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

Abstract

Intense coherent edge radiation (CER) with a power of 0.10 mW was observed in a straight line with an undulator of Kyoto University Free Electron Laser (KU-FEL). To investigate the evolution of a bunch length in a macropulse of an electron beam, a technique was developed using the CER beam. The measured air-extracted CER beam profile has a hollow structure resembling the first-order Laguerre–Gaussian mode with asymmetric intensity in horizontal direction. It roughly agrees with the profile calculated considering the effective area of a deflection mirror employed to extract the beam from the FEL optical cavity into air. The root-mean-squared (RMS) bunch length determined from a measured spectrum of the CER beam is 60 μm for an electron-beam macropulse. Changes in the CER intensity in the electron-beam macropulse were measured by diode detectors with short time constants at two frequencies. The evolution of the RMS bunch length was determined using the ratio of the CER intensity at the two frequencies. It could be concluded that the evolution correlated with the macropulse structure of the FEL power. Therefore, measuring the evolution of the CER intensity at multiple frequencies proved useful to control FEL macropulses.