On the reliability of measured results by non-destructive beam profile monitor

In KEK-PS, there is a pair of horizontal (H) and vertical (V) non-destructive beam profile monitor (NDPM) in the Booster, and two H- and one V-NDPM in the main ring. All NDPMs can measure the time dependence of beam profile within one acceleration period without beam destruction, and the combination of two H-NDPMs in the main ring can measure the time dependence of momentum spread. By the system with a scraper and orbit bump magnets we crosschecked the beam size. And by measuring the longitudinal bunch length and the RF characteristics, we also crosschecked the momentum spread. The half width by NDPM is apt to be wider than the width by the scraper method to the extent of some mm. The error becomes severe in the measurement of narrow beam width