Presentation on theme: "A high-power, beam-based, coherently enhanced THz radiation source Yuelin Li, Yin-E Sun, and Kwang-Je Kim Accelerator Systems Division Argonne Accelerator."— Presentation transcript:
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.
CLEO 2008, San Jose, May 4-9, 2008 2 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
CLEO 2008, San Jose, May 4-9, 2008 3 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.
CLEO 2008, San Jose, May 4-9, 2008 4 The matter of coherence Coherent radiation Radiation power from a electron bunch Coherence factor dE/d : electron radiation energy into per spectral frequency N:total number of electrons S(t): electron temporal distribution Incoherent radiation
CLEO 2008, San Jose, May 4-9, 2008 5 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).
CLEO 2008, San Jose, May 4-9, 2008 6 Degradation of coherence factors in electron bunches The degradation is due to space charge force. Energy from zero to 8 MeV (see later)
CLEO 2008, San Jose, May 4-9, 2008 7 Effect of the space charge force Q: total charge z, r :longi and trans beam sizes : 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.
CLEO 2008, San Jose, May 4-9, 2008 8 Preserve the coherence factor by bunch trains Coherence factor for a bunch train coh : coherence factor for individual bunched b :bunch spacing, to be set as 2 / N b: Number of bunches
CLEO 2008, San Jose, May 4-9, 2008 9 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
CLEO 2008, San Jose, May 4-9, 2008 10 Laser pulse train generation Number of pulses= 2 n, n is the number of birefringence crystals (Credit: Cialdi et al., Appl. Phys. 46, 4959 (2007))
CLEO 2008, San Jose, May 4-9, 2008 11 Rf photoinjector Need high duty factor, kHz to MHz Laser power of 100 W Klystron power: 10 kW Laser Electrons Gun Laser Klystron L/S band gun
CLEO 2008, San Jose, May 4-9, 2008 12 Simulation for an rf gun: bunch coherent factor Coherence fator at harmonics
CLEO 2008, San Jose, May 4-9, 2008 13 Smith-Purcell radiation (Credit: Scott Berg, http://www.cap.bnl.gov/spexp/) Resonant wavelength Radiation power per electron N g, g : number of grating grooves and grating period. e: 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).
CLEO 2008, San Jose, May 4-9, 2008 14 Putting things together: radiation powers at 1 MHz, for 0.5 THz Laser Electrons Gun 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.
CLEO 2008, San Jose, May 4-9, 2008 15 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.
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