研究目的
To investigate the evolution of a bunch length in a macropulse of an electron beam using coherent edge radiation (CER) and develop a technique for this purpose.
研究成果
The developed technique using CER intensity ratios at multiple frequencies effectively measured the evolution of RMS bunch length, which correlated with the FEL power macropulse structure. This method is useful for controlling FEL oscillations without complex equipment, though improvements are needed for higher electron energies and FEL oscillation conditions.
研究不足
The electron energy was low (27.5 MeV), leading to a large divergence angle of the CER beam and insufficient extraction efficiency by the de?ection mirror. The technique cannot measure the longitudinal distribution in the electron bunch like an RF de?ector. The CER intensity was weak, and measurements were affected by diffraction losses and water vapor absorption in air.
1：Experimental Design and Method Selection:
The study used coherent edge radiation (CER) generated at the downstream bending magnet of the FEL straight line to measure the bunch length evolution. A technique involving CER spectra and intensity ratios at two frequencies was developed.
2：Sample Selection and Data Sources:
The electron beam from the Kyoto University Free Electron Laser (KU-FEL) facility was used, with specific RF phase differences between the RF gun and accelerator tube.
3：List of Experimental Equipment and Materials:
Equipment included a de?ection mirror, vacuum window (fused quartz or crystal quartz), THz camera (Pyrocam IV), polytetra?uoroethylene lens, Michelson-type interferometer with nitrocellulose pellicle beam splitter, pyroelectric energy meter (THz10), diode detectors (DXP-06 and Ultrafast-150), bandpass ?lters, and parabolic mirrors.
4：Experimental Procedures and Operational Workflow:
The CER beam was extracted from the optical cavity using a de?ection mirror, transported through optical elements, and measured for spatial distribution, spectra using an interferometer, and temporal intensity evolution using diode detectors. Data were analyzed to determine RMS bunch length.
5：Data Analysis Methods:
Spectra were analyzed using least squares approximation to extract bunch length, and temporal data were processed using linear regression based on intensity ratios.
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THz camera
Pyrocam IV
Ophir Optronics Solutions Ltd.
Measured the spatial distribution of the CER beam.
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Pyroelectric energy meter
THz10
Sensor- und Lasertechnik Inc.
Measured the autocorrelation signal from the interferometer.
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Diode detector
DXP-06
Millitech Inc.
Measured CER intensity at 0.10 THz with high temporal resolution.
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Diode detector
Ultrafast-150
TeraSEnse Group Inc.
Measured CER intensity at 0.35 THz with high temporal resolution.
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Bandpass filter
Filtered specific frequency bands for diode detectors.
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Polytetra?uoroethylene lens
Converged the CER beam for measurement with the THz camera.
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Nitrocellulose pellicle
Used as a beam splitter in the Michelson-type interferometer.
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Parabolic mirror
Concentrated the autocorrelation signal and CER beam for detectors.
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Concave mirror
Transported the CER beam to the interferometer.
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De?ection mirror
Extracted the CER beam from the optical cavity into air.
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Vacuum window
Allowed extraction of the CER beam into air.
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Black polyethylene film
Suppressed unwanted infrared radiation.
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