1. Coherent THz radiation source driven by pre-bunched electron beam
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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

2. Outline Introduction Electron beam modulation Coherent THz radiation
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Outline
Introduction
Electron beam modulation
Coherent THz radiation
Summary and future work
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3. Electromagnetic spectrum
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Introduction
Electromagnetic spectrum
30um
1mm
(10 THz THz)
Applications: communication technology, biology and medical sciences, non-destructive detection, and security check
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4. Introduction THz radiation sources
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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 ( THz and W)
Compact gyrotrons with moderate mobility
High magnetic field is required ( THz and W )
Stationary accelerator-based sources
J. H. Booske et al, IEEE Trans. THz Sci. Technol., 2011
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5. Introduction THz radiation sources
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Introduction
THz radiation sources
FEL
Stationary accelerator-based sources
FELs and beamline sources: CTR, CSR, CDR
( THz and 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
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6. Introduction Coherent radiation enhancement
For a bunch with Ne electrons:
Incoherent radiation
Coherent radiation
Coherence conditions:
Bunch length  operating wavelength
Bunch periodicity ≈ operating wavelength
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7. 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
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8. 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
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9. Electron beam modulation Beam modulation simulation
Beam parameters (based on ATF BNL)
single particle energy MeV;
total bunch charge nC;
bunch length mm (5ps);
bunch transverse σr μm;
bunch longitudinal profile - trapezoidal with equal rise and decay time (50 μm/0.17ps)
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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.
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11. Electron beam modulation
λp=200um
λp=100um
0.5λp
The beam charge density on axis along the beam after different propagation distances
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12. 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
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13. 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)
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14. 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)
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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.
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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
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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 ).