Performance study of Fast Beam Current Transformer calibration in the LHC

Fast Beam Current Transformers (FBCTs) are essential devices for measuring bunched beam intensities in the Large Hadron Collider (LHC). A cross-calibration of these devices against DC Beam Current Transformers (DC-BCTs) is used in the LHC ring, but this method relies on assumptions that introduce inaccuracies. FBCTs are also used in the LHC transfer lines, where DC-BCTs are unavailable, making absolute calibration crucial for transmission studies. To allow absolute calibration, the FBCTs are equipped with a pulsed current source to perform a calibration before each fill. However, a systematic underestimation in the calculated scaling factors has been observed when comparing them to the cross-calibration with the DC-BCTs. This work characterizes the calibrator performance, evaluates different algorithms for calculating the scaling factors and reports on issues found linked to processing chain modeling inaccuracies, ghost/satellite bunches, and the FPGA-based digital integration. A revised model of the processing chain improved calibration accuracy by 0.4%, while an initial test of a batch-by-batch baseline in the digital integration seems to explain the remaining 3% underestimation.