Ultrafast Science

at SLAC

Aaron Lindenberg Stanford Dept. of Materials Science and Engineering

Stanford Linear Accelerator Center June 26, 2008

Ourunderstandingoftheworldislimitedbywhatwecanobserve.Forthousandsofyears,thismeantwithourowneyes...

1608-HansLippersheyinventstelescope1609-Galileousesthetelescopetoobservethemoon’sofJupiter,cratersonthemoon,sunspots,galacticnebulae)

1595-ZachariasJansenninventsmicroscopeImprovedbyAntonvanLeeuwenhoek.Heobservesbloodcells,bacteria,nematodes...

1665(RobertHooke)

X-rays,withwavelengthsontheorderofaninteratomicspacing,provideameansofseeingontheatomic-scale.

Apowerfultoolforprobingmaterialsstructure,molecularstructure,nanoscaleobjects...andcreatinghavocinthestreets.

E[eV]≅12398/λ[Å]

TheFatherofUltrafastScience,circa1890

Whenahorsegallops,doallfourfeetleavethegroundatonetime?

Takingsnapshotsofdynamicalphenomena

H.Edgerton

Control of materials structure with light

EricMazur,HarvardUniversity

Thelightsponge

Mazuretal.

SKawata,etal.,Nature412,697,2001.

Femtosecondsub-micronsculpting

Shortlength-scalesimpliesultrafasttime-scales

Short-pulsex-raysallowonetomeasurebothtechnologicallyrelevantlength-scalesandtechnologicallyrelevanttime-scales...

Ultrasmall Ultrafast

WhyX-rays?

crystallography imaging

(Roentgen1895)

microscopy

ThestructureofDNA,solvedthroughx-raydiffraction

Watson,Crick,Franklinetal.-1953

Visualizingatomic-scalemotion:Howdoatomsmove?

UltraslowMammoth-scaleMicroscopy

harpo 10-27

yocto 10-24

milli 10-3

micro 10-6

nano 10-9

pico 10-12

femto 10-15

atto 10-18

zepto 10-21 Pulse duration (seconds)

lasers (and laser-driven sources)

Synchrotrons

Condensed Matter Physics Chemistry and Biology HEP,

Cosmology

Ultrafast X-ray Sources and Science:

Particle Collisions Atomic Physics

LCLS SPPS

(fromP.Bucksbaum)

HowdoesanRNAmoleculefold?

Phase-changematerials:Switchingdynamics

Whatarethefluctuationsthatoccurinthestructureofaliquid?

equilibrium...

andnon-equilibrium...

Example2.Imagingnanoscalenucleationdynamicsatfemtosecondresolution

Formationofsub-criticalnuclei,fluctuatingonpstime-scales

Simulations(Lorazoetal):

Directvisualizationofmotiononafreeenergysurface

StanfordLinearAcceleratorCenter(SLAC)

Twomainstepstodoingultrafastx-rayscienceatSLAC

1.Makerelativisticultrafastelectronbunches

2.Turnelectronsintox-rays

What’sanFEL?

Answer:It’sarelativisticelectronbeamandanelectromagneticwaveco-propagatingthroughaperiodicmagneticfield.

Sometimeduring2009,arevolutionarynewtoolwillbecomeavailableatSLAC...

TheLCLS(LinacCoherentLightSource)(Theworld’sfirsthardx-rayFEL)

Howdoesanundulatorwork?

1.  Fromtheelectron’spointofview,asithurtlestowardstheundulator,theundulatorperiodisLorentzcontractedbyγ

2.  Theelectronwigglesinthisfieldandradiateslikeadipoleatwavelength

3.  Anobserversittingattheoutputoftheundulatorseestheemittedlightblue-shiftedbyroughlyanotherfactorofγ.

!U/"

How does an undulator work?

Consider electrons moving at almost the speed of light through a periodic magnetic field (undulator).

The periodically-accelerating electrons create periodic radiation wave fronts…

Resonance condition: In the time it takes an electron to travel the length of one undulator period, the light wavefront travels one light wavelength further.

This gives constructive interference and results in narrowband intense emission of x-rays.

Lightproducedondifferentundulatorbendsfromthesameelectronareinphase

Butlightproducedbydifferentelectronswithinthebunchareoutofphase

Intensityscaleswiththenumberofelectrons

What happens if the radiated field acts back on the electrons?

• Depending on the relative phase of EM field with respect to an electron, it either gains transverse momentum, loses it, or feels no net effect

Electron path in undulator field

EM field adds momentum

EM field removes momentum

EM effect neutral

Undulator FEL

The Free-Electron Laser instability •  Electrons that gain momentum swing wider, take longer path length

•  Electrons that lose momentum take shorter path length •  All electrons converge on each other at the neutral phase position: they

bunch together with spacing equal to undulator fundamental wavelength

•  The bunched electrons radiate in phase, adding to the strength of the EM field

•  This makes the interaction between field and electrons stronger

•  Causes more bunching •  Etc.

Intensity scales like N2!

Self-Amplified Spontaneous Emission (SASE)

log

(out

put p

ower

)

Length along undulator

Undulator regime (unbunched)

Exponential gain regime (bunching)

Saturation regime

ChladniPatterns

2-3 mJ FEL

20 mJ Spontaneous

Spatial distribution of SASE FEL radiation

Fluence, viewed at 100m, fundamental λ = 1.5Å

SLACResearchYard

LCLSParameters

Answer:1033ph/sec/mrad2/mm2/0.1%BW

Thanks,buthowbrightistheLCLS?

1mJin100fs=1010W(totalUSenergyconsumptionrateisabout1012W)

focusedto1um:1018W/cm2(roughlytheearth’ssolarfluxfocusedtotheheadofapin)

E-field:3x1010V/cm(orderofmagnitudelargerthantheatomicunitoffield)

HowbrightistheLCLS?