THz Imaging by a Wide-band Compact FEL

Transcription

1 FEL-2004, Trieste, Italy THz Imaging by a Wide-band Compact FEL 2 Sep Young Uk Jeong, Grigori M. Kazakevitch b, Hyuk Jin Cha, Seong Hee Park, and Byung Cheol Lee Korea Atomic Energy Research Institute (KAERI), Daejon, Korea b Budker Institute of Nuclear Physics (BINP), Novosibirsk, Russia

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3 Peongyang Seoul Daejon Pohang Busan

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5 Undulator High Power IR KAERI RF Generator RF Generator Control Room Cryogenic System Electron Gun SC Cavity e-beam Extractor Controller 2-MeV Injector 2-MeV Injector SC Cavity

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8 Why Terahertz Radiation (T-Ray)?! New Tool for Science and Technology - Probing tool for hyperfine electron structure of atoms and molecules Semiconductor, superconductor, IR/FIR sensors, and so on - High resolution detection of gases - Non-destructive test of materials Anthrax, narcotics, semiconductor package, fast foods, and so on Hu and Nuss, 1995

9 History of KAERI THz FEL! Brief Histories : Start Project : Development of the FEL system : Upgrade the system for µm wavelength : First lasing at µm : Wide-band operation at µm : System stabilization for users application facility : THz applications on imaging, SEW, spectroscopy, and so on

10 Main Features of KAERI THz FEL! Frequency Stabilization of Magnetron : Cheap and Compact Machine - Magnetron frequency is stabilized by coupling a RF cavity with higher Q-value and the magnetron (frequency pulling effect) - Stabilized frequency : f/f ~ 10-5! Energy Variable Microtron : Wide-band Operation in THz Range - By changing the RF cavity position inside Microtron magnet, electron beam energy is tuned from 4 MeV to 6.5 MeV, which corresponds the wavelength of µm! High Performance Oscillator : Stable Operation with Enough Gain - Complex resonator Small-gap 1-D waveguide : gap distance of 2 mm along 3 m Small mode size : 10 mm x 1 mm - Hybrid EM undulator Strong field strength : kg Extremely low field error : 5x10-4 Compact, easy tuning, and cheap

11 KAERI Compact THz FEL! THz FEL Using a Classical Microtron Accelerator driven by a Magnetron as RF Driver! Laboratory Size (4 m 2 ), Low Cost, but Good Performances - Wide-band and stable FEL operation - Fourier-transform-limited linewidth of spectrum - Diffraction-limited spatial distribution

12 KAERI Compact THz FEL BEAMLINE MICROTRON UNDULATOR! FEL Beam Specification - Wavelength Range : µm ( µm) - Macropulse Pulsewidth : 4 µs Power : 10 W at the experimental stage - Micropulse Pulsewidth : ps Power : 100 W at the experimental stage - Pulse Energy Fluctuation : <10% rms

13 THz Transport Line for Application Experimental Vacuum Chamber Experimental Room Shielding Wall FEL Room FIR Beam for Applications He-Ne Laser Alignment Target Vacuum Line 3 m 5 m FEL Resonator FEL Resonator Experimental Stage

14 Experimental Stages for FIR Applications Vacuum Exp. Chamber FIR BEAM Pyroelectric Detectors Fabry-Perot Interferometer Liquid He-Cooled Ga:Ge Detector

15 THz FEL Output 3.0 B w =5.5 kg, E=6.5 MeV, Wavelength=118 um Laser Beam Electron Beam Spontaneous Emission FIR Power (Arb. Units) ma Time (µs)

16 FEL Pulse Energy Fluctuation FEL Output Pulse Energy (Arb. Units) Pulse Repetition Rate : 1 Hz Standard Deviation = 9.4% Time (second)

17 Relative deviations of the Reference signal Relative deviations of the cooling water temperature FEL Pulse Energy Fluctuation Relative Fluctuation Time (second)

18 FEL Pulse Energy Fluctuation FEL Pulse Energy (Arb. Units) Interferogram measured by 0.5 mm scanning Diatance (mm)

19 FEL Pulse Energy Fluctuation FEL Pulse Energy (Arb. Units) Interferogram measured by 0.1 mm scanning Diatance (mm)

20 FEL Pulse Energy Fluctuation Normalized Pulse Energy Fluctuation < 1% FEL Pulse Energy (Arb. Units) Diatance (mm)

21 Highly Polarized THz Light Transmitted Ratio through Polarizer Polarizer : grid (spacing 20 µm) Linearly polarized > 98% Polarizer Angle (degree)

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23 FEL Pulse Energy/Power depending on Detuning 4000 FEL Pulse Power/Energy (Arb. Units) Pulse Energy Pulse Power Detuning Length (mm)

24 FEL Spectrum depending on Detuning Spectral power (Arb. Units) λ/λ = 3 x 10-3 Detuning=-0.2 mm Detuning=-0.3 mm Detuning=-0.4 mm Detuning=-0.5 mm Detuning=-0.6 mm Detuning=-0.7 mm Wavelength (µm)

25 FEL Spectrum depending on Detuning 100 Spectral Power (Arb. Units) [THPOS31] Behavior of Power Spectrum of the Compact FIR FEL at KAERI, S. H. Park et al. Detuning (mm) Wavelength (µm)

26 Layout of THz Imaging Experiment Oscilloscope PC Monitoring Detector Signal Detector THz FEL Aperture Sample Aperture Beamsplitter Parabola Mirror (f=50 mm) Parabola Mirror (f=50 mm) 2-D Scanning stage

27 Experimental Stage for THz Imaging THz FEL BEAM Liquid He-Cooled Ga:Ge Detector Pulse Energy Monitor 2-D Scanning Stage Sample

28 Spatial Resolution of THz Imaging THz Pulse Energy (Arb. Units) mm λ=110 µm Distance (mm)

29 THz Image of a Microchip 0 Scale (µm) Scanning : 0.2 mm/step Wavelength : 165 µm Dynamic range of THz intensity > 10 4

30 THz Image of a metal and o-ring inside a paper box Scanning : 0.4 mm/step Wavelength : 165 µm Scale (mm) Metal ring O-ring

31 THz Image of a Gingko Leaf ZOOM 0 Scanning : 0.08 mm/step Wavelength : 110 µm Scale (mm)

32 Future Plans on THz Imaging! Higher Repetition Rate Operation of the FEL - 10 Hz 300 Hz - Increase shielding for higher X-ray! Development of a FTIR for THz Microspectroscopy - Upgrade a FTIR spectrometer for THz range! Development of a Single-Shot Imaging System - Electro-optic method with the visible/ir reading of FIR image! Development of TCT (Terahertz Coherence Tomography) Technology - Check the possibility of THz tomography with holographic method

33 Single-shot THz Imaging with EO Method FEL Beam Expander He-Ne Laser Polarizer EO Crystal Sample BS Analyzer CCD

34 Layout of Coherence Length Measurement Monitoring Detector Wedged Beamsplitter THz FEL Mirror Beamsplitter Linear Translator [THPOS63] Coherence Length and Pulsewidth Measurement of the KAERI THz FEL, H. J. Cha et al.

35 Experimental Stage for Coherence Length Measurement Pulse Energy Monitor THz FEL BEAM Liquid He-Cooled Ga:Ge Detector B.S. 1-D Scanning Stage

36 Layout of Coherence Length Measurement 6 Interference Signal (Arb. Units) λ = 110 µm λ = 0.4 µm L c,fwhm = 0.83L 0 = 10 mm (30 ps) L 0 = 12 mm Optical Delay (mm)

37 FEL Spectrum depending on Detuning Spectral Power (Arb. Units) Interference Signal (Arb. Units) FWHM~25 ps Optical Delay (mm) Detuning (mm) Interference Signal (Arb. Units) FWHM~40 ps Optical Delay (mm) Wavelength (µm)

38 Coherent THz Imaging for Tomography Coherent THz Tomography by combining pulse holography technique Sample Coherence Length THz 2-D Detector B.S. Scanning over several periods of the THz wavelengths Modulated intensity (phase) Information on the strong background signal Mirror Hologram THz FEL

39 Other Application Works! Surface Electromagnetic Wave for Solid-State Physics - Study for nano-thickness surfaces of materials! THz Spectroscopy - Gas Sensing Best Fingerprint Range of Gas Molecules - Study on Solid-state physics Hyperfine structure of semiconductor, superconductor, and so on - Study on Dielectric Constant of Materials Less-known Information on the FIR Characteristics of Materials

40 Transmittance of THz Materials 100 Wavelength : 110 µm 80 Transmittance (%) θ E Plastic Wedge 5mm Plastic Wedge 2mm Crystal Quartz 5mm Crystal Quartz 0.5mm Polarization (degree)

41 Transmittance of THz Materials 100 Wavelength : 110 µm Transmittance (%) ϕ E Plastic Wedge 5t Plastic Wedge 2t Crystal Quartz 5t Crystal Quartz 0.5t S-Polarization Angle (degree)

42 Transmittance of THz Materials 100 Wavelength : 110 µm 80 Transmittance (%) ψ E Plastic Wedge 5t Plastic Wedge 2t Crystal Quartz 5t Crystal Quartz 0.5t P-Polarization Angle (degree)

43 Acknowledgements FEL upgrade Nikolay Gavrilov, Pavel Vobly, Eduard Kuper, Vladimir Ovchar, Yuri Tokarev, Sergey Miginsky, Mikhail Kondaurov, Viatcheslav Pavlov, Boris Gudkov Budker Institute of Nuclear Physics Sung Oh Cho Korea Advanced Institute of Science and Technology THz detector and spectrometer Vitaly V. Kubarev Budker Institute of Nuclear Physics Application on surface electromagnetic wave Guerman N. Zhizhin, Science and Technology Center of Unique Instrumentation of RAS Vitaliy V. Zavyalov, Gennady D. Bogomolov P. L. Kapitza Institute for Physical Problems of RAS Alexey K. Nikitin, People's Friendship University of Russia