Teravolt-per-meterplasmawakefieldsfromlow-charge,femtosecondelectronbeams
J.B.RosenzweigUCLADept.ofPhysicsandAstronomy
FACETIIScienceWorkshopOctober19,2017
BrightnessBegetsBrightness:ElectronsandPhotons
• Lightsourcerevolutionduetoe- beamimprovements
• Twoordersofmagnitudeproducequalitativelynewlightsource,theX-rayFree-electronLaser– Intensecoldbeam;instability
– 8ordersofmagnitudephotonbrightness
• Forneedede-beam,mustcompresstofsscale• Ultra-bright,Å,coherent,fseclightsource
– X-rayFEL:manyordersofmagnitudeleapforward
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Be =2Iεn
2 →2 ×1014 A/m2
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Erad ∝ exp z /Lg( ); Lg ∝ Be−1/ 3
Ultra-shortXFELpulses:motivation• Toolatatomicelectron spatio-temporalscales
– Angstroms-nanometers(~Bohrradius)– Femtoseconds (e- motion,Bohrperiod;femto-chemistry,etc.)
• 100fsaccessiblewithstandardapproaches• Promisingpath:ultra-short,lowQelectronbeam
– MyriadofadvantagesinFELand beamphysics• Mitigatecollectiveeffectsdramatically
– Robustinapplication:XFEL,coherentoptical/IRsource• Canalsousemicrobunching…• Spin-offtoultra-highfieldPWFA!
Beamphysics:fromplasmatoplasma• Beamatlowerenergyissinglecomponentrelativisticplasma
• Preserveoptimizeddynamics:changeQ,keepingplasmafrequency(n,aspectratio)same
• Dimensionsscaleas– Shorterbeam,easiertocompress– Bigemittance reduction,easytofocus– Result:ultra-highbrightnessbeam
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σ i ∝Q1/ 3
J.B. Rosenzweig and E. Colby, Advanced Accelerator Concepts p. 724 (AIP Conf. Proc. 335, 1995).
Ultra-shortpulsesatSPARX(LNF)
• Chicanebunchingaftervelocitybunching• Use~1pC beamforsinglespike
– SS:cooperationlength=bunchlength• Short,lowemittancebeamatfinalenergy2.1GeV
• Veryhighfinalbrightness– 2ordersofmagnitude!
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εnx ≅ 7.5 ×10−8 m - rad
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B = 2 ×1017 A/m2
sz =4.7 µm after velocity bunchingFinal ongitudinal phase space Final current profile
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σ t ≅ 600 attoseconds(!)
SingleSpikeX-rayFEL
• Singlespike,>1GWpeakpower• 480attosecond rmspulseat2nm• 1st timeinX-rayregime
s (µm)
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ΔωΔt = 1.67
l (nm)z (m)
z
z
Example:LCLSw/sub-fspulse• Useevenshorter0.25pCbeam,150aspulse
– Singlespikew/standardLCLSundulator
• Obtainultra-compact“LCLS” at4.3GeV• Extendenergyreachto83keV(0.15Å)
Gain evolution at 13.6 GeV; 0.15 Å, 83 kV FEL Gain evolution for 1.5 Å 4.3 GeV (0.25 pC)
Short undulator!
HighinterestinFELcommunity…low-QexplorationsatLCLS
20 pC, 135 MeV, 0.6-mm spot diameter, 400 µm rms bunch length (5 A)
OTR screen with transverse
deflector ON
0.14 µm
Emittance near calculated thermal emittance limit
Low emittances at LCLS with 20 pC. Diagnostic limited
sz » 0.6 µm
8 kA?
LiTrack(no CSR)
Photo-diode signal on OTR screen after BC2, bestcompression at L2-linac phase of -34.5 deg.
Horizontal projected emittance measured at 10 GeV LCLS FEL simulation at 1.5 Å; not single spike.
1.5 Å,3.6´1011 photonsIpk = 4.8 kAge » 0.4 µm
2fsbeamsattemporalmeasurementresolutionlimit
• Coherenttransitionradiation(destructive)• Non-destructive:coherentedgeradiation(CER)
Combined ER and SR0
0.2
0.4
0.6
0.8
1
1.2
0 50 100 150 200
Spec
tral i
nten
sity
( µJ/T
Hz)
f (THz)
QUINDI simulation FACET II case
Advancedacceleratorphysics:focusingultra-shortbeams
• 2fs(600nm)beampredictedtohaveIp=8kA• Focustosr<200nm(lowemittance enables…)• Surfacefields
• TV/m(100V/Å!)infsunipolar(1/2-cycle)pulse– Newtoolforhighfield-matterinteraction(AMO,nuclear)– FACETIlimit~100GV/m– emittance toohigh!
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eEr ≈ remec2Ip /ecσ r
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Er ≈1 TV/m!
Howtofocus?• Veryshortfocallengthfinalfocus• Useultra-highfieldpermanentmagnetquads
– mitigatechromaticaberrations– FF-DD-Ftriplet,adjustthroughquadplacement
• Developed570T/mPMQfields- Needslightlystronger,noproblem(Pr gives>1kT/m)
Small aperture PMQ B’=570 T/m Halbach PMQ as built
Final beam sizes: ~130 nm
CollectiveBeamField-inducedTunnelingIonization
¡ “Weaker” fields:tunneling¡ Regimewellunderstood
¡ ADK perturbationtheory¡ Developedforlasers¡ ADK-basedsimulation
(OOPIC,Vsim)¡ Benchmarkedtoe-beam
experiments(E167andsuccessors)
Tunneling Ionization
1TV/mReachestheBarrierSuppressionRegime(BSI)
¡ BSI:e- classicallyescapesatom¡ Previouslyonlyreached
experimentalbylasers¡ Theoryconcentratesonlasers
¡ BSInotwellunderstood¡ Non-perturbative¡ Empiricalformulas
¡ Fundamentalatomicphysicstool¡ Plasmawakefields…
BSIionizationoccursin2fscase• Extensiontounipolarfieldpulse
– approachofBauer,etal.inlasercontext
• BSIimportantabove40GV/m,buttunnelinghasalreadybeenaccomplished…
• FortotalionizationtrustOOPIC
0
0.2
0.4
0.6
0.8
1
1.2
-6 -4 -2 0 2 4 6
Normalized EIonization fraction
Frac
tiona
l Ion
izat
ion,
Nor
mal
ized
Ele
ctric
Fie
ld
t (fs)
Fractional ionization due to BSI, 800 GV/m peak, 2 fs gaussian pulse
TV/mPlasmaWakefieldAccelerator• Ultra-highbrightness,fsbeamsinplasma• Use20pCLCLSbeaminhighn plasma• In“blowout” regime:totalrarefactionofplasmae-s
– Beamdenserthanplasma– Verynonlinearplasmadynamics– Pureioncolumnfocusingfore-s– EMacceleration,independentofr– Generalmeasureofnonlinearity:
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˜ Q ≡Nbkp
3
n0
= 4πkpreNb
<< 1, linear regime> 1, nonlinear "blowout" ⎧ ⎨ ⎩
MAGIC simulation of blowout PWFA case
-1.5
-1
-0.5
0
0.5
1
1.5
0 0.5 1 1.5 2 2.5
.
F/m
Wakes in blown out region
SinglebunchexcitationatFACETII
• Beammustbeshortandnarrowcomparedtoplasmaskindepth
• Inthiscaseimplies,blowout• With2fsFACETIIbeamwechoose• For20pCbeam,wehave• Linear“Cerenkov” scaling
• 1TV/mfields,convertedEr)• Collaborationinitiated(authors)
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nb > n0
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σ r < kp−1
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σ z < kp−1
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˜ Q > 1
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n0 = 7 ×1019 cm-3
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˜ Q = 7OOPIC simulation of LCLS case
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eEz,dec ≅4e2Nb
σ z2
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eEz,dec =e2Nbn k( ) −1n k( )
dk∫ ⇒=e2Nbkp2
Beam-fieldinducedionization• Focusbeamto<200nmrms• RadialE-field>TV/m• IonizationstudiedinLi,Hgas(ADKmodel,whichappliesinbeamhead…)
Complete ionization in LiHydrogen, ionization complete inside beamOOPIC study, 3.15 atm hydrogen ionized by beam
Well-focused,denseelectronbeamcanleadtoioncollapse
• Positiveions“focused” byultra-densee-beamfields
• Nonlinearfields,emittancegrowth.Badforlinearcolliderapplications
• Detect10-100keVions(hydrogen)
Non-uniform ion density enhancement Beam mismatch and growth (e-growth)
Increasedbrightnesswithinreach
20
250 MV/m peak field S-band cryogenic gun with cryostat, focusing magnets
Brightnessatphotocathode
• Brightness atcathode:• In1Dlimit,peakcurrentfromapulsedphotocathodeis
• Brightnessis
• Loweremissiontemperatureand/or…• Lesson:increaselaunchfield
21
Be =2Iεn2 =
2Jmaxmec2
kBTc
Jz,b ≈ecε0mec
2 E0 sinϕ0( )2
Be,b ≈2ecε0kBTc
E0 sinϕ0( )2
Dramaticallyhighergradientsinhigheryieldstrengthmaterials
• SLACX-bandstudiesonhardCu,CuAgalloyshowgreatimprovement
• Cryogenicstructures giveyethighergradients,and lowerdissipation
22Game changing technique for high launch fieldsPractical limit (dark current) ~300 MV/m presently
500 MV/m beforebreakdown
TopGun
GPTsimulationof200pCcase• Uselongcigar-like beam(10ps)• Emittanceen=45nm-rad
– TentimessmallerthanpreviousexampleCurrent I=20 ABrightness a recordfor this charge
Be = 2×1016 A/m2
This is six times what is available ata reoptimized LCLS
Compression is hard. Use ESASEmicrobunching
ESASEresultsat100pC• 100pC(10ps),36nmemittance• Shortperiodcryo-undulator, l=9mm,K=1.8• Operationat14GeVgives80keVX-ray• Saturationin<20m,with70GWpeak
24Current profile (10 kA) Energy evolution
Using10kApeakmicrobunchedbeaminPWFA
• Utilizequasi-nonlinear(QNL)regimeperiodicexcitation(ubunchingperiod=plasmaperiod)
• Highlyfocusedbeam(22nm),peakEr>5TV/mn0 = 2.5 ×1020 cm−3
Resonant excitation to >3 TV/m4 drive pulses + witness
Experimentalimplementation• Beamfocusing
– Few-100nmbeamdemandsmini-betaPMQs
• Plasmasection– ~3-30atm gasjet,withBSI.Startwithtenuousgas
– Length~0.5mmgives~GeV DE,“perturbative”• Beamdiagnosticsinentirelynewregime
– Longitudinal:coherentedge/transitionradiation– Transverse:
• Ionization,appearanceintensity• coherence
Sub- µmbeamtransversediagnosis
• Measuresub-µmbeamsizeswithcoherentimaging(borrowedfromXFEL).Coherentinformationdownto100nm?
Coherent transition radiation imaging reconstruction expt., A. Marinelli et al., PRL 110, 094802 (2013)
Reconstructed phase Reconstructed profile Incoherent (detuned) OTR
Conclusions• Attosecond e-beamscanbereachedatlowQ• Greatlyenhancedbeambrightness
– Singlespike,compactFELs– Newhighfieldsources,enhancewavelengthrange
• Frontierregimeforbeams;coherentopticalradiation,ionization,newdiagnostics
• Enablesnewfrontiers:– Extremeplasmawakefield accelerators
• TeV/mat1atm• Resonantlydrivenoptical/IRwavelengths(beamiseasier,better!)• Ultra-highfieldatomic-physics– TV/munipolarfield
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