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Possibility of THz Light Generation by using SW/TW Hybrid Photoinjector 11/16-19, 2009, HBEB, Maui Atsushi Fukasawa, James Rosenzweig, David Schiller, UCLA, Los Angeles, CA, USA

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Hybrid of Standing-Wave and Travelling-Wave Structures Produce short bunches without a magnetic chicane.

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Bunching vs. Injection Phase Peak Current Bunch Form Factor @1THz Emittance Q = 500pC -The peak current reaches as high as 2.3 kA. - The bunch form factor at 1 THz is 0.43. - The emttance will get worse by increasing bunching force.

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Full Bunching Case Charge500 pC Energy21.04 MeV Xrms, Yrms1.48, 1.48 mm Emitx,y 2.1, 2.1 mm.mrad Trms (T_FWHM), Erms 210 fs (54 fs), 1.3% Bunch Form Factor (1 THz) 0.18 Emit (t- ) 2.74 ps Strong energy modulation by the space charge. Strong spike. (54 fs, FWHM) Bad around the spike. Large energy modulation. “Swan” shape shows strong modulation due to space charge. 3.7 mm.mrad Bunch shape Slice emittance t-E phase spaceEnergy Spectrum Bunch Form Factor

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Coherent Cherenkov Radiation - The mode whose v ph = v b will be excited well. - b-a is most important to determine the resonant frequency. - The energy of the beam does not matter.

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CCR Experiment at UCLA a = 250 mm Fourier Transform Beam: Q=200pC, t =270fs, E=10MeV Dielectric tube: r =3.8 (SiO 2 ), L=1cm

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OOPIC Simulations Parameters: 1 THz caseParameters: 1.5 THz case Rms beam length100 μmRms beam length50 μm Rms beam radius30 μmRms beam radius22 μm Beam total charge Q1 nCBeam total charge Q0.5 nC Fundamental frequency1 THzFundamental frequency1.5 THz Bunch Form Factor0.012Bunch Form Factor0.085 Outer radius b115 μmOuter radius b84.6μm Inner radius a77 μmInner radius a60.2μm Peak power21 MWPeak power52 MW Peak long. electric field294 MV/mPeak long. electric field619 MV/m Pulse length69 psPulse length60 ps Total CCR energy1.47 mJTotal CCR energy1.55 mJ

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Scaled to the beam from the hybrid photoinjector Beam total charge Q0.5 nCBeam total charge Q0.5 nC Fundamental frequency1 THzFundamental frequency1.5 THz Bunch Form Factor0.18Bunch Form Factor0.042 Peak power79 MWPeak power26 MW Peak long. electric field570 MV/mPeak long. electric field310 MV/m Total CCR energy5.5 mJTotal CCR energy1.1 mJ

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Coherent Edge Radiation - The edge radiation is produced at the interface of the dipole field: the entrance and the exit. - The property of the edge radiation is similar to that of the transition radiation; the radial polarization and the hollow distribution.

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QUINDI Flow chart of QUINDI QUINDI is a first principles beam diagnostics simulator which calculates the radiative spectrum from a relativistic electron bunch passing through a magnetic array. (From QUINDI’s home page.) Postprocess on Matlab. Lienard-Wiechelt potentials Parallelized with MPI HDF5

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QUINDI Example G. Andonian et al., ”Observation of coherent terahertz edge radiation from compressed electron beams”, Phys. Rev. ST AB 12, 030701 (2009) peaks coincide. CER spectra CER, coherent edge radiation CSR, coherent synchrotron radiation Coherent THz Edge Radiation - Measured at BNL ATF. - Used vertical four-bend magnet array. ( = 20 o and = 1.2 m, or B = 1.7kGauss at 61 MeV) - Bunch length = 100 – 150 fs. - Initial distribution for QUINDI input: UCLA PARMELA and Elegant Vertical Chicane

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QUINDI Example (continued) Polarization of CER + CSR -polarization of CER Far-field radiation intensity distribution of CER + CSR with various polarizer’s angles. Dots: measured. Solid line: QUINDI. Color map: measured. Black contour line: QUINDI. Modest agreement. G. Andonian et al., ”Observation of coherent terahertz edge radiation from compressed electron beams”, Phys. Rev. ST AB 12, 030701 (2009) CSR: -polarization CER: - + -polarization

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CER at NEPTUNE PMQ (Triplet) PM Dipole (90 deg) - 90-deg bending ( = 4 cm) enables to watch the radiation mainly from CER. - CER signal was as large as Cherenkov radiation at the beam dump. - Radial polarization, which is characteristic to CER was observed. CER Spectrum Polarization property CER CER + CSR Beam (11 MeV) CER

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Other Method Beam Lorentz factor 44 Rms bunch length 100 m Rms bunch width 200 m Undulator wavelength6.9 cm Number of undulator periods15 Undulator strength K5.6 On-axis radiation wavelength 299 m (1 THz) Super-radiant FEL QUINDI is updating to solve this problem.

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Summary - SW/TW hybrid photoinjector can produce the high brightness beam. Q = 500 pC, x =2.1 mm.mrad, trms = 210 fs, bunch form factor @1THz = 0.18. - Coherent Cherenkov radiation was simulated. (Scaled from OOPIC results) @1THz: P = 79 MW (5.1mJ), Ez = 570 MV/m @1.5THz: P = 26 MW (1.1mJ), Ez=360MV/m - Coherent edge radiation was being investigated from the experiment and simulation. - QUINDI is being developed. - Demonstrated at BNL ATF, and being commenced at Neptune, UCLA. - Super-radiant FEL will be simulated on QUINDI.

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