Generation of THz radiation at KEK LUCX facility

K. Lekomtsev on behalf of LUCX collaboration

JAI Seminar: Introduction Seminar by Recently Started Research Staff

Personal Introduction

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KonstantinLekomtsev

Currently:Marie– CurieresearchfellowatRoyalHollowayUniversityofLondon.

2012– 2016:PostdoctoralresearcheratHighEnergyAcceleratorResearchOrganization(KEK),Tsukuba,Japan.

• AnalyticalstudiesandSimulationsoftheradiativephenomenainelectronaccelerators(Transition,Diffraction,Cherenkov,Smith-Purcelletc.).

• Highpowerfslasersystemtuningandmaintenance,experimentalstudiesattheLaserUndulator CompactX-rayfacility(LUCX).• etc.

2009– 2012:Marie– Curieearlystageresearcher(PhDstudent)atRoyalHollowayUniversityofLondon

• AnalyticalstudiesofcoherentDiffractionradiation.• ExperimentalstudyatCLICTestFacility3atCERN.• etc.

Earlier: NationalResearchNuclearUniversity(MoscowEngineeringPhysicsInstitute)

• MasterdegreeinAppliedMathematics.

Introduction

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1.OverviewoftheLUCXfacility.a.Beamparameters.b.Multi-bunchbeamgeneration.

2.MonochromaticityofcoherentTHzSmith-Purcellradiation(SPR).a.Brieftheoreticalbackground.b.ParticleInCellsimulationsofSPRspectrum.c.Discussionofexperimentaldata.

3.CherenkovSmith– Purcellradiation(ChSPR)fromcorrugatedcapillary.a.Brieftheoreticalbackgroundandcomparisonwithsimulations.b.ParticleInCellsimulationsoftheradiationfrommulti-bunchbeamandcapillarywithreflector.c.Discussionofexperimentaldata.

LUCX facility: Overview

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“Femtosecondmode”n Ti:Sa lasern e-bunchRMSlength~100fsn e-bunchcharge<100pCn Singlebunchtrain,Micro-bunching4-16(4isconfirmed)n TypicalRep.rate3.13Hzn Experiments:THzprogram

“Picosecondmode”n Q-switchNd:YAG lasern e-bunchRMSlength~10psn e-bunchcharge<0.5nCn Multi-bunchtrain2- few103

n MaxRep.rate12.5Hzn Experiments:Compton,CDR

• TheLaserUndulator CompactX-rayfacility(LUCX)isamultipurposelinearacceleratorwhichwasinitiallyconstructedasanRFguntestbenchandlaterextendedtofacilitateComptonscatteringandcoherentradiationgenerationexperiments.

LUCX laser system

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• FemtoseconddurationelectronbuncheswithTHzrepetitionfrequency illuminateaphotocathodeandandelectronbeamisgeneratedonasingleRFacceleratingfieldcycle.

• Titanium– Sapphire“ChirpedPulseAmplificationtechnique”lasersystemisusedtogenerateasequenceofmicrobunches.

Micro-bunch beam generation and characterization

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• TheRMSelectronbunchlengthismeasuredbythezero-phasingmethod*.Timecorrelatedmomentumdeviation imposedonthebunch ifweoperateatthezerocrossingoftheacceleratingwave.

• ThebeamisthendispersedbyadipolemagnetBH1GsothatthedifferenttimeslicesoftheelectronbunchareprojectedontoascintillatingYAGscreenatdifferenthorizontalpositions, andthus beamimageonthescreenshows theintensity distributionoftheelectronbunchalongitstemporalprofile.

• Thecorrelation oftheRFphasewiththeYAGbeamimageshiftismeasured.• ThelinearcorrelationofthisapproximationgivesthescaleofthehorizontalimagesizeinRFdegrees,whichcanberecalculatedtotimescale.

*D.X.Wangetal,Phys.Rev.E57,2283 (1998).

Monochromaticity of SPR

7

Smith-Purcellradiationappearswhenchargedparticlesmoveaboveandparalleltoadiffractiongrating.Spectrallinespositionsaredefinedbythedispersionrelation*:

𝜆" =$"

%& − 𝑐𝑜𝑠𝜃 ; (1)

where 𝜆"isthewavelengthoftheresonanceorder𝑘, 𝑑 isthegratingperiod, 𝛽 istheparticleintheunitsofthespeedoflight,and 𝜃 istheobservationangle.

Thespectral-angulardistributionofthecoherentSPR** producedfromgratingwithfinitenumberofperiodsN:

$/0$1$2 =

$/03$1$2

456/ 78456/ 8 𝑁: + 𝑁: 𝑁: − 1 𝐹 ; (2)

𝜑 = 𝑑 ?1@ 𝛽A% − 𝑐𝑜𝑠𝜃 - phaseassociatedwithstripsperiodicity, $

/03$1$2 isthespectralangulardistributionfromasinglegratingperiod,

𝜈 isradiationfrequency,𝑁 isthenumberofgratingperiods,𝑁: isthebunchpopulation,and𝐹 isthebunchform-factor.

From(2),ifFWHMistakenasanabsolutespectrallinewidth,themonochromaticity isdefinedas:

∆DD =

E.GH"7 . (3)

*S.J.SmithandE.M.Purcell,Visiblelightfromlocalizedsurfacechargesmovingacrossagrating,Phys.Rev.92,1069 (1953).**A.P.Potylitsynetal.,DiffractionRadiationfromRelativisticParticles,Springer(2010).

Monochromaticity of SPR

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Measurementslimitations:

ThewidthofSPRspectrallinescanbecomelargeriftheyaremeasuredbythedetectorplacedinthesocalled“pre-wavezone”.IfthegratingtodetectordistanceisL,thenthefar-fieldzone(orwavezone)condition isdeterminedby*:

𝐿 ≫ 𝐿KK = 𝑘𝑁L𝑑 1 + 𝑐𝑜𝑠𝜃 .

Monochromaticityoftheradiationgeneratedfromaninfinitegrating(𝑁 →∞) andmeasuredwithafiniteaperturedetector∆𝜃:

∆DD =

456OPQA@R4O

∆𝜃.

Assumingthatthereallineshape𝛿𝜆T andthespectrometerresolution𝛿𝜆4U canbeapproximatedbyaGaussiandistribution,theFWHMofthemeasuredline:

𝛿𝜆 = 𝛿𝜆TL + 𝛿𝜆4U

L�.

*D.V.KarlovetsandA.P.Potylitsyn,JETPLetters84,489 (2006).

Particle in Cell simulations (SPR)

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Simulation parameter Value

L 500mm

D 60mm

h 0.6mm

d 4 mm

𝛼 30deg.

Bunch length 0.5ps

Bunchtransversesize 250𝜇m

Beamenergy 8MeV

• SimulationswereperformedinCSTParticleStudio,ParticleInCellSolver.• Consideredtwocalculationdomainsinordertoshowthe influenceof

thepre-wavezoneeffectforthefirstdiffractionorderofSPR.

• SPRspectrumobtainedbyrecordingelectricfieldcomponentsasfunctionsoftime andthenbyperformingFouriertransformofthetimedependenceofthedominantcomponent.

Particle in Cell simulations (SPR)

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• Boththesimulationandthetheoryshowthathigherorderspectrallinesbecomemoremonochromatic.

• Whencomparingthelinewidthsforthetheoryandthesimulation,itisimportanttorememberthatthetheorywasdevelopedfor𝑁 → ∞andnottakingintoaccountrealshapeofthegrating.

ComparisonoftheSPRspectrallinewidths:

Experimental study (SPR)

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• Vacuumwindow:12mmthick2deg.wedgedsapphire,witheffectiveapertureof145mm.

• 5-axismanipulatorsystemwasinstalledonthetopofthevacuumchamber.Usedforfineadjustment ofthegratingpositions in3orthogonal directionsandalsoforthecontrolforthe2rotationalangles.

• Thegratingwasalignedwithrespecttotheelectronbeamusing theforwardbremsstrahlungappearingduetodirectinteractionoftheelectronbeamwiththetargetmaterial.

Experimental study (SPR)

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MeasurednormalizedTransitionRadiationspectrumcanbeusedasspectralefficiencyoftheentiremeasurementsystem,including:

ü spectraltransmission efficiencyofthevacuumwindow,ü detectorwavelengthefficiency,ü splitterefficiency,ü reflectioncharacteristicsofthemirrorsandabsorption inair.

SpectralresolutionofFourierspectrometer: 𝛿𝜆 defined asFWHMofthespectralpeakfromamonochromatic source:

YDD= 1.21 D

L[\]^,

where 𝐿56_ istheinterferometermaximumopticalpathdifferencefromzeroposition.

TransitionRadiationangularscanusing SBD320- 460

TransitionRadiationspectralmeasurements: Applying thiscriteriontotheinterferograms,thespectrometerresolution:

YDPDP

= 15%; YDbDb

= 6%

ThemeasuredpeaksFWHM:

YDPd

DP= 16%; YDb

d

Db= 6.1%

SBD60– 90GHzSBD320– 460GHz

Theory and simulation of THz radiation from corrugated channel

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Spectral– angulardistribution *oftheradiationgeneratedasaresultofthepointlikeelectronpassing throughcorrugatedchannelininfinite dielectric(𝑅 → ∞):

CST Particle In Cell simulation:

𝑐𝑜𝑠 𝛳 = L?g"$

+ %& h i� ;

ThediffractionordersofCherenkov andSmith-Purcellradiationpeakssatisfy the dispersion relation:

where𝛳 ispolarangle,𝛽 istheelectronspeed intermsofthespeedoflight,𝑘 isthewavenumberindielectric,𝑑 isthecorrugationperiod,𝑚 isadiffractionorder,𝜀 𝜔 isdielectricpermittivityasafunctionoffrequency.

*A.A.Ponomarenko et.al,Terahertzradiationfromelectronsmovingthroughawaveguidewithvariableradius,basedonSmith-PurcellandCherenkovmechanisms,NIMB309,223 (2013).

Bluecurve– corrugatedchannel.Reddashedcurve– channelwithconstantradius.

PIC simulations (ChSPR)

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x

y

• 3Dpowerpatternandthecorrespondingazimuthalcross-sections(𝜃 = 90 deg.)fortheradiationat300GHzfortheoff-centralbeampropagationx=0,y=1mm.

• Cherenkovradiationisreflectedbytheoutside boundariesofthecorrugatedcapillaryanddirectedatsmallangles𝜃 ≈ 10 deg.

𝑃U:q" =r i∆_ ,

∆𝑡 =0.13ns(simulationtime)

Experiment (ChSPR)

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15

x

z

y

zDet.

220

mm

θ≈ 40𝑑𝑒𝑔.

beam

x

y

xDet.beam

manX

θ≈ 40𝑑𝑒𝑔.

20 mm180 mm

20 mm

180 mm

Bunchsize:200x200𝜇𝑚

Bunchcharge:1bunch 25pC

Detector:SBD320– 460GHz

Quartzvacuumwindow:100mm(effectivediameter)

manY

Experiment (ChSPR)

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Cross-checkwithTransition radiation:

Positioning ofthecapillarieswithrespecttothebeam(bremsstrahlung, appearing duetodirectinteractionoftheelectronbeamwiththetargetmaterial):

Experiment (ChSPR)

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Thepolardistribution oftheradiation:

• The simulated geometry was identical to the realisticone with the exception of holders, which were nottaken into account in the simulations.

• The beam parameters were chosen to be the sameas during the time of the experiment, and the beamwas moving at 0.6 mm from the corrugation to allowfor 3𝜎 beam – corrugation separation.

• The power distribution was obtained by, first,calculating the power spectrum of emitted radiationin the frequency range 240 – 360 GHz. The radiationdirectivity pattern for each frequency was calculatedas well, hence it was possible to convert powerspectrum into the power distribution at the detectorlocations on the translation stage.

• After the converted power distribution wasobtained, the beam contribution in the powerspectrum was subtracted.

• The power spectrum of the radiation emittedthrough the surface of the outside boundary A ofthe calculation domain during the simulation time∆𝑡 was calculated as:

𝑃(𝜔) = ∫ ∯𝑺�� 𝜔 ∗ 𝒏𝑑𝐴𝑑𝑡∆_E ;

where 𝑺(𝜔) is the Poynting vector, n is a unity vector in theoutward normal direction from the boundary 𝐴of thecalculation domain.

Experiment (ChSPR)

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• Azimuthaldistributionoftheradiationwassimulatedusingthefar-fieldmonitorofCSTParticleStudio,whichextrapolatestheelectricfieldvaluesattheborderofthecalculationdomaintoobtaintheelectricfields inthefar-field(distances≫ 𝛾L𝜆)atasinglefrequency.

• Accordingtothedispersion relation:𝑐𝑜𝑠 𝛳 = L?g

"$+ %

& h i� the

frequencyat𝛳 = 90deg.(detZ =0)is300GHz(𝜆 = 1mm).

• Theredcurve:measurementoftheazimuthaldistribution oftheradiationat300GHz.

Theazimuthaldistributionoftheradiation:

Overview and outlook

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• ThemainobjectiveoftheSPRstudywastodemonstratethefeasibilitytogenerateSPRwithmonochromaticitybetterthan1-2%,bychoosingahigherdiffractionorderandrelativelysmallnumberofperiods(intheorderof10).

• Themeasurementswerelimitedbyseveralfactors,includingtheresolutionofthespectrometer,thequalityofitsalignment,angularacceptanceofthedetector.Allofthesefactorscontributetothewideningofthemeasuredspectrallines.

• Furthermonochromaticityimprovementsareexpected ifamulti-bunchbeamisused.

• TheobjectivesoftheChSPR studyweretocross-checktheradiationwithotherwellknownradiationmechanism(TR),tostudytheeffectofcorrugationandmeasuretheradiationdistributions.

• Measured10-foldincreaseoftheradiationintensityforthecorrugatecapillaryincomparisontotheblankcapillary.• Confirmed thatthemaximumoftheradiationintensity isachievedfortheoff-centralbeampropagation.• Thecompositedesignofthecorrugatedcapillaryallowsforflexibility toeasilychangethegeometry,whichcanbeveryuseful

foravarietyofstudiesincludingradiationgeneration,beamenergyandchargemodulationetc.

Marie Sklodowska-Curie Horizon 2020 project

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Researchobjectives:

• toinvestigateCherenkovandSmith- PurcellmechanismsforTHzradiationgenerationusingEMsimulationtoolsandproposethemostefficienttargetconfigurationandobservationgeometry;

• tomanufacturearadiationsourcethatprovideshighpeakpower levelsandbasedonacompactlinearacceleratortechnologywithfs-durationbeam;

• toevaluatethepossibleapplicationsoftheinvestigatedradiationmechanismsforbeampositionandbunchlengthdiagnostics;• Toexpandresearchinterestsinthedirectionswhichallowforeffectiveskillsandknowledgetransfer(dielectricwake-field

acceleration,accelerator-scalewake-fieldsimulations,evaluationofpositiveandnegativeeffectsofwake-fieldsinaccelerator).• toprovidetheknowledgeexchangebetweenthepartnerorganizationsandotherinterestedpartiesviaseminars,satelliteand

progressmeetings,conferencesandworkshops;• toimprovethepublicawarenessabouttheongoingresearchviaoutreachactivities;• tofurtherdevelopprojectmanagementskills(financial,workflow,deliverablesplanningetc.)

Acknowledgements:

Thisprojecthasreceivedfunding fromtheEuropeanUnion’sHorizon2020researchandinnovation programme undertheMarieSklodowska-Curie grantagreementNo655179.

PIC simulations (capillary with reflector)

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Parameter Value

beamLorentz- factor,𝛾 16

frequency up to700 GHz

bunch length,𝜎�R6� 0.03mm

bunch transversesize,𝜎_Tq64�

0.3 mm

micro-bunch charge 0.1nC

Nofmicro-bunches 4

distancebetweenmicro-bunches

variable(0.25– 1mm)

capillarymaterial Fusedquartz

holdermaterial Copper

numberofperiods 30

cylindrical ringwidth,l 0.5mm

corrugationperiod,2l 1mm

groovedepth, r2– r1 0.2 mm

internal radius, r1 2mm

outer radius, r3 2.7mm

Simulationgeometry(generalview):

Simulationgeometry(meshviewinOyzplane):

z

y

x

Probe

• Electricfieldprobeislocatedoutsideofthecalculationdomain;• Thefieldvaluesattheprobelocationareobtainedbasedonthe

extrapolationofthefieldvaluesattheborderofthecalculationdomain*.

beam

*Yee K S, Ingham D and Shlager K 1991 Time-Domain Extrapolation to the far field based on FDTD calculations IEEE Trans. of Ant. and Prop. 39 410

PIC simulations (capillary with reflector)

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The power radiated through the surface of the outsideboundary 𝐴 of the calculation domain during thesimulation time ∆𝑡 at each frequency is given by thefollowing expression:

𝑃(𝜔) = ∫ ∯𝑺�� 𝜔 ∗ 𝒏𝑑𝐴𝑑𝑡∆_E ;

where 𝑺(𝜔) is the Poynting vector, n is a unity vector inthe outward normal direction from the boundary 𝐴 of thecalculation domain.

Single bunch:relatively flat response at all frequencies.

Blank capillary:power spectral modulation due to Cherenkov radiation.

Corrugated capillary:even larger spectral modulation if the distance betweenbunches is equal to the corrugation period.

Resonantcondition:

bunchdistance=corrugationperiod

PIC simulations (capillary with reflector)

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Beamposition:x=0mm,y=1mm

x

y

Form-factorsoffourbuncheswithdifferentbunchspacing,andtheformfactorofasinglebunch:

Spectralmodulationoftheradiationdependsontheperiodicityofthecorrugationaswellasthedistancebetweenbunches.

Assembly of the capillary with holders

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Capillarywithholders andradiationreflectorassembledonthebaseplate:

Laserscanswereperformedontheoutside surfaceofthecapillaryinstalledintheholders:

• Laserbeamscannedalongtheoutersurfaceofcorrugatedandblankcapillaries.

• Lightreflectedfromthesurfacedetectedbyanarraydetectorandtheverticaloffsetofthelaserbeamrecorder.

• Asaresultobtainedtheverticaloffsetasafunctionofthehorizontaltravelrange.

Both corrugated and blank capillaries are constructed as sets of cylindrical rings:

Experiment (capillary with reflector)

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detY

detX

beamscan

holder

Target – beam separation and the corresponding radiation yield: