Characterization, signal estimation and analyzing of cold button BPMs for a low-[formula omitted] helium/proton superconducting LINAC

Abstract

CAFe is a low-β helium/proton superconducting LINAC. It is as a demo LINAC for China initiative Accelerator-Driven System and constructed at the Institute of Modern Physics, Chinese Academy of Sciences. In this paper, an off-site characterization focusing on the capacitance measurements of cold button BPMs is presented firstly. Furthermore, a signal estimation and analysis of cold button BPMs is performed with regard to two kinds of beam commissioning, helium and proton beams. A good agreement between the calculation and the measurement proves that the developed theoretical model could accurately estimate the output signal of cold button BPMs. It considers the influence of low-β effect and long cable issues. Based on the transformation of the estimated signal from the time domain to the frequency domain, it is found that the amplitude spectra of cold button BPMs expand with the energy increasing and the bunch length shortening. The amplitude at the first harmonic frequency decreases, which causes the summed values from BPM electronics to be declined in the LINAC. However, the decline in summed values is not proportional to the decreasing of beam current. This is the reason why BPMs only give relative intensity and not absolute value for low-β beams when BPMs’ electronics using a narrow-band digital signal processing. These developments will be used for the cold BPM system and the fast machine protection system. It is dedicated to the driven LINAC of China initiative Accelerator-Driven System, which will deliver a 500 MeV, 5 mA proton beam in continuous wave operation mode.