Estimation for the intensity of beam loss by the optical fiber based Cherenkov beam loss monitor at SACLA

Abstract

From the issues of radiation safety and radiation damage of accelerator equipments, it is useful to detect the beam loss immediately at high-energy and high intensity particle accelerator facilities. In the undulator section of Spring-8 Angstrom Compact free electron LAser (SACLA) [1], beam loss monitors have been installed in order to control beam losses and to prevent demagnetization of permanent magnets due to electron hitting [2]. Since the commissioning phase of SACLA, this monitor has been successfully working. This beam loss monitor consists of a quartz fiber with 400 μm core diameter and a photomultiplier tube (PMT) which is connected with the upstream end of the fiber. The Cherenkov radiation which is produced in the fiber due to the passing of secondary electrons and positrons (Cherenkov emitters) is detected by the photomultiplier tube. With the detection time of the monitor signals, the position of the beam loss point can be measured. In addition, the use of two monitors with different gains allows measuring intensity of the beam loss over the wide dynamic range. While it is expected that the waveform of the monitor signal corresponds to the spatial distribution of the electron flux or absorbed dose due to the beam loss, if this monitor is used for the beam loss control, it is necessary to calibrate the magnitude of the monitor signal to a value such as the number of electrons or the absorbed dose in order to set an appropriate threshold level. In this study, the electron beam loss was measured by the beam loss monitor and thermo luminescent dosimeters (TLD), and the correlation among the signal of the beam loss monitor, the absorbed dose and electron flux was investigated.