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.
2CLEO 2008, San Jose, May 4-9, 2008
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
3CLEO 2008, San Jose, May 4-9, 2008
Current sources
Broadband, THz TDS, <650 W 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
~W, 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
Coherent radiation
2
2
NNd
d
d
dI
E
2
)(
)cos()()(
dttS
dtttScoh
Radiation power from a electron bunch
Coherence factor
dE/d: electron radiation energy into per spectral frequencyN: total number of electrons
S(t): electron temporal distribution
Incoherent radiation
5CLEO 2008, San Jose, May 4-9, 2008
Coherence factor as a function of bunch length
0.0 0.2 0.4 0.60.0
0.5
1.0
C
oh
/
Ellipsoid Flat Gauss
Short bunch is the key for high coherent factor!Y.Li and K.-J. Kim, Appl. Phys. Lett. 92, 014101 (2008).
6CLEO 2008, San Jose, May 4-9, 2008
Degradation of coherence factors in electron bunches
0 40 80 1200.00
0.10
0.20
0.30
0 40 80 1200.0
0.5
1.0
z (cm)
31 pC 62 pC 125 pC
z/(b) (c)
coh
z (cm)
The degradation is due to space charge force.
Energy from zero to 8 MeV (see later)
7CLEO 2008, San Jose, May 4-9, 2008
Effect of the space charge force
22rz
SC
QF
Q: total chargez, r: longi and trans beam sizes: relativistic factor
To solve the problemHigher beam energy, costly on $$$$Less charge, costly on photons
How about bunch train? Reduced space charge but preserved coherence factor.
8CLEO 2008, San Jose, May 4-9, 2008
Preserve the coherence factor by bunch trains
.22
2sin
2sin1
)()(
2
2
b
coh
b
b
bb
b
coh
N
N
Coherence factor for a bunch train
coh: coherence factor for individual bunchedb: bunch spacing, to be set as 2/Nb: Number of bunches
9CLEO 2008, San Jose, May 4-9, 2008
Preserve the coherence factor by bunch trains
Same coherence factor but narrower band width
0 50 100 150 200 2500
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Coherent factor as a function of frequency for 1-16 bunches
10CLEO 2008, San Jose, May 4-9, 2008
Laser pulse train generation
Number of pulses= 2n, n is the number of birefringence crystals
(Credit: Cialdi et al., Appl. Phys. 46, 4959 (2007))
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Rf photoinjector
Need high duty factor, kHz to MHz Laser power of 100 W Klystron power: 10 kWLaser
Electrons
Gun
Laser KlystronL/S band gun
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Simulation for an rf gun: bunch coherent factor
0.0 0.5 1.0 1.50.0
0.4
0.8
0 2 4 6 8
-0.5
0.0
0.5
-0.6 -0.3 0.0 0.3 0.60.0
0.5
1.0
(c)
coh
(THz)
z (mm)
(a)
x
(mm
)
(b)
f (a.
u.)
x (mm)
0.5 1.5 2.5 3.5
0.0
0.5
1.0
0.5 THz 1 THz 1.5 THz 2 THz
(b)
Q (nC)
Particle distribution in the x-t plane as seen by a screen at z=60 cm; (b) the transverse beam profile (dashed) and the fitting to a Lorentz distribution with a 200-m width (solid); and (c) the normalized Fourier spectrum averaged over z=40-70 cm. The total charge is 1 nC.
Coherence fator at harmonics
13CLEO 2008, San Jose, May 4-9, 2008
Smith-Purcell radiation
(Credit: Scott Berg, http://www.cap.bnl.gov/spexp/)
,cos1
nGResonant wavelength
dx
c
Ne
d
dI
e
GG 2
20
2
22
sin]exp[8
sin Radiation power per electron
Ng, g: number of grating grooves and grating period.e: evanescent wavelengthn: 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).
14CLEO 2008, San Jose, May 4-9, 2008
Putting things together: radiation powers at 1 MHz, for 0.5 THz
Laser
Electrons
Gun
grating
THz
0.0 0.1 0.2 0.3 0.4 0.50.0
0.2
0.4
0.6 10 20 50 100
(c)
P (kW
)
h (mm)
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.
15CLEO 2008, San Jose, May 4-9, 2008
Summary
http://www.tfot.info/news/1051/boeing-tests-avenger-solid-state-laser-weapon.html
Can we make a THz source like this?
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.
16CLEO 2008, San Jose, May 4-9, 2008
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