OTR monitor for ATF linac

Abstract

A bunch by bunch profile monitor system using optical transition radiation (OTR) is developed for an Accelerator Test Facility (ATF) linac. The ATF consists of a 1.5 GeV linac and a damping ring is now under constructing in KEK. The linac accelerates a multi-bunch beam (20 bunches/pulse, 2 × 10 electrons/bunch, 2.8 ns spacing between bunches). The energy spread of the multi-bunch caused by the transient beam loading is a significant problem for the injection of the damping ring. The linac has energy compensation system to compensate the energy spread of the multi-bunch. In order to measure the energy and energy spread of each bunch, we developed the monitor system. The system and the measurement result are reported. I n t r o d u c t i o n The Accelerator Test Facility (ATF) consists of a 1.5 GeV linac and a damping ring(DR) is now under constructing in KEK. The DR is designed to realize a small vertical emittance, eny = ~30 nm, for future Linear Collider. The commissioning of the ATF linac had been started from November 1995 and the commissioning of the DR will be started in the end of this year. The linac accelerates multi-bunch beam. The beam has 20 bunches of 2 × 10 electrons with 2.8 ns spacing. In order to reduce the energy spread of the multi-bunch beam, due to the transient beam loading, Energy Compensation System (ECS) were installed and the preliminary experiment was carried out [1]. The measurement system of the energy and the energy spread of each bunch is needed for tuning the ECS. The optical transition radiation (OTR) monitor was already developed for the 80 MeV injector section [2]. The OTR is emitted when the charged particles go through the interface which have different dielectric constants. The polished stain less steal was employed as the emitter for the OTR monitor. A fast gate camera (Hamamatsu C2925) is used for observed the bunch by bunch profile in the multi-bunch beam when apply the gate signal to each beam timing. This monitor could measure the beam emittance, energy and energy spread of each bunch at the 80 MeV injector section. The OTR monitor at 1.5 GeV section is designed and tested for the above purpose. The spot size limit of the OTR monitor according to ~γλ /2π [3]. At 1.5 GeV section, the spot size limit is 0.24 mm for 500nm wavelength. This value is assumed that is not so affected the beam size measurement. Recently, the spot size limit was discussed and tested [4, 5]. OTR monitor system The monitor setup is shown in Fig. 1. The OTR monitor is located at the downstream of the first bending magnet and the first quadrupole magnet of the beam transport line. The position deviation at the place is calculated by ∆x=η ∆E E , where h is the dispersion function. Fig. 1. Location of the OTR monitor for ATF linac. The actuator system has two screens and can be stopped each screen position at the beam line. One is fluorescent screen, the other is the OTR emitter made by polished stain less steal which has 1 mm thickness and 1/2λ flatness. The emitted light (fluorescent light/OTR) is reflected by a mirror to avoid the x-ray and fed to a gate camera and a CCD camera by a half mirror. The fluorescent light is observed by the CCD camera and the OTR is observed by the gate camera. Both profile by these monitors can compare each other. Trigger control and video analyze system The control system of the gate camera is shown in Fig. 2. The gate camera can observed the bunch by bunch beam profile when apply the appropriate gate width and timing. The timing signal is created from the beam trigger. The signal is delayed in 2.8 ns step and met to each beam timing by the delay module. The delay module makes delay by count the reference clock from start signal. The reference clock is synchronized to the accelerating frequency. The trigger jitter of the delay module is less than 10 ps. The fine delay C1097 (Hamamatsu) adjusts the gate timing to the center of the beam timing. The LINAC 96