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

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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

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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).

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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.

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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

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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

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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).

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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.

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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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