Suppression of nonlinear-response in differential-CT system for SACLA

A differential current-transformer (CT) system was developed to measure an electron charge without substantial noise coming from a klystron power source in the x-ray free electron laser, SACLA. At a place downstream of the third bunch-compressor (BC3) in SACLA, nonlinearity was found in a measured electron-charge value with the differential- CT system. The reason for this uncertainty was confirmed to be the nonlinear response of a preamplifier in the CT system due to a short pulse duration of 400 ps. Therefore, a 15 dB attenuator or a 30m-long coaxial cable as a low pass filter was inserted before the preamplifier input. The linearity of the differential-CT system was improved to be about 10% for the attenuator case, which is an allowable error in the operation monitoring of SACLA. The linearity for the coaxial cable case was 4%, which is sufficient accuracy to estimate the accelerated electron-charge. (author)