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Coherent THz radiation source driven by pre-bunched electron beam

H. Zhang, F. Bakkali Taheri, G. Doucas, I. V. Konoplev John Adams Institute, Department of Physics, Oxford University. Coherent THz radiation source driven by pre-bunched electron beam. Outline. Introduction. Electron beam modulation. Coherent THz radiation. Summary and future work.

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Coherent THz radiation source driven by pre-bunched electron beam

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  1. H. Zhang, F. Bakkali Taheri, G. Doucas, I. V. Konoplev John Adams Institute, Department of Physics, Oxford University Coherent THz radiation source driven by pre-bunched electron beam

  2. Outline Introduction Electron beam modulation Coherent THz radiation Summary and future work

  3. Introduction 30um 1mm (10 THz - 0.3 THz) Electromagnetic spectrum

  4. Introduction • Solid-state electronic (Gun diodes, transistors, …) Low power • Lasers (Gas, quantum cascade lasers, …) • Vacuum Electronic Terahertz Sources High power • Compact sources with high mobility BWOs and their relatives, EIKs, TWTs (0.1-1.0 THz and 103 -10-3 W) • Compact gyrotrons with moderate mobility High magnetic field is required (0.1-1.0 THz and 106 -10-3 W ) • Stationary accelerator-based sources J. H. Booske et al, IEEE Trans. THz Sci. Technol., 2011 THz radiation sources

  5. Introduction FEL Stationary accelerator-based sources FELs and beamline sources: CTR, CSR, CDR (0.1-10 THz and 10-109 W) Free electron lasers (FEL) Coherent transition radiation sources (CTR) Coherent synchrotron radiation sources (CSR) Coherent diffraction radiation sources (CDR) Advantage: tunability, high radiation power CSR CDR THz radiation sources

  6. Introduction For a bunch with Ne electrons: Incoherent radiation Coherent radiation Coherence conditions: Bunch length operating wavelength Bunch periodicity ≈ operating wavelength Coherent radiation enhancement

  7. Electron beam modulation Self-modulation instability (SMI) W . Lu et al, PRL 2006 N. Kumar et al, PRL 2010 Electron beam modulation period equals to the plasma wavelength Microbunch trains generation: plasma wakefield

  8. Electron beam modulation Beam modulation after passing through a 5cm capillary (by 3D PIC code VSim) 0.5THz 1) 1.0 THz 0.5THz 1.5mm 2) 1 THz 2.0 THz 3) 1.5mm 2 THz 1.5mm Tunability of beam modulation

  9. Electron beam modulation • Beam parameters (based on ATF BNL) single particle energy - 50 MeV; total bunch charge - 0.5 nC; bunch length - 1.5 mm (5ps); bunch transverse σr- 80 μm; bunch longitudinal profile - trapezoidal with equal rise and decay time (50 μm/0.17ps) Beam modulation simulation

  10. Electron beam modulation Bunch front 0.5λp 1.0λp Beam density distribution versus plasma density. (a) Initial beam distribution; (b)-(d) Beam density distribution after propagating 1.8 cm in plasma with plasma density of 1.24×1016/cm3, 2.84×1016/cm3 and 11.2×1016/cm3 respectively, the corresponding plasma wavelength is 300μm, 200μm and 100μm.

  11. Electron beam modulation λp=200um λp=100um 0.5λp The beam charge density on axis along the beam after different propagation distances

  12. THz coherent radiation Dispersion relation l: grating period n: harmonic order β: bunch velocity/c θ: observation angle (far-field region) 1/ x0 is the distance between beam and the periodic structure 2/ e is the electron beam - EM wave coupling parameter 3/ further electron beam is away smaller energy transfer to EM wave Smith-Purcell radiation

  13. THz coherent radiation SP signal spectrum from the beams with No modulation; 1ps; 2ps; 4ps (500m grating ) I(dB) Frequency (GHz) Tunability of beam modulation

  14. THz coherent radiation Bunch moving direction Output pulse from the right port Grating Electron bunch Spectrum of the output pulse Peak at 1THz Current(A) Frequency(THz) 9 micro-bunches were generated THz Radiation from a micro-bunch train

  15. Conclusion • A micro-bunch train can be generated when long picosecond electron beams propagate in plasma; • By changing the plasma density, the variation of micro-bunch train modulation frequency can be obtained; • Using such a pre-bunched beam, with well controlled periodicity, a tuneable coherent THz radiation can be generated and controlled by using periodic structures or dielectric material.

  16. Future work • Analyse different THz radiation mechanisms • Smith-Purcell radiation • Cherenkov radiation • Diffraction radiation • Develop theory and simulation to • Study of dispersion • Study of generation in case of finite ohmic and radiation losses in more details • Design a novel periodic structure and build THz radiation source

  17. Acknowledgements This work was supported (in parts) by the: • UK Science and Technology Facilities Council (STFC) through grant ST/M003590/1; • The Leverhulme Trust through the International Network Grant (IN-2015-012).

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