Peak Shift of Coherent Edge Radiation Spectrum Depending on Radio Frequency Field Phase of Accelerator

Abstract

Spectra of coherent edge radiation (CER) were observed at the S-band linac facility of Kyoto University Free Electron Laser. A local maximum was observed in the CER spectrum on-crest operation of the radio frequency (RF) field. As the phase of the RF field was shifted from the crest, the frequency of the maximum decreased, and the CER spectrum approached a spectrum of Gaussian-distributed electrons in a bunch. It was found that this strange spectrum can be explained by a model in which a satellite pulse exists around a main pulse in the electron bunch. Furthermore, it demonstrated that CER is an effective tool for monitoring the shape of the electron bunch.

Deformation of an electron bunch caused by free-electron lasers

“…Figures 2(c) and (d) show the frequency dependence of ratios of the CER power P on /P off , where P on is the CER power with the FEL at ΔL = +2 μm and P off is the CER power without the FEL at ΔL = +23 μm. It has been confirmed that the electron distribution in a bunch can be well approximated by a Gaussian when the FEL is not oscillating [23,25]. In the frequency region of 0.3 − 1.0 THz, where the CER power was strong sufficiently, it is noted that the CER power ratios had the following relationship [26]:…”

“…Figures 2(a) and (b) show the measured CER spectra at the detuning length of the optical cavity (ΔL) of +2 μm (where the CER power was minimum) and +23 μm. Because the spectra were measured in the atmosphere, absorption because of water vapor was observed at the frequencies of 0.56, 0.75, 0.99, 1.1-1.3, 1.4, and 1.6 THz [25]. The CER spectra rapidly decreased below 0.3 THz owing to a diffraction loss of the optical system for the CER beam.…”

“…These experimental results indicate that the FEL interaction increased the RMS bunch length without significantly affecting the outline of the electron-bunch shape. Using the CER spectra at frequencies not absorbed by atmospheric water vapor in a region of 0.3-1.5 THz [25], the RMS bunch lengths were 52.6 ± 1.5 and 45.8 ± 1.9 μm at the electron energies of 27.1 and 30.5 MeV, respectively. Thereafter, by oscillating the FEL around the perfect synchronism condition, they increased up to 55.6 ± 2.0 and 50.0 ± 1.9 μm at the electron energies of 27.1 and 30.5 MeV, respectively.…”