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
~mW, 8 min
H. B. Liu et al, Proc. IEEE 95, 1515 (2007).
Higher power is needed field application.
Radiation power from a electron bunch
dE/dw: electron radiation energy into per spectral frequency
N: total number of electrons
S(t): electron temporal distribution
Short bunch is the key for high coherent factor!
Y.Li and K.-J. Kim, Appl. Phys. Lett. 92, 014101 (2008).
Energy from zero to 8 MeV (see later)
The degradation is due to 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.
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
Same coherence factor but narrower band width
Coherent factor as a function of frequency for 1-16 bunches
(Credit: Cialdi et al., Appl. Phys. 46, 4959 (2007))
Number of pulses= 2n, n is the number of birefringence crystals
L/S band gun
Coherence fator at harmonics
(Credit: Scott Berg, http://www.cap.bnl.gov/spexp/)
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).
GunPutting 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.
Can we make a THz source like this?