Signal transmission analysis and noise reduction technique for current monitor in linear accelerators

A signal transmission and its characteristics of a beam-current monitor (CM) have been investigated for long-pulsed electron beams with a pulse width of ∼1 µs. The monitor is based on a conventional current transformer, which comprises an induction coil wound around a ferrite core. Three CMs of this type are used in the compact electron linac of the Slow Positron Facility (SPF) of the Institute of Materials and Structure Science (IMSS) at KEK. It was found that only the signal measured by one of these CMs was very noisy, and however, the signals by the other CMs seemed to be normal. This result gave me strong motivation to investigate this work, because it seems that there are no differences among these CMs. After experimental tests and simulations based on balanced and unbalanced circuit theory, it was found that it was important where the grounded points should be located in the signal transmission line of CM. The generation and transmission mechanism of the noise signal from CM and its suppression technique are discussed from a physical point of view based on a signal transmission model. (author)