ultrafast science at slac - stanford university · 2008. 7. 1. · slac aaron lindenberg stanford...
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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?