Coherent THz radiation source driven by pre-bunched electron beam 4/28/2017 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

Outline Introduction Electron beam modulation Coherent THz radiation 4/28/2017 Outline Introduction Electron beam modulation Coherent THz radiation Summary and future work 28 April 2017

Electromagnetic spectrum 4/28/2017 Introduction Electromagnetic spectrum 30um 1mm (10 THz - 0.3 THz) Applications: communication technology, biology and medical sciences, non-destructive detection, and security check 28 April 2017

Introduction THz radiation sources 4/28/2017 Introduction THz radiation sources 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 28 April 2017

Introduction THz radiation sources 4/28/2017 Introduction THz radiation sources 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 28 April 2017

Introduction Coherent radiation enhancement For a bunch with Ne electrons: Incoherent radiation Coherent radiation Coherence conditions: Bunch length  operating wavelength Bunch periodicity ≈ operating wavelength 28 April 2017

Electron beam modulation Microbunch trains generation: plasma wakefield 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 28 April 2017

Electron beam modulation Tunability of 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 28 April 2017

Electron beam modulation Beam modulation simulation 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) 28 April 2017

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. 28 April 2017

Electron beam modulation λp=200um λp=100um 0.5λp The beam charge density on axis along the beam after different propagation distances 28 April 2017

THz coherent radiation Smith-Purcell 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 28 April 2017

THz coherent radiation Tunability of beam modulation SP signal spectrum from the beams with No modulation; 1ps; 2ps; 4ps (500m grating ) I(dB) Frequency (GHz) 28 April 2017

THz coherent radiation THz Radiation from a micro-bunch train Bunch moving direction Output pulse from the right port Grating Electron bunch Spectrum of the output pulse Peak at 1THz Current(A) 9 micro-bunches were generated Frequency(THz) 28 April 2017

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. 28 April 2017

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 28 April 2017

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).