A high power beam based coherently enhanced thz radiation source
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A high-power, beam-based, coherently enhanced THz radiation source.

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A high-power, beam-based, coherently enhanced THz radiation source

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A high-power, beam-based, coherently enhanced THz radiation source

We propose a Smith-Purcell radiation device that can potentially generate high average power THz radiation with very high conversion efficiency. The source is based on a train of short electron bunches from an rf photoemission gun at an energy of a few MeV. Particle tracking simulation and analysis show that with a beam current of 1 mA, it is feasible to generate hundreds of Watts of narrow-band THz radiation at a repetition rate of 1 MHz.

Yuelin Li, Yin-E Sun, and Kwang-Je Kim

Accelerator Systems Division

Argonne Accelerator Institute

Argonne National Laboratory, Argonne, IL 60439


  • Power of THz imaging

  • Capability of current available source

  • Our Approach of THz generation

    • Coherence enhancement

    • Laser pulse train generation

    • E-beam generation and dynamics

    • Smith-Purcell radiation

    • Putting together

  • Challenges

  • Summary

Current sources

  • Broadband, THz TDS, <650 mW

  • CW

    • Gunn diode/Back wave oscillators, <200 mW

    • THz-wave parametric oscillators, <100 mW

    • THz gas lasers, <180 mW

    • QCL, <100 mW

    • FEL, >20 W, but bulky

~mW, 8 min

H. B. Liu et al, Proc. IEEE 95, 1515 (2007).

Higher power is needed field application.

The matter of coherence

Radiation power from a electron bunch

Coherent radiation

Incoherent radiation

dE/dw: electron radiation energy into per spectral frequency

N:total number of electrons

Coherence factor

S(t): electron temporal distribution

Coherence factor as a function of bunch length

Short bunch is the key for high coherent factor!

Y.Li and K.-J. Kim, Appl. Phys. Lett. 92, 014101 (2008).

Degradation of coherence factors in electron bunches

Energy from zero to 8 MeV (see later)

The degradation is due to space charge force.

Effect of the space charge force

Q: total charge

sz, sr:longi and trans beam sizes

g: relativistic factor

To solve the problem

Higher beam energy, costly on $$$$

Less charge, costly on photons

How about bunch train? Reduced space charge but preserved coherence factor.

Preserve the coherence factor by bunch trains

Coherence factor for a bunch train

scoh:coherence factor for individual bunched

tb:bunch spacing, to be set as 2p/w

Nb:Number of bunches

Preserve the coherence factor by bunch trains

Same coherence factor but narrower band width

Coherent factor as a function of frequency for 1-16 bunches

Laser pulse train generation

(Credit: Cialdi et al., Appl. Phys. 46, 4959 (2007))

Number of pulses= 2n, n is the number of birefringence crystals




Rf photoinjector

  • Need high duty factor, kHz to MHz

  • Laser power of 100 W

  • Klystron power: 10 kW

L/S band gun



Simulation for an rf gun: bunch coherent factor

Coherence fator at harmonics

Smith-Purcell radiation

(Credit: Scott Berg, http://www.cap.bnl.gov/spexp/)

Resonant wavelength

Radiation power per electron

Ng, lg: number of grating grooves and grating period.

le:evanescent wavelength

n: diffraction order

S.J. Smith and E. M. Purcell, Phys. Rev. 92, 1069 (1953).

P.M. van den Berg, J. Opt. Soc. Am. 63, 1588 (1973).




Putting things together: radiation powers at 1 MHz, for 0.5 THz



total radiation power as a function of the beam center-grating distance with a beam scraper height D in mm measured from the grating surface.


  • We showed that with coherence enhancement, a beam based source delivering hundreds of watts of THz power is possible and may be made compact for field application tools.

Can we make a THz source like this?



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