A 476 MHz resonant stripline beam position monitor(BPM)is planned to be installed in an infrared free electron laser machine at National Synchrotron Radiation Laboratory.This type of BPM was developed based on a standard stripline BPM by moving the coupling feedthrough closer to the short end downstream,which introduces a resonance and therefore a capability for higher resolution compared with broadband BPMs.The design and offline measurement results of the prototype are shown in this paper.The design goal is the optimization of the central frequencies and corresponding quality factors of the three intrinsic transverse electromagnetic modes to roughly476 MHz and 30,respectively,the fulfillment of which is demonstrated by a transmission parameter test via a network analyzer.Induced voltage signal modeling and an estimation of the position resolution of the designed BPM are shown in detail.Furthermore,a calibration test of the prototype using the stretched wire method is presented,including a description of the test stand and the evaluation of position sensitivities.
Xiao-Yu LiuFang-Fang WuTian-Yu ZhouPing LuBao-Gen Sun
For a high-energy electron facility, estimates of induced radioactivity in materials are of considerable importance to ensure that the exposure of personnel and the environment remains as low as reasonably achievable. In addition, accurate predictions of induced radioactivity are essential to the design, operation, and decommissioning of a high-energy electron linear accelerator. In the case of the 200-MeV electron linac of the National Synchrotron Radiation Laboratory(NSRL), the electrons are accelerated by five acceleration tubes and collimated by copper scrapers. The scrapers, which play a vital role in protecting the acceleration cavity, are bombarded by many electrons over a long-term operation, which causes a significant amount of induced radioactivity. Recently, the NSRL Linac is the first high-energy electron linear accelerator in China to be out of commission.Its decommissioning is highly significant for obtaining decommissioning experience. This paper focuses on the measurement of induced radioactivity on the fourth scraper, where the electron energy was 158 MeV. The radionuclides were classified according to their half-lives. Such a classification provides a reliable basis for the formulation of radiation protection and facility decommissioning. To determine the high-radioactivity area and to facilitate the decommissioning process, the slicing method was applied in this study. The specific activity of60 Co in each slice was measured at a cooling time of ten months, and the results were compared with the predictions generated by Monte Carlo program FLUKA. The trend of the measured results is in good agreement with the normalized simulation results. The slicing simulation using Monte Carlo method is useful for the determination of high-radiation areas and proper material handling protocols and, therefore, lays a foundation for the accumulation of decommissioning experience.
Tile photoelectron spectroscopy beamline at National Synchrotron Radiation Laboratory （NSRL） is equipped with a spherical grating monochromator with the included angle of 174. Three gratings with line density of 200, 700 and 1200 lines/mm are used to cover the energy region from 60 eV to 1000 eV. After several years＇ operation, the spectral resolution and flux throughput were deteriorated, and realignment was necessary to improve the performance. First, the wavelength scanning mechanism, the optical components position and the exit slit guide direction are aligned according to tile design value. Second, the gratings are checked by Atomic Force Microscopy （AFM） and then the gas absorption spectrum is measured to optimize the focusing condition of the monoehromator. The spectral resolving power E/AE is recovered to the designed value of 1000@244 eV. The flux at the end station for the 200 lines/ram grating is about 1010 photons/sec/200 mA, which is in accordance with the design. The photon flux for the 700 lines/mm grating is about 5 108 photons/sec/200mA, which is lower than expected. This poor flux throughput may be caused by carbon contamination on the optical components. The 1200 lines/ram grating has roughness much higher than expected so the diffraction efficiency is too low to detect any signal. A new grating would be ordered. After the alignment, the beamline has significant performance improvements in both the resolving power aim the flux throughput for 200 and 700 lines/ram gratings and is provided to users.
