Beam position monitor system of J-PARC RCS

The J-PARC RCS is a 25Hz Rapid-Cycling proton Synchrotron and its designed beam power is 1MW. The beam position monitor (BPM) system at J-PARC RCS is described in this paper. The pre-defined diameter of the BPM detectors is larger than 250mm, however, the system has to measure the beam position very accurately. In addition, it is necessary to have a large dynamic range. The system should work not only for the high intensity but also for low intensity, such as during beam commissioning, when the intensity is below 1% of the design intensity. There are 54 BPM detectors around the ring and most of them are placed inside steering magnets because of quite limited space. The BPM detector is an electro-static type and it has four electrodes, and a pair of electrodes gives a good linear response with a diagonal cut shape to detect the charge center precisely. The signal processing units, which are equipped with 14-bit 40MS/s ADC and 600MHz DSP, have been developed. They are accessed via shared memory space and controlled by EPICS. Such a processing unit is capable of recording the full 25Hz pulse data for the so-called “COD mode” (averaged beam position calculation) and it can also store the whole waveform data for further analysis, like turn-by-turn position calculation. The resolution was estimated to be 20μm for “COD mode” and to be 0.3mm for the turn-by-turn mode with relatively low intensity of 8×1011ppp. The position accuracy is estimated to be about 0.5mm using a newly developed Beam Based Alignment (BBA) method.