Improvement of laser wire monitor at NewSUBARU

The laser wire monitor developed in NewSUBARU can measure the vertical beam size accurately in decay mode. In top-up mode, however, spike noises in the γ measurement are observed in synchronism with beam injection and impair the measurement accuracy. To resolve this problem we have made a masking circuit, which masks the γ signal just after beam injection by stopping the counter for several ms. By setting mask width comparable to damping time of betatron oscillation, the spike noise disappears and the vertical beam size can be accurately measured even in top-up operation. Measured beam size and emittance are in good agreement with calculations. In the laser wire monitor, Bremsstrahlung is the origin of both background noise and spike noise. We found that, in background noise, γ yield by the collision of lost electrons with vacuum chamber is comparable to that by the collision of circulating electrons with residual gas molecules. On the other hand, in spike noise, γ yield is mainly from the former. (author)