Coherent radiation studies for beam diagnostics and highintensity

Coherent radiation studies for beam diagnostics and high-intensity THz sources at CLEAR A. Curcio EAAC 2019 on behalf of and in collaboration with CLEAR team, BI group (CERN), University of Rome ‘La Sapienza’, INFN, John Adams Institute, Royal Holloway University, Tomsk University

Outline Introduction to Cherenkov-Diffraction radiation A Coherent Cherenkov-Diffraction-based Beam Position Monitor A Coherent Cherenkov-Diffraction-based Bunch Length Monitor Other radiation mechanisms explored so far at CLEAR The ‘Electromagnetic Shadowing’ problem Conclusions

Cherenkov-Diffraction Radiation Cherenkov-Diffraction because impact parameter ρ>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 Ch. DR is refracted according to Snell’s law (courtesy of J. Gardelle)

Designing a Ch. DR prism Radiator response to single electron excitation (Transfer Function) for Radiator response to single electron different radiated frequencies and impact parameters. Comparison excitation (Transfer Function) vs radiated between MAGIC and a model based on Maxwell equations frequency averaged on impact parameters. Comparison between MAGIC and a model based on Maxwell’s equations Coherent Cherenkov-Diffraction Radiation spectra simulated with Vsim and calculated with a model based on Maxwell’s equations for different bunch lengths, extending into the THz region

Longitudinal diagnostics, high-intensity field production and studies on electromagnetic shadowing CCh. DR-based BPM And Bunch Length Monitor Experiments on high-intensity THz generation and Electromagnetic Shadowing

General digression on BPMs Electric field of a charged particle in the proximity of a BPM coupler Standard BPMs work in the RF range, exploiting the 1/ρ dependence of the particles’ electric field BPMs based on Coherent Ch. DR work in the (sub-)THZ range, exploiting a mixture between the 1/ ρ dependence of the particles’ electric field and the exponential dependence on the impact parameter BPMs based on Incoherent Ch. DR work in the IR-visible range, exploiting the exponential dependence on the impact parameter

A Coherent Cherenkov-Diffraction-based Beam Position Monitor BPM formula Beam centered Important note: This BPM, based on coherent radiation, is sensitive only to bunches shorter than a certain threshold bunch length. This means that it can be used to distinguish between bunches of different length, or even to make bunch length measurements. Beam not centered New design for vacuum (courtesy of K. Lekomtsev)

Schematic of the bunch length measurement experiment with CCh. DR at CLEAR

Longitudinal diagnostics with CCh. DR Using two diodes (84 GHz and 113. 5 GHz) Using three diodes (60 GHz, 84 GHz and 113. 5 GHz) Measurement made by exploiting a oneparameter formula for a gaussian bunch (100 p. C) Measurement made by exploiting a twoparameter system for a skew-gaussian bunch (300 p. C) Important note: distance between the prism and the diodes around 10 cm

A THz source based on Coherent Transition Radiation (CTR) Spectrally and angularly characterized CTR source, by means of band-pass filtered Schottky diodes Application: bunch length diagnostics Experiment Simulation Source characterized both in near and far-field by means of a THz camera, angular distribution/polarization shaping by different beam focusing at the radiator plane 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) .

3 D Coherent Smith-Purcell Radiation 3 D grating with walls Angular scan with grating width 1 mm W (mm) F 1 (GHz) K 1 (m-1) Q 1 (deg) Q 2 (deg) Grating width First Harmonic frequency First Harmonic wavenumber First Harmonic Emission angle Second harmonic Emission angle 1 175 3680 135 82 1. 2 156 3270 157 88 1. 3 149 3118 Imaginary 90 1. 5 135 2857 Imaginary 96 2 116 2426 Imaginary 107

Comparison among different radiation mechanisms for the CLEAR THz source Comparison among Coherent Smith-Purcell Radiation (CSPR), Coherent Transition Radiation (CTR), Coherent Diffraction Radiation (CDR) and Coherent Cherenkov-Diffraction Radiation (CCh. DR). The values refer to a single bunch with parameters: 100 p. C (charge) 0. 5 ps (rms bunch length)

Electromagnetic Shadowing Measurements performed at 0. 17 THz with a band-pass-filtered Schottky diode studying the interaction between an arbitrary source of forward THz radiation with a CTR source An overview of all radiators tested scanning the distance between the sources and the CTR mirror Above: Diffraction Radiation from Al, Si and PTFE Below: Shadowing of Diffraction Radiation with Al Even if the formation length is expected hundreds meters, Electromagnetic Shadowing in the THz range is not observed already at fractions of meter…. Electromagnetic shadowing: the THZ radiation field is diffracted by the boundary conditions and it needs time/space to interfere with backward radiation at the plane of the second source

Conclusions and perspectives We have set up and fully characterized a new THz source @CLEAR based on different mechanisms (CTR, CDR, CCh. DR, CSPR) We have succesfully tested a Cherenkov-Diffraction dielectric radiator both for transverse and longitudinal diagnostics; We have explored different targets for high-intensity THz generation but also for Electromagnetic Shadowing experiments; We are going to possibly test new radiators and enhance the beam performances for high-intensity THz generation, in order to go towards the application of THz for acceleration at CLEAR; First application of the CLEAR THz source: beam diagnostics