 Eacc ~ Prf/λ L A ps DWA S 10-11 E RPulse Length [s] Breakdown Limits 10 -9 Conventional Structure Du (1996) ns ~GV/m 10-7 us 10-5 Conventional RF 10-3 10-1 100 102 104 106 108 1010 1012 1014 Frequency [Hz] in metals...
Of available power sources at wavelengths shorter than microwaves, lasers are the most capablelack of sources, materials and fabrication technology force us to make a leap from Microwave to Optical
Optical-scale dielectric-structures promise GeV/mgradients 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 Tolerances: Gradients x10-x100 metal PWFA: ~300nm Structural control of ﬁelds LWFA: ~30nm Many possible geometries MAP: ~10nm Scalable fabrication
The choice of accelerator technology impacts thepossible light source conﬁgurations... McGuinness RF Optical Gradient 10-100 MeV/m 1-10 GeV/m Energy gain per period 1 MeV 1 keV Repetition Rate 100 Hz 10-100 MHz Charge per Bunch 0.1 - 1+ nC 0.01-1 pC Bunch Length 1-100 ps 1-100 fs key: charge and time scale; not gradient
Optical structures naturally have sub-fs timestructures and favor high rep. rate operation Micropulse Optical Cycles femtosec Laser Cycle 3.3 fs charge capture < 1 fs Macropulse Laser Pulse // picosec Fill Time ~ 1 ps Fill Time ~ 1-5 ps 100-1000 ns (1-10 MHz) Emitter Pulse // nanosec Emission Time ~1 ns
An example of a soft x-ray FEL-based sourcereveals the need for new undulator approaches 106 electrons; 108 photons 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 Lcoop/σL<1: 1-2 spikes 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 Pellegrini and Travish
... the undulator technology has at least as muchimpact on the FEL design. PM Micro/Pulsed RF Optical Period >1 cm 0.1 - 1 mm 0.1-1 cm 1-20µmParameter 1-10 <1 ~1 ~1 Gap 5 mm 1 mm 1+ cm 20-100µm? Status Mature some SC work stalled paper Focusing is an addition issue: # n 4% !opt "3 Lg $ &
At SLAC, the E-163 AARD team is producing a setof laser-driven dielectric micro-accelerators PBG HC-1060 Fiber 10 µm Woodpile 4 Layer Structure (10/08) 2 Layer Structure (6/08)
PBG-ﬁber-based structures afford large apertures and scalability to HEP-length structures input port! X. E. Lin “Photonic bandgap ﬁber accelerator,” PRSTAB 4, 051301 (2001) Efﬁcient coupling to the accelerating mode of a PBG ﬁber is absorbing boundary! complicated by various issues: ~2.5 GV/m ➡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%HFSS: custom dielectric waveguide coupler
Planar structures offer beam dynamics advantagesas well as ease of coupling power MAP Logpile Grating Flat beam LS: modes? coherence? undulator?
The MAP structure consists of a diffractive optic coupling structure and a partial reﬂector For gap a and dielectric b-a idealized resonance: ( )cot # k z ! " 1 b " a % = k z a ! " 1 ! ! $ &
The design of the relativistic structure is matureand includes realistic material properties. laser Ez = E0 cos(! z c) !
Simulations including acceleration and beamdynamics are underway. Resonant Fields (@ t = 7 ps) Input laser source Incident laser • can correspond to actual Ti:Al2O3 laser Ex (V/m) Ex (V/m)y(m) t(s) Ex (V/m) x(m)Energy Distributions t(s) Energy Gain
Prototype structures are starting to be produced. Full scale structure DBR Structure Dimension: 300nmX250μmX1000 96.2nm 130.8nm 287.6nm 96.2nm 134.6nm 92.4nm
Integration of a full structure has been developed.Process control improvements of fabrication is ongoing.
We are planning a ß=1 MAP beam de/accelerationexperiment at SLAC’s E163
How can we produce a low-beta structure? at 1 GeV/m, each period only produces 1KeV 1000 periods only yields 1 MeV 1 TeV requires 1 billion periods
Creating a sub-relativistic MAP is hard: the coupling and periodicity are one and the sametapered structure two-color operation rapid change in velocity laser light 1 Thick Glass Substrate 0.65 β 0.3 0 0.5 1 z (cm)DTL-like Solutions The accelerating ﬁeldperiodicity variation may die off before the !/" particle fullly dephases ! periodicity skipping ! 2! !
The low beta structure is now the critical technical risk. Multiple approaches are being tried. 800 nm incident laser 800 nm incident laser DBR Periodic metal matching layer lets FP leakacceleration out, but reinforces Reflective DBR standing wave is short enough to let F-P modes leak out 800 nm 800 nm 400 nm 400 nm
Beam dynamics are challenging in optical scale structures due to large transverse forcesAcceleration: coupling slot separation of βλ. Causes strong divergent force. cannot achieve simultaneous transverse focusing and longitudinal stability e-FODO scheme proposed for focusing, stability (being studied)
RF & Laser based undulators offer advantages butdemand excellent uniformity and are undevelopedGood: Bad: Ugly:large aperture betatron motion δ aUhigh ﬁelds power loss along waveguide << ρsmooth bore (wakeﬁelds) modes and cutoffs aUtunable Beating can create larger periodsRF waveguide undulators can work Issues: 800nm + 1µm = 20µm Readily available laser technology Efﬁcient path to longer periods Better than OPO/OPA? Ripples ok?
A grating based undulator can produce anintermediate-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 beenconsidered: Image charge undulator (Wakeﬁeld)Issues:Another beam?Advantage over RF?Energy loss?Acronym challenged (ICU) Y. Zhang et al., NIM A 507 (2003) 459–463
A MAP-based undulator structure has beendesigned Undulator Period = Laser Phase Flip waveplate E-field…… …… λu >> λlaser For E=3 GV/m, Beqv=10 Tesla
Good mode quality has beenfound but phase ﬂips are hard laser
It is possible to have an all-laser-powered x-raysource using optical accelerator structures... low energy high energy + + optical undulator conventional undulator = = QFEL FEL but long ... but compromises must be made
A hard x-ray light source powered entirely by lasersand on a laptop scale will be a Quantum FEL Parameter Optical Und. Conventional FEL Wavelength ~0.1 Å (10 keV) Beam energy 10s MeV 100s MeV Emittance (norm.) 0.06 µm Current 2000 A Charge 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 because !! / E " 6 # 10 $4 one photon emitted recoils > FEL bandwidth, ρ
We have the opportunity to develop a suite of on- chip particle beam tools guns sub-relativistic structures undulatorsmonolithic structures muons, protons, ions coherent THz/x-ray sources IFEL accelerator deﬂecting cavities focusing ultra-fast sources ICS Gamma-Ray Source all using laser-driven dielectric structure
AcknowledgmentsFunding: Team:NNSA Rodney YoderDTRA Jianyun Zhou (Postdoc - Fabrication)UCLA Josh McNeur (Grad - Simulations)DOE Hristo Badakov (Engineer) Several past and present students...