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

1
A high-power, beam-based, coherently enhanced
THz radiation source

• Yuelin Li, Yin-E Sun, and Kwang-Je Kim
• Accelerator Systems Division
• Argonne Accelerator Institute
• Argonne National Laboratory, Argonne, IL 60439

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.
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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

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Current sources

• Broadband, THz TDS, lt650 mW
• CW
• Gunn diode/Back wave oscillators, lt200 mW
• THz-wave parametric oscillators, lt100 mW
• THz gas lasers, lt180 mW
• QCL, lt100 mW
• FEL, gt20 W, but bulky

mW, 8 min H. B. Liu et al, Proc. IEEE 95, 1515
(2007).
Higher power is needed field application.
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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
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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).
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Degradation of coherence factors in electron
bunches
Energy from zero to 8 MeV (see later)
The degradation is due to space charge force.
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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.
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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
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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
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Laser pulse train generation
(Credit Cialdi et al., Appl. Phys. 46, 4959
(2007))
Number of pulses 2n, n is the number of
birefringence crystals
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Rf photoinjector
L/S band gun
Klystron
Laser

• Need high duty factor, kHz to MHz
• Laser power of 100 W
• Klystron power 10 kW

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Simulation for an rf gun bunch coherent factor
Coherence fator at harmonics
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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).
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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.
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Summary
Can we make a THz source like this?
http//www.tfot.info/news/1051/boeing-tests-avenge
r-solid-state-laser-weapon.html

• 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.

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