Measurement of bunch length evolution in electron beam macropulse of S-band linac using coherent edge radiation

Observation of terahertz coherent edge radiation amplified by infrared free-electron laser oscillations

“…1b. Figure 2 shows the spatial distributions of CER beams calculated using the CER theory for electron energies of (a) 58 MeV and (b) 75 MeV on a concave mirror 9,14 . In these calculations, the root-mean-squared (RMS) bunch length and charge of the electron bunch were assumed to be 0.060 mm and 27.6 pC, respectively.…”

“…This decrease was caused by the diffraction loss of the transport optical system of the CER beam. Although all optical elements in the transport optical system caused the diffraction loss, it is typically sufficient to consider a diffraction loss where the ratio of the diameter of the optical element to the CER beam size is the smallest 14 . The observed spectrum O(ω) expressed as the following approximate expression using the CER spectrum U(ω):…”

“…The dotted curves shown in Fig. 4 were obtained by applying the least-squares approximation to the spectrum measured using the hollow concave mirror at the frequencies of 0.37, 0.47, 0.64, 0.84, 0.94, 1.04, 1.28, and 1.48 THz 14 . The measured data were not affected by the water vapor absorption at these frequencies.…”

“…Although the THz detectors used in this study were suitable for observing the macro-temporal properties of the CER, they could not measure the micropulse of the CER because of the long time constant. To observe the longitudinal electron distribution for each micropulse, the CER spectra should be measured using multiple narrow-band diode detectors with high temporal resolutions, as demonstrated by the corresponding author at the Kyoto University Free Electron Laser 14 . We plan to measure the evolution of the CER spectra in a macropulse with narrow-band diode detectors while monitoring the FEL oscillation.…”

“…When the spectra of the CER beam were measured, the flat mirror that transported the CER beam to the THz-wave power meter was replaced with a beam splitter, which was a nitrocellulose pellicle of a thickness 2 μm and diameter 108 mm. This pellicle was coated with a nickel-based alloy, and the transmittance and reflectance in the THz region were approximately 60% and 10%, respectively 14 . The CER beam transmitted through the pellicle was injected into a Michelson interferometer with another pellicle as a beam splitter.…”