Coherent Radiation Studies for Beam Diagnostics and High-Intensity THz Sources at CLEAR

1. Coherentradiationstudies for beamdiagnostics and high-intensityTHzsourcesat CLEARA. Curcio EAAC 2019on behalf of and in collaboration withCLEAR team,BI group (CERN),University of Rome ‘La Sapienza’,INFN, John Adams Institute, RoyalHollowayUniversity, Tomsk University

2. Outline Introduction to Cherenkov-Diffractionradiation A CoherentCherenkov-Diffraction-basedBeam Position Monitor A CoherentCherenkov-Diffraction-basedBunchLengthMonitor Otherradiationmechanismsexplored so far atCLEAR The ‘ElectromagneticShadowing’ problem Conclusions

3. Cherenkov-Diffraction Radiation Cherenkov-Diffractionbecause impact parameterρ>0 The electric field is diffracted by both the internal and the external aperture of the radiator since γλ>>a, where a is any characteristic linear size of the radiator ChDRisrefractedaccording to Snell’s law (courtesy of J. Gardelle)

4. Designing a ChDR prism Radiatorresponse to single electron excitation (Transfer Function) for differentradiatedfrequencies and impact parameters. ComparisonbetweenMAGIC and a model based on Maxwell equations Radiatorresponse to single electron excitation (Transfer Function) vs radiatedfrequencyaveraged on impact parameters. ComparisonbetweenMAGIC and a model basedon Maxwell’sequations CoherentCherenkov-DiffractionRadiationspectrasimulated with Vsimand calculated with a model basedon Maxwell’sequationsfor differentbunchlengths, extendinginto the THzregion

5. Longitudinal diagnostics, high-intensityfield production and studies on electromagnetic shadowing CChDR-based BPM And BunchLength Monitor Experiments on high-intensity THz generation and ElectromagneticShadowing

6. General digression on BPMs Electricfield of a chargedparticle in the proximity of a BPM coupler StandardBPMs work in the RFrange, exploiting the 1/ρdependence of the particles’ electricfield BPMsbased on CoherentChDRwork in the (sub-)THZ range, exploitinga mixturebetween the 1/ ρdependence of the particles’ electricfield and the exponentialdependence on the impact parameter BPMsbased on IncoherentChDRwork in the IR-visiblerange, exploitingthe exponentialdependence on the impact parameter

7. A CoherentCherenkov-Diffraction-basedBeamPosition Monitor BPM formula Important note: This BPM, based on coherentradiation, issensitive only to bunches shorterthan a certainthresholdbunchlength.Thismeansthatit can be used to distinguishbetweenbunches of differentlength, or evento makebunchlengthmeasurements. Beamnotcentered Beamcentered New design for vacuum (courtesy of K. Lekomtsev) (courtesy of K. Lekomtsev)

8. Schematic of the bunchlengthmeasurementexperiment with CChDRat CLEAR

9. Longitudinaldiagnostics with CChDR Using twodiodes (84 GHz and 113.5 GHz) Using threediodes (60 GHz, 84 GHz and 113.5 GHz) Measurement made by exploitingaone-parameter formula for a gaussianbunch (100 pC) Measurement made by exploiting a two- parametersystem for a skew-gaussianbunch (300 pC) Important note: distancebetween the prism and the diodesaround 10 cm

10. A THz source based on CoherentTransitionRadiation (CTR) Spectrallyandangularly characterized CTR source, by means of band-pass filteredSchottkydiodes Application: bunchlengthdiagnostics Experiment Simulation Source characterizedbothin nearandfar-field by means of a THz camera, angulardistribution/polarizationshaping bydifferentbeamfocusingat the radiatorplane See Ref. Curcio, A., et al. "A beam-based (sub-)THz source at the CERN Linear Electron Accelerator for Research"Physical Review Accelerators and Beams(2019).

11. 3D Coherent Smith-PurcellRadiation 3D grating with walls Angularscan with gratingwidth 1 mm

12. Comparisonamongdifferentradiationmechanisms for the CLEAR THz source ComparisonamongCoherent Smith-PurcellRadiation (CSPR), CoherentTransitionRadiation (CTR), CoherentDiffractionRadiation (CDR) and CoherentCherenkov-DiffractionRadiation (CChDR). The valuesrefer to a single bunch with parameters: 100 pC(charge) 0.5 ps(rmsbunchlength)

13. ElectromagneticShadowing Measurementsperformedat0.17 THzwith a band-pass-filteredSchottkydiode studying the interactionbetween an arbitrary source of forwardTHzradiation with a CTR source An overview of allradiatorstested scanning the distancebetweenthe sources and the CTR mirror Above: DiffractionRadiation from Al, Si and PTFE Below: Shadowing of DiffractionRadiation with Al Evenif the formationlengthisexpectedhundreds meters, ElectromagneticShadowing in the THzrange isnotobservedalreadyatfractions of meter…. Electromagneticshadowing: the THZ radiationfieldisdiffracted by the boundaryconditions and itneeds time/space to interfere with backwardradiationat the plane of the second source

14. Conclusions and perspectives Wehaveset up and fullycharacterized a new THz source@CLEAR basedon differentmechanisms (CTR, CDR, CChDR, CSPR) Wehavesuccesfullytested a Cherenkov-Diffractiondielectricradiator both for transverse and longitudinaldiagnostics; Wehaveexploreddifferent targets for high-intensityTHzgeneration butalso for ElectromagneticShadowingexperiments; We are going to possiblytest new radiatorsand enhance the beam performances for high-intensityTHzgeneration, in order to go towardsthe application of THz for accelerationat CLEAR; First application of the CLEAR THz source: beamdiagnostics