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

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


Content

Content

  • 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

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

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

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

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

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

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 trains1

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

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

Electrons

Laser

Gun

Rf photoinjector

  • Need high duty factor, kHz to MHz

  • Laser power of 100 W

  • Klystron power: 10 kW

L/S band gun

Klystron

Laser


Simulation for an rf gun bunch coherent factor

Simulation for an rf gun: bunch coherent factor

Coherence fator at harmonics


Smith purcell radiation

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

Electrons

Laser

Gun

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

grating

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.


Summary

Summary

  • 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?

http://www.tfot.info/news/1051/boeing-tests-avenger-solid-state-laser-weapon.html


References

References

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