Beam profile monitor for intense, negative, hydrogen-ion beams in the J-PARC linac

Tracking the beam halo and its growth are essential to mitigate the beam dynamics problem in high-current accelerators. In the J-PARC linac, a beam halo has been reported to appear at the place of a mismatched magnet and/or an inaccurately set cavity. The beam loss becomes more significant as the beam’s output power increases. We developed a sensitive wire scanner monitor (WSM) to measure the transverse root-mean-squared (RMS) size of a negative hydrogen-ion beam for the transverse matching between quadrupole magnets. Although the dynamic range of 10 +2 is sufficient to evaluate the RMS beam size, we set a dynamic range goal of 10 +4 because we intend to observe the beam halo at the same time. We achieved a dynamic range over 10 +4 by selecting a proper combination of wire material and diameter by considering the interactions between the beam and the wires. This enabled a measurement of the beam halo evolution in the linac. We discuss the mechanism that enabled the wider dynamic range compared with proton-beam WSMs. In addition, we propose a new chopper-phase-tuning method, which is realized because of the wide dynamic range of the WSM. This method may provide an accurate tuning scheme for not only the present operation but also the high-peak beam-current operation of the linac.